Patent Publication Number: US-2023156183-A1

Title: Method for reference picture processing in video coding

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This disclosure claims the benefits of priority to U.S. application Ser. No. 17/327,572, filed on May 21, 2021, which claims the benefits of priority to U.S. Provisional Application No. 63/028,509, filed on May 21, 2020, both of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to video processing, and more particularly, to methods, apparatus and a non-transitory computer-readable storage medium for processing reference pictures. 
     BACKGROUND 
     A video is a set of static pictures (or “frames”) capturing the visual information. To reduce the storage memory and the transmission bandwidth, a video can be compressed before storage or transmission and decompressed before display. The compression process is usually referred to as encoding and the decompression process is usually referred to as decoding. There are various video coding formats which use standardized video coding technologies, most commonly based on prediction, transform, quantization, entropy coding and in-loop filtering. The video coding standards, such as the High Efficiency Video Coding (HEVC/H.265) standard, the Versatile Video Coding (VVC/H.266) standard, and AVS standards, specifying the specific video coding formats, are developed by standardization organizations. With more and more advanced video coding technologies being adopted in the video standards, the coding efficiency of the new video coding standards get higher and higher. 
     SUMMARY OF THE DISCLOSURE 
     Embodiments of the present disclosure provide a method for video processing. In some embodiments, the method includes: deriving a total number by summing a number of reference picture list structures in sequence parameter set (SPS) and one; allocating memory for the total number of reference picture list structures in response to a reference picture list structure being signaled in a picture header of a current picture or a slice header of a current slice; and processing a current picture or a current slice using the allocated memory. 
     In some embodiments, the method includes: signaling a first flag in a picture parameter set (PPS) to indicate whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture referring to the PPS; wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1; determining whether the first index and a second index to be signaled, wherein the second index is an index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0; in response to the second index not to be signaled, determining a value of the second index comprising: when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0; in response to the first index not to be signaled, determining a value of the first index comprising: when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; deriving the reference picture list based on the first index and the second index; and encoding the current picture based on the reference picture list. 
     In some embodiments, the method includes: receiving a video bitstream; determining a value of a first flag indicating whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture, wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1; determining whether the first index and a second index being present, wherein the second index is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0; in response to the second index being not present, determining a value of the second index comprising: when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0; in response to the first index being not present, determining a value of the first index comprising: when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; and decoding a current picture based on the first index and the second index. 
     In some embodiments, the method includes: signaling a first flag in a slice header to indicate whether an active reference index number is present in a slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that is used to encode a current slice; in response to the first flag indicating the active reference index number is present in the slice header, determining a number of entries of reference picture list 0, and signaling the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and determining a number of entries of reference picture list 1, and signaling the active reference index number of reference picture list 1 in the slice header for B slice when the number of entries of reference picture list 1 is greater than 1. 
     In some embodiments, the method includes: receiving a video bitstream including a slice header and a picture header syntax; determining a value of a first flag signaled in the slice header that indicates whether an active reference index number is present in the slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that is used to decode a current slice; in response to the first flag indicating the active reference index number is present, determining a number of entries of reference picture list 0, and decoding the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and determining a number of entries of reference picture list 1, and decoding the active reference index number of reference picture list 1 in slice header for B slice when a number of entries of reference picture list 1 is greater than 1. 
     In some embodiments, the method includes: determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all non-I slices of a current picture; and processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction. 
     Embodiments of the present disclosure provide an apparatus for performing video processing. In some embodiments, the apparatus comprising: a memory configured to store instructions; and one or more processors configured to execute the instructions to cause the apparatus to perform: deriving a total number by summing a number of reference picture list structures in sequence parameter set (SPS) and one; allocating memory for the total number of reference picture list structures in response to a reference picture list structure being signaled in a picture header of a current picture or a slice header of a current slice; and processing a current picture or a current slice using the allocated memory. 
     In some embodiments, the apparatus comprising: a memory configured to store instructions; and one or more processors configured to execute the instructions to cause the apparatus to perform: signaling a first flag in a picture parameter set (PPS) to indicate whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture referring to the PPS; wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1; determining whether the first index and a second index to be signaled, wherein the second index is an index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0; in response to the second index not to be signaled, determining a value of the second index comprising: when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0; in response to the first index not to be signaled, determining a value of the first index comprising: when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; deriving the reference picture list based on the first index and the second index; and encoding the current picture based on the reference picture list. 
     In some embodiments, the apparatus comprising: a memory configured to store instructions; and one or more processors configured to execute the instructions to cause the apparatus to perform: receiving a video bitstream; determining a value of a first flag indicating whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture, wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1; determining whether the first index and a second index being present, wherein the second index is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0; in response to the second index being not present, determining a value of the second index comprising: when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0; in response to the first index being not present, determining a value of the first index comprising: when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; and decoding a current picture based on the first index and the second index. 
     In some embodiments, the apparatus comprising: a memory configured to store instructions; and one or more processors configured to execute the instructions to cause the apparatus to perform: signaling a first flag in a slice header to indicate whether an active reference index number is present in a slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that is used to encode a current slice; in response to the first flag indicating the active reference index number is present in the slice header, determining a number of entries of reference picture list 0, and signaling the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and determining a number of entries of reference picture list 1, and signaling the active reference index number of reference picture list 1 in the slice header for B slice when the number of entries of reference picture list 1 is greater than 1. 
     In some embodiments, the apparatus comprising: a memory configured to store instructions; and one or more processors configured to execute the instructions to cause the apparatus to perform: receiving a video bitstream including a slice header and a picture header syntax; determining a value of a first flag signaled in the slice header that indicates whether an active reference index number is present in the slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that is used to decode a current slice; in response to the first flag indicating the active reference index number is present, determining a number of entries of reference picture list 0, and decoding the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and determining a number of entries of reference picture list 1, and decoding the active reference index number of reference picture list 1 in slice header for B slice when a number of entries of reference picture list 1 is greater than 1. 
     In some embodiments, the apparatus comprising: a memory configured to store instructions; and one or more processors configured to execute the instructions to cause the apparatus to perform: determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all non-I slices of a current picture; and processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction. 
     Embodiments of the present disclosure provide a non-transitory computer-readable storage medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video processing. In some embodiments, the method includes: deriving a total number by summing a number of reference picture list structures in sequence parameter set (SPS) and one; allocating memory for the total number of reference picture list structures in response to a reference picture list structure being signaled in a picture header of a current picture or a slice header of a current slice; and processing a current picture or a current slice using the allocated memory. 
     In some embodiments, the method includes: signaling a first flag in a picture parameter set (PPS) to indicate whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture referring to the PPS; wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1; determining whether the first index and a second index to be signaled, wherein the second index is an index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0; in response to the second index not to be signaled, determining a value of the second index comprising: when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0; in response to the first index not to be signaled, determining a value of the first index comprising: when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; deriving the reference picture list based on the first index and the second index; and encoding the current picture based on the reference picture list. 
     In some embodiments, the method includes: receiving a video bitstream; determining a value of a first flag indicating whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture, wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1; determining whether the first index and a second index being present, wherein the second index is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0; in response to the second index being not present, determining a value of the second index comprising: when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0; in response to the first index being not present, determining a value of the first index comprising: when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; and decoding a current picture based on the first index and the second index. 
     In some embodiments, the method includes: signaling a first flag in a slice header to indicate whether an active reference index number is present in a slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that is used to encode a current slice; in response to the first flag indicating the active reference index number is present in the slice header, determining a number of entries of reference picture list 0, and signaling the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and determining a number of entries of reference picture list 1, and signaling the active reference index number of reference picture list 1 in the slice header for B slice when the number of entries of reference picture list 1 is greater than 1. 
     In some embodiments, the method includes: receiving a video bitstream including a slice header and a picture header syntax; determining a value of a first flag signaled in the slice header that indicates whether an active reference index number is present in the slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that is used to decode a current slice; in response to the first flag indicating the active reference index number is present, determining a number of entries of reference picture list 0, and decoding the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and determining a number of entries of reference picture list 1, and decoding the active reference index number of reference picture list 1 in slice header for B slice when a number of entries of reference picture list 1 is greater than 1. 
     In some embodiments, the method includes: determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all non-I slices of a current picture; and processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments and various aspects of the present disclosure are illustrated in the following detailed description and the accompanying figures. Various features shown in the figures are not drawn to scale. 
         FIG.  1    is a schematic diagram illustrating structures of an exemplary video sequence, according to some embodiments of the present disclosure. 
         FIG.  2 A  is a schematic diagram illustrating an exemplary encoding process of a hybrid video coding system, consistent with embodiments of the disclosure. 
         FIG.  2 B  is a schematic diagram illustrating another exemplary encoding process of a hybrid video coding system, consistent with embodiments of the disclosure. 
         FIG.  3 A  is a schematic diagram illustrating an exemplary decoding process of a hybrid video coding system, consistent with embodiments of the disclosure. 
         FIG.  3 B  is a schematic diagram illustrating another exemplary decoding process of a hybrid video coding system, consistent with embodiments of the disclosure. 
         FIG.  4    is a block diagram of an exemplary apparatus for encoding or decoding a video, according to some embodiments of the present disclosure. 
         FIG.  5 A  shows an exemplary syntax including syntax structure for reference picture lists, according to some embodiments of the present disclosure. 
         FIG.  5 B  shows an exemplary pseudocode including derivation of variable FullPocLt[i][j], according to some embodiments of the present disclosure. 
         FIG.  6 A  shows an exemplary syntax including syntax structure for reference picture list, according to some embodiments of the present disclosure. 
         FIG.  6 B  shows an exemplary pseudocode including derivation for variable NumLtrpEntries[listIdx][rplsIdx], according to some embodiments of the present disclosure. 
         FIG.  6 C  shows an example pseudocode including derivation for variable AbsDeltaPocSt[listIdx][rplsIdx][i], according to some embodiments of the present disclosure. 
         FIG.  6 D  shows an example pseudocode including derivation for variable DeltaPocValSt[listIdx][rplsIdx], according to some embodiments of the present disclosure. 
         FIG.  7    shows an exemplary syntax including syntax structure for reference picture list in sequence parameter set, according to some embodiments of the present disclosure. 
         FIG.  8    shows an exemplary syntax including syntax structure for reference picture list in picture parameter set, according to some embodiments of the present disclosure. 
         FIG.  9 A  shows an exemplary syntax including syntax structure for reference picture list in picture header structure, according to some embodiments of the present disclosure. 
         FIG.  9 B  shows an example pseudocode including derivation for variable MaxNumSubblockMergeCand, according to some embodiments of the present disclosure. 
         FIG.  10 A  shows an exemplary syntax including syntax structure for reference picture list in slice header, according to some embodiments of the present disclosure. 
         FIG.  10 B  shows an exemplary pseudocode including derivation for variable NumRefIdxActive[i], according to some embodiments of the present disclosure. 
         FIG.  11 A  shows a flow-chart of an exemplary video encoding method for signaling flags in PH syntax structure, according to some embodiments of the present disclosure. 
         FIG.  11 B  shows a flow-chart of an exemplary video decoding method for signaling flags in PH syntax structure, according to some embodiments of the present disclosure. 
         FIG.  11 C  shows an exemplary syntax including updated signaling of ph_collocated_from_l0_flag and ph_mvd_l1_zero_flag, according to some embodiments of the present disclosure. 
         FIG.  12 A  shows a flow-chart of an exemplary video encoding method for indicating a collocated picture using picture order count, according to some embodiments of the present disclosure. 
         FIG.  12 B  shows a flow-chart of an exemplary video encoding method for indicating a collocated picture using picture order count, according to some embodiments of the present disclosure. 
         FIG.  12 C  shows another flow-chart of an exemplary video encoding method for indicating a collocated picture, according to some embodiments of present disclosure. 
         FIG.  12 D  shows a flow-chart of an exemplary video decoding method for indicating a collocated picture using picture order count, according to some embodiments of the present disclosure. 
         FIG.  12 E  shows a flow-chart of an exemplary video decoding method for indicating a collocated picture using picture order count, according to some embodiments of the present disclosure. 
         FIG.  12 F  shows an exemplary syntax including updated reference picture list in picture parameter set, according to some embodiments of the present disclosure. 
         FIG.  12 G  shows an exemplary syntax including updated slice header, according to some embodiments of the present disclosure. 
         FIG.  12 H  shows an exemplary pseudocode including derivation of AbsDeltaPocStCol, according to some embodiments of the present disclosure. 
         FIG.  12 I  shows an exemplary pseudocode including derivation of DeltaPocValStCol, according to some embodiments of the present disclosure. 
         FIG.  12 J  shows an exemplary pseudocode for deriving the collocated picture used in a decoding method, according to some embodiments of the present disclosure. 
         FIG.  13 A  shows a flow-chart of an exemplary video encoding method for inferring the index of collocated picture in SH using the number of active entries in reference picture list, according to some embodiments of the present disclosure. 
         FIG.  13 B  shows a flow-chart of an exemplary video decoding method for inferring the index of collocated picture in SH using the number of active entries in reference picture list, according to some embodiments of the present disclosure. 
         FIG.  13 C  shows an exemplary semantics for updated syntax element sh_collocated_ref_idx, according to some embodiments of the present disclosure. 
         FIG.  14 A  shows a flow-chart of an exemplary video processing method for a decoder allocating memory, according to some embodiments of the present disclosure. 
         FIG.  14 B  shows an exemplary semantics for allocating memory, according to some embodiments of the present disclosure. 
         FIG.  15 A  shows a flow-chart of an exemplary video encoding method for inferring the index in the reference picture list, according to some embodiments of the present disclosure. 
         FIG.  15 B  shows a flow-chart of an exemplary video decoding method for inferring the index in the reference picture list, according to some embodiments of the present disclosure. 
         FIG.  15 C  shows an exemplary semantics for updated variable rpl_idx[i], according to some embodiments of the present disclosure. 
         FIG.  16 A  shows a flow-chart of an exemplary video encoding method for indicating whether an override number of active reference index in slice header present, according to some embodiments of the present disclosure. 
         FIG.  16 B  shows a flow-chart of an exemplary video decoding method for indicating whether an override number of active reference index in slice header present, according to some embodiments of the present disclosure. 
         FIG.  16 C  shows an exemplary semantics for updated syntax element sh_num_ref_idx_active_override_flag, according to some embodiments of the present disclosure. 
         FIG.  17 A  shows a flow-chart of an exemplary video processing method for defining an index of collocated picture in SH for I slices, according to some embodiments of the present disclosure. 
         FIG.  17 B  shows an exemplary semantics for updated bitstream conformance constraint for syntax element sh_collocated_ref_idx, according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations consistent with the invention. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the invention as recited in the appended claims. Particular aspects of the present disclosure are described in greater detail below. The terms and definitions provided herein control, if in conflict with terms and/or definitions incorporated by reference. 
     The Joint Video Experts Team (JVET) of the ITU-T Video Coding Expert Group (ITU-T VCEG) and the ISO/IEC Moving Picture Expert Group (ISO/IEC MPEG) is currently developing the Versatile Video Coding (VVC/H.266) standard. The VVC standard is aimed at doubling the compression efficiency of its predecessor, the High Efficiency Video Coding (HEVC/H.265) standard. In other words, VVC&#39;s goal is to achieve the same subjective quality as HEVC/H.265 using half the bandwidth. 
     To achieve the same subjective quality as HEVC/H.265 using half the bandwidth, the JVET has been developing technologies beyond HEVC using the joint exploration model (JEM) reference software. As coding technologies were incorporated into the JEM, the JEM achieved substantially higher coding performance than HEVC. 
     The VVC standard has been developed recently, and continues to include more coding technologies that provide better compression performance. VVC is based on the same hybrid video coding system that has been used in modern video compression standards such as HEVC, H.264/AVC, MPEG2, H.263, etc. 
     A video is a set of static pictures (or “frames”) arranged in a temporal sequence to store visual information. A video capture device (e.g., a camera) can be used to capture and store those pictures in a temporal sequence, and a video playback device (e.g., a television, a computer, a smartphone, a tablet computer, a video player, or any end-user terminal with a function of display) can be used to display such pictures in the temporal sequence. Also, in some applications, a video capturing device can transmit the captured video to the video playback device (e.g., a computer with a monitor) in real-time, such as for surveillance, conferencing, or live broadcasting. 
     For reducing the storage space and the transmission bandwidth needed by such applications, the video can be compressed before storage and transmission and decompressed before the display. The compression and decompression can be implemented by software executed by a processor (e.g., a processor of a generic computer) or specialized hardware. The module for compression is generally referred to as an “encoder,” and the module for decompression is generally referred to as a “decoder.” The encoder and decoder can be collectively referred to as a “codec.” The encoder and decoder can be implemented as any of a variety of suitable hardware, software, or a combination thereof. For example, the hardware implementation of the encoder and decoder can include circuitry, such as one or more microprocessors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), discrete logic, or any combinations thereof. The software implementation of the encoder and decoder can include program codes, computer-executable instructions, firmware, or any suitable computer-implemented algorithm or process fixed in a computer-readable medium. Video compression and decompression can be implemented by various algorithms or standards, such as MPEG-1, MPEG-2, MPEG-4, H.26x series, or the like. In some applications, the codec can decompress the video from a first coding standard and re-compress the decompressed video using a second coding standard, in which case the codec can be referred to as a “transcoder.” 
     The video encoding process can identify and keep useful information that can be used to reconstruct a picture and disregard unimportant information for the reconstruction. If the disregarded, unimportant information cannot be fully reconstructed, such an encoding process can be referred to as “lossy.” Otherwise, it can be referred to as “lossless.” Most encoding processes are lossy, which is a tradeoff to reduce the needed storage space and the transmission bandwidth. 
     The useful information of a picture being encoded (referred to as a “current picture”) include changes with respect to a reference picture (e.g., a picture previously encoded and reconstructed). Such changes can include position changes, luminosity changes, or color changes of the pixels, among which the position changes are mostly concerned. Position changes of a group of pixels that represent an object can reflect the motion of the object between the reference picture and the current picture. 
     A picture coded without referencing another picture (i.e., it is its own reference picture) is referred to as an “I-picture.” A picture is referred to as a “P-picture” if some or all blocks (e.g., blocks that generally refer to portions of the video picture) in the picture are predicted using intra prediction or inter prediction with one reference picture (e.g., uni-prediction). A picture is referred to as a “B-picture” if at least one block in it is predicted with two reference pictures (e.g., bi-prediction). 
       FIG.  1    illustrates structures of an exemplary video sequence  100 , according to some embodiments of the present disclosure. Video sequence  100  can be a live video or a video having been captured and archived. Video sequence  100  can be a real-life video, a computer-generated video (e.g., computer game video), or a combination thereof (e.g., a real-life video with augmented-reality effects). Video sequence  100  can be inputted from a video capture device (e.g., a camera), a video archive (e.g., a video file stored in a storage device) containing previously captured video, or a video feed interface (e.g., a video broadcast transceiver) to receive video from a video content provider. 
     As shown in  FIG.  1   , video sequence  100  can include a series of pictures arranged temporally along a timeline, including pictures  102 ,  104 ,  106 , and  108 . Pictures  102 - 106  are continuous, and there are more pictures between pictures  106  and  108 . In  FIG.  1   , picture  102  is an I-picture, the reference picture of which is picture  102  itself. Picture  104  is a P-picture, the reference picture of which is picture  102 , as indicated by the arrow. Picture  106  is a B-picture, the reference pictures of which are pictures  104  and  108 , as indicated by the arrows. In some embodiments, the reference picture of a picture (e.g., picture  104 ) can be not immediately preceding or following the picture. For example, the reference picture of picture  104  can be a picture preceding picture  102 . It should be noted that the reference pictures of pictures  102 - 106  are only examples, and the present disclosure does not limit embodiments of the reference pictures as the examples shown in  FIG.  1   . 
     Typically, video codecs do not encode or decode an entire picture at one time due to the computing complexity of such tasks. Rather, they can split the picture into basic segments, and encode or decode the picture segment by segment. Such basic segments are referred to as basic processing units (“BPUs”) in the present disclosure. For example, structure  110  in  FIG.  1    shows an example structure of a picture of video sequence  100  (e.g., any of pictures  102 - 108 ). In structure  110 , a picture is divided into 4×4 basic processing units, the boundaries of which are shown as dash lines. In some embodiments, the basic processing units can be referred to as “macroblocks” in some video coding standards (e.g., MPEG family, H.261, H.263, or H.264/AVC), or as “coding tree units” (“CTUs”) in some other video coding standards (e.g., H.265/HEVC or H.266/VVC). The basic processing units can have variable sizes in a picture, such as 128×128, 64×64, 32×32, 16×16, 4×8, 16×32, or any arbitrary shape and size of pixels. The sizes and shapes of the basic processing units can be selected for a picture based on the balance of coding efficiency and levels of details to be kept in the basic processing unit. 
     The basic processing units can be logical units, which can include a group of different types of video data stored in a computer memory (e.g., in a video frame buffer). For example, a basic processing unit of a color picture can include a luma component (Y) representing achromatic brightness information, one or more chroma components (e.g., Cb and Cr) representing color information, and associated syntax elements, in which the luma and chroma components can have the same size of the basic processing unit. The luma and chroma components can be referred to as “coding tree blocks” (“CTBs”) in some video coding standards (e.g., H.265/HEVC or H.266/VVC). Any operation performed to a basic processing unit can be repeatedly performed to each of its luma and chroma components. 
     Video coding has multiple stages of operations, examples of which are shown in  FIGS.  2 A- 2 B  and  FIGS.  3 A- 3 B . For each stage, the size of the basic processing units can still be too large for processing, and thus can be further divided into segments referred to as “basic processing sub-units” in the present disclosure. In some embodiments, the basic processing sub-units can be referred to as “blocks” in some video coding standards (e.g., MPEG family, H.261, H.263, or H.264/AVC), or as “coding units” (“CUs”) in some other video coding standards (e.g., H.265/HEVC or H.266/VVC). A basic processing sub-unit can have the same or smaller size than the basic processing unit. Similar to the basic processing units, basic processing sub-units are also logical units, which can include a group of different types of video data (e.g., Y, Cb, Cr, and associated syntax elements) stored in a computer memory (e.g., in a video frame buffer). Any operation performed to a basic processing sub-unit can be repeatedly performed to each of its luma and chroma components. It should be noted that such division can be performed to further levels depending on processing needs. It should also be noted that different stages can divide the basic processing units using different schemes. 
     For example, at a mode decision stage (an example of which is shown in  FIG.  2 B ), the encoder can decide what prediction mode (e.g., intra-picture prediction or inter-picture prediction) to use for a basic processing unit, which can be too large to make such a decision. The encoder can split the basic processing unit into multiple basic processing sub-units (e.g., CUs as in H.265/HEVC or H.266/VVC), and decide a prediction type for each individual basic processing sub-unit. 
     For another example, at a prediction stage (an example of which is shown in  FIGS.  2 A- 2 B ), the encoder can perform prediction operation at the level of basic processing sub-units (e.g., CUs). However, in some cases, a basic processing sub-unit can still be too large to process. The encoder can further split the basic processing sub-unit into smaller segments (e.g., referred to as “prediction blocks” or “PBs” in H.265/HEVC or H.266/VVC), at the level of which the prediction operation can be performed. 
     For another example, at a transform stage (an example of which is shown in  FIG.  2 A  and  FIG.  2 B ), the encoder can perform a transform operation for residual basic processing sub-units (e.g., CUs). However, in some cases, a basic processing sub-unit can still be too large to process. The encoder can further split the basic processing sub-unit into smaller segments (e.g., referred to as “transform blocks” or “TBs” in H.265/HEVC or H.266/VVC), at the level of which the transform operation can be performed. It should be noted that the division schemes of the same basic processing sub-unit can be different at the prediction stage and the transform stage. For example, in H.265/HEVC or H.266/VVC, the prediction blocks and transform blocks of the same CU can have different sizes and numbers. 
     In structure  110  of  FIG.  1   , basic processing unit  112  is further divided into 3×3 basic processing sub-units, the boundaries of which are shown as dotted lines. Different basic processing units of the same picture can be divided into basic processing sub-units in different schemes. 
     In some implementations, to provide the capability of parallel processing and error resilience to video encoding and decoding, a picture can be divided into regions for processing, such that, for a region of the picture, the encoding or decoding process can depend on no information from any other region of the picture. In other words, each region of the picture can be processed independently. By doing so, the codec can process different regions of a picture in parallel, thus increasing the coding efficiency. Also, when data of a region is corrupted in the processing or lost in network transmission, the codec can correctly encode or decode other regions of the same picture without reliance on the corrupted or lost data, thus providing the capability of error resilience. In some video coding standards, a picture can be divided into different types of regions. For example, H.265/HEVC and H.266/VVC provide two types of regions: “slices” and “tiles.” It should also be noted that different pictures of video sequence  100  can have different partition schemes for dividing a picture into regions. 
     For example, in  FIG.  1   , structure  110  is divided into three regions  114 ,  116 , and  118 , the boundaries of which are shown as solid lines inside structure  110 . Region  114  includes four basic processing units. Each of regions  116  and  118  includes six basic processing units. It should be noted that the basic processing units, basic processing sub-units, and regions of structure  110  in  FIG.  1    are only examples, and the present disclosure does not limit embodiments thereof. 
       FIG.  2 A  illustrates a schematic diagram of an exemplary encoding process  200 A, consistent with embodiments of the disclosure. For example, the encoding process  200 A can be performed by an encoder. As shown in  FIG.  2 A , the encoder can encode video sequence  202  into video bitstream  228  according to process  200 A. Similar to video sequence  100  in  FIG.  1   , video sequence  202  can include a set of pictures (referred to as “original pictures”) arranged in a temporal order. Similar to structure  110  in  FIG.  1   , each original picture of video sequence  202  can be divided by the encoder into basic processing units, basic processing sub-units, or regions for processing. In some embodiments, the encoder can perform process  200 A at the level of basic processing units for each original picture of video sequence  202 . For example, the encoder can perform process  200 A in an iterative manner, in which the encoder can encode a basic processing unit in one iteration of process  200 A. In some embodiments, the encoder can perform process  200 A in parallel for regions (e.g., regions  114 - 118 ) of each original picture of video sequence  202 . 
     In  FIG.  2 A , the encoder can feed a basic processing unit (referred to as an “original BPU”) of an original picture of video sequence  202  to prediction stage  204  to generate prediction data  206  and predicted BPU  208 . The encoder can subtract predicted BPU  208  from the original BPU to generate residual BPU  210 . The encoder can feed residual BPU  210  to transform stage  212  and quantization stage  214  to generate quantized transform coefficients  216 . The encoder can feed prediction data  206  and quantized transform coefficients  216  to binary coding stage  226  to generate video bitstream  228 . Components  202 ,  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  226 , and  228  can be referred to as a “forward path.” During process  200 A, after quantization stage  214 , the encoder can feed quantized transform coefficients  216  to inverse quantization stage  218  and inverse transform stage  220  to generate reconstructed residual BPU  222 . The encoder can add reconstructed residual BPU  222  to predicted BPU  208  to generate prediction reference  224 , which is used in prediction stage  204  for the next iteration of process  200 A. Components  218 ,  220 ,  222 , and  224  of process  200 A can be referred to as a “reconstruction path.” The reconstruction path can be used to ensure that both the encoder and the decoder use the same reference data for prediction. 
     The encoder can perform process  200 A iteratively to encode each original BPU of the original picture (in the forward path) and generate predicted reference  224  for encoding the next original BPU of the original picture (in the reconstruction path). After encoding all original BPUs of the original picture, the encoder can proceed to encode the next picture in video sequence  202 . 
     Referring to process  200 A, the encoder can receive video sequence  202  generated by a video capturing device (e.g., a camera). The term “receive” used herein can refer to receiving, inputting, acquiring, retrieving, obtaining, reading, accessing, or any action in any manner for inputting data. 
     At prediction stage  204 , at a current iteration, the encoder can receive an original BPU and prediction reference  224 , and perform a prediction operation to generate prediction data  206  and predicted BPU  208 . Prediction reference  224  can be generated from the reconstruction path of the previous iteration of process  200 A. The purpose of prediction stage  204  is to reduce information redundancy by extracting prediction data  206  that can be used to reconstruct the original BPU as predicted BPU  208  from prediction data  206  and prediction reference  224 . 
     Ideally, predicted BPU  208  can be identical to the original BPU. However, due to non-ideal prediction and reconstruction operations, predicted BPU  208  is generally slightly different from the original BPU. For recording such differences, after generating predicted BPU  208 , the encoder can subtract it from the original BPU to generate residual BPU  210 . For example, the encoder can subtract values (e.g., greyscale values or RGB values) of pixels of predicted BPU  208  from values of corresponding pixels of the original BPU. Each pixel of residual BPU  210  can have a residual value as a result of such subtraction between the corresponding pixels of the original BPU and predicted BPU  208 . Compared with the original BPU, prediction data  206  and residual BPU  210  can have fewer bits, but they can be used to reconstruct the original BPU without significant quality deterioration. Thus, the original BPU is compressed. 
     To further compress residual BPU  210 , at transform stage  212 , the encoder can reduce spatial redundancy of residual BPU  210  by decomposing it into a set of two-dimensional “base patterns,” each base pattern being associated with a “transform coefficient.” The base patterns can have the same size (e.g., the size of residual BPU  210 ). Each base pattern can represent a variation frequency (e.g., frequency of brightness variation) component of residual BPU  210 . None of the base patterns can be reproduced from any combinations (e.g., linear combinations) of any other base patterns. In other words, the decomposition can decompose variations of residual BPU  210  into a frequency domain. Such a decomposition is analogous to a discrete Fourier transform of a function, in which the base patterns are analogous to the base functions (e.g., trigonometry functions) of the discrete Fourier transform, and the transform coefficients are analogous to the coefficients associated with the base functions. 
     Different transform algorithms can use different base patterns. Various transform algorithms can be used at transform stage  212 , such as, for example, a discrete cosine transform, a discrete sine transform, or the like. The transform at transform stage  212  is invertible. That is, the encoder can restore residual BPU  210  by an inverse operation of the transform (referred to as an “inverse transform”). For example, to restore a pixel of residual BPU  210 , the inverse transform can be multiplying values of corresponding pixels of the base patterns by respective associated coefficients and adding the products to produce a weighted sum. For a video coding standard, both the encoder and decoder can use the same transform algorithm (thus the same base patterns). Thus, the encoder can record only the transform coefficients, from which the decoder can reconstruct residual BPU  210  without receiving the base patterns from the encoder. Compared with residual BPU  210 , the transform coefficients can have fewer bits, but they can be used to reconstruct residual BPU  210  without significant quality deterioration. Thus, residual BPU  210  is further compressed. 
     The encoder can further compress the transform coefficients at quantization stage  214 . In the transform process, different base patterns can represent different variation frequencies (e.g., brightness variation frequencies). Because human eyes are generally better at recognizing low-frequency variation, the encoder can disregard information of high-frequency variation without causing significant quality deterioration in decoding. For example, at quantization stage  214 , the encoder can generate quantized transform coefficients  216  by dividing each transform coefficient by an integer value (referred to as a “quantization scale factor”) and rounding the quotient to its nearest integer. After such an operation, some transform coefficients of the high-frequency base patterns can be converted to zero, and the transform coefficients of the low-frequency base patterns can be converted to smaller integers. The encoder can disregard the zero-value quantized transform coefficients  216 , by which the transform coefficients are further compressed. The quantization process is also invertible, in which quantized transform coefficients  216  can be reconstructed to the transform coefficients in an inverse operation of the quantization (referred to as “inverse quantization”). 
     Because the encoder disregards the remainders of such divisions in the rounding operation, quantization stage  214  can be lossy. Typically, quantization stage  214  can contribute the most information loss in process  200 A. The larger the information loss is, the fewer bits the quantized transform coefficients  216  can need. For obtaining different levels of information loss, the encoder can use different values of the quantization syntax element or any other syntax element of the quantization process. 
     At binary coding stage  226 , the encoder can encode prediction data  206  and quantized transform coefficients  216  using a binary coding technique, such as, for example, entropy coding, variable length coding, arithmetic coding, Huffman coding, context-adaptive binary arithmetic coding, or any other lossless or lossy compression algorithm. In some embodiments, besides prediction data  206  and quantized transform coefficients  216 , the encoder can encode other information at binary coding stage  226 , such as, for example, a prediction mode used at prediction stage  204 , syntax elements of the prediction operation, a transform type at transform stage  212 , syntax elements of the quantization process (e.g., quantization syntax elements), an encoder control syntax element (e.g., a bitrate control syntax element), or the like. The encoder can use the output data of binary coding stage  226  to generate video bitstream  228 . In some embodiments, video bitstream  228  can be further packetized for network transmission. 
     Referring to the reconstruction path of process  200 A, at inverse quantization stage  218 , the encoder can perform inverse quantization on quantized transform coefficients  216  to generate reconstructed transform coefficients. At inverse transform stage  220 , the encoder can generate reconstructed residual BPU  222  based on the reconstructed transform coefficients. The encoder can add reconstructed residual BPU  222  to predicted BPU  208  to generate prediction reference  224  that is to be used in the next iteration of process  200 A. 
     It should be noted that other variations of the process  200 A can be used to encode video sequence  202 . In some embodiments, stages of process  200 A can be performed by the encoder in different orders. In some embodiments, one or more stages of process  200 A can be combined into a single stage. In some embodiments, a single stage of process  200 A can be divided into multiple stages. For example, transform stage  212  and quantization stage  214  can be combined into a single stage. In some embodiments, process  200 A can include additional stages. In some embodiments, process  200 A can omit one or more stages in  FIG.  2 A . 
       FIG.  2 B  illustrates a schematic diagram of another exemplary encoding process  200 B, consistent with embodiments of the disclosure. Process  200 B can be modified from process  200 A. For example, process  200 B can be used by an encoder conforming to a hybrid video coding standard (e.g., H.26x series). Compared with process  200 A, the forward path of process  200 B additionally includes mode decision stage  230  and divides prediction stage  204  into spatial prediction stage  2042  and temporal prediction stage  2044 . The reconstruction path of process  200 B additionally includes loop filter stage  232  and buffer  234 . 
     Generally, prediction techniques can be categorized into two types: spatial prediction and temporal prediction. Spatial prediction (e.g., an intra-picture prediction or “intra prediction”) can use pixels from one or more already coded neighboring BPUs in the same picture to predict the current BPU. That is, prediction reference  224  in the spatial prediction can include the neighboring BPUs. The spatial prediction can reduce the inherent spatial redundancy of the picture. Temporal prediction (e.g., an inter-picture prediction or “inter prediction”) can use regions from one or more already coded pictures to predict the current BPU. That is, prediction reference  224  in the temporal prediction can include the coded pictures. The temporal prediction can reduce the inherent temporal redundancy of the pictures. 
     Referring to process  200 B, in the forward path, the encoder performs the prediction operation at spatial prediction stage  2042  and temporal prediction stage  2044 . For example, at spatial prediction stage  2042 , the encoder can perform the intra prediction. For an original BPU of a picture being encoded, prediction reference  224  can include one or more neighboring BPUs that have been encoded (in the forward path) and reconstructed (in the reconstructed path) in the same picture. The encoder can generate predicted BPU  208  by extrapolating the neighboring BPUs. The extrapolation technique can include, for example, a linear extrapolation or interpolation, a polynomial extrapolation or interpolation, or the like. In some embodiments, the encoder can perform the extrapolation at the pixel level, such as by extrapolating values of corresponding pixels for each pixel of predicted BPU  208 . The neighboring BPUs used for extrapolation can be located with respect to the original BPU from various directions, such as in a vertical direction (e.g., on top of the original BPU), a horizontal direction (e.g., to the left of the original BPU), a diagonal direction (e.g., to the down-left, down-right, up-left, or up-right of the original BPU), or any direction defined in the used video coding standard. For the intra prediction, prediction data  206  can include, for example, locations (e.g., coordinates) of the used neighboring BPUs, sizes of the used neighboring BPUs, syntax elements of the extrapolation, a direction of the used neighboring BPUs with respect to the original BPU, or the like. 
     For another example, at temporal prediction stage  2044 , the encoder can perform the inter prediction. For an original BPU of a current picture, prediction reference  224  can include one or more pictures (referred to as “reference pictures”) that have been encoded (in the forward path) and reconstructed (in the reconstructed path). In some embodiments, a reference picture can be encoded and reconstructed BPU by BPU. For example, the encoder can add reconstructed residual BPU  222  to predicted BPU  208  to generate a reconstructed BPU. When all reconstructed BPUs of the same picture are generated, the encoder can generate a reconstructed picture as a reference picture. The encoder can perform an operation of “motion estimation” to search for a matching region in a scope (referred to as a “search window”) of the reference picture. The location of the search window in the reference picture can be determined based on the location of the original BPU in the current picture. For example, the search window can be centered at a location having the same coordinates in the reference picture as the original BPU in the current picture and can be extended out for a predetermined distance. When the encoder identifies (e.g., by using a pel-recursive algorithm, a block-matching algorithm, or the like) a region similar to the original BPU in the search window, the encoder can determine such a region as the matching region. The matching region can have different dimensions (e.g., being smaller than, equal to, larger than, or in a different shape) from the original BPU. Because the reference picture and the current picture are temporally separated in the timeline (e.g., as shown in  FIG.  1   ), it can be deemed that the matching region “moves” to the location of the original BPU as time goes by. The encoder can record the direction and distance of such a motion as a “motion vector.” When multiple reference pictures are used (e.g., as picture  106  in  FIG.  1   ), the encoder can search for a matching region and determine its associated motion vector for each reference picture. In some embodiments, the encoder can assign weights to pixel values of the matching regions of respective matching reference pictures. 
     The motion estimation can be used to identify various types of motions, such as, for example, translations, rotations, zooming, or the like. For inter prediction, prediction data  206  can include, for example, locations (e.g., coordinates) of the matching region, the motion vectors associated with the matching region, the number of reference pictures, weights associated with the reference pictures, or the like. 
     For generating predicted BPU  208 , the encoder can perform an operation of “motion compensation.” The motion compensation can be used to reconstruct predicted BPU  208  based on prediction data  206  (e.g., the motion vector) and prediction reference  224 . For example, the encoder can move the matching region of the reference picture according to the motion vector, in which the encoder can predict the original BPU of the current picture. When multiple reference pictures are used (e.g., as picture  106  in  FIG.  1   ), the encoder can move the matching regions of the reference pictures according to the respective motion vectors and average pixel values of the matching regions. In some embodiments, if the encoder has assigned weights to pixel values of the matching regions of respective matching reference pictures, the encoder can add a weighted sum of the pixel values of the moved matching regions. 
     In some embodiments, the inter prediction can be unidirectional or bidirectional. Unidirectional inter predictions can use one or more reference pictures in the same temporal direction with respect to the current picture. For example, picture  104  in  FIG.  1    is a unidirectional inter-predicted picture, in which the reference picture (e.g., picture  102 ) precedes picture  104 . Bidirectional inter predictions can use one or more reference pictures at both temporal directions with respect to the current picture. For example, picture  106  in  FIG.  1    is a bidirectional inter-predicted picture, in which the reference pictures (e.g., pictures  104  and  108 ) are at both temporal directions with respect to picture  104 . 
     Still referring to the forward path of process  200 B, after spatial prediction  2042  and temporal prediction stage  2044 , at mode decision stage  230 , the encoder can select a prediction mode (e.g., one of the intra prediction or the inter prediction) for the current iteration of process  200 B. For example, the encoder can perform a rate-distortion optimization technique, in which the encoder can select a prediction mode to minimize a value of a cost function depending on a bit rate of a candidate prediction mode and distortion of the reconstructed reference picture under the candidate prediction mode. Depending on the selected prediction mode, the encoder can generate the corresponding predicted BPU  208  and predicted data  206 . 
     In the reconstruction path of process  200 B, if intra prediction mode has been selected in the forward path, after generating prediction reference  224  (e.g., the current BPU that has been encoded and reconstructed in the current picture), the encoder can directly feed prediction reference  224  to spatial prediction stage  2042  for later usage (e.g., for extrapolation of a next BPU of the current picture). The encoder can feed prediction reference  224  to loop filter stage  232 , at which the encoder can apply a loop filter to prediction reference  224  to reduce or eliminate distortion (e.g., blocking artifacts) introduced during coding of the prediction reference  224 . The encoder can apply various loop filter techniques at loop filter stage  232 , such as, for example, deblocking, sample adaptive offsets, adaptive loop filters, or the like. The loop-filtered reference picture can be stored in buffer  234  (or “decoded picture buffer (DPB)”) for later use (e.g., to be used as an inter-prediction reference picture for a future picture of video sequence  202 ). The encoder can store one or more reference pictures in buffer  234  to be used at temporal prediction stage  2044 . In some embodiments, the encoder can encode syntax elements of the loop filter (e.g., a loop filter strength) at binary coding stage  226 , along with quantized transform coefficients  216 , prediction data  206 , and other information. 
       FIG.  3 A  illustrates a schematic diagram of an exemplary decoding process  300 A, consistent with embodiments of the disclosure. Process  300 A can be a decompression process corresponding to the compression process  200 A in  FIG.  2 A . In some embodiments, process  300 A can be similar to the reconstruction path of process  200 A. A decoder can decode video bitstream  228  into video stream  304  according to process  300 A. Video stream  304  can be very similar to video sequence  202 . However, due to the information loss in the compression and decompression process (e.g., quantization stage  214  in  FIG.  2 A  and  FIG.  2 B ), generally, video stream  304  is not identical to video sequence  202 . Similar to processes  200 A and  200 B in  FIG.  2 A  and  FIG.  2 B , the decoder can perform process  300 A at the level of basic processing units (BPUs) for each picture encoded in video bitstream  228 . For example, the decoder can perform process  300 A in an iterative manner, in which the decoder can decode a basic processing unit in one iteration of process  300 A. In some embodiments, the decoder can perform process  300 A in parallel for regions (e.g., regions  114 - 118 ) of each picture encoded in video bitstream  228 . 
     In  FIG.  3 A , the decoder can feed a portion of video bitstream  228  associated with a basic processing unit (referred to as an “encoded BPU”) of an encoded picture to binary decoding stage  302 . At binary decoding stage  302 , the decoder can decode the portion into prediction data  206  and quantized transform coefficients  216 . The decoder can feed quantized transform coefficients  216  to inverse quantization stage  218  and inverse transform stage  220  to generate reconstructed residual BPU  222 . The decoder can feed prediction data  206  to prediction stage  204  to generate predicted BPU  208 . The decoder can add reconstructed residual BPU  222  to predicted BPU  208  to generate predicted reference  224 . In some embodiments, predicted reference  224  can be stored in a buffer (e.g., a decoded picture buffer in a computer memory). The decoder can feed predicted reference  224  to prediction stage  204  for performing a prediction operation in the next iteration of process  300 A. 
     The decoder can perform process  300 A iteratively to decode each encoded BPU of the encoded picture and generate predicted reference  224  for encoding the next encoded BPU of the encoded picture. After decoding all encoded BPUs of the encoded picture, the decoder can output the picture to video stream  304  for display and proceed to decode the next encoded picture in video bitstream  228 . 
     At binary decoding stage  302 , the decoder can perform an inverse operation of the binary coding technique used by the encoder (e.g., entropy coding, variable length coding, arithmetic coding, Huffman coding, context-adaptive binary arithmetic coding, or any other lossless compression algorithm). In some embodiments, besides prediction data  206  and quantized transform coefficients  216 , the decoder can decode other information at binary decoding stage  302 , such as, for example, a prediction mode, syntax elements of the prediction operation, a transform type, syntax elements of the quantization process (e.g., quantization syntax elements), an encoder control syntax element (e.g., a bitrate control syntax element), or the like. In some embodiments, if video bitstream  228  is transmitted over a network in packets, the decoder can depacketize video bitstream  228  before feeding it to binary decoding stage  302 . 
       FIG.  3 B  illustrates a schematic diagram of another exemplary decoding process  300 B, consistent with embodiments of the disclosure. Process  300 B can be modified from process  300 A. For example, process  300 B can be used by a decoder conforming to a hybrid video coding standard (e.g., H.26x series). Compared with process  300 A, process  300 B additionally divides prediction stage  204  into spatial prediction stage  2042  and temporal prediction stage  2044 , and additionally includes loop filter stage  232  and buffer  234 . 
     In process  300 B, for an encoded basic processing unit (referred to as a “current BPU”) of an encoded picture (referred to as a “current picture”) that is being decoded, prediction data  206  decoded from binary decoding stage  302  by the decoder can include various types of data, depending on what prediction mode was used to encode the current BPU by the encoder. For example, if intra prediction was used by the encoder to encode the current BPU, prediction data  206  can include a prediction mode indicator (e.g., a flag value) indicative of the intra prediction, syntax elements of the intra prediction operation, or the like. The syntax elements of the intra prediction operation can include, for example, locations (e.g., coordinates) of one or more neighboring BPUs used as a reference, sizes of the neighboring BPUs, syntax elements of extrapolation, a direction of the neighboring BPUs with respect to the original BPU, or the like. For another example, if inter prediction was used by the encoder to encode the current BPU, prediction data  206  can include a prediction mode indicator (e.g., a flag value) indicative of the inter prediction, syntax elements of the inter prediction operation, or the like. The syntax elements of the inter prediction operation can include, for example, the number of reference pictures associated with the current BPU, weights respectively associated with the reference pictures, locations (e.g., coordinates) of one or more matching regions in the respective reference pictures, one or more motion vectors respectively associated with the matching regions, or the like. 
     Based on the prediction mode indicator, the decoder can decide whether to perform a spatial prediction (e.g., the intra prediction) at spatial prediction stage  2042  or a temporal prediction (e.g., the inter prediction) at temporal prediction stage  2044 . The details of performing such spatial prediction or temporal prediction are described in  FIG.  2 B  and will not be repeated hereinafter. After performing such spatial prediction or temporal prediction, the decoder can generate predicted BPU  208 . The decoder can add predicted BPU  208  and reconstructed residual BPU  222  to generate prediction reference  224 , as described in  FIG.  3 A . 
     In process  300 B, the decoder can feed predicted reference  224  to spatial prediction stage  2042  or temporal prediction stage  2044  for performing a prediction operation in the next iteration of process  300 B. For example, if the current BPU is decoded using the intra prediction at spatial prediction stage  2042 , after generating prediction reference  224  (e.g., the decoded current BPU), the decoder can directly feed prediction reference  224  to spatial prediction stage  2042  for later usage (e.g., for extrapolation of a next BPU of the current picture). If the current BPU is decoded using the inter prediction at temporal prediction stage  2044 , after generating prediction reference  224  (e.g., a reference picture in which all BPUs have been decoded), the decoder can feed prediction reference  224  to loop filter stage  232  to reduce or eliminate distortion (e.g., blocking artifacts). The decoder can apply a loop filter to prediction reference  224 , in a way as described in  FIG.  2 B . The loop-filtered reference picture can be stored in buffer  234  (e.g., a decoded picture buffer (DPB) in a computer memory) for later use (e.g., to be used as an inter-prediction reference picture for a future encoded picture of video bitstream  228 ). The decoder can store one or more reference pictures in buffer  234  to be used at temporal prediction stage  2044 . In some embodiments, prediction data can further include syntax elements of the loop filter (e.g., a loop filter strength). In some embodiments, prediction data includes syntax elements of the loop filter when the prediction mode indicator of prediction data  206  indicates that inter prediction was used to encode the currOent BPU. 
       FIG.  4    is a block diagram of an exemplary apparatus  400  for encoding or decoding a video, consistent with embodiments of the disclosure. As shown in  FIG.  4   , apparatus  400  can include processor  402 . When processor  402  executes instructions described herein, apparatus  400  can become a specialized machine for video encoding or decoding. Processor  402  can be any type of circuitry capable of manipulating or processing information. For example, processor  402  can include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), a neural processing unit (“NPU”), a microcontroller unit (“MCU”), an optical processor, a programmable logic controller, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field-Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), or the like. In some embodiments, processor  402  can also be a set of processors grouped as a single logical component. For example, as shown in  FIG.  4   , processor  402  can include multiple processors, including processor  402   a , processor  402   b , and processor  402   n.    
     Apparatus  400  can also include memory  404  configured to store data (e.g., a set of instructions, computer codes, intermediate data, or the like). For example, as shown in  FIG.  4   , the stored data can include program instructions (e.g., program instructions for implementing the stages in processes  200 A,  200 B,  300 A, or  300 B) and data for processing (e.g., video sequence  202 , video bitstream  228 , or video stream  304 ). Processor  402  can access the program instructions and data for processing (e.g., via bus  410 ), and execute the program instructions to perform an operation or manipulation on the data for processing. Memory  404  can include a high-speed random-access storage device or a non-volatile storage device. In some embodiments, memory  404  can include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, a hard drive, a solid-state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or the like. Memory  404  can also be a group of memories (not shown in  FIG.  4   ) grouped as a single logical component. 
     Bus  410  can be a communication device that transfers data between components inside apparatus  400 , such as an internal bus (e.g., a CPU-memory bus), an external bus (e.g., a universal serial bus port, a peripheral component interconnect express port), or the like. 
     For ease of explanation without causing ambiguity, processor  402  and other data processing circuits are collectively referred to as a “data processing circuit” in this disclosure. The data processing circuit can be implemented entirely as hardware, or as a combination of software, hardware, or firmware. In addition, the data processing circuit can be a single independent module or can be combined entirely or partially into any other component of apparatus  400 . 
     Apparatus  400  can further include network interface  406  to provide wired or wireless communication with a network (e.g., the Internet, an intranet, a local area network, a mobile communications network, or the like). In some embodiments, network interface  406  can include any combination of any number of a network interface controller (NIC), a radio frequency (RF) module, a transponder, a transceiver, a modem, a router, a gateway, a wired network adapter, a wireless network adapter, a Bluetooth adapter, an infrared adapter, an near-field communication (“NFC”) adapter, a cellular network chip, or the like. 
     In some embodiments, optionally, apparatus  400  can further include peripheral interface  408  to provide a connection to one or more peripheral devices. As shown in  FIG.  4   , the peripheral device can include, but is not limited to, a cursor control device (e.g., a mouse, a touchpad, or a touchscreen), a keyboard, a display (e.g., a cathode-ray tube display, a liquid crystal display, or a light-emitting diode display), a video input device (e.g., a camera or an input interface coupled to a video archive), or the like. 
     It should be noted that video codecs (e.g., a codec performing process  200 A,  200 B,  300 A, or  300 B) can be implemented as any combination of any software or hardware modules in apparatus  400 . For example, some or all stages of process  200 A,  200 B,  300 A, or  300 B can be implemented as one or more software modules of apparatus  400 , such as program instructions that can be loaded into memory  404 . For another example, some or all stages of process  200 A,  200 B,  300 A, or  300 B can be implemented as one or more hardware modules of apparatus  400 , such as a specialized data processing circuit (e.g., an FPGA, an ASIC, an NPU, or the like). 
     In video coding, pictures need to be identified for multiple purposes, including for being identified as reference pictures in inter prediction, as pictures to be output from the DPB, as temporal collocated picture for motion vector prediction, etc. The most common way to identify a picture is using picture order count (“POC”). 
     For identifying reference pictures in inter prediction, temporal collocated picture in motion vector (“MV”) temporal prediction and scaling, the reference picture lists (usually two as in AVC, HEVC and VVC) can be derived. For example, reference picture list 0 and reference picture list1 can be derived, each of which includes a list of reconstructed pictures in the DPB to be used as the reference pictures. And reference indices to the reference picture lists can be signaled at a block level for identifying the reference picture for the current block. To correctly maintain the reference pictures in the DPB without requiring unnecessarily large amount of DPB memory, reference picture marking is needed. 
     In VVC (e.g., VVC draft 9), two Reference Picture Lists (“RPL”), reference picture list 0 and reference picture list 1, are used. They are directly signaled and derived. Information on the two reference picture lists is signaled by syntax elements and syntax structures in a Sequence Parameter Set (“SPS”), a Picture Parameter Set (“PPS”), a Picture Header (“PH”), and a Slice Header (“SH”). Predefined reference picture list structures are signaled in the SPS, for use by referencing in the PH or SH. New reference picture list structure can also be signaled in PH or SH, for derivation of reference picture list 0 and reference picture list 1. Whether the reference picture list information is signaled in PH or SH is determined by a flag signaled in PPS. 
     In VVC (e.g., VVC draft 9), two reference picture lists are generated for all types of slices (e.g., B, P, and I slice). For I slices, neither of the two reference picture lists, reference picture list 0 nor reference picture list 1, may be used for decoding. For P slices, only reference picture list 0 may be used for decoding. For B slices, both reference picture lists, reference picture list 0 and reference picture list 1, may be used for decoding. The two reference picture lists are constructed without using a reference picture list initialization process or a reference picture list modification process. 
     Not all pictures in the reference picture list are used as the reference picture for the current picture or slice. Only the active entries of a reference picture list may be used in the decoding process of the slice data. The default number of active entries is signaled in PPS in VVC (e.g., VVC draft 9) and can be overridden by slice header for the current slice. 
     To identify the pictures in DPB to construct the RPL, the POC comprising most significant bits (“MSB”) and least significant bits (“LSB”) are used. In VVC (e.g., VVC draft 9), LSB of POC is signaled in PH and MSB may be explicitly signaled in PH or derived by comparing POC LSB of the current picture and that of one or more preceding pictures. 
     In VVC (e.g., VVC draft 9), a decoded picture in the DPB can be marked as “unused for reference,” “used for short-term reference,” or “used for long-term reference.” The decoded picture can be marked as only one among these three at any given moment during the operation of the decoding process. Assigning one of these markings to a picture implicitly removes other markings when applicable. When a picture is referred to as being marked as “used for reference,” this also refers to the picture being marked as “used for short-term reference” or “used for long-term reference,” but not both. 
     Short-term reference pictures (“STRP”) and inter-layer reference pictures (“ILRP”) are identified by their NAL (Network Abstraction Layer) unit ID and POC values. Long-term reference pictures (“LTRP”) are identified by their NAL unit ID and a number of LSBs of their POC values. 
       FIG.  5 A  illustrates an exemplary syntax including syntax structure for reference picture lists, according to some embodiments of the present disclosure. The syntax shown in  FIG.  5 A  can be a part of the VVC standard (e.g., VVC draft 9) or in other video coding technologies. 
     As shown in  FIG.  5 A , the syntax structure  500 A for reference picture lists (e.g., ref_pic_lists( )) may be present in the PH syntax structure or the SH. 
     As shown in  FIG.  5 A , syntax element  510 A (e.g., rpl_sps_flag[i]) equal to 1 specifies that reference picture list i (e.g., i can be 0 or 1) in the syntax structure for reference picture lists (e.g., ref_pic_lists( )) is derived based on one of the synatx structures for reference picture list structure (e.g. ref_pic_list_struct(listIdx, rplsIdx) with listIdx equal to i) in the SPS. Syntax element  510 A being equal to 0 specifies that reference picture list i (e.g., i can be 0 or 1) is derived based on the synatx structure for reference picture list structure (e.g., ref_pic_list_struct(listIdx, rplsIdx) with listIdx equal to i) that is directly included in the syntax structure for reference picture lists (e.g., ref_pic_lists( )). 
     When syntax element  510 A is not present, the following applies. First, if the number of reference picture lists in SPS (e.g., sps_num_ref_pic_lists[i]) is equal to 0, the value of syntax element  510 A is inferred to be equal to 0. Second, if the number of reference picture lists in SPS (e.g., sps_num_ref_pic_lists[i]) is not equal to 0 (e.g., if the number of reference picture lists in SPS is greater than 0), when syntax element  520 A (e.g. pps_rpl1_idx_present_flag) is equal to 0 and i is equal to 1, the value of syntax element  510 A for reference picture list 1 in SPS (e.g. rpl_sps_flag[1]) is inferred to be equal to the value of syntax element  510 A for reference picture list 0 in SPS (e.g. rpl_sps_flag[0]). 
     Syntax element  530 A (e.g., rpl_idx[i]) specifies the index, to the list of the ref_pic_list_struct(listIdx, rplsIdx) with listIdx equal to i included in the SPS, of the ref_pic_list_struct(listIdx, rplsIdx) with listIdx equal to i that is used for derivation of reference picture list i of the current picture. The length of syntax element  530 A is a number of bits with a smallest integer greater than or equal to a base-2 logarithm of the number of the ref_pic_list_struct(listIdx, rplsIdx) syntax structures in SPS, which can be represented by Ceil(Log 2 (sps_num_ref_pic_lists[i])) bits. The value of syntax element  530 A can be in an inclusive range of 0 to the number of reference picture lists in SPS −1 (e.g., sps_num_ref_pic_lists[i]−1). When syntax element  530 A is not present, if syntax element  510 A is equal to 1 and syntax element  520 A is equal to 0, the value of rpl_idx[1] is inferred to be equal to the value of rpl_idx[0], otherwise the value of rpl_idx[1] is inferred to be equal to 0. 
     The variable RplsIdx[i] can be derived as follows: 
       RplsIdx[ i ]=rpl_sps_flag[ i ]?rpl_idx[ i ]:sps_num_ref_pic_lists[ i ]. 
     Syntax element  540 A (e.g. poc_lsb_lt[i][j]) specifies the value of the picture order count modulo MaxPicOrderCntLsb of the j-th LTRP entry in the i-th reference picture list in the ref_pic_lists( ). The length of syntax element  540 A is equal to base-2 logarithm of the maximum LSB in POC bits (e.g., sps_log 2_max_pic_order_cnt_lsb_minus4+4 bits). 
     The variable PocLsbLt[i][j] can be derived as follows: 
       PocLsbLt[ i ][ j ]=ltrp_in_header_flag[ i ][RplsIdx[ i ]]?poc_lsb_lt[ i ][ j ]:rpls_poc_lsb_lt[listIdx][RplsIdx[ i ]][ j ]. 
     Syntax element  550 A (e.g. delta_poc_msb_cycle_present_flag[i][j]) equal to 1 specifies that syntax element  560 A (e.g. delta_poc_msb_cycle_lt[i][j]) is present. Syntax element  550 A being equal to 0 specifies that syntax element  560  is not present. 
     The previous picture in decoding order having nuh_layer_id that is the same as the slice header or picture header referring to the ref_pic_lists( ) syntax structure and having TemporalID and ph_non_ref_pic_flag both equal to 0 and is not a RASL (Radom Access Skipped Leading) or RADL (Random Access Decodable Leading) picture can be described as prevTid0Pic. nuh_layer_id is a syntax element specifies the identifier of the layer to which a VCL (Video Coding Layer) NAL (Network Abstraction Layer) unit belongs or the identifier of a layer to which a non-VCL NAL unit applies. TemporalID is a temporal identifier of the picture. A set of previous POC values described as setOfPrevPocVals is a set comprising the following:
         the POC value (e.g. PicOrderCntVal) of prevTid0Pic;   the POC value (e.g. PicOrderCntVal) of each picture that is referred to by entries in reference picture list 0 (e.g.RefPicList[0]) or reference picture list 1 (e.g. RefPicList[1]) of prevTid0Pic and has nuh_layer_id the same as the current picture;   the POC value (e.g. PicOrderCntVal) of each picture that follows prevTid0Pic in decoding order, has nuh_layer_id the same as the current picture, and precedes the current picture in decoding order.       

     When there is more than one value in setOfPrevPocVals for which the value modulo MaxPicOrderCntLsb is equal to variable PocLsbLt[i][j], the value of syntax element  550 A (e.g. delta_poc_msb_cycle_present_flag[i][j]) is equal to 1. 
       FIG.  5 B  shows an exemplary pseudocode including derivation of variable FullPocLt [i][j], according to some embodiments of the present disclosure. Syntax element  560 A (e.g. delta_poc_msb_cycle_lt[i][j]) specifies the value of the variable FullPocLt[i][j] as shown in  FIG.  5 B . The value of syntax element  560 A (e.g., delta_poc_msb_cycle_lt[i][j]) can be in an inclusive range of 0 to 2 (32-sps_log 2_max_pic_order_cnt_lsb_minus4 -4) . When syntax element  560  is not present, the value of syntax element  560  is inferred to be equal to 0. 
       FIG.  6 A  illustrates an exemplary syntax including syntax structure for reference picture list structure, according to some embodiments of the present disclosure. The syntax structure shown in  FIG.  6 A  can be a part of the VVC standard (e.g., VVC draft 9) or in other video coding technologies. As shown in  FIG.  6 A , the ref_pic_list_struct(listIdx, rplsIdx) may be present in an SPS, in a PH syntax structure, or in an SH. Depending on whether the syntax structure is included in an SPS, a PH syntax structure, or an SH, the following applies:
         If ref_pic_list_struct(listIdx, rplsIdx) is present in a PH syntax structure or an SH, the ref_pic_list_struct(listIdx, rplsIdx) syntax structure specifies reference picture list listIdx of the current picture (e.g., the coded picture containing the PH syntax structure or SH).   If ref_pic_list_struct(listIdx, rplsIdx) is not present in a PH syntax structure or an SH (e.g., present in an SPS), the ref_pic_list_struct(listIdx, rplsIdx) syntax structure specifies a candidate for reference picture list listIdx, and the term “the current picture” in the semantics specified in the remainder of this clause refers to each picture that 1) has a PH syntax structure or one or more slices containing rpl_idx[listIdx] equal to an index into the list of the ref_pic_list_struct(listIdx, rplsIdx) syntax structures included in the SPS, and 2) is in a Coded Layer-wise Video Sequence (CLVS) that refers to the SPS.       

     As shown in  FIG.  6 A , syntax element  610 A (e.g., num_ref_entries[listIdx][rplsIdx]) specifies the number of entries in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure. The value of parameter  610 A can be in an inclusive range of 0 to MaxDpbSize+13, where MaxDpbSize is as specified in a level of a video coding standard (e.g., VVC draft 9). 
     Syntax element  620 A (e.g., ltrp_in_header_flag[listIdx][rplsIdx]) being equal to 0 specifies that the POC LSBs of the LTRP entries indicated in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure are present in the same syntax structure. Syntax element  620 A being equal to 1 specifies that the POC LSBs of the LTRP entries indicated in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure are not present in the same syntax structure. When sps_long_term_ref_pics_flag is equal to 1 and the rplsIdx is equal to sps_num_ref_pic_lists[listIdx], the value of syntax element  620 A is inferred to be equal to 1. 
     Syntax element  630 A (e.g., inter_layer_ref_pic_flag[listIdx][rplsIdx][i]) being equal to 1 specifies that the i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure is an ILRP entry. Syntax element  630 A being equal to 0 specifies that the i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure is not an ILRP entry. When the syntax element  630 A is not present, the value of syntax element  630 A is inferred to be equal to 0. 
     Syntax element  640 A (e.g., st_ref_pic_flag[listIdx][rplsIdx][i]) being equal to 1 specifies that the i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure is an STRP entry. Syntax element  640 A being equal to 0 specifies that the i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure is an LTRP entry. When syntax element  630 A is equal to 0 and syntax element  640 A is not present, the value of syntax element  640 A is inferred to be equal to 1. 
       FIG.  6 B  shows an exemplary pseudocode including derivation for the number of LTRP entries (e.g., variable NumLtrpEntries[listIdx][rplsIdx]), according to some embodiments of the present disclosure. The variable NumLtrpEntries[listIdx][rplsIdx] (e.g. variable  570 A in  FIG.  5 A ) can be derived as shown in  FIG.  6 B . 
       FIG.  6 C  shows an exemplary pseudocode including derivation for variable AbsDeltaPocSt[listIdx][rplsIdx][i], according to some embodiments of the present disclosure. Syntax element  650 A (e.g., abs_delta_poc_st[listIdx][rplsIdx][i]) specifies the value of the variable AbsDeltaPocSt[listIdx][rplsIdx][i] (e.g. variable  690 A) as shown in  FIG.  6 C . The value of syntax element  650 A (e.g., abs_delta_poc_st[listIdx][rplsIdx][i]) can be in an inclusive range of 0 to 2 15 −1. 
     Syntax element  660 A (e.g., strp_entry_sign_flag[listIdx][rplsIdx][i]) equal to 1 specifies that i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure has a value greater than or equal to 0. Syntax element  660 A equal to 0 specifies that the i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure has a value less than 0. When the syntax element  660 A is not present, the value of the syntax element  660 A is inferred to be equal to 1. 
       FIG.  6 D  shows an exemplary pseudocode including derivation for variable DeltaPocValSt[listIdx][rplsIdx], according to some embodiments of the present disclosure. The DeltaPocValSt[listIdx][rplsIdx] can be derived as shown in  FIG.  6 D . 
     Referring back to  FIG.  6 A , syntax element  670 A (e.g., rpls_poc_lsb_lt[listIdx][rplsIdx][i]) specifies the value of the picture order count modulo MaxPicOrderCntLsb of the picture referred to by the i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure. The length of the syntax element  670 A is sps_log 2_max_pic_order_cnt_lsb_minus4+4 bits. 
     Syntax element  680 A (e.g. ilrp_idx[listIdx][rplsIdx][i]) specifies the index, to the list of the direct reference layers, of the ILRP of the i-th entry in the ref_pic_list_struct(listIdx, rplsIdx) syntax structure. The value of the syntax element  680 A can be in an inclusive range of 0 to NumDirectRefLayers[GeneralLayerIdx[nuh_layer_id]]−1, where NumDirectRefLayers[LayerIdx] means the number of direct reference layers of a layer with index equal to LayerIdx. 
       FIG.  7    shows an exemplary syntax including syntax structure for reference picture list structure in sequence parameter set, according to some embodiments of the present disclosure. The syntax shown in  FIG.  7    can be a part of the VVC standard (e.g., VVC draft 9) or in other video coding technologies. 
     As shown in  FIG.  7   , syntax element  710  (e.g., sps_long_term_ref_pics_flag) being equal to 0 specifies that no LTRP is used for inter prediction of any coded picture in the CLVS. Syntax element  710  being equal to 1 specifies that LTRPs may be used for inter prediction of one or more coded pictures in the CLVS. 
     Syntax element  720  (e.g., sps_inter_layer_ref_pics_present_flag) being equal to 0 specifies that no ILRP is used for inter prediction of any coded picture in the CLVS. Syntax element  720  being equal to 1 specifies that ILRPs may be used for inter prediction of one or more coded pictures in the CLVS. When sps_video_syntax element_set_id is equal to 0, that is, the SPS does not refer to a VPS (Video Parameter Set), and no VPS is referred to when decoding each CLVS referring to the SPS (there is only one layer), the value of syntax element  720  is inferred to be equal to 0. When vps_independent_layer_flag[GeneralLayerIdx[nuh_layer_id]] is equal to 1, that is, the layer with index GeneralLayerIdx[nuh_layer_id] does not use inter-layer prediction, the value of syntax element  720  is equal to 0. 
     Syntax element  730  (e.g., sps_idr_rpl_present_flag) being equal to 1 specifies that reference picture list syntax elements are present in slice headers of IDR (Instantaneous Decoding Refresh) pictures. Syntax element  730  equal to 0 specifies that reference picture list syntax elements are not present in slice headers of IDR pictures. 
     Syntax element  740  (e.g., sps_rpl1_same_as_rpl0_flag) being equal to 1 specifies that the syntax element sps_num_ref_pic_lists[1] and the syntax structure ref_pic_list_struct(1, rplsIdx) are not present and the following applies: the value of sps_num_ref_pic_lists[1] is inferred to be equal to the value of sps_num_ref_pic_lists[0]; and the value of each of syntax elements in ref_pic_list_struct(1, rplsIdx) is inferred to be equal to the value of corresponding syntax element in ref_pic_list_struct(0, rplsIdx) for rplsIdx ranging from 0 to sps_num_ref_pic_lists[0]−1. 
     Syntax element  750  (e.g., sps_num_ref_pic_lists[i]) specifies the number of the ref_pic_list_struct(listIdx, rplsIdx) syntax structures with listIdx equal to i included in the SPS. The value of syntax element  750  can be in an inclusive range of 0 to 64. For each value of listIdx (equal to 0 or 1), a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) can allocate memory for ref_pic_list_struct(listIdx, rplsIdx) syntax structures with a total number of the number of RPLs in SPS plus 1 (e.g., sps_num_ref_pic_lists[i]+1), since there may be one ref_pic_list_struct(listIdx, rplsIdx) syntax structure directly signaled in the slice headers of a current picture. 
       FIG.  8    shows an exemplary syntax including syntax structure for reference picture list in picture parameter set, according to some embodiments of the present disclosure. The syntax shown in  FIG.  8    can be a part of the VVC standard (e.g., VVC draft 9) or in other video coding technologies. 
     As shown in  FIG.  8   , syntax element  810  (e.g., pps_num_ref_idx_default_active_minus1[i]) plus 1, when i is equal to 0, that is for reference picture list 0, specifies the inferred value of the variable NumRefIdxActive[0] for P or B slices with sh_num_ref_idx_active_override_flag equal to 0. When i is equal to 1, that is for reference picture list 1, syntax element  810  plus 1 specifies the inferred value of variable NumRefIdxActive[1] for B slices with sh_num_ref_idx_active_override_flag equal to 0. The value of syntax element  810  can be in an inclusive range of 0 to 14. 
     Syntax element  820  (e.g., pps_rpl1_idx_present_flag) being equal to 0 specifies that rpl_sps_flag[1] and rpl_idx[1] are not present in the PH syntax structures or the slice headers for pictures referring to the PPS. Syntax element  820  being equal to 1 specifies that rpl_sps_flag[1] and rpl_idx[1] may be present in the PH syntax structures or the slice headers for pictures referring to the PPS. 
     Syntax element  830  (e.g., pps_rpl_info_in_ph_flag) being equal to 1 specifies that reference picture list information is present in the PH syntax structure and not present in slice headers referring to the PPS that do not contain a PH syntax structure. Syntax element  830  being equal to 0 specifies that reference picture list information is not present in the PH syntax structure and may be present in slice headers referring to the PPS. When the syntax element  830  is not present, the value of the syntax element  830  is inferred to be equal to 0. 
       FIG.  9 A  shows an exemplary syntax including syntax structure for reference picture list in picture header structure, according to some embodiments of the present disclosure. The syntax shown in  FIG.  9 A  can be a part of the VVC standard (e.g., VVC draft 9) or in other video coding technologies. 
     As shown in  FIG.  9 A , syntax element  910 A (e.g., ph_pic_output_flag) affects the decoded picture output and removal processes. When the syntax element  910 A is not present, it is inferred to be equal to 1. There is no picture in the bitstream that has ph_non_reference_picture_flag equal to 1 and syntax element  910 A equal to 0. Element ph_non_reference_picture_flag being equal to 1 specifies that the current picture is never used as a reference picture. Element ph_non_ref_pic_flag being equal to 0 specifies that the current picture might or might not be used as a reference picture. 
     Syntax element  920 A (e.g., ph_temporal_mvp_enabled_flag) being equal to 0 specifies that temporal motion vector predictor is disabled and not used in decoding of the slices in the current picture. Syntax element  920 A being equal to 1 specifies that temporal motion vector predictors is enabled and may be used in decoding of the slices in the current picture. When syntax element  920 A is not present, the value of syntax element  920 A is inferred to be equal to 0. Due to other existing constraints, the value of syntax element  920 A can only be equal to 0 in a conforming bitstream when one or more of the following conditions are true: 1) no reference picture in the DPB has the same spatial resolution and the same scaling window offsets as the current picture, and 2) no reference picture in the DPB exists in the active entries of the RPLs of all slices in the current picture. There can be other situations, complicated conditions under which syntax element  920 A can only be equal to 0 that are not listed. 
       FIG.  9 B  shows an exemplary pseudocode including derivation for variable MaxNumSubblockMergeCand, according to some embodiments of the present disclosure. As shown in  FIG.  9 B , the value of MaxNumSubblockMergeCand refers to the maximum number of subblock-based merging MVP (Motion Vector Predictor) candidates. The value of MaxNumSubblockMergeCand can be in an inclusive range of 0 to 5. 
     Referring back to  FIG.  9 A , syntax element  930 A (e.g., ph_collocated_from_l0_flag) being equal to 1 specifies that the collocated picture used for temporal motion vector prediction is derived from reference picture list 0. Syntax element  930 A being equal to 0 specifies that the collocated picture used for temporal motion vector prediction is derived from reference picture list 1. When syntax element  920 A and syntax element  830 A (e.g., pps_rpl_info_in_ph_flag) are both equal to 1 and num_ref_entries[1][RplsIdx[1]] is equal to 0, the value of syntax element  930 A is inferred to be equal to 1. 
     Syntax element  940 A (e.g., ph_collocated_ref_idx) specifies the reference index of the collocated picture used for temporal motion vector prediction. When syntax element  930 A is equal to 1, syntax element  940 A refers to an entry in reference picture list 0, and the value of syntax element  940 A can be in an inclusive range of 0 to num_ref_entries[0][RplsIdx[0]]−1. When syntax element  930 A is equal to 0, syntax element  940 A refers to an entry in reference picture list 1, and the value of syntax element  940 A can be in an inclusive range of 0 to num_ref_entries[1][RplsIdx[1]]−1. When syntax element  940 A is not present, the value of syntax element  940 A is inferred to be equal to 0. 
     Syntax element  950 A (e.g., ph_mvd_l1_zero_flag) being equal to 1 specifies that the motion vector difference (e.g., mvd_coding(x0, y0, 1, cpIdx)) syntax structure is not parsed and MvdL1[x0][y0][compIdx] and MvdCpL1[x0][y0][cpIdx][compIdx] are set equal to 0 for compIdx=0 . . . 1 and cpIdx=0 . . . 2. Syntax element  950 A being equal to 0 specifies that the mvd_coding(x0, y0, 1, cpIdx) syntax structure is parsed. When the syntax element  950 A is not present, the value of syntax element  950 A is inferred to be 1. MvdL1 is the motion vector difference decoded from the bitstream associated with the reference picture in the reference picture list1. MvdCpL1 is the control point motion vector difference decoded from the bitstream associated with the reference picture in the reference picture list 1. It is for a coding block using affine motion compensation. x0, y0 are the top-left position of the current coding block, compIdx is the component index, and cpIdx is the index of control point. 
       FIG.  10 A  shows an exemplary syntax including syntax structure for reference picture list in slice header, according to some embodiments of the present disclosure. The syntax shown in  FIG.  10 A  can be a part of the VVC standard (e.g., VVC draft 9) or in other video coding technologies. 
     As shown in  FIG.  10 A , syntax element  1010 A (e.g., sh_num_ref_idx_active_override_flag) equal to 1 specifies that the syntax element sh_num_ref_idx_active_minus1[0] is present for P and B slices and the syntax element sh_num_ref_idx_active_minus1[1] is present for B slices. Syntax element  1010 A equal to 0 specifies that the syntax elements sh_num_ref_idx_active_minus1[0] and sh_num_ref_idx_active_minus1[1] are not present. When the syntax element  1010 A is not present, the value of syntax element  1010 A is inferred to be equal to 1. 
     Syntax element  1020 A (e.g., sh_num_ref_idx_active_minus1[i]) is used for the derivation of the variable NumRefIdxActive[i]. The value of syntax element  1020 A can be in an inclusive range of 0 to 14. For i equal to 0 or 1, when the current slice is a B slice, syntax element  1010 A is equal to 1, and when syntax element  1020 A is not present, the syntax element  1020 A is inferred to be equal to 0. 
       FIG.  10 B  shows an exemplary pseudocode including derivation for variable NumRefIdxActive[i], according to some embodiments of the present disclosure. As shown in  FIG.  10 B , value of NumRefIdxActive[i]−1 specifies the maximum reference index for reference picture list i that may be used to decode the slice. Syntax element  1020 A is used for the derivation of NumRefIdxActive[i] as shown by Equation (1) in  FIG.  10 B . When the value of NumRefIdxActive[i] is equal to 0, no reference index for reference picture list i may be used to decode the slice. When the current slice is a P slice, the value of NumRefIdxActive[0] is greater than 0. When the current slice is a B slice, both NumRefIdxActive[0] and NumRefIdxActive[1] is greater than 0. 
     As shown in  FIG.  10 A , syntax element  1030 A (e.g., sh_cabac_init_flag) specifies the method for determining the initialization table used in the initialization process for context variables. When syntax element  1030 A is not present, it is inferred to be equal to 0. 
     Syntax element  1040 A (e.g., sh_collocated_from_l0_flag) being equal to 1 specifies that the collocated picture used for temporal motion vector prediction is derived from reference picture list 0. Syntax element  1040 A being equal to 0 specifies that the collocated picture used for temporal motion vector prediction is derived from reference picture list 1. When sh_slice_type is equal to B or P, syntax element  920 A (e.g., ph_temporal_mvp_enabled_flag) is equal to 1, and syntax element  1040 A is not present, the following applies: if sh_slice_type is equal to B, syntax element  1040 A is inferred to be equal to syntax element  930 A (e.g., ph_collocated_from_l0_flag); and if sh_slice_type is not equal to P (e.g., sh_slice_type is equal to P), the value of syntax element  1040 A is inferred to be equal to 1. 
     Syntax element  1050 A (e.g., sh_collocated_ref_idx) specifies the reference index of the collocated picture used for temporal motion vector prediction. When sh_slice_type is equal to P or when sh_slice_type is equal to B and syntax element  1040 A is equal to 1, syntax element  1050 A refers to an entry in reference picture list 0, and the value of syntax element  1050 A can be in an inclusive range of 0 to NumRefIdxActive[0]−1. When sh_slice_type is equal to B and syntax element  1040 A is equal to 0, syntax element  1050 A refers to an entry in reference picture list 1, and the value of syntax element  1050 A can be in an inclusive range of 0 to NumRefIdxActive[1]−1. When syntax element  1050 A is not present, the following applies: if syntax element  830  (e.g., pps_rpl_info_in_ph_flag) is equal to 1, the value of syntax element  1050 A is inferred to be equal to syntax element  940 A (e.g., ph_collocated_ref_idx); and if syntax element  830  is not equal to 1 (e.g., syntax element  830  is equal to 0), the value of syntax element  1050 A is inferred to be equal to 0. It is a requirement of bitstream conformance that the picture referred to by syntax element  1050 A is the same for all slices of a coded picture and RprConstraintsActive[sh_collocated_from_l0_flag ? 0:1][sh_collocated_ref_idx] is equal to 0. This constraint requires the collocated picture to have the same spatial resolution and the same scaling window offsets as the current picture. 
     In VVC (e.g., VVC draft 9), syntax element  930 A (e.g., ph_collocated_from_l0_flag) and syntax element  950 A (e.g., ph_mvd_l1_zero_flag) are two flags signaled in PH. Syntax element  930 A indicates which reference picture list the collocated picture used for temporal motion vector prediction is from. Syntax element  950 A indicates whether the mvd_coding( ) syntax structure is parsed for reference picture list 1. As a result, these two flags are only relevant when the number of active entries in reference picture list 1 is larger than 0. However, as shown in  FIG.  10 A , since the number of active entries in reference picture list are overridden in slice header by sh_num_ref_idx_active_minus1[i], when syntax element  930 A and syntax element  950 A are signaled in PH, the decoder has no knowledge of the exact number of active entries of reference picture list 1. Therefore, in VVC (e.g., VVC draft 9), the total number of entries in reference picture list 1 is used as a condition to signal these two flags, as shown in  FIG.  9 A . 
     It is appreciated that while the present disclosure refers to various syntax elements providing inferences based on the value being equal to 0 or 1, the values can be configured in any way (e.g., 1 or 0) for providing the appropriate inference. 
     In VVC (e.g., VVC draft 9), it is guaranteed that for an I slice, the number of active entries of both two reference picture lists are equal to 0. For a P slice, the number of active entries in reference picture list 0 is greater than 0 and the number of active entries in reference picture list 1 is equal to 0. For a B slice, the number of active entries in both two reference picture lists are greater than 0. There is no guarantee for the total number of entries in the reference picture list. For example, for an I slice, the number of entries in any of two reference picture lists may be greater than 0. As a result, the signaling condition that total number of entries in reference picture list 1 is greater than 0 is too relaxed for syntax element  930 A and syntax element  950 A, which causes unnecessary signaling of these two syntax elements. 
     To overcome this deficiency with conventional coding technologies, in some embodiments of the present disclosure (such as provided below in  FIG.  11 A  to  FIG.  11 C ), an unnecessary signaling in case the number of entries in reference picture list 0 equals to 0 is avoided. 
       FIG.  11 A  illustrates a flow-chart of an exemplary video encoding method  1100 A for signaling flags in PH syntax structure, according to some embodiments of the disclosure. Method  1100 A can be performed by an encoder (e.g., by process  200 A of  FIG.  2 A or  200 B  of  FIG.  2 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1100 A. In some embodiments, method  1100 A can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  11 A , method  1100 A may include the following steps  1102 A and  1104 A. 
     At step  1102 A, the encoder encodes a current picture based on a collocated picture. The reference pictures can be derived, for example, by reference picture 0 and reference picture list 1, each of which includes a list of reconstructed pictures in the DPB (e.g., buffer  234  in  FIG.  3 B ) to be used as the reference pictures. The current picture is used for temporal motion vector prediction. 
     At step  1104 A, syntax element ph_collocated_from_l0_flag (e.g., syntax element  930 A) and syntax element ph_mvd_l1_zero_flag (e.g., syntax element  950 A) are signaled, when the number of entries in reference picture lists 0 and the number of entries in reference picture lists 1 are both greater than 0. The syntax element ph_collocated_from_l0_flag indicates which reference picture list a collocated picture used for temporal motion vector prediction is from, that is, the collocated picture used for temporal motion vector prediction is from a reference picture list that is indicated by the first flag. The syntax element ph_mvd_l1_zero_flag indicates whether a motion vector difference syntax structure associated with reference picture list 1 is signaled. In this way, the entries in both reference picture list 1 and reference picture 0 are guaranteed when signaling the two flags. Therefore, an unnecessary signaling in case the number of entries in reference picture list 0 equals to 0 is avoided, and the efficiency of decoding is improved. 
       FIG.  11 B  illustrates a flow-chart of an exemplary video decoding method  1100 B for decoding flags in PH syntax structure, according to some embodiments of the disclosure. Method  1100 B can be performed by a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1100 B. In some embodiments, method  1100 B can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  11 B , method  1100 B may include the following steps  1102 B- 1106 B. 
     At step  1102 B, the decoder receives a video bitstream (e.g., video bitstream  228  in  FIG.  3 B ) and the video bitstream may be coded using inter prediction. 
     At step  1104 B, syntax element ph_collocated_from_l0_flag (e.g., syntax element  930 A) and syntax element ph_mvd_l1_zero_flag (e.g., syntax element  950 A) are decoded from the bitstream by a decoder, when the number of entries in reference picture lists 0 and the number of entries in reference picture lists 1 are both greater than 0. The syntax element ph_collocated_from_l0_flag indicates which reference picture list a collocated picture used for temporal motion vector prediction is from, that is, the collocated picture used for temporal motion vector prediction is from a reference picture list that is indicated by the first flag. The syntax element ph_mvd_l1_zero_flag indicates whether a motion vector difference syntax structure associated with reference picture list 1 is present in the bitstream. In this way, the entries in both reference picture list 1 and reference picture 0 are guaranteed when signaling the two flags. 
     At step  1106 B, a current picture is decoded based on the collocated picture. Therefore, an unnecessary signaling in case the number of entries in reference picture list 0 equals to 0 is avoided, and the efficiency is improved. 
       FIG.  11 C  illustrates a portion of an exemplary picture header syntax structure  1100 C, according to some embodiments of the present disclosure. The picture header (PH) syntax structure  1100 C can be used in method  1100 A. PH syntax structure  1100 B is modified based on syntax structure  900 A of  FIG.  9 A , and changes from the previous VVC are shown in italic in block  1110 C and  1120 C. 
     Referring to  1110 C, in some embodiments, syntax element ph_collocated_from_l0_flag (e.g., syntax element  930 A) is signaled when num_ref_entries[0][RplsIdx[0]] is greater than 0 and num_refe_entries[1][RplsIdx[1]] is greater than 0. Referring to  1120 C, syntax element ph_mvd_l1_zero_flag (e.g., syntax element  950 A) is signaled when pps_rpl_info_in_ph_flag is not equal to 0 or num_ref_entries[0][RplsIdx[0]] is greater than 0 with num_refe_entries[1][RplsIdx[1]] is greater than 0. Therefore, syntax element  930 A and syntax element  950 A can be signaled when the number of entries in reference picture list 0 and the number of reference picture list 1 are both greater than 0. An unnecessary signaling in case the number of entries in reference picture list 0 equals to 0 is avoided, and the coding efficiency is improved. 
     In VVC (e.g., VVC draft 9), the collocated picture can be indicated in PH or SH. If reference picture list information is signaled in PH, collocated picture is indicated in PH by syntax element  930 A (e.g., ph_collocated_from_l0_flag) and syntax element  940 A (e.g., ph_ollocated_ref_idx). If the reference picture list information is signaled in SH, collocated picture is indicated in SH by syntax element  1040 A (e.g., sh_collocated_from_l0_flag) and syntax element  1050 A (e.g., sh_collocated_ref_idx). Syntax element  930 A being equal to 1 specifies that the collocated picture used for temporal motion vector prediction is derived from reference picture list 0. Syntax element  930 A being equal to 0 specifies that the collocated picture used for temporal motion vector prediction is derived from reference picture list 1. When syntax element  930 A is signaled in PH, the signaling condition is that the number of entries in reference picture list 1 is greater than 0. However, the number of active entries in reference picture list can be overridden in slice level. Therefore, even if syntax element  930 A is signaled to be 0, it cannot be guaranteed that the collocated picture can be selected from reference picture list 1, since SH may override the number of active entries in reference picture list 1 to be 0. Similarly, when syntax element  940 A is signaled in PH, the maximum allowed value is the number of entries in the reference picture list minus 1. If SH overrides the number of active entries to be a value less than syntax element  940 A, then it is an illegal bitstream. 
     To avoid such illegal scenarios, VVC (e.g., VVC draft 9) imposes several bitstream conformance constraints. However, it gives a burden for encoder to satisfy all the constraints. And practically the decoder should also consider how to deal with the bitstream when such illegal cases happen. 
     To overcome this deficiency with conventional coding technologies, in some embodiments of the present disclosure (such as provided below in  FIGS.  12 A- 12 J ), the collocated picture is indicated without signaling the index to the reference picture list, such that the illegal scenarios are avoided in a more robust way. 
       FIG.  12 A  illustrates a flow-chart of an exemplary video encoding method  1200 A for indicating a collocated picture without signaling the index to the reference picture list, according to some embodiments of present disclosure. Method  1200 A can be performed by an encoder (e.g., by process  200 A of  FIG.  2 A or  200 B  of  FIG.  2 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1200 A. In some embodiments, method  1200 A can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  12 A , method  1200 A may include the following steps  1202 A and  1204 A. 
     At step  1202 A, the encoder encodes a current picture to a bitstream based on a collocated picture, wherein the collocated picture is used for temporal motion vector prediction. At step  1204 A, the collocated picture in the bitstream is indicated without signaling an index of a reference picture list. Since the collocated picture is indicated without referring to an entry in the reference picture list via the index, the collocated picture can be legally indicated even if SH overrides the number of active entries in reference picture list 1 to be 0. Therefore, the robustness of encoding process is improved. 
       FIG.  12 B  shows an exemplary flowchart of an encoding method  1200 B, according to some embodiments of present disclosure. It is appreciated that method  1200 B can be part of step  1204 A in method  1200 A of  FIG.  12 A .  FIG.  12 C  shows another flow-chart of an exemplary video encoding method  1200 B for indicating a collocated picture, according to some embodiments of present disclosure. Referring to  FIG.  12 B  and  FIG.  12 C , in some embodiments, the method  1200 B may further include the following steps  1202 B- 1206 B. 
     At step  1202 B, when the collocated picture is an inter-layer reference picture, a first parameter is signaled to indicate the collocated picture. The first parameter indicates the index of the collocated picture to the list of direct reference layers of the layer where the current picture is in. For example, the index could be syntax element inter_layer_col_pic_idx. Therefore, the collocated picture is indicated without using the reference picture list. The illegal scenarios can be avoided when the SH overrides the number of active entries in the reference picture list. Prior to step  1202 B, a flag to indicate whether the collocated picture is an inter-layer reference picture can be signaled. The step  1202 B may also be referred to  1201 C and  1202 C in  FIG.  12 C . 
     At step  1204 B, when the collocated picture is a short-term reference picture (STRP), a delta picture order count (delta POC) is signaled. Furthermore, a POC can be derived by the delta POC. In this scenario, the collocated picture is indicated using the POC, without using the reference picture list. Therefore, the illegal scenarios can be avoided when the SH overrides the number of active entries in the reference picture list. The step  1204 B may also be referred to  1203 C and  1204 C in  FIG.  12 C . 
     At step  1206 B, when the collocated picture is a long-term reference picture (LTRP), a least significant bits (LSB) of POC and a most significant bits (MSB) of POC is signaled. Furthermore, a POC can be derived by the LSB and MSB. In this scenario, the collocated picture is indicated using the POC, without using the reference picture list. Therefore, the illegal scenarios can be avoided when the SH overrides the number of active entries in the reference picture list. The step  1206 B may also be referred to  1203 C and  1205 C in  FIG.  12 C . Indicating the collocated picture using the POC can efficiently enhance the robustness for determining the collocated picture. In some embodiments, prior to steps  1204 B and  1206 B, a flag to indicate whether the collocated picture is a short-term reference picture can be signaled. 
       FIG.  12 D  illustrates a flow-chart of an exemplary video decoding method  1200 D for indicating a collocated picture without decoding the index to the reference picture list, according to some embodiments of present disclosure. Method  1200 D can be performed by a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1200 D. In some embodiments, method  1200 D can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  12 D , method  1200 D may include the following steps  1202 D- 1206 D. 
     At step  1202 D, the decoder receives a video bitstream (e.g., video bitstream  228  in  FIG.  3 B ) for processing and the video bitstream may be coded using inter prediction. The reference pictures can be derived, for example, by reference picture 0 and reference picture list 1, each of which includes a list of reconstructed pictures in the DPB (e.g., buffer  234  in  FIG.  3 B ) to be used as the reference pictures. 
     At step  1204 D, a collocated picture used for temporal motion vector prediction is determined based on the bitstream but without decoding an index to a reference picture list. 
     At step  1206 D, a current picture is decoded based on the collocated picture. Since the collocated picture is indicated without using the reference picture list structure, the collocated picture can be legally indicated even if SH overrides the number of active entries in reference picture list 1 to be 0. Therefore, the robustness of decoding process is improved. 
     Since the collocated picture is indicated without using the reference picture list structure, the collocated picture can be legally indicated even if SH overrides the number of active entries in reference picture list 1 to be 0. Therefore, the robustness of decoding process is improved. 
       FIG.  12 E  shows an exemplary flowchart of a decoding method  1200 E, according to some embodiments of present disclosure. It is appreciated that method  1200 E can be part of step  1204 D in method  1200 D of  FIG.  12 D . 
     At step  1202 E, when the collocated picture is an inter-layer reference picture, a first parameter is decoded to indicate the collocated picture. The first parameter indicates the index of the collocated picture to the list of direct reference layers of the layer where the current picture is in. For example, the index could be syntax element inter_layer_col_pic_idx. Therefore, the collocated picture is indicated without using the reference picture list. The illegal scenarios can be avoided when the SH overrides the number of active entries in the reference picture list. In some embodiments, prior to step  1202 E, a first flag that indicates whether the collocated picture is an inter-layer reference picture is decoded, and whether the collocated picture is an inter-layer reference picture is determined based on the first flag. 
     At step  1204 E, when the collocated picture is a short-term reference picture (STRP), a delta picture order count (delta POC) is decoded. Furthermore, a POC can be derived by the delta POC. In this scenario, the collocated picture is indicated using the POC, without using the reference picture list. Therefore, the illegal scenarios can be avoided when the SH overrides the number of active entries in the reference picture list. 
     At step  1206 E, when the collocated picture is a long-term reference picture (LTRP), a least significant bits (LSB) of POC and a most significant bits (MSB) of POC is decoded. Furthermore, a POC can be derived by the LSB and MSB. In this scenario, the collocated picture is indicated using the POC, without using the reference picture list. Therefore, the illegal scenarios can be avoided when the SH overrides the number of active entries in the reference picture list. In some embodiment, prior to step  1204 E and  1206 E, a second flag that indicates whether the collocated picture is a short-term reference picture is decoded, and whether the collocated picture is a short-term reference picture is determined based on the second flag. 
       FIG.  12 F  and  FIG.  12 G  illustrate a portion of an exemplary picture parameter set syntax structure  1200 F and a portion of an exemplary slice header syntax structure  1200 G, according to some embodiments of the present disclosure. The picture parameter set syntax structure  1200 F together with the slice header syntax structure  1200 G can be used in methods  1200 A,  1200 B,  1200 D and  1200 E. Picture parameter set syntax structure  1200 F is modified based on a portion  960 A of syntax structure  900 A of  FIG.  9 A , and changes from the previous VVC are shown in italic, with proposed deleted syntax being further shown in strikethrough. Slice header syntax structure  1200 G is modified based on a portion  1060 A of syntax structure  1000 A of  FIG.  10 A , with proposed deleted syntax being further shown in strikethrough. As shown in  FIG.  12 F  and  FIG.  12 G , the syntax element ph_collocated_from_l0_flag, ph_ollocated_ref_idx, sh_collocated_from_l0_flag and sh_collocated_ref_idx are no longer signaled in PPS nor in SH. 
     As shown in  FIG.  12 F , syntax element  1210 F (e.g., inter_layer_col_pic_flag) being equal to 1 specifies that the collocated picture used for temporal motion vector prediction is referred to by an ILRP entry in the reference picture list. Syntax element  1210 F being equal to 0 specifies that collocated picture used for temporal motion vector prediction is not referred to by an ILRP entry in the reference picture list. When syntax element  1210 F is not present, the value of syntax element  1210 F is inferred to be equal to 0. The syntax element  1210 F can be signaled in  1201 C for determining whether the collocated picture is an inter-layer reference picture. 
     Syntax element  1220 F (e.g., st_col_pic_flag) being equal to 1 specifies that collocated picture used for temporal motion vector prediction is referred to by an STRP entry in the reference picture list. Syntax element  1220 F being equal to 0 specifies that collocated picture used for temporal motion vector prediction is referred to by an LTRP entry in the reference picture list. When syntax element  1210 F is equal to 0 and syntax element  1220 F is not present, the value of syntax element  1220 F is inferred to be equal to 1. The syntax element  1220 F can be signaled in  1203 C for determining whether the collocated picture is a short-term reference picture. If the syntax element  1220 F is equal to 1 (e.g.,  1203 C—true in  FIG.  12 C ), then step  1204 B (as shown in  FIG.  12 B ) is processed, and a delta picture order count (delta POC) is signaled (e.g., in  1204 C in  FIG.  12 C ). If the syntax element  1220 F equals to 0 (e.g.,  1203 C—false in  FIG.  12 C ), then step  1206 B (as shown in  FIG.  12 B ) is processed, and a least significant bits (LSB) of POC and a most significant bits (MSB) of POC is signaled (e.g., in  1205 C in  FIG.  12 C ). 
     Syntax element  1230 F (e.g., abs_delta_poc_st_col) specifies the value of the variable AbsDeltaPocStCol.  FIG.  12 H  shows an example pseudocode including derivation of AbsDeltaPocStCol  1210 H, according to some embodiments of the present disclosure. The value of syntax element  1230 F (e.g., abs_delta_poc_st_col) can be in an inclusive range of 0 to 2 15 −1. 
     Referring back to  FIG.  12 F , syntax element  1240 F (e.g., sign_delta_poc_st_col_flag) being equal to 1 specifies that the value of variable DeltaPocValStCol is greater than or equal to 0. Syntax element  1240 F being equal to 0 specifies that the value of variable DeltaPocValStCol is less than 0. When syntax element  1240 F is not present, the value of syntax element  1240 F is inferred to be equal to 1.  FIG.  12 I  shows an example pseudocode including derivation of DeltaPocValStCol, according to some embodiments of the present disclosure. The variable DeltaPocValStCol can be derived as shown in  FIG.  12 I . 
     Referring back to  FIG.  12 F , in some embodiments, syntax element  1250 F (e.g., poc_lsb_lt_col) specifies the value of the picture order count modulo MaxPicOrderCntLsb of the collocated picture used for temporal motion vector prediction. The length of the syntax element  1250 F is sps_log 2_max_pic_order_cnt_lsb_minus4+4 bits. 
     Syntax element  1260 F (e.g., delta_poc_msb_cycle_lt_col) specifies the value of the variable FullPocLtCol as follows: 
       FullPocLtCol=PicOrderCntVal−delta_poc_msb_cycle_lt_col*MaxPicOrderCntLsb−(PicOrderCntVal &amp;(MaxPicOrderCntLsb−1))+poc_lsb_lt_col
 
     Syntax element  1270 F (e.g., delta_poc_msb_cycle_col_present_flag) being equal to 1 specifies that syntax element  1260 F (e.g., delta_poc_msb_cycle_lt_col) is present. Syntax element  1270 B being equal to 0 specifies that syntax element  1260 F is not present. 
     Further for syntax element  1270 F, let prevTid0Pic be the previous picture in decoding order that has nuh_layer_id the same as the slice or picture header referring to the ref_pic_lists ( ) syntax structure, has TemporalId equal to 0, and is not a RASL or RADL picture. Let setOfPrevPocVals be a set consisting of the following:
         the PicOrderCntVal of prevTid0Pic,   the PicOrderCntVal of each picture that is referred to by entries in RefPicList[0] or RefPicList[1] of prevTid0Pic and has nuh_layer_id the same as the current picture,   the PicOrderCntVal of each picture that follows prevTid0Pic in decoding order, has nuh_layer_id the same as the current picture, and precedes the current picture in decoding order.       

     When there is more than one value in setOfPrevPocVals for which the value modulo MaxPicOrderCntLsb is equal to syntax element  1250 F (e.g., poc_lsb_lt_col), the value of delta_poc_msb_cycle_present_flag[i]I[j] shall be equal to 1. 
     Syntax element  1280 F (e.g., inter_layer_col_pic_idx) specifies the index, to the list of the direct reference layers, of the collocated picture used for temporal motion vector when the collocated picture used for temporal motion vector prediction is referred to by an ILRP entry in the reference picture list. The value of syntax element  1280 F can be in an inclusive range of 0 to NumDirectRefLayers[GeneralLayerIdx[nuh_layer_id]]−1. 
     As shown in  FIG.  12 F , when sps_inter_layer_ref_pics_present_flag (e.g., syntax element  720 ) is equal to 1, syntax element  1210 F is signaled, that is, ILRPs may be used for inter prediction of one or more coded pictures in the CLVS, an index (e.g., syntax element  1280 F inter_layer_col_pic_idx) is signaled to indicate which inter-layer reference picture is treated as collocated picture, which is corresponding to step  1202 B in  FIG.  12 B . If the collocated picture is a short-term reference picture, that is, syntax element  1220 F (e.g., st_col_pic_flag) is equal to 1, a delta POC (e.g., syntax element  1230 F) is signaled, which is corresponding to step  1204 B in  FIG.  12 B . If the collocated picture is a long-term reference picture, that is syntax element  1220 F (e.g., st_col_pic_flag) is equal to 0, a LSB of POC (e.g., syntax element  1250 F and a delta MSB of POC (e.g., syntax element  1260 F) are signaled, which is corresponding to step  1206 B in  FIG.  12 B . Furthermore, a MSB of POC can be derived by a delta MSB, and a POC can be derived by a MSB and an LSB. Therefore, the collocated picture can be indicated independently from the reference picture list structure. 
     Considering the fact that VVC (e.g., VVC draft 9) has a constraint that the collocated picture referred to by all the slices within a picture should be a same picture, according to the updated syntax structure  1200 F and  1200 G, the collocated picture can be only indicated in PH, and not in SH. As a result, all the slices within a picture can be guaranteed to have the same collocated picture and the constraint is not needed, therefore the efficiency and robustness for indicating the collocated picture is enhanced. 
       FIG.  12 J  illustrates an example pseudocode for deriving the collocated picture denoted as colPic and the flag colPicFlag used in methods  1200 A,  1200 B,  1200 C and  1200 D, according to some embodiments of the present disclosure. As shown in  FIG.  12 J , for different scenarios of the collocated picture, such as the collocated picture being referred to by an STRP entry in the reference picture list (as shown in scenario  1210 J), the collocated picture being referred to by an LTRP entry in the reference picture list (as shown in scenario  1220 J), or the collocated picture being referred to by an ILPR entry in the reference picture list (as shown in scenario  1230 J), all the slices within a picture have the same collocated picture (e.g., picA). Therefore, the robustness for determining the collocated picture is improved. 
     In some embodiments, there is a requirement of bitstream conformance that the following constraints apply: colPic is not be “no reference picture” and is referred to by an active entry in RefPicList[0] or RefPicList[1] and colPicFlag is equal to 0, when ph_temporal_mvp_enabled_flag is equal to 1. The “no reference picture” can be regarded as a marker to indicate that there is no reference picture in RPL. The colPicFlag being equal to 0 indicates that the current picture and the collocated picture have a same picture size and same scaling window. In another word, when the temporal MVP is enabled, the collocated picture should exist in the reference picture list and is referred to by an active entry in reference picture list 0 or reference picture list 1. Therefore, the robustness for the collocated picture is improved. 
     In VVC (e.g., VVC draft 9), ref_pic_ist_struct( ) and the syntax elements that are used to identify the collocated picture (syntax element  930 A (e.g., ph_collocated_from_l0_flag) and syntax element  940 A (e.g., ph_collocated_ref_idx) in PH and syntax element  1040 A (e.g., sh_collocated_from_l0_flag) and syntax element  1050 A (e.g., sh_colocated_ref_idx) in SH)) may be signaled in PH or SH dependent on the value of pps_rpl_info_ph_flag. When the value of pps_rpl_info_ph_flag is equal to 1, syntax element  930 A, syntax element  940 A and ref_pic_list_struct( ) are signaled in PH, and syntax element  1040 A and syntax element  1050 A are not signaled. In this case, the value of syntax element  1040 A and syntax element  1050 A are inferred according to the value of syntax element  930 A, syntax element  940 A and slice type of the current slice. If it is a B slice, syntax element  1040 A is inferred to be equal to syntax element  930 A. If it is a P slice, syntax element  1040 A is directly inferred to be equal to 1 regardless of the value of syntax element  930 A. And syntax element  1050 A is inferred to be equal to syntax element  940 A for both P and B slices. However, for syntax element  940 A that is signaled in PH, the maximum allowed value is the number of entries in reference picture list minus 1, but for syntax element  1050 A, the maximum allowed value is the number of active entries in reference picture list minus 1 which may be overridden in slice header. As a result, when syntax element  1050 A is inferred to be equal to syntax element  940 A, it might violate the maximum value constraint. 
     For example, when syntax element  930 A is signaled as 0, the number of entries in reference picture list 1 (num_ref_entries[1] signaled in ref_pic_list_structure( )) is N and ph_colocated_ref_idx is signaled as N−1, and then in this case, syntax element  1040 A is inferred to be equal to 0 and syntax element  1050 A is inferred to be equal to N−1. But the number of active entries in reference picture list 1 may be overridden as a number less than N. In that case, the bitstream is illegal. 
     In another example, when syntax element  930 A is signaled as 0, the number of entries in reference picture list 1 (num_ref_entries[1] signaled in ref_pic_list_structure( )) is N, syntax element  940 A is signaled as N−1, and the number of active entries is not overridden in slice header (assume the number of active entries is the same as the number of entries in both reference picture lists). But if the current slice is a P slice, then syntax element  940 A is inferred to be equal to 1 and syntax element  1050 A is inferred to be equal to N−1. However, the number of entries in reference picture list 0 (num_ref_entries[0] signaled in ref_pic_list_structure( )) may be less than N. As a result, the bitstream is also illegal. 
     To overcome this deficiency with conventional coding technologies, in some embodiments of the present disclosure (such as provided below in  FIGS.  13 A- 13 C ), the collocated picture in SH is inferred also based on the number of active entries in reference picture list. 
       FIG.  13 A  shows a flow-chart of an exemplary video encoding method  1300 A for determining the index of collocated picture in SH using the number of active entries in reference picture list, according to some embodiments of the disclosure. Method  1300 A can be performed by an encoder (e.g., by process  200 A of  FIG.  2 A or  200 B  of  FIG.  2 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1300 A. In some embodiments, method  1300 A can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  13 A , method  1300 A may include the following steps  1302 A- 1306 A. 
     At step  1302 A, whether to signal a parameter to indicate a reference index of the collocated picture in a slice header is determined. In VVC, the parameter to indicate a reference index of the collocated picture in a slice header could be syntax element sh_collocated_ref_idx. 
     At step  1304 A, when the parameter not being signaled in the slice header, the collocated picture is determined as the picture referred to by an index with the value equal to the smaller one between a value of a reference index of the collocated picture signaled in a picture header (e.g., ph_collocated_ref_idx) and a number of active entries in a target reference picture list minus 1 (e.g., NumRefIdxActive[!sh_collocated_from_l0_flag]−1). The target reference picture list in reference picture lists is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived. Therefore, the number of active entries in the reference picture is taken into consideration when inferring the value of syntax element  1050 A (e.g., sh_collocated_ref_idx). If the value of syntax element  940 A (e.g., ph_ollocated_ref_idx) signaled in PH is greater than or equal to the number of active entries in the target reference picture list, the inferred value of syntax element  1050 A (e.g., sh_collocated_ref_idx) is clipped to less than the number of active entries in the target reference picture list. The target reference picture list in the reference picture list is indicated by syntax element  1040 A (e.g., sh_collocated_from_l0_flag). 
     At step  1306 A, a current picture is encoded based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction. Therefore, illegal bitstreams are avoided and the robustness of the collocated picture is improved. 
       FIG.  13 B  shows a flow-chart of an exemplary video decoding method  1300 B for determining the index of collocated picture in SH using the number of active entries in reference picture list, according to some embodiments of the disclosure. Method  1300 B can be performed by a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1300 B. In some embodiments, method  1300 B can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  13 B , method  1300 B may include the following steps  1302 B- 1310 B. 
     At step  1302 B, the decoder receives a video bitstream (e.g., video bitstream  228  in  FIG.  3 B  and the video bitstream may be coded using inter prediction. Therefore the reference pictures can be derived, for example, by reference picture 0 and reference picture list 1, each of which includes a list of reconstructed pictures in the DPB (e.g., buffer  234  in  FIG.  3 B ) to be used as the reference pictures. 
     At step  1304 B, whether a parameter indicating a reference index of the collocated picture used for temporal motion vector prediction being present in a slice header is determined. In VVC, the parameter to indicate a reference index of the collocated picture in the slice header could be syntax element sh_collocated_ref_idx. 
     At step  1306 B, when the parameter being not present, a value of the parameter is determined to be equal to the smaller one between a value of a reference index of the collocated picture used for temporal motion vector prediction present in picture header (e.g., ph_ollocated_ref_idx) and a number of active entries in a target reference picture list minus 1 (e.g., NumRefIdxActive[!sh_collocated_from_l0_flag]−1). The target reference picture list in reference picture lists is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived. Therefore, the number of active entries in the reference picture is taken into consideration when determining the value of syntax element  1050 A (e.g., sh_collocated_ref_idx). If the value of syntax element  940 A (e.g., ph_ollocated_ref_idx) signaled in PH is greater than or equal to the number of active entries in the target reference picture list, the determined value of syntax element  1050 A (e.g., sh_collocated_ref_idx) is clipped to less than the number of active entries in the target reference picture list. The target reference picture list in the reference picture list is indicated by syntax element  1040 A (e.g., sh_collocated_from_l0_flag). Therefore, illegal bitstreams are avoided. 
     At step  1308 B, the collocated picture is determined as a picture referred to by an index with a value equal to the parameter in the target reference picture list. The robustness of the collocated picture is improved. 
     At step  1310 B, a current picture is decoded based on the collocated picture. The reliability of the decoding process is improved. 
       FIG.  13 C  illustrates a portion of an exemplary semantics  1300 C, according to some embodiments of the present disclosure. The semantics  1300 C can be used in method  1300 A and method  1300 B. As shown in  FIG.  13 C , changes from the previous VVC are shown in italic, with proposed deleted syntax being further shown in strikethrough in syntax  1310 C. The syntax  1310 C is corresponding to the step  1306 A in  FIG.  13 A  and step  1306 B in  FIG.  13 B . If pps_rpl_info_in_ph_flag (e.g., syntax element  830 ) is equal to 1, that means the reference picture list information is present in the PH syntax structure and not present in SH referring to the PPS that do not contain a PH syntax structure, the value of sh_collocated_ref_idx (e.g., syntax element  1050 A) is inferred to be equal to min(ph_collocated_ref_idx, NumRefIdxActive[!sh_collocated_from_l0_flag]−1), that is, the value of sh_collocated_ref_idx is set equal to a smaller one of a value of reference index of the collocated picture used for temporal motion vector prediction in picture header (e.g., ph_ollocated_ref_idx) and a number of active entries in a target reference picture list minus 1 (e.g., NumRefIdxActive[!sh_collocated_from_l0_flag]−1). The target reference picture list is indicated by syntax element  1040 A (e.g., sh_collocated_from_l0_flag), which is the reference picture list that the collocated picture used for temporal motion vector prediction is derived from. When the collocated picture used for temporal MVP is derived from reference picture list 0, the target reference picture list is reference picture list 0. When the collocated picture used for temporal MVP is derived from reference picture list 1, the target reference picture list is reference picture list 1. 
     In VVC (e.g., VVC draft 9), ref_pic_ist_struct( ) may be signaled in SPS or included in syntax structure ref_pic_lists( ). When ref_pic_list_structure( ) that are signaled in SPS are not selected in PH or SH, another ref_pic_list_structure( ) may be directly signaled in ref_pic_lists( ), which is signaled in PH or SH. However, the VVC (e.g., VVC draft 9) provides the following: each value of listIdx (equal to 0 or 1), a decoder should allocate memory for a total number of sps_num_ref_pic_lists[i] plus one ref_pic_list_struct(listIdx, rplsIdx) syntax structures since there may be one ref_pic_list_struct(listIdx, rplsIdx) syntax structure directly signalled in the slice headers of a current picture. This is not accurate in view of the above. 
     To overcome this deficiency with conventional coding technologies, in some embodiments of the present disclosure (such as provided below in  FIG.  14 A  and  FIG.  14 B ), for each value of listIdx (equal to 0 or 1), a decoder allocate memory for a total number of sps_num_ref_pic_lists[i] plus one ref_pic_list_struct(listIdx, rplsIdx) syntax structures for the case that one ref_pic_list_struct(listIdx, rplsIdx) syntax structure directly signalled in the picture headers or slice headers of a current picture. 
       FIG.  14 A  illustrates a flow-chart of an exemplary video processing method  1400 A for allocating memory, according to some embodiments of the disclosure. Method  1400 A can be performed by an encoder (e.g., by process  200 A of  FIG.  2 A or  200 B  of  FIG.  2 B ), a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processor (e.g., processor  402  of  FIG.  4   ) can perform method  1400 A. In some embodiments, method  1400 A can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  14 A , method  1400 A may include the following steps  1402 A- 1406 A. 
     At step  1402 A, a total number by summing a number of reference picture list structures in sequence parameter set (SPS) and one is derived. Since there is a possibility that one additional RPL is signaled later (in picture header or slice header), the additional number one is added to number of reference picture list structure in SPS to get a total number. 
     At step  1404 A, memory for the total number of reference picture list structures in response to a reference picture list structure being signaled in a picture header of a current picture or a slice header of a current slice is allocated. Therefore, more memory is allocated for the additional RPL that is signaled in a picture header of a current picture or in a slice header of a current slice by an encoder/decoder before encoding/decoding, which will be helpful for video processing. 
     At step  1406 A, a current picture or a current slice is processed using the allocated memory. Since the allocated memory is more reliable for the additional RPL, the encoding/decoding process can be more accurate and robust. 
       FIG.  14 B  illustrates a portion of an exemplary semantics  1400 B, according to some embodiments of the present disclosure. The semantics  1400 B can be used in method  1400 A, changes from the previous VVC are shown in italic (refer to block  1410 B). More memory is allocated for additional RPL for a possibility that one additional RPL is signaled later (in picture header or slice header). 
     In VVC (e.g., VVC draft 9), syntax element  530 A (e.g., rpl_idx[i]) specifies the index, into the list of the ref_pic_list_struct(listIdx, rplsIdx) syntax structures with listIdx equal to i included in the SPS, of the ref_pic_list_struct(listIdx, rplsIdx) syntax structure with listIdx equal to i that is used for derivation of reference picture list i of the current picture. This semantics may not be accurate since reference picture list may be derived for a picture or a slice. 
     In VVC (e.g., VVC draft 9), when syntax element  530 A is not present, there is a inference rule to infer the value of syntax element  530 A: if syntax element  510 A (e.g., rpl_sps_flag[i]) is equal to 1 and syntax element  520 A (e.g., pps_rpl1_idx_present_flag) is equal to 0, the value of rpl_idx[1] is inferred to be equal to rpl_idx[0], otherwise the value of rpl_idx[1] is inferred to be equal to 0. The inference rule has some problems. First, there is only an inference rule for rpl_idx[1], but no inference rule for rpl_idx[0]. Second, when syntax element  510 A is equal to 1 and syntax element  520 A is equal to 0, there is no guarantee that rpl_idx[0] is signaled. So inferring the value of rpl_idx[1] to be equal to rpl_idx[0] may be problematic in this case. In a word, the inference rule in VVC (e.g., VVC draft 9) cannot guarantee that both rpl_idx[0] and rpl_idx[1] get a proper value in decoder side when they are not present. 
     To overcome this deficiency with conventional coding technologies, in some embodiments of the present disclosure (such as provided below in  FIGS.  15 A- 15 C ), an updated semantics for syntax element  530 A (e.g., rpl_idx[i]) is provided. 
       FIG.  15 A  illustrates a flow-chart of an exemplary video encoding method  1500 A for determining the index in the reference picture list, according to some embodiments of the disclosure. Method  1500 A can be performed by an encoder (e.g., by process  200 A of  FIG.  2 A or  200 B  of  FIG.  2 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1500 A. In some embodiments, method  1500 A can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  15 A , method  1500 A may include the following steps  1502 A- 1514 A. 
     At step  1502 A, a first flag (e.g., pps_rpl1_idx_present_flag) in a picture parameter set (PPS) is signaled to indicate whether a second flag (e.g., rpl_sps_flag[1]) and a first index (e.g., rpl_idx[1]) being present in a picture header syntax or a slice header for a current picture referring to the PPS. The first flag (e.g., pps_rpl1_idx_present_flag) indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index (e.g., rpl_idx[1]) is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1. Then the second flag (e.g., rpl_sps_flag[1]) can be signaled. 
     At step  1504 A, whether the first index (e.g., rpl_idx[1]) and a second index (e.g., rpl_idx[0]) to be signaled is determined. The second index (e.g., rpl_idx[0]) is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0. 
     When the second index (e.g., rpl_idx[0]) is not to be signaled, a value of the second index (e.g., rpl_idx[0]) can be determined by step  1506 A. 
     At step  1506 A, the value of the second index (e.g., rpl_idx[0]) is determined to be equal to 0, when at most one reference picture list structure associated with reference picture list 0 is included in SPS. Referring to  FIG.  5 A , when the sps_num_ref_pic_lists[0] is less than or equal to one, the rpl_idx[0] is not signaled. Therefore, with step  1506 A, the value of rpl_idx[0] is determined for the situation that the rpl_idx[0] is not signaled, enhancing the reliability for inferring rpl_idx[0]. 
     When the first index (e.g., rpl_idx[1]) is not to be signaled, the value of the first index (e.g., rpl_idx[0]) can be determined by the step  1508 A and  1510 A. 
     At step  1508 A, the value of the first index (e.g., rpl_idx[1]) is determined to be equal to 0 when at most one reference picture list structure associated with reference picture list 1 is included in SPS. Referring to  FIG.  5 A , when the sps_num_ref_pic_lists[1] is less than or equal to one, the rpl_idx[1] is not signaled. Therefore, with step  1508 A, the value of rpl_idx[1] is determined for the situation that the rpl_idx[1] is not signaled, enhancing the reliability for inferring rpl_idx[1]. 
     At step  1510 A, the value of the first index (e.g., rpl_idx[1]) is determined to be equal to the value of the second index (e.g., rpl_idx[0]), when the first flag (e.g., pps_rpl1_idx_present_flag) is equal to 0 and the second flag (e.g., rpl_sps_flag[1]) is equal to 1. Since the value of rpl_idx[0] is set to 0 if sps_num_ref_pic_lists[0] is less than or equal to one (in step  1508 A) and otherwise (e.g., sps_num_ref_pic_list[0]&gt;1), the rpl_idx[0] is signaled (referring to  FIG.  5 A ), the value of rpl_idx[0] is determined for all the scenarios. Therefore, in this case, the value of rpl_idx[1] is set equal to the value of rpl_idx[0], which is determined. Thus, for all the scenarios (e.g., no matter rpl_idx[0] is signaled or not), the value of rpl_idx[1] is determined. The value of rpl_idx[i] (both rpl_idx[0] and rpl_idx[1]) can be guaranteed to get a proper value if the rpl_idx[i] is not signaled. 
     After the determination of values of the first index (e.g., rpl_idx[1]) and the second index (e.g., rpl_idx[0]), at step  1512 A, the reference picture list is determined based on the first index and the second index. Since the determination of the values of the first index (e.g., rpl_idx[1]) and the second index (e.g., rpl_idx[0]) is guaranteed for the cases no matter the first index (e.g., rpl_idx[1]) and the second index (e.g., rpl_idx[0]) being signaled or not, the determination for the reference picture list can be more reliable. 
     At step  1514 A, a current picture is encoded based on the reference picture list. Therefore, the robustness for the encoding process is improved. 
     In some embodiments, the step  1510 A can be replaced by “rpl_idx[i] is determined to be equal to 0 in response to one reference picture list structure for reference picture list i being present in SPS”, as rpl_idx[0] is inferred to be equal to 0 when one reference picture list structure of reference picture list 0 being present in SPS (referring to step  1508 A). The efficiency of encoding process can be further improved. 
       FIG.  15 B  illustrates a flow-chart of an exemplary video decoding method  1500 B for determining the index in the reference picture list, according to some embodiments of the disclosure. Method  1500 B can be performed by a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1500 B. In some embodiments, method  1500 B can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  15 B , method  1500 B may include the following steps  1502 B- 1514 B. 
     At step  1502 B, the decoder receives a video bitstream (e.g., video bitstream  228  in  FIG.  3 B ) and the video bitstream may be coded using inter prediction. the reference pictures can be derived, for example, by reference picture 0 and reference picture list 1, each of which includes a list of reconstructed pictures in the DPB (e.g., buffer  234  in  FIG.  3 B ) to be used as the reference pictures. 
     At step  1504 B, a value of a first flag (e.g., pps_rpl1_idx_present_flag) indicating whether a second flag (e.g., rpl_sps_flag[1]) and a first index (e.g., rpl_idx[1]) is present in a picture header syntax or a slice header for a current picture is determined. The second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1. Then the value of the second flag (e.g., rpl_sps_flag[1]) can be determined. 
     At step  1506 B, whether the first index (e.g., rpl_idx[1]) and a second index (e.g., rpl_idx[0]) being present is determined. The second index (e.g., rpl_idx[0]) is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0. 
     When the second index (e.g., rpl_idx[0]) is not present, a value of the second index (e.g., rpl_idx[0]) can be determined by step  1508 B. 
     At step  1508 B, the value of the second index (e.g., rpl_idx[0]) is determined to be equal to 0, when at most one reference picture list structure associated with reference picture list 0 is included in SPS. Referring to  FIG.  5 A , when the sps_num_ref_pic_lists[0] is less than or equal to one, the rpl_idx[0] is not signaled, thus the rpl_idx[0] is not present. In this case, the rpl_idx[0] is set to be equal to 0. Therefore, with step  1508 B, the value of rpl_idx[0] is determined for the situation that the rpl_idx[0] is not present, enhancing the reliability for inferring rpl_idx[0]. 
     When the first index (e.g., rpl_idx[1]) is not present, the value of the first index (e.g., rpl_idx[1]) can be determined by the step  1510 B and  1512 B. 
     At step  1510 B, the value of the first index (e.g., rpl_idx[1]) is determined to be equal to 0 when at most one reference picture list structure associated with reference picture list 1 is included in SPS. Referring to  FIG.  5 A , when the sps_num_ref_pic_lists[1] is less than or equal to one, the rpl_idx[1] is not signaled, thus the rpl_idx[1] is not present. Therefore, with step  1510 B, the value of rpl_idx[1] is determined for the situation that the rpl_idx[1] is not signaled, enhancing the reliability for inferring rpl_idx[1]. 
     At step  1512 B, the value of the first index (e.g., rpl_idx[1]) is determined to be equal to the value of the second index (e.g., rpl_idx[0]), when the first flag (e.g., pps_rpl1_idx_present_flag) is equal to 0 and the second flag (e.g., rpl_sps_flag[1]) is equal to 1 Since the value of rpl_idx[0] is set to 0 if sps_num_ref_pic_lists[0] is less than or equal to one (in step  1508 A) and otherwise (e.g., sps_num_ref_pic_list[0]&gt;1), the rpl_idx[0] is signaled (referring to  FIG.  5 A ), the value of rpl_idx[0] is determined for all the scenarios. Therefore, in this case, the value of rpl_idx[1] is set equal to the value of rpl_idx[0], which is determined. Thus, for all the scenarios (e.g., no matter rpl_idx[0] is present or not), the value of rpl_idx[1] is determined. The value of rpl_idx[i] (both rpl_idx[0] and rpl_idx[1]) can be guaranteed to get a proper value if the rpl_idx[i] is not present. 
     At  1514 B, a current picture is decoded based on the first index (e.g., rpl_idx[1]) and the second index (e.g., rpl_idx[0]). As the determination of the values of the first index (e.g., rpl_idx[1]) and the second index (e.g., rpl_idx[0]) is guaranteed for the cases no matter the first index (e.g., rpl_idx[1]) and the second index (e.g., rpl_idx[0]) being present or not, the determination for the reference picture list can be more reliable. 
     In some embodiments, the step  1514 B can be replaced by “rpl_idx[i] is inferred to be equal to 0 in response to one reference picture list structure for reference picture list i being present in SPS”, as rpl_idx[0] is inferred to be equal to 0 when one reference picture list structure of reference picture list 0 being present in SPS (referring to step  1508 B). The efficiency of decoding process can be further improved. 
       FIG.  15 C  illustrates a portion of an exemplary semantics  1500 C, according to some embodiments of the present disclosure. The semantics  1500 C can be used in methods  1500 A and  1500 B. As shown in  FIG.  15 C , changes from the previous VVC are shown in italic, with proposed deleted syntax being further shown in strikethrough (referring to block  1510 C and  1520 C). Two alternative derivation descriptions are provided. In some embodiments, as shown in block  1510 C, for the case that the rpl_idx[i] is not present, if there is at most one reference picture list structure for reference picture list i (e.g., sps_num_ref_pic_list[i] is less than or equal to 1), the value of rpl_idx[i] is inferred to equal to 0; otherwise (there is more than one reference picture list structure for reference picture i, that is sps_num_ref_pic_list[i] is greater than 1), and i is equal to 1, that is sps_num_ref_pic_list[1] is greater than 1, the value of rpl_idx[1] is inferred to be equal to rpl_idx[0]. The difference between block  1520 C and  1510 C is that the expression of “otherwise” and “i is equal to 1” are interpreted in detail as “sps_num_ref_pic_list[1] is greater than 1.” In some embodiments, the condition “if sps_num_ref_pic_list[i] is less than or equal to 1” (referring to block  1511 C and block  1521 C) can be replaced by “if sps_num_ref_pic_list[i] is equal to 1.” 
     In VVC (e.g., VVC draft 9), syntax element  1010 A (e.g., sh_num_ref_idx_active_override_flag) being equal to 1 specifies that the syntax element sh_num_ref_idx_active_minus1[0] is present for P and B slices and the syntax element sh_num_ref_idx_active_minus1[1] is present for B slices. Syntax element  1010 A being equal to 0 specifies that the syntax elements sh_num_ref_idx_active_minus1[0] and sh_num_ref_idx_active_minus1[1] are not present. However, as shown in  FIG.  10 A , when syntax element  1010 A is equal to 1, the value of num_ref_entries[i][RplsIdx[i]] is further checked for signaling sh_num_ref_idx_active_minus1[i]. Syntax element sh_num_ref_idx_active_minus1[i] is signaled only when syntax element  1010 A is equal to 1 and num_ref_entries[i][RplsIdx[i]] is greater than 1. As a result, syntax element  1010 A equal to 1 does not necessarily mean sh_num_ref_idx_active_minus1[i] is signaled. 
     To overcome this deficiency with conventional coding technologies, in some embodiments of the present disclosure (such as provided below in  FIGS.  16 A- 16 C ), an updated semantics for syntax element  1010 A is provided to improve the efficiency of the encoding/decoding process. 
       FIG.  16 A  illustrates a flow-chart of an exemplary video encoding method  1600 A for indicating active reference index number in slice header present, according to some embodiments of the disclosure. Method  1600 A can be performed by an encoder (e.g., by process  200 A of  FIG.  2 A or  200 B  of  FIG.  2 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1600 A. In some embodiments, method  1600 A can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  16 A , method  1600 A may include the following steps  1602 A- 1608 A. 
     At step  1602 A, a first flag is signaled in a slice header to indicate whether an active reference index number is present in a slice header. For example, syntax element sh_num_ref_idx_active_override_flag is signaled to indicate whether the active reference index number of reference picture list i (e.g., sh_num_ref_idx_active_minus1[i]) (i equals to 0 or 1) is present in the slice header or not. The active reference index number is used to derive maximum reference index for a corresponding reference picture list that may be used to encode a current slice. The number of reference index used for encoding the current slice can be less than or equal to the maximum number derived from the active reference index number. 
     At step  1604 A, whether the active reference index number being present is determined. When the first flag indicates the active reference index number is present, the syntax element sh_num_ref_idx_active_minus1[0] is present for P and B slices and the syntax element sh_num_ref_idx_active_minus1[1] is present for B slices. Then, step  1606 A and step  1608 A are performed. 
     At step  1606 A, a number of entries of reference picture list 0 is determined first and if the number of entries of reference picture list 0 (e.g., num_ref_entries[0] [RplsIdx[0]]) is determined to be greater than 1, an active reference index number of reference picture list 0 (e.g., sh_num_ref_idx_active_minus1[0]) is signaled in slice header for P and B slice. 
     At step  1608 A, a number of entries of reference picture list 1 is determined first and if the number of entries of reference picture list 1 (e.g., num_ref_entries[1][RplsIdx[1]]) is determined to be greater than 1, an active reference index number of reference picture list 1 (e.g., sh_num_ref_idx_active_minus1[1]) is signaled in slice header for B slice. 
     With step  1606 A and step  1608 A, the active reference index number of reference picture list i (e.g., sh_num_ref_idx_active_minus1[i]) is signaled in the slice level when a number of entries of reference picture list i (e.g., num_ref_entries[i][RplsIdx[i]]) is greater than 1. 
     Therefore, the uncertainty for sh_num_ref_idx_active_minus1[i] signaled when the sh_num_ref_idx_active_override_flag being equals to 1 is eliminated, and the accuracy and robustness for encoding process can be improved. 
     In some embodiments, the method  1600 A can further include step  1610 A and  1612 A. When the first flag indicates the active reference index number is not present, the syntax element sh_num_ref_idx_active_minus1[i] is not present. Then, step  1610 A and step  1612 A are performed. 
     At step  1610 A, signaling the active reference index number of reference picture list 0 (e.g., sh_num_ref_idx_active_minus1[0]) is skipped in slice header for P and B slice. In another word, there is no sh_num_ref_idx_active_minus1[0] signaled in slice header for P and B slice. 
     At step  1612 A, signaling the active reference index number of reference picture list 1 is skipped in slice header for B slice. In another word, there is no sh_num_ref_idx_active_minus1[1] signaled in slice header for B slice. 
     Therefore, when the active reference index number is not present, by skipping signaling the active reference number, the encoding process can be more efficient. 
       FIG.  16 B  illustrates a flow-chart of an exemplary video decoding method  1600 B for indicating the active reference index number in slice header, according to some embodiments of the disclosure. Method  1600 B can be performed by a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1600 B. In some embodiments, method  1600 B can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  16 A , method  1600 B may include the following steps  1602 B- 1608 B. 
     At step  1602 B, the decoder receives a video bitstream (e.g., Video bitstream  228  in  FIG.  3 B ) including a slice header and a picture header syntax and the video bitstream may be coded using inter prediction. The reference pictures can be derived, for example, by reference picture 0 and reference picture list 1, each of which includes a list of reconstructed pictures in the DPB (e.g., buffer  234  in  FIG.  3 B ) to be used as the reference pictures. 
     At step  1604 B, a value of the first flag signaled in the slice header that indicates whether an active reference index number is present is determined. In some embodiments, the first flag is the syntax element sh_num_ref_idx_active_override_flag, which can indicate whether an active reference index of reference picture list i (e.g., sh_num_ref_idx_active_minus1[i]) (i equals to 0 or 1) is present or not. The active reference index number is used to derive maximum reference index for a corresponding reference picture list that may be used to decode a current slice. The number of reference index used for decoding the current slice can be less than or equal to the maximum number derived from the active reference index number. 
     When the value of the first flag is determined to a value indicating the active reference index number is present, the syntax element sh_num_ref_idx_active_minus1[0] is present for P and B slices and the syntax element sh_num_ref_idx_active_minus1[1] is present for B slices. Then, step  1606 B and step  1608 B are performed. 
     At step  1606 B, a number of entries of reference picture list 0 (e.g., num_ref_entries[0][RplsIdx[0]]) is determined and if the number of entries of reference picture list 0 is determined to be greater than 1, an active reference index number of reference picture list 0 (e.g., sh_num_ref_idx_active_minus1[0]) is decoded in slice header for P and B slice. 
     At step  1608 B, a number of entries of reference picture list 1 (e.g., num_ref_entries[1][RplsIdx[1]]) is determined and if the number of entries of reference picture list 1 is determined to be greater than 1, an active reference index number of reference picture list 1 (e.g., sh_num_ref_idx_active_minus1[1]) is decoded in slice header for B slice. 
     With step  1606 B and step  1608 B, the active reference index number of reference picture list i (e.g., sh_num_ref_idx_active_minus1[i]) is signaled when a number of entries of reference picture list i (e.g., num_ref_entries[i][RplsIdx[i]]) is greater than 1. Therefore, the uncertainty for sh_num_ref_idx_active_minus1[i] signaled when the sh_num_ref_idx_active_override_flag being equals to 1 is eliminated. 
     In some embodiments, the method  1600 B can further include step  1610 B and step  1612 B. When the value of the first flag is determined to be a value indicating the active reference index number is not present, the syntax element sh_num_ref_idx_active_minus1[i] is not signaled. Then, step  1610 B and step  1612 B are performed. 
     At step  1610 B, decoding the active reference index number of reference picture list 0 (e.g., sh_num_ref_idx_active_minus1[0]) is skipped in slice header for P and B slice. In another word, there is no sh_num_ref_idx_active_minus1[0] in slice header for B slice 
     At step  1612 B, decoding the active reference index number of reference picture list 1 (e.g., sh_num_ref_idx_active_minus1[1]) is skipped in slice header for B slice. In another word, there is no sh_num_ref_idx_active_minus1[1] present in slice header for B slice. Therefore, the efficiency of decoding process can be improved. 
       FIG.  16 C  illustrates a portion of an exemplary semantics  1600 C, according to some embodiments of the present disclosure. The semantics  1600 B can be used in method  1600 A and  1600 B. As shown in  FIG.  16 C , changes from the previous VVC are shown in italic, with proposed deleted syntax being further shown in strikethrough (referring to block  1610 C and  1620 C). Two alternative descriptions are provided. As shown in block  1610 C, sh_num_ref_idx_active_override_flag being equal to 1 doesn&#39;t necessarily specify the syntax element sh_num_ref_idx_active_minus1[0] is present for P and B slices or the syntax element sh_num_ref_idx_active_minus1[1] is present for B slices. As shown in block  1620 C, a condition of “when num_ref_entries[0][RplsIdx[0] is greater than 1” is added for the syntax element sh_num_ref_idx_active_minus1[0] is present for P and B slices and a condition of “when num_ref_entries[1][RplsIdx[1] is greater than 1” is added for the syntax element sh_num_ref_idx_active_minus1 [1] is present for B slices, for sh_num_ref_idx_active_override_flag being equal to 1. Therefore, the accuracy and robustness for decoding process can be improved. 
     In VVC (e.g., VVC draft 9), there is a bitstream conformance constraint that the picture referred to by syntax element  1050 A (e.g., sh_collocated_ref_idx) is the same for all slices of a coded picture and RprConstraintsActive[sh_collocated_from_l0_flag ? 0:1][sh_collocated_ref_idx] is equal to 0. To identify the picture referred to by syntax element  1050 A, the value of syntax element  1040 A (e.g., sh_collocated_from_l0_flag) and syntax element  1050 A (e.g., sh_collocated_ref_idx) need to be decided first. However, as shown in  FIG.  10 A , syntax element  1040 A is only signaled for B slice and syntax element  1050 A is only signaled for P and B slice. For I slices, syntax element  1040 A and syntax element  1050 A are not signaled. And there is also no inferred value for these two syntax elements for I slices. As a result, for I slices, the value of syntax element  1050 A is undefined. Therefore, the encoder/decoder cannot identify the picture referred to by syntax element  1050 A and cannot perform the conformance constraint check. 
     To overcome this deficiency with conventional coding technologies, in some embodiments of the present disclosure (such as provided below in  FIG.  17 A  and  FIG.  17 B ), updated semantics are provided to improve the accuracy and robustness for video processing. 
     For example,  FIG.  17 A  illustrates a flow-chart of an exemplary video processing method  1700 A for picture processing. Method  1700 A can be performed by an encoder (e.g., by process  200 A of  FIG.  2 A or  200 B  of  FIG.  2 B ), a decoder (e.g., by process  300 A of  FIG.  3 A or  300 B  of  FIG.  3 B ) or performed by one or more software or hardware components of an apparatus (e.g., apparatus  400  of  FIG.  4   ). For example, one or more processors (e.g., processor  402  of  FIG.  4   ) can perform method  1700 A. In some embodiments, method  1700 A can be implemented by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers (e.g., apparatus  400  of  FIG.  4   ). Referring to  FIG.  24 A , method  1700 A may include the step  1702 A and  1704 A. 
     At step  1702 A, determining a collocated picture referred to by a reference index of the collocated picture in slice level (e.g., sh_collocated_ref_idx), wherein the collocated picture is determined to be a same picture for all non-I slices of a current picture. Therefore, the uncertainty for the values of sh_collocated_ref_idx and sh_collocated_from_l0_flag is avoided. 
     At step  1704 A, the current picture is processed based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction. Thus, the robustness for video processing can be improved. 
     That is picture used for temporal motion vector prediction which is referred to by a reference index of collocated picture is determined to be the same for all non-I slices of a coded picture. In some embodiments, picture used for temporal motion vector prediction that is referred to by a reference index of collocated picture is determined to be the same for all P slices and B slices of a current picture. 
       FIG.  17 B  illustrates a portion of an exemplary semantics  1700 B, according to some embodiments of the present disclosure. The semantics  1700 B can be used in method  1700 A. As shown in  FIG.  17 B , changes from the previous VVC are shown in italic, with proposed deleted syntax being further shown in strikethrough (referring to block  1710 B and  1720 B). Two alternative description are provided. As shown in block  1710 B, the requirement of bitstream conformance is further detailed to “all non-I slices” instead of “all slices”. Therefore, the efficiency and robustness for decoding process is improved. The difference between block  1720 B and  1710 B is that the expression of “non-I slices” is replaced by “P slices and B slices” to be more accurate. 
     In some embodiments, a non-transitory computer-readable storage medium including instructions is also provided, and the instructions may be executed by a device (such as the disclosed encoder and decoder), for performing the above-described methods. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM or any other flash memory, NVRAM, a cache, a register, any other memory chip or cartridge, and networked versions of the same. The device may include one or more processors (CPUs), an input/output interface, a network interface, and/or a memory. 
     It should be noted that, the relational terms herein such as “first” and “second” are used only to differentiate an entity or operation from another entity or operation, and do not require or imply any actual relationship or sequence between these entities or operations. Moreover, the words “comprising,” “having,” “containing,” and “including,” and other similar forms are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. 
     As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a database may include A or B, then, unless specifically stated otherwise or infeasible, the database may include A, or B, or A and B. As a second example, if it is stated that a database may include A, B, or C, then, unless specifically stated otherwise or infeasible, the database may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. 
     It is appreciated that the above-described embodiments can be implemented by hardware, or software (program codes), or a combination of hardware and software. If implemented by software, it may be stored in the above-described computer-readable media. The software, when executed by the processor can perform the disclosed methods. The computing units and other functional units described in this disclosure can be implemented by hardware, or software, or a combination of hardware and software. One of ordinary skill in the art will also understand that multiple ones of the above-described modules/units may be combined as one module/unit, and each of the above-described modules/units may be further divided into a plurality of sub-modules/sub-units. 
     The embodiments may further be described using the following clauses:
         1. A computer-implemented method for encoding video, comprising:
           encoding a current picture based on a collocated picture, wherein the collocated picture is used for temporal motion vector prediction; and   signaling a first flag and a second flag in response to a number of entries in a reference picture list 0 and a number of entries in a reference picture list 1 being both greater than 0, wherein the first flag indicates that the collocated picture is derived from the reference picture list 0 or the reference picture list 1, and the second flag indicates whether a motion vector difference syntax structure is signaled.   
           2. A computer-implemented method for decoding video, comprising:
           receiving a video bitstream;   decoding a first flag and a second flag in response to a number of entries in a reference picture list 0 and a number of entries in a reference picture list 1 being both greater than 0, wherein the first flag indicates a collocated picture used for temporal motion vector prediction is derived from the reference picture list 0 or the reference picture list 1, and the second flag indicates whether a motion vector difference syntax structure is present in the bitstream for a current picture; and   decoding the current picture based on the collocated picture.   
           3. A computer-implemented method for encoding video, comprising:
           encoding a current picture based on a collocated picture, wherein the collocated picture is used for temporal motion vector prediction; and   indicating the collocated picture in the bitstream without signaling an index to a reference picture list.   
           4. The method of clause 3, wherein indicating the collocated picture in the bitstream without signaling an index to a reference picture list further comprises:
           signaling a first flag to indicate whether the collocated picture is an inter-layer reference picture; and   in response to the collocated picture being an inter-layer reference picture, signaling a first parameter to indicate the collocated picture, wherein the first parameter indicates the index of the collocated picture to the list of direct reference layers of the layer where the current picture is in.   
           5. The method of clause 4, wherein indicating the collocated picture in the bitstream without signaling an index to a reference picture list further comprises:
           signaling a second flag to indicate whether the collocated picture is a short-term reference picture or a long-term reference picture; and   in response to the collocated picture being the short-term reference picture, signaling a second parameter to indicate the collocated picture, wherein the second parameter indicates a difference between a picture order count of the collocated picture and a picture order count of the current picture.   
           6. The method of clause 5, further comprising:
           in response to the collocated picture being the long-term reference picture, signaling a third parameter and a fourth parameter to indicate the collocated picture, wherein the third parameter indicates a least significant bit (LSB) of picture order count (POC) of the collocated picture and the fourth parameter indicates a delta most significant bit (MSB) of picture order count (POC) of the collocated picture.   
           7. The method of clause 6, wherein the first flag, the second flag, the first parameter, the second parameter, the third parameter and the fourth parameter are signaled in a picture header, and all slices within a picture have a same collocated picture.   8. The method of clause 3, wherein the reference picture list is reference picture list 0 or reference picture list 1.   9. A computer-implemented method for decoding video, comprising:
           receiving a video bitstream;   determining a collocated picture used for temporal motion vector prediction without decoding an index to a reference picture list; and   decoding a current picture based on the collocated picture.   
           10. The method of clause 9, wherein determining the collocated picture used for temporal motion vector prediction without decoding an index to the reference picture list further comprises:
           decoding a first flag that indicates whether the collocated picture is an inter-layer reference picture;   determining whether the collocated picture is an inter-layer reference picture based on the first flag; and   in response to the collocated picture being an inter-layer reference picture, decoding a first parameter and determining the collocated picture based on the first parameter, wherein the first parameter indicates an index of the collocated picture to the list of direct reference layers of the layer where the current picture is in.   
           11. The method of clause 10, wherein determining the collocated picture used for temporal motion vector prediction without decoding an index to a reference picture list structure further comprises:
           decoding a second flag that indicates whether the collocated picture is a short-term reference picture or a long-term reference picture;   determining whether the collocated picture is the short-term reference picture or the long-term reference picture based on the second flag; and   in response to the collocated picture being the short-term reference picture, decoding a second parameter and determining the collocated picture based on the second parameter, wherein the second parameter indicates a difference between a picture order count of the collocated picture and a picture order count of the current picture.   
           12. The method of clause 11, further comprising:
           in response to the collocated picture being the long-term reference picture, decoding a third parameter and a fourth parameter and determining the collocated picture based on the third and the fourth parameter, wherein the third parameter indicates a least significant bit (LSB) of picture order count (POC) of the collocated picture and the fourth parameter indicates a delta most significant bit (MSB) of picture order count (POC) of the collocated picture.   
           13. The method of clause 12, wherein the first flag, the second flag, the first parameter, the second parameter, the third parameter and the fourth parameter are present in a picture header, and all slices within a picture have a same collocated picture.   14. The method of clause 9, wherein the reference picture list is reference picture list 0 or reference picture list 1.   15. A computer-implemented method for encoding video, comprising:
           determining whether to signal a parameter to indicate a reference index of a collocated picture in a slice header;   in response to the parameter not being signaled in the slice header, determining the collocated picture as the picture referred to by an index with a value equal to a smaller one between a value of a reference index of the collocated picture signaled in a picture header and a number of active entries in a target reference picture list minus 1; and   encoding a current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
           16. The method of clause 15, wherein the target reference picture list is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived.   17. A computer-implemented method for decoding video, comprising:
           receiving a video bitstream;   determining whether a parameter indicating a reference index of a collocated picture used for temporal motion vector prediction is present in a slice header;   in response to the parameter being not present, determining a value of the parameter to be equal to a smaller one between a value of a reference index of the collocated picture used for temporal motion vector prediction present in picture header and a number of active entries in a target reference picture list minus 1;   determining the collocated picture as a picture referred to by an index with a value equal to the value of the parameter in the target reference picture list; and   decoding a current picture based on the collocated picture.   
           18. The method of clause 17, wherein the target reference picture list is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived.   19. A computer-implemented method for video processing, comprising:
           deriving a total number by summing a number of reference picture list structures in sequence parameter set (SPS) and one;   allocating memory for the total number of reference picture list structures in response to a reference picture list structure being signaled in a picture header of a current picture or a slice header of a current slice; and   processing a current picture or a current slice using the allocated memory.   
           20. A computer-implemented method for encoding video, comprising:
           signaling a first flag in a picture parameter set (PPS) to indicate whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture referring to the PPS; wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1;   determining whether the first index and a second index to be signaled, wherein the second index is an index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0;   in response to the second index not to be signaled, determining a value of the second index comprising:
               when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0;   
               in response to the first index not to be signaled, determining a value of the first index comprising:
               when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and   when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index;   
               deriving the reference picture list based on the first index and the second index; and   encoding the current picture based on the reference picture list.   
           21. A computer-implemented method for decoding video, comprising:
           receiving a video bitstream;   determining a value of a first flag indicating whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture, wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1;   determining whether the first index and a second index being present, wherein the second index is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0;   in response to the second index being not present, determining a value of the second index comprising:
               when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0;   
               in response to the first index being not present, determining a value of the first index comprising:
               when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and   when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; and   
               decoding a current picture based on the first index and the second index.   
           22. A computer-implemented method for encoding video, comprising:
           signaling a first flag in a slice header to indicate whether an active reference index number is present in a slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that can be used to encode a current slice;   in response to the first flag indicating the active reference index number is present in the slice header,
               determining a number of entries of reference picture list 0, and signaling the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and   determining a number of entries of reference picture list 1, and signaling the active reference index number of reference picture list 1 in the slice header for B slice when the number of entries of reference picture list 1 is greater than 1.   
               
           23. A method of clause 22, further comprising:
           in response to the first flag indicating the active reference index number is not present in the slice header,
               skipping signaling the active reference index number of reference picture list 0 in the slice header for P and B slice; and   skipping signaling the active reference index number reference picture list 1 in slice header for B slice.   
               
           24. A computer-implemented method for decoding video, comprising:
           receiving a video bitstream including a slice header and a picture header syntax;   determining a value of a first flag signaled in the slice header that indicates whether an active reference index number is present in the slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that can be used to decode a current slice;   in response to the first flag indicating the active reference index number is present,
               determining a number of entries of reference picture list 0, and decoding the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and   determining a number of entries of reference picture list 1, and decoding the active reference index number of reference picture list 1 in slice header for B slice when a number of entries of reference picture list 1 is greater than 1.   
               
           25. The method of clause 24, further comprising:
           in response to the first flag indicating the active reference index number is not present,
               skipping decoding the active reference index number of reference picture list 0 in the slice header for P and B slice; and   skipping decoding the active reference index number of reference picture list 1 in slice header for B slice.   
               
           26. A computer-implemented method for video processing, comprising:
           determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all non-I slices of a current picture; and   processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
           27. A computer-implemented method for video processing, comprising:
           determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all P slices and B slices of a current picture; and   processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
           28. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               encoding a current picture based on a collocated picture, wherein the collocated picture is used for temporal motion vector prediction; and   signaling a first flag and a second flag in response to a number of entries in a reference picture list 0 and a number of entries in a reference picture list 1 being both greater than 0, wherein the first flag indicates that the collocated picture is derived from the reference picture list 0 or the reference picture list 1, and the second flag indicates whether a motion vector difference syntax structure is signaled.   
               
           29. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               receiving a video bitstream;   decoding a first flag and a second flag in response to a number of entries in a reference picture list 0 and a number of entries in a reference picture list 1 being both greater than 0, wherein the first flag indicates a collocated picture used for temporal motion vector prediction is derived from the reference picture list 0 or the reference picture list 1, and the second flag indicates whether a motion vector difference syntax structure is present in the bitstream for a current picture; and   decoding the current picture based on the collocated picture.   
               
           30. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               encoding a current picture based on a collocated picture, wherein the collocated picture is used for temporal motion vector prediction; and   indicating the collocated picture in the bitstream without signaling an index to a reference picture list.   
               
           31. The apparatus of clause 30, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           signaling a first flag to indicate whether the collocated picture is an inter-layer reference picture; and   in response to the collocated picture being an inter-layer reference picture, signaling a first parameter to indicate the collocated picture, wherein the first parameter indicates the index of the collocated picture to the list of direct reference layers of the layer where the current picture is in.   
           32. The apparatus of clause 31, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           signaling a second flag to indicate whether the collocated picture is a short-term reference picture or a long-term reference picture; and   in response to the collocated picture being the short-term reference picture, signaling a second parameter to indicate the collocated picture, wherein the second parameter indicates a difference between a picture order count of the collocated picture and a picture order count of the current picture.   
           33. The apparatus of clause 32, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           in response to the collocated picture being the long-term reference picture, signaling a third parameter and a fourth parameter to indicate the collocated picture, wherein the third parameter indicates a least significant bit (LSB) of picture order count (POC) of the collocated picture and the fourth parameter indicates a delta most significant bit (MSB) of picture order count (POC) of the collocated picture.   
           34. The apparatus of clause 33, wherein the first flag, the second flag, the first parameter, the second parameter, the third parameter and the fourth parameter are signaled in a picture header, and all slices within a picture have a same collocated picture.   35. The apparatus of clause 30, wherein the reference picture list is reference picture list 0 or reference picture list 1.   36. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               receiving a video bitstream;   determining a collocated picture used for temporal motion vector prediction without decoding an index to a reference picture list; and   decoding a current picture based on the collocated picture.   
               
           37. The apparatus of clause 36, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           decoding a first flag that indicates whether the collocated picture is an inter-layer reference picture;   determining whether the collocated picture is an inter-layer reference picture based on the first flag; and   in response to the collocated picture being an inter-layer reference picture, decoding a first parameter and determining the collocated picture based on the first parameter, wherein the first parameter indicates an index of the collocated picture to the list of direct reference layers of the layer where the current picture is in.   
           38. The apparatus of clause 37, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           decoding a second flag that indicates whether the collocated picture is a short-term reference picture or a long-term reference picture;   determining whether the collocated picture is the short-term reference picture or the long-term reference picture based on the second flag; and   in response to the collocated picture being the short-term reference picture, decoding a second parameter and determining the collocated picture based on the second parameter, wherein the second parameter indicates a difference between a picture order count of the collocated picture and a picture order count of the current picture.   
           39. The apparatus of clause 38, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           in response to the collocated picture being the long-term reference picture, decoding a third parameter and a fourth parameter and determining the collocated picture based on the third and the fourth parameter, wherein the third parameter indicates a least significant bit (LSB) of picture order count (POC) of the collocated picture and the fourth parameter indicates a delta most significant bit (MSB) of picture order count (POC) of the collocated picture.   
           40. The apparatus of clause 39, wherein the first flag, the second flag, the first parameter, the second parameter, the third parameter and the fourth parameter are present in a picture header, and all slices within a picture have a same collocated picture.   41. The apparatus of clause 36 wherein the reference picture list is reference picture list 0 or reference picture list 1.   42. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               determining whether to signal a parameter to indicate a reference index of a collocated picture in a slice header;   in response to the parameter not being signaled in the slice header, determining the collocated picture as the picture referred to by an index with a value equal to a smaller one between a value of a reference index of the collocated picture signaled in a picture header and a number of active entries in a target reference picture list minus 1; and   encoding a current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
               
           43. The apparatus of clause 42, wherein the target reference picture list is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived.   44. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:   
           receiving a video bitstream;
           determining whether a parameter indicating a reference index of a collocated picture used for temporal motion vector prediction is present in a slice header; and   in response to the parameter being not present, determining a value of the parameter to be equal to a smaller one between a value of a reference index of the collocated picture used for temporal motion vector prediction present in picture header and a number of active entries in a target reference picture list minus 1;   determining the collocated picture as a picture referred to by an index with a value equal to the value of the parameter in the target reference picture list; and   decoding a current picture based on the collocated picture.   
           45. The apparatus of clause 44, wherein the target reference picture list is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived.   46. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               deriving a total number by summing a number of reference picture list structures in sequence parameter set (SPS) and one;   allocating memory for the total number of reference picture list structures in response to a reference picture list structure being signaled in a picture header of a current picture or a slice header of a current slice; and   processing a current picture or a current slice using the allocated memory.   
               
           47. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               signaling a first flag in a picture parameter set (PPS) to indicate whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture referring to the PPS; wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1;   determining whether the first index and a second index to be signaled, wherein the second index is an index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0;   in response to the second index not to be signaled, determining a value of the second index comprising:
                   when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0;   
                   in response to the first index not to be signaled, determining a value of the first index comprising:
                   when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and   when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index;   
                   deriving the reference picture list based on the first index and the second index; and   encoding the current picture based on the reference picture list.   
               
           48. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               receiving a video bitstream;   determining a value of a first flag indicating whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture, wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1;   determining whether the first index and a second index being present, wherein the second index is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0;   in response to the second index being not present, determining a value of the second index comprising:
                   when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0;   
                   in response to the first index being not present, determining a value of the first index comprising:
                   when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and   when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; and   
                   decoding a current picture based on the first index and the second index.   
               
           49. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               signaling a first flag in a slice header to indicate whether an active reference index number is present in a slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that can be used to encode a current slice;   in response to the first flag indicating the active reference index number is present in the slice header,
                   determining a number of entries of reference picture list 0, and signaling the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and   determining a number of entries of reference picture list 1, and signaling the active reference index number of reference picture list 1 in the slice header for B slice when the number of entries of reference picture list 1 is greater than 1.   
                   
               
           50. The apparatus of clause 49, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           in response to first flag indicating the active reference index number is not present in the slice header,
               skipping signaling the active reference index number of reference picture list 0 in the slice header for P and B slice; and   skipping signaling the active reference index number reference picture list 1 in slice header for B slice.   
               
           51. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:   receiving a video bitstream including a slice header and a picture header syntax;
               determining a value of a first flag signaled in the slice header that indicates whether an active reference index number is present in the slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that can be used to decode a current slice;   in response to the first flag indicating the active reference index number is present,
                   determining a number of entries of reference picture list 0, and decoding the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and   determining a number of entries of reference picture list 1, and decoding the active reference index number of reference picture list 1 in slice header for B slice when a number of entries of reference picture list 1 is greater than 1.   
                   
               
           52. The apparatus of clause 50, wherein the processor is further configured to execute the instructions to cause the apparatus to perform:
           in response to the first flag indicating the active reference index number is not present,
               skipping decoding the active reference index number of reference picture list 0 in the slice header for P and B slice; and   skipping decoding the active reference index number of reference picture list 1 in slice header for B slice.   
               
           53. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all non-I slices of a current picture; and   processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
               
           54. An apparatus for performing video data processing, the apparatus comprising:
           a memory configured to store instructions; and   one or more processors configured to execute the instructions to cause the apparatus to perform:
               determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all P slices and B slices of a current picture; and   processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
               
           55. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           encoding a current picture based on a collocated picture, wherein the collocated picture is used for temporal motion vector prediction; and   signaling a first flag and a second flag in response to a number of entries in a reference picture list 0 and a number of entries in a reference picture list 1 being both greater than 0, wherein the first flag indicates that the collocated picture is derived from the reference picture list 0 or the reference picture list 1, and the second flag indicates whether a motion vector difference syntax structure is signaled.   
           56. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           receiving a video bitstream;   decoding a first flag and a second flag in response to a number of entries in a reference picture list 0 and a number of entries in a reference picture list 1 being both greater than 0, wherein the first flag indicates a collocated picture used for temporal motion vector prediction is derived from the reference picture list 0 or the reference picture list 1, and the second flag indicates whether a motion vector difference syntax structure is present in the bitstream for a current picture; and   decoding the current picture based on the collocated picture.   
           57. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           encoding a current picture based on a collocated picture, wherein the collocated picture is used for temporal motion vector prediction; and   indicating the collocated picture in the bitstream without signaling an index to a reference picture list.   
           58. The non-transitory computer readable medium of clause 57, wherein the method further comprises:
           signaling a first flag to indicate whether the collocated picture is an inter-layer reference picture; and   in response to the collocated picture being an inter-layer reference picture, signaling a first parameter to indicate the collocated picture, wherein the first parameter indicates the index of the collocated picture to the list of direct reference layers of the layer where the current picture is in.   
           59. The non-transitory computer readable medium of clause 58, wherein the method further comprises:
           signaling a second flag to indicate whether the collocated picture is a short-term reference picture or a long-term reference picture; and   in response to the collocated picture being the short-term reference picture, signaling a second parameter to indicate the collocated picture, wherein the second parameter indicates a difference between a picture order count of the collocated picture and a picture order count of the current picture.   
           60. The non-transitory computer readable medium of clause 59, wherein the method further comprises:
           in response to the collocated picture being the long-term reference picture, signaling a third parameter and the fourth parameter to indicate the collocated picture, wherein the third parameter indicates a least significant bit (LSB) of picture order count (POC) of the collocated picture and a fourth parameter indicates a delta most significant bit (MSB) of picture order count (POC) of the collocated picture.   
           61. The non-transitory computer readable medium of clause 60, wherein the first flag, the second flag, the first parameter, the second parameter, the third parameter and the fourth parameter are signaled in a picture header, and all slices within a picture have a same collocated picture.   62. The non-transitory computer readable medium of clause 57, wherein the reference picture list is reference picture list 0 or reference picture list 1.   63. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           receiving a video bitstream;   determining a collocated picture used for temporal motion vector prediction without decoding an index to a reference picture list; and   decoding a current picture based on the collocated picture.   
           64. The non-transitory computer readable medium of clause 63, wherein the method further comprises:
           decoding a first flag that indicates whether the collocated picture is an inter-layer reference picture;   determining whether the collocated picture is an inter-layer reference picture based on the first flag; and   in response to the collocated picture being an inter-layer reference picture, decoding a first parameter and determining the collocated picture based on the first parameter, wherein the first parameter indicates an index of the collocated picture to the list of direct reference layers of the layer where the current picture is in.   
           65. The non-transitory computer readable medium of clause 64, wherein the method further comprises:
           decoding a second flag that indicates whether the collocated picture is a short-term reference picture or a long-term reference picture;   determining whether the collocated picture is the short-term reference picture or the long-term reference picture based on the second flag; and   in response to the collocated picture being the short-term reference picture, decoding a second parameter and determining the collocated picture based on the second parameter, wherein the second parameter indicates a difference between a picture order count of the collocated picture and a picture order count of the current picture.   
           66. The non-transitory computer readable medium of clause 65, wherein the method further comprises:
           in response to the collocated picture being the long-term reference picture, decoding a third parameter and a fourth parameter and determining the collocated picture based on the third and the fourth parameter, wherein the third parameter indicates a least significant bit (LSB) of picture order count (POC) of the collocated picture and the fourth parameter indicates a delta most significant bit (MSB) of picture order count (POC) of the collocated picture.   
           67. The non-transitory computer readable medium of clause 66, wherein the first flag, the second flag, the first parameter, the second parameter, the third parameter and the fourth parameter are present in a picture header, and all slices within a picture have a same collocated picture.   68. The non-transitory computer readable medium of clause 63, wherein the reference picture list is reference picture list 0 or reference picture list 1.   69. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           determining whether to signal a parameter to indicate a reference index of a collocated picture in a slice header;   in response to the parameter not being signaled in the slice header, determining the collocated picture as the picture referred to by an index with a value equal to a smaller one between a value of a reference index of the collocated picture signaled in a picture header and a number of active entries in a target reference picture list minus 1; and   encoding a current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
           70. The non-transitory computer readable medium of clause 69, wherein the target reference picture list is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived.   71. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           receiving a video bitstream;   determining whether a parameter indicating a reference index of a collocated picture used for temporal motion vector prediction is present in a slice header;   in response to the parameter being not present, determining a value of the parameter to be equal to a smaller one between a value of a reference index of the collocated picture used for temporal motion vector prediction present in picture header and a number of active entries in a target reference picture list minus 1;   determining the collocated picture as a picture referred to by an index with a value equal to the value of the parameter in the target reference picture list; and   decoding a current picture based on the collocated picture.   
           72. The non-transitory computer readable medium of clause 71, wherein the target reference picture list is indicated by a flag that indicates from which reference picture list the collocated picture used for temporal motion vector prediction is derived.   73. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           deriving a total number by summing a number of reference picture list structures in sequence parameter set (SPS) and one;   allocating memory for the total number of reference picture list structures in response to a reference picture list structure being signaled in a picture header of a current picture or a slice header of a current slice; and   processing a current picture or a current slice using the allocated memory.   
           74. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           signaling a first flag in a picture parameter set (PPS) to indicate whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture referring to the PPS; wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1;   determining whether the first index and a second index to be signaled, wherein the second index is an index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0;   in response to the second index not to be signaled, determining a value of the second index comprising:
               when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0;   
               in response to the first index not to be signaled, determining a value of the first index comprising:
               when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and   when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index;   
               deriving the reference picture list based on the first index and the second index; and   encoding the current picture based on the reference picture list.   
           75. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           receiving a video bitstream;   determining a value of a first flag indicating whether a second flag and a first index is present in a picture header syntax or a slice header for a current picture, wherein the second flag indicates whether reference picture list 1 is derived based on one of the reference picture list structures associated with reference picture list 1 signaled in a sequence parameter set (SPS) and the first index is the index, to the list of the reference picture list structures associated with reference picture list 1 included in the SPS, of the reference picture list structure associated with reference picture list 1 that is used for derivation of reference picture list 1;   determining whether the first index and a second index being present, wherein the second index is the index, to the list of the reference picture list structures associated with reference picture list 0 included in the SPS, of the reference picture list structure associated with reference picture list 0 that is used for derivation of reference picture list 0;   in response to the second index being not present, determining a value of the second index comprising:
               when at most one reference picture list structure associated with reference picture list 0 is included in SPS, determining the value of the second index to be equal to 0;   
               in response to the first index being not present, determining a value of the first index comprising:
               when at most one reference picture list structure associated with reference picture list 1 is included in SPS, determining the value of the first index to be equal to 0; and   when the first flag is equal to 0 and the second flag is equal to 1, determining the value of the first index to be equal to the value of the second index; and   
               decoding a current picture based on the first index and the second index.   
           76. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           signaling a first flag in a slice header to indicate whether an active reference index number is present in a slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that can be used to encode a current slice;   in response to the first flag indicating the active reference index number is present in the slice header,
               determining a number of entries of reference picture list 0, and signaling the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and   determining a number of entries of reference picture list 1, and signaling the active reference index number of reference picture list 1 in the slice header for B slice when the number of entries of reference picture list 1 is greater than 1.   
               
           77. The non-transitory computer readable medium of clause 76, wherein the method further comprises:
           in response to the first flag indicating the active reference index number is not present in the slice header,
               skipping signaling the active reference index number of reference picture list 0 in the slice header for P and B slice; and   skipping signaling the active reference index number reference picture list 1 in slice header for B slice.   
               
           78. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           receiving a video bitstream including a slice header and a picture header syntax;   determining a value of a first flag signaled in the slice header that indicates whether an active reference index number is present in the slice header, wherein the active reference index number is used to derive maximum reference index for a corresponding reference picture list that can be used to decode a current slice;   in response to the first flag indicating the active reference index number is present,
               determining a number of entries of reference picture list 0, and decoding the active reference index number of reference picture list 0 in the slice header for P and B slice when the number of entries of reference picture list 0 is greater than 1; and   determining a number of entries of reference picture list 1, and decoding the active reference index number of reference picture list 1 in slice header for B slice when a number of entries of reference picture list 1 is greater than 1.   
               
           79. The non-transitory computer readable medium of clause 78, wherein the method further comprises:   in response to the first flag indicating the active reference index number is not present,
           skipping decoding the active reference index number of reference picture list 0 in the slice header for P and B slice; and   skipping decoding the active reference index number of reference picture list 1 in slice header for B slice.   
           80. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all non-I slices of a current picture; and   processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
           81. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of an apparatus to cause the apparatus to initiate a method for performing video data processing, the method comprising:
           determining a collocated picture referred to by a reference index of the collocated picture in slice level, wherein the collocated picture is determined to be a same picture for all P slices and B slices of a current picture; and   processing the current picture based on the collocated picture, wherein the collocated picture is used for temporal motion vector prediction.   
               

     In the foregoing specification, embodiments have been described with reference to numerous specific details that can vary from implementation to implementation. Certain adaptations and modifications of the described embodiments can be made. Other embodiments can be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. It is also intended that the sequence of steps shown in figures are only for illustrative purposes and are not intended to be limited to any particular sequence of steps. As such, those skilled in the art can appreciate that these steps can be performed in a different order while implementing the same method. 
     In the drawings and specification, there have been disclosed exemplary embodiments. However, many variations and modifications can be made to these embodiments. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.