Patent Publication Number: US-2022224883-A1

Title: Method and apparatus for video coding

Description:
INCORPORATION BY REFERENCE 
     The present application claims the benefit of priority to U.S. Provisional Application No. 63/137,350, “MIXED-RESOLUTION PREDICTION FOR CU-BASED SUPER-RESOLUTION CODING,” filed on Jan. 14, 2021, which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure describes embodiments generally related to video coding. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Video coding and decoding can be performed using inter-picture prediction with motion compensation. Uncompressed digital video can include a series of pictures, each picture having a spatial dimension of, for example, 1920×1080 luminance samples and associated chrominance samples. The series of pictures can have a fixed or variable picture rate (informally also known as frame rate) of, for example, 60 pictures per second or 60 Hz. Uncompressed video has significant bitrate requirements. For example, 1080p60 4:2:0 video at 8 bit per sample (1920×1080 luminance sample resolution at 60 Hz frame rate) requires close to 1.5 Gbit/s bandwidth. An hour of such video requires more than 600 GBytes of storage space. 
     One purpose of video coding and decoding can be the reduction of redundancy in the input video signal, through compression. Compression can help reduce the aforementioned bandwidth or storage space requirements, in some cases by two orders of magnitude or more. Both lossless and lossy compression, as well as a combination thereof can be employed. Lossless compression refers to techniques where an exact copy of the original signal can be reconstructed from the compressed original signal. When using lossy compression, the reconstructed signal may not be identical to the original signal, but the distortion between original and reconstructed signals is small enough to make the reconstructed signal useful for the intended application. In the case of video, lossy compression is widely employed. The amount of distortion tolerated depends on the application; for example, users of certain consumer streaming applications may tolerate higher distortion than users of television distribution applications. The compression ratio achievable can reflect that: higher allowable/tolerable distortion can yield higher compression ratios. 
     A video encoder and decoder can utilize techniques from several broad categories, including, for example, motion compensation, transform, quantization, and entropy coding. 
     Video codec technologies can include techniques known as intra coding. In intra coding, sample values are represented without reference to samples or other data from previously reconstructed reference pictures. In some video codecs, the picture is spatially subdivided into blocks of samples. When all blocks of samples are coded in intra mode, that picture can be an intra picture. Intra pictures and their derivations such as independent decoder refresh pictures, can be used to reset the decoder state and can, therefore, be used as the first picture in a coded video bitstream and a video session, or as a still image. The samples of an intra block can be exposed to a transform, and the transform coefficients can be quantized before entropy coding. Intra prediction can be a technique that minimizes sample values in the pre-transform domain. In some cases, the smaller the DC value after a transform is, and the smaller the AC coefficients are, the fewer the bits that are required at a given quantization step size to represent the block after entropy coding. 
     Traditional intra coding such as known from, for example MPEG-2 generation coding technologies, does not use intra prediction. However, some newer video compression technologies include techniques that attempt to predict sample values, from, for example, surrounding sample data and/or metadata obtained during the encoding and/or decoding of spatially neighboring, and preceding in decoding order, blocks of data. Such techniques are henceforth called “intra prediction” techniques. Note that in at least some cases, intra prediction is only using reference data from the current picture under reconstruction and not from reference pictures. 
     There can be many different forms of intra prediction. When more than one of such techniques can be used in a given video coding technology, the technique in use can be coded in an intra prediction mode. In certain cases, modes can have submodes and/or parameters, and those can be coded individually or included in the mode codeword. Which codeword to use for a given mode, submode, and/or parameter combination can have an impact in the coding efficiency gain through intra prediction, and so can the entropy coding technology used to translate the codewords into a bitstream. 
     A certain mode of intra prediction was introduced with H.264, refined in H.265, and further refined in newer coding technologies such as joint exploration model (JEM), versatile video coding (VVC), and benchmark set (BMS). A predictor block can be formed using neighboring sample values belonging to already available samples. Sample values of neighboring samples are copied into the predictor block according to a direction. A reference to the direction in use can be coded in the bitstream or may be predicted itself. 
     Referring to  FIG. 1A , depicted in the lower right is a subset of nine predictor directions known from H.265&#39;s 33 possible predictor directions (corresponding to the 33 angular modes of the 35 intra modes). The point where the arrows converge ( 101 ) represents the sample being predicted. The arrows represent the direction from which the sample is being predicted. For example, arrow ( 102 ) indicates that sample ( 101 ) is predicted from a sample or samples to the upper right, at a 45 degree angle from the horizontal. Similarly, arrow ( 103 ) indicates that sample ( 101 ) is predicted from a sample or samples to the lower left of sample ( 101 ), in a 22.5 degree angle from the horizontal. 
     Still referring to  FIG. 1A , on the top left there is depicted a square block ( 104 ) of 4×4 samples (indicated by a dashed, boldface line). The square block ( 104 ) includes 16 samples, each labeled with an “S”, its position in the Y dimension (e.g., row index) and its position in the X dimension (e.g., column index). For example, sample S21 is the second sample in the Y dimension (from the top) and the first (from the left) sample in the X dimension. Similarly, sample S44 is the fourth sample in block ( 104 ) in both the Y and X dimensions. As the block is 4×4 samples in size, S44 is at the bottom right. Further shown are reference samples that follow a similar numbering scheme. A reference sample is labeled with an R, its Y position (e.g., row index) and X position (column index) relative to block ( 104 ). In both H.264 and H.265, prediction samples neighbor the block under reconstruction; therefore no negative values need to be used. 
     Intra picture prediction can work by copying reference sample values from the neighboring samples as appropriated by the signaled prediction direction. For example, assume the coded video bitstream includes signaling that, for this block, indicates a prediction direction consistent with arrow ( 102 )—that is, samples are predicted from a prediction sample or samples to the upper right, at a 45 degree angle from the horizontal. In that case, samples S41, S32, S23, and S14 are predicted from the same reference sample R05. Sample S44 is then predicted from reference sample R08. 
     In certain cases, the values of multiple reference samples may be combined, for example through interpolation, in order to calculate a reference sample; especially when the directions are not evenly divisible by 45 degrees. 
     The number of possible directions has increased as video coding technology has developed. In H.264 (year 2003), nine different direction could be represented. That increased to 33 in H.265 (year 2013), and JEM/VVC/BMS, at the time of disclosure, can support up to 65 directions. Experiments have been conducted to identify the most likely directions, and certain techniques in the entropy coding are used to represent those likely directions in a small number of bits, accepting a certain penalty for less likely directions. Further, the directions themselves can sometimes be predicted from neighboring directions used in neighboring, already decoded, blocks. 
       FIG. 1B  shows a schematic ( 105 ) that depicts 65 intra prediction directions according to JEM to illustrate the increasing number of prediction directions over time. 
     The mapping of intra prediction directions bits in the coded video bitstream that represent the direction can be different from video coding technology to video coding technology; and can range, for example, from simple direct mappings of prediction direction to intra prediction mode, to codewords, to complex adaptive schemes involving most probable modes, and similar techniques. In all cases, however, there can be certain directions that are statistically less likely to occur in video content than certain other directions. As the goal of video compression is the reduction of redundancy, those less likely directions will, in a well working video coding technology, be represented by a larger number of bits than more likely directions. 
     Motion compensation can be a lossy compression technique and can relate to techniques where a block of sample data from a previously reconstructed picture or part thereof (reference picture), after being spatially shifted in a direction indicated by a motion vector (MV henceforth), is used for the prediction of a newly reconstructed picture or picture part. In some cases, the reference picture can be the same as the picture currently under reconstruction. MVs can have two dimensions X and Y, or three dimensions, the third being an indication of the reference picture in use (the latter, indirectly, can be a time dimension). 
     In some video compression techniques, an MV applicable to a certain area of sample data can be predicted from other MVs, for example from those related to another area of sample data spatially adjacent to the area under reconstruction, and preceding that MV in decoding order. Doing so can substantially reduce the amount of data required for coding the MV, thereby removing redundancy and increasing compression. MV prediction can work effectively, for example, because when coding an input video signal derived from a camera (known as natural video) there is a statistical likelihood that areas larger than the area to which a single MV is applicable move in a similar direction and, therefore, can in some cases be predicted using a similar MV derived from MVs of a neighboring area. That results in the MV found for a given area to be similar or the same as the MV predicted from the surrounding MVs, and that in turn can be represented, after entropy coding, in a smaller number of bits than what would be used if coding the MV directly. In some cases, MV prediction can be an example of lossless compression of a signal (namely: the MVs) derived from the original signal (namely: the sample stream). In other cases, MV prediction itself can be lossy, for example because of rounding errors when calculating a predictor from several surrounding MVs. 
     Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec. H.265, “High Efficiency Video Coding”, December 2016). Out of the many MV prediction mechanisms that H.265 offers, the present embodiments are directed to a technique henceforth referred to as “spatial merge.” 
     Referring to  FIG. 1C , a current block ( 111 ) can include samples that have been found by the encoder during the motion search process to be predictable from a previous block of the same size that has been spatially shifted. Instead of coding that MV directly, the MV can be derived from metadata associated with one or more reference pictures, for example from the most recent (in decoding order) reference picture, using the MV associated with either one of five surrounding samples, denoted A0, A1, and B0, B1, B2 ( 112  through  116 , respectively). In H.265, the MV prediction can use predictors from the same reference picture that the neighboring block is using. 
     SUMMARY 
     Aspects of the disclosure provide apparatuses for video encoding/decoding. An apparatus includes processing circuitry that decodes a video bitstream to obtain a reduced-resolution residual block for a current block. The processing circuitry determines that a block level flag is set to a pre-defined value. The pre-defined value indicates that the current block is coded in reduced-resolution coding. Based on the block level flag, the processing circuitry generates a reduced-resolution prediction block for the current block by down-sampling a full-resolution reference block of the current block. The processing circuitry generates a reduced-resolution reconstruction block for the current block based on the reduced-resolution prediction block and the reduced-resolution residual block. The processing circuitry generates a full-resolution reconstruction block for the current block by up-sampling the reduced-resolution reconstruction block. 
     In an embodiment, the processing circuitry determines a size of the reduced-resolution prediction block based on a size of the full-resolution reference block and down-sampling factors of the current block. 
     In an embodiment, the processing circuitry decodes the block level flag for the current block from the video bitstream. The block level flag indicates that the current block is coded in the reduced-resolution coding. 
     In an embodiment, the processing circuitry decodes one of a filter coefficient or an index of the filter coefficient from the video bitstream. The filter coefficient is used in the up-sampling the reduced-resolution reconstruction block. 
     In an embodiment, the processing circuitry scales a motion vector of a first neighboring block of the current block based on a scaling factor that is a ratio of down-sampling factors of the current block and the first neighboring block. The processing circuitry constructs a first motion vector candidate list for the current block. The first motion vector candidate list includes the scaled motion vector of the first neighboring block. 
     In an embodiment, the processing circuitry determines the scaled motion vector based on a shift operation in response to the scaling factor being a power of two. In an example, when the scaling factor is 2 N , then the lower N bits of the horizontal component of the motion vector and the lower N bits of the vertical component of the motion vector are discarded to get the scaled motion vector. In another example, when the scaling factor is 2 N , the motion vector is first added with a rounding factor (e.g., 2 N−1 ), then the lower N bits of the horizontal component of the motion vector and the lower N bits of the vertical component of the motion vector are discarded to get the scaled motion vector. 
     In an embodiment, the processing circuitry determines the scaled motion vector based on a look-up table in response to the scaling factor not being a power of two. 
     In an embodiment, the processing circuitry determines a priority of the scaled motion vector in the first motion vector candidate list based on the down-sampling factors of the current block and the first neighboring block. 
     In an embodiment, the processing circuitry constructs a second motion vector candidate list for the current block based on one or more second neighboring blocks of the current block. Each of the one or more second neighboring blocks has the same down-sampling factors as the current block. The processing circuitry constructs a third motion vector candidate list for the current block based on one or more third neighboring blocks of the current block. Each of the one or more third neighboring blocks has different down-sampling factors from the current block. 
     In an embodiment, the processing circuitry scans the third motion vector candidate list based on a number of motion vector candidates in the second motion vector candidate list being less than a designated number. 
     In an embodiment, the processing circuitry determines a fourth motion vector candidate list for the current block by merging the second motion vector candidate list and the third motion vector candidate list in an interleaved manner. 
     In an embodiment, the processing circuitry determines an affine parameter of the current block based on down-sampling factors of the current block. 
     Aspects of the disclosure provide methods for video encoding/decoding. In the method, a video bitstream is decoded to obtain a reduced-resolution residual block for a current block. A block level flag that is set to a pre-defined value is determined. The pre-defined value indicates that the current block is coded in reduced-resolution coding. Based on the block level flag, a reduced-resolution prediction block is generated for the current block by down-sampling a full-resolution reference block of the current block. A reduced-resolution reconstruction block is generated for the current block based on the reduced-resolution prediction block and the reduced-resolution residual block. A full-resolution reconstruction block is generated for the current block by up-sampling the reduced-resolution reconstruction block. 
     Aspects of the disclosure also provide non-transitory computer-readable mediums storing instructions which when executed by at least one processor cause the at least one processor to perform any one or a combination of the methods for video encoding/decoding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, the nature, and various advantages of the disclosed subject matter will be more apparent from the following detailed description and the accompanying drawings in which: 
         FIG. 1A  is a schematic illustration of an exemplary subset of intra prediction modes; 
         FIG. 1B  is an illustration of exemplary intra prediction directions; 
         FIG. 1C  is a schematic illustration of a current block and its surrounding spatial merge candidates in one example; 
         FIG. 2  is a schematic illustration of a simplified block diagram of a communication system in accordance with an embodiment; 
         FIG. 3  is a schematic illustration of a simplified block diagram of a communication system in accordance with an embodiment; 
         FIG. 4  is a schematic illustration of a simplified block diagram of a decoder in accordance with an embodiment; 
         FIG. 5  is a schematic illustration of a simplified block diagram of an encoder in accordance with an embodiment; 
         FIG. 6  shows a block diagram of an encoder in accordance with another embodiment; 
         FIG. 7  shows a block diagram of a decoder in accordance with another embodiment; 
         FIG. 8  shows exemplary block partitions according to some embodiments of the disclosure; 
         FIGS. 9A and 9B  show an exemplary block partitioning using quad-tree plus binary-tree (QTBT) and a corresponding tree structure according to an embodiment of the disclosure; 
         FIG. 10  shows exemplary nominal angles according to an embodiment of the disclosure; 
         FIG. 11  shows positions of top, left, and top-left samples for one pixel in a current block according to an embodiment of the disclosure; 
         FIG. 12  shows an exemplary recursive filter intra mode according to an embodiment of the disclosure; 
         FIG. 13  shows an exemplary multi-layer reference frame structure according to an embodiment of the disclosure; 
         FIG. 14  shows an exemplary candidate motion vector list building process according to an embodiment of the disclosure; 
         FIG. 15  shows an exemplary motion-field estimation process according to an embodiment of the disclosure; 
         FIGS. 16A and 16B  show exemplary overlapped regions (shaded regions) to be predicted using a top-neighboring block and a left-neighboring block, respectively; 
         FIG. 17  shows an exemplary two-step warping process in which a horizontal-shearing is followed by a vertical-shearing; 
         FIG. 18  shows an overall loop-filtering pipeline including a frame-level super-resolution in AV1; 
         FIG. 19  shows an example implementation using the block-level flag according to an embodiment of the disclosure; 
         FIG. 20  shows an exemplary spatially neighboring motion vector referencing according to an embodiment of the disclosure; 
         FIG. 21  shows an exemplary temporally neighboring motion vector referencing according to an embodiment of the disclosure; 
         FIG. 22  shows an exemplary spatially neighboring motion vector referencing for affine motion prediction according to an embodiment of the disclosure; 
         FIG. 23  shows an exemplary flowchart in accordance with an embodiment; and 
         FIG. 24  is a schematic illustration of a computer system in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     I. Video Decoder and Encoder Systems 
       FIG. 2  illustrates a simplified block diagram of a communication system ( 200 ) according to an embodiment of the present disclosure. The communication system ( 200 ) includes a plurality of terminal devices that can communicate with each other, via, for example, a network ( 250 ). For example, the communication system ( 200 ) includes a first pair of terminal devices ( 210 ) and ( 220 ) interconnected via the network ( 250 ). In the  FIG. 2  example, the first pair of terminal devices ( 210 ) and ( 220 ) performs unidirectional transmission of data. For example, the terminal device ( 210 ) may code video data (e.g., a stream of video pictures that are captured by the terminal device ( 210 )) for transmission to the other terminal device ( 220 ) via the network ( 250 ). The encoded video data can be transmitted in the form of one or more coded video bitstreams. The terminal device ( 220 ) may receive the coded video data from the network ( 250 ), decode the coded video data to recover the video pictures and display video pictures according to the recovered video data. Unidirectional data transmission may be common in media serving applications and the like. 
     In another example, the communication system ( 200 ) includes a second pair of terminal devices ( 230 ) and ( 240 ) that performs bidirectional transmission of coded video data that may occur, for example, during videoconferencing. For bidirectional transmission of data, in an example, each terminal device of the terminal devices ( 230 ) and ( 240 ) may code video data (e.g., a stream of video pictures that are captured by the terminal device) for transmission to the other terminal device of the terminal devices ( 230 ) and ( 240 ) via the network ( 250 ). Each terminal device of the terminal devices ( 230 ) and ( 240 ) also may receive the coded video data transmitted by the other terminal device of the terminal devices ( 230 ) and ( 240 ), and may decode the coded video data to recover the video pictures and may display video pictures at an accessible display device according to the recovered video data. 
     In the  FIG. 2  example, the terminal devices ( 210 ), ( 220 ), ( 230 ) and ( 240 ) may be illustrated as servers, personal computers and smart phones but the principles of the present disclosure may be not so limited. Embodiments of the present disclosure find application with laptop computers, tablet computers, media players and/or dedicated video conferencing equipment. The network ( 250 ) represents any number of networks that convey coded video data among the terminal devices ( 210 ), ( 220 ), ( 230 ) and ( 240 ), including for example wireline (wired) and/or wireless communication networks. The communication network ( 250 ) may exchange data in circuit-switched and/or packet-switched channels. Representative networks include telecommunications networks, local area networks, wide area networks and/or the Internet. For the purposes of the present discussion, the architecture and topology of the network ( 250 ) may be immaterial to the operation of the present disclosure unless explained herein below. 
       FIG. 3  illustrates, as an example for an application for the disclosed subject matter, the placement of a video encoder and a video decoder in a streaming environment. The disclosed subject matter can be equally applicable to other video enabled applications, including, for example, video conferencing, digital TV, storing of compressed video on digital media including CD, DVD, memory stick, and the like. 
     A streaming system may include a capture subsystem ( 313 ) that can include a video source ( 301 ), for example a digital camera, creating for example a stream of video pictures ( 302 ) that are uncompressed. In an example, the stream of video pictures ( 302 ) includes samples that are taken by the digital camera. The stream of video pictures ( 302 ), depicted as a bold line to emphasize a high data volume when compared to encoded video data ( 304 ) (or coded video bitstreams), can be processed by an electronic device ( 320 ) that includes a video encoder ( 303 ) coupled to the video source ( 301 ). The video encoder ( 303 ) can include hardware, software, or a combination thereof to enable or implement aspects of the disclosed subject matter as described in more detail below. The encoded video data ( 304 ) (or encoded video bitstream ( 304 )), depicted as a thin line to emphasize the lower data volume when compared to the stream of video pictures ( 302 ), can be stored on a streaming server ( 305 ) for future use. One or more streaming client subsystems, such as client subsystems ( 306 ) and ( 308 ) in  FIG. 3  can access the streaming server ( 305 ) to retrieve copies ( 307 ) and ( 309 ) of the encoded video data ( 304 ). A client subsystem ( 306 ) can include a video decoder ( 310 ), for example, in an electronic device ( 330 ). The video decoder ( 310 ) decodes the incoming copy ( 307 ) of the encoded video data and creates an outgoing stream of video pictures ( 311 ) that can be rendered on a display ( 312 ) (e.g., display screen) or other rendering device (not depicted). In some streaming systems, the encoded video data ( 304 ), ( 307 ), and ( 309 ) (e.g., video bitstreams) can be encoded according to certain video coding/compression standards. Examples of those standards include ITU-T Recommendation H.265. In an example, a video coding standard under development is informally known as Versatile Video Coding (VVC). The disclosed subject matter may be used in the context of VVC. 
     It is noted that the electronic devices ( 320 ) and ( 330 ) can include other components (not shown). For example, the electronic device ( 320 ) can include a video decoder (not shown) and the electronic device ( 330 ) can include a video encoder (not shown) as well. 
       FIG. 4  shows a block diagram of a video decoder ( 410 ) according to an embodiment of the present disclosure. The video decoder ( 410 ) can be included in an electronic device ( 430 ). The electronic device ( 430 ) can include a receiver ( 431 ) (e.g., receiving circuitry). The video decoder ( 410 ) can be used in the place of the video decoder ( 310 ) in the  FIG. 3  example. 
     The receiver ( 431 ) may receive one or more coded video sequences to be decoded by the video decoder ( 410 ); in the same or another embodiment, one coded video sequence at a time, where the decoding of each coded video sequence is independent from other coded video sequences. The coded video sequence may be received from a channel ( 401 ), which may be a hardware/software link to a storage device which stores the encoded video data. The receiver ( 431 ) may receive the encoded video data with other data, for example, coded audio data and/or ancillary data streams, that may be forwarded to their respective using entities (not depicted). The receiver ( 431 ) may separate the coded video sequence from the other data. To combat network jitter, a buffer memory ( 415 ) may be coupled in between the receiver ( 431 ) and an entropy decoder/parser ( 420 ) (“parser ( 420 )” henceforth). In certain applications, the buffer memory ( 415 ) is part of the video decoder ( 410 ). In others, it can be outside of the video decoder ( 410 ) (not depicted). In still others, there can be a buffer memory (not depicted) outside of the video decoder ( 410 ), for example to combat network jitter, and in addition another buffer memory ( 415 ) inside the video decoder ( 410 ), for example to handle playout timing. When the receiver ( 431 ) is receiving data from a store/forward device of sufficient bandwidth and controllability, or from an isosynchronous network, the buffer memory ( 415 ) may not be needed, or can be small. For use on best effort packet networks such as the Internet, the buffer memory ( 415 ) may be required, can be comparatively large and can be advantageously of adaptive size, and may at least partially be implemented in an operating system or similar elements (not depicted) outside of the video decoder ( 410 ). 
     The video decoder ( 410 ) may include the parser ( 420 ) to reconstruct symbols ( 421 ) from the coded video sequence. Categories of those symbols include information used to manage operation of the video decoder ( 410 ), and potentially information to control a rendering device such as a render device ( 412 ) (e.g., a display screen) that is not an integral part of the electronic device ( 430 ) but can be coupled to the electronic device ( 430 ), as was shown in  FIG. 4 . The control information for the rendering device(s) may be in the form of Supplemental Enhancement Information (SEI messages) or Video Usability Information (VUI) parameter set fragments (not depicted). The parser ( 420 ) may parse/entropy-decode the coded video sequence that is received. The coding of the coded video sequence can be in accordance with a video coding technology or standard, and can follow various principles, including variable length coding, Huffman coding, arithmetic coding with or without context sensitivity, and so forth. The parser ( 420 ) may extract from the coded video sequence, a set of subgroup parameters for at least one of the subgroups of pixels in the video decoder, based upon at least one parameter corresponding to the group. Subgroups can include Groups of Pictures (GOPs), pictures, tiles, slices, macroblocks, Coding Units (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) and so forth. The parser ( 420 ) may also extract from the coded video sequence information such as transform coefficients, quantizer parameter values, MVs, and so forth. 
     The parser ( 420 ) may perform an entropy decoding/parsing operation on the video sequence received from the buffer memory ( 415 ), so as to create symbols ( 421 ). 
     Reconstruction of the symbols ( 421 ) can involve multiple different units depending on the type of the coded video picture or parts thereof (such as: inter and intra picture, inter and intra block), and other factors. Which units are involved, and how, can be controlled by the subgroup control information that was parsed from the coded video sequence by the parser ( 420 ). The flow of such subgroup control information between the parser ( 420 ) and the multiple units below is not depicted for clarity. 
     Beyond the functional blocks already mentioned, the video decoder ( 410 ) can be conceptually subdivided into a number of functional units as described below. In a practical implementation operating under commercial constraints, many of these units interact closely with each other and can, at least partly, be integrated into each other. However, for the purpose of describing the disclosed subject matter, the conceptual subdivision into the functional units below is appropriate. 
     A first unit is the scaler/inverse transform unit ( 451 ). The scaler/inverse transform unit ( 451 ) receives a quantized transform coefficient as well as control information, including which transform to use, block size, quantization factor, quantization scaling matrices, etc. as symbol(s) ( 421 ) from the parser ( 420 ). The scaler/inverse transform unit ( 451 ) can output blocks comprising sample values that can be input into aggregator ( 455 ). 
     In some cases, the output samples of the scaler/inverse transform ( 451 ) can pertain to an intra coded block; that is: a block that is not using predictive information from previously reconstructed pictures, but can use predictive information from previously reconstructed parts of the current picture. Such predictive information can be provided by an intra picture prediction unit ( 452 ). In some cases, the intra picture prediction unit ( 452 ) generates a block of the same size and shape of the block under reconstruction, using surrounding already reconstructed information fetched from the current picture buffer ( 458 ). The current picture buffer ( 458 ) buffers, for example, partly reconstructed current picture and/or fully reconstructed current picture. The aggregator ( 455 ), in some cases, adds, on a per sample basis, the prediction information that the intra prediction unit ( 452 ) has generated to the output sample information as provided by the scaler/inverse transform unit ( 451 ). 
     In other cases, the output samples of the scaler/inverse transform unit ( 451 ) can pertain to an inter coded, and potentially motion compensated block. In such a case, a motion compensation prediction unit ( 453 ) can access reference picture memory ( 457 ) to fetch samples used for prediction. After motion compensating the fetched samples in accordance with the symbols ( 421 ) pertaining to the block, these samples can be added by the aggregator ( 455 ) to the output of the scaler/inverse transform unit ( 451 ) (in this case called the residual samples or residual signal) so as to generate output sample information. The addresses within the reference picture memory ( 457 ) from where the motion compensation prediction unit ( 453 ) fetches prediction samples can be controlled by MVs, available to the motion compensation prediction unit ( 453 ) in the form of symbols ( 421 ) that can have, for example X, Y, and reference picture components. Motion compensation also can include interpolation of sample values as fetched from the reference picture memory ( 457 ) when sub-sample exact MVs are in use, MV prediction mechanisms, and so forth. 
     The output samples of the aggregator ( 455 ) can be subject to various loop filtering techniques in the loop filter unit ( 456 ). Video compression technologies can include in-loop filter technologies that are controlled by parameters included in the coded video sequence (also referred to as coded video bitstream) and made available to the loop filter unit ( 456 ) as symbols ( 421 ) from the parser ( 420 ), but can also be responsive to meta-information obtained during the decoding of previous (in decoding order) parts of the coded picture or coded video sequence, as well as responsive to previously reconstructed and loop-filtered sample values. 
     The output of the loop filter unit ( 456 ) can be a sample stream that can be output to the render device ( 412 ) as well as stored in the reference picture memory ( 457 ) for use in future inter-picture prediction. 
     Certain coded pictures, once fully reconstructed, can be used as reference pictures for future prediction. For example, once a coded picture corresponding to a current picture is fully reconstructed and the coded picture has been identified as a reference picture (by, for example, the parser ( 420 )), the current picture buffer ( 458 ) can become a part of the reference picture memory ( 457 ), and a fresh current picture buffer can be reallocated before commencing the reconstruction of the following coded picture. 
     The video decoder ( 410 ) may perform decoding operations according to a predetermined video compression technology in a standard, such as ITU-T Rec. H.265. The coded video sequence may conform to a syntax specified by the video compression technology or standard being used, in the sense that the coded video sequence adheres to both the syntax of the video compression technology or standard and the profiles as documented in the video compression technology or standard. Specifically, a profile can select certain tools as the only tools available for use under that profile from all the tools available in the video compression technology or standard. Also necessary for compliance can be that the complexity of the coded video sequence is within bounds as defined by the level of the video compression technology or standard. In some cases, levels restrict the maximum picture size, maximum frame rate, maximum reconstruction sample rate (measured in, for example megasamples per second), maximum reference picture size, and so on. Limits set by levels can, in some cases, be further restricted through Hypothetical Reference Decoder (HRD) specifications and metadata for HRD buffer management signaled in the coded video sequence. 
     In an embodiment, the receiver ( 431 ) may receive additional (redundant) data with the encoded video. The additional data may be included as part of the coded video sequence(s). The additional data may be used by the video decoder ( 410 ) to properly decode the data and/or to more accurately reconstruct the original video data. Additional data can be in the form of, for example, temporal, spatial, or signal noise ratio (SNR) enhancement layers, redundant slices, redundant pictures, forward error correction codes, and so on. 
       FIG. 5  shows a block diagram of a video encoder ( 503 ) according to an embodiment of the present disclosure. The video encoder ( 503 ) is included in an electronic device ( 520 ). The electronic device ( 520 ) includes a transmitter ( 540 ) (e.g., transmitting circuitry). The video encoder ( 503 ) can be used in the place of the video encoder ( 303 ) in the  FIG. 3  example. 
     The video encoder ( 503 ) may receive video samples from a video source ( 501 ) (that is not part of the electronic device ( 520 ) in the  FIG. 5  example) that may capture video image(s) to be coded by the video encoder ( 503 ). In another example, the video source ( 501 ) is a part of the electronic device ( 520 ). 
     The video source ( 501 ) may provide the source video sequence to be coded by the video encoder ( 503 ) in the form of a digital video sample stream that can be of any suitable bit depth (for example: 8 bit, 10 bit, 12 bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ), and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb 4:4:4). In a media serving system, the video source ( 501 ) may be a storage device storing previously prepared video. In a videoconferencing system, the video source ( 501 ) may be a camera that captures local image information as a video sequence. Video data may be provided as a plurality of individual pictures that impart motion when viewed in sequence. The pictures themselves may be organized as a spatial array of pixels, wherein each pixel can comprise one or more samples depending on the sampling structure, color space, etc. in use. A person skilled in the art can readily understand the relationship between pixels and samples. The description below focuses on samples. 
     According to an embodiment, the video encoder ( 503 ) may code and compress the pictures of the source video sequence into a coded video sequence ( 543 ) in real time or under any other time constraints as required by the application. Enforcing appropriate coding speed is one function of a controller ( 550 ). In some embodiments, the controller ( 550 ) controls other functional units as described below and is functionally coupled to the other functional units. The coupling is not depicted for clarity. Parameters set by the controller ( 550 ) can include rate control related parameters (picture skip, quantizer, lambda value of rate-distortion optimization techniques, . . . ), picture size, group of pictures (GOP) layout, maximum MV allowed reference area, and so forth. The controller ( 550 ) can be configured to have other suitable functions that pertain to the video encoder ( 503 ) optimized for a certain system design. 
     In some embodiments, the video encoder ( 503 ) is configured to operate in a coding loop. As an oversimplified description, in an example, the coding loop can include a source coder ( 530 ) (e.g., responsible for creating symbols, such as a symbol stream, based on an input picture to be coded, and a reference picture(s)), and a (local) decoder ( 533 ) embedded in the video encoder ( 503 ). The decoder ( 533 ) reconstructs the symbols to create the sample data in a similar manner as a (remote) decoder also would create (as any compression between symbols and coded video bitstream is lossless in the video compression technologies considered in the disclosed subject matter). The reconstructed sample stream (sample data) is input to the reference picture memory ( 534 ). As the decoding of a symbol stream leads to bit-exact results independent of decoder location (local or remote), the content in the reference picture memory ( 534 ) is also bit exact between the local encoder and remote encoder. In other words, the prediction part of an encoder “sees” as reference picture samples exactly the same sample values as a decoder would “see” when using prediction during decoding. This fundamental principle of reference picture synchronicity (and resulting drift, if synchronicity cannot be maintained, for example because of channel errors) is used in some related arts as well. 
     The operation of the “local” decoder ( 533 ) can be the same as of a “remote” decoder, such as the video decoder ( 410 ), which has already been described in detail above in conjunction with  FIG. 4 . Briefly referring also to  FIG. 4 , however, as symbols are available and encoding/decoding of symbols to a coded video sequence by an entropy coder ( 545 ) and the parser ( 420 ) can be lossless, the entropy decoding parts of the video decoder ( 410 ), including the buffer memory ( 415 ) and the parser ( 420 ) may not be fully implemented in the local decoder ( 533 ). 
     An observation that can be made at this point is that any decoder technology except the parsing/entropy decoding that is present in a decoder also necessarily needs to be present, in substantially identical functional form, in a corresponding encoder. For this reason, the disclosed subject matter focuses on decoder operation. The description of encoder technologies can be abbreviated as they are the inverse of the comprehensively described decoder technologies. Only in certain areas a more detail description is required and provided below. 
     During operation, in some examples, the source coder ( 530 ) may perform motion compensated predictive coding, which codes an input picture predictively with reference to one or more previously-coded picture from the video sequence that were designated as “reference pictures”. In this manner, the coding engine ( 532 ) codes differences between pixel blocks of an input picture and pixel blocks of reference picture(s) that may be selected as prediction reference(s) to the input picture. 
     The local video decoder ( 533 ) may decode coded video data of pictures that may be designated as reference pictures, based on symbols created by the source coder ( 530 ). Operations of the coding engine ( 532 ) may advantageously be lossy processes. When the coded video data may be decoded at a video decoder (not shown in  FIG. 5 ), the reconstructed video sequence typically may be a replica of the source video sequence with some errors. The local video decoder ( 533 ) replicates decoding processes that may be performed by the video decoder on reference pictures and may cause reconstructed reference pictures to be stored in the reference picture cache ( 534 ). In this manner, the video encoder ( 503 ) may store copies of reconstructed reference pictures locally that have common content as the reconstructed reference pictures that will be obtained by a far-end video decoder (absent transmission errors). 
     The predictor ( 535 ) may perform prediction searches for the coding engine ( 532 ). That is, for a new picture to be coded, the predictor ( 535 ) may search the reference picture memory ( 534 ) for sample data (as candidate reference pixel blocks) or certain metadata such as reference picture MVs, block shapes, and so on, that may serve as an appropriate prediction reference for the new pictures. The predictor ( 535 ) may operate on a sample block-by-pixel block basis to find appropriate prediction references. In some cases, as determined by search results obtained by the predictor ( 535 ), an input picture may have prediction references drawn from multiple reference pictures stored in the reference picture memory ( 534 ). 
     The controller ( 550 ) may manage coding operations of the source coder ( 530 ), including, for example, setting of parameters and subgroup parameters used for encoding the video data. 
     Output of all aforementioned functional units may be subjected to entropy coding in the entropy coder ( 545 ). The entropy coder ( 545 ) translates the symbols as generated by the various functional units into a coded video sequence, by lossless compressing the symbols according to technologies such as Huffman coding, variable length coding, arithmetic coding, and so forth. 
     The transmitter ( 540 ) may buffer the coded video sequence(s) as created by the entropy coder ( 545 ) to prepare for transmission via a communication channel ( 560 ), which may be a hardware/software link to a storage device which would store the encoded video data. The transmitter ( 540 ) may merge coded video data from the video coder ( 503 ) with other data to be transmitted, for example, coded audio data and/or ancillary data streams (sources not shown). 
     The controller ( 550 ) may manage operation of the video encoder ( 503 ). During coding, the controller ( 550 ) may assign to each coded picture a certain coded picture type, which may affect the coding techniques that may be applied to the respective picture. For example, pictures often may be assigned as one of the following picture types: 
     An Intra Picture (I picture) may be one that may be coded and decoded without using any other picture in the sequence as a source of prediction. Some video codecs allow for different types of intra pictures, including, for example Independent Decoder Refresh (“IDR”) Pictures. A person skilled in the art is aware of those variants of I pictures and their respective applications and features. 
     A predictive picture (P picture) may be one that may be coded and decoded using intra prediction or inter prediction using at most one MV and reference index to predict the sample values of each block. 
     A bi-directionally predictive picture (B Picture) may be one that may be coded and decoded using intra prediction or inter prediction using at most two MVs and reference indices to predict the sample values of each block. Similarly, multiple-predictive pictures can use more than two reference pictures and associated metadata for the reconstruction of a single block. 
     Source pictures commonly may be subdivided spatially into a plurality of sample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 samples each) and coded on a block-by-block basis. Blocks may be coded predictively with reference to other (already coded) blocks as determined by the coding assignment applied to the blocks&#39; respective pictures. For example, blocks of I pictures may be coded non-predictively or they may be coded predictively with reference to already coded blocks of the same picture (spatial prediction or intra prediction). Pixel blocks of P pictures may be coded predictively, via spatial prediction or via temporal prediction with reference to one previously coded reference picture. Blocks of B pictures may be coded predictively, via spatial prediction or via temporal prediction with reference to one or two previously coded reference pictures. 
     The video encoder ( 503 ) may perform coding operations according to a predetermined video coding technology or standard, such as ITU-T Rec. H.265. In its operation, the video encoder ( 503 ) may perform various compression operations, including predictive coding operations that exploit temporal and spatial redundancies in the input video sequence. The coded video data, therefore, may conform to a syntax specified by the video coding technology or standard being used. 
     In an embodiment, the transmitter ( 540 ) may transmit additional data with the encoded video. The source coder ( 530 ) may include such data as part of the coded video sequence. Additional data may comprise temporal/spatial/SNR enhancement layers, other forms of redundant data such as redundant pictures and slices, SEI messages, VUI parameter set fragments, and so on. 
     A video may be captured as a plurality of source pictures (video pictures) in a temporal sequence. Intra-picture prediction (often abbreviated to intra prediction) makes use of spatial correlation in a given picture, and inter-picture prediction makes uses of the (temporal or other) correlation between the pictures. In an example, a specific picture under encoding/decoding, which is referred to as a current picture, is partitioned into blocks. When a block in the current picture is similar to a reference block in a previously coded and still buffered reference picture in the video, the block in the current picture can be coded by a vector that is referred to as a MV. The MV points to the reference block in the reference picture, and can have a third dimension identifying the reference picture, in case multiple reference pictures are in use. 
     In some embodiments, a bi-prediction technique can be used in the inter-picture prediction. According to the bi-prediction technique, two reference pictures, such as a first reference picture and a second reference picture that are both prior in decoding order to the current picture in the video (but may be in the past and future, respectively, in display order) are used. A block in the current picture can be coded by a first MV that points to a first reference block in the first reference picture, and a second MV that points to a second reference block in the second reference picture. The block can be predicted by a combination of the first reference block and the second reference block. 
     Further, a merge mode technique can be used in the inter-picture prediction to improve coding efficiency. 
     According to some embodiments of the disclosure, predictions, such as inter-picture predictions and intra-picture predictions are performed in the unit of blocks. For example, according to the HEVC standard, a picture in a sequence of video pictures is partitioned into coding tree units (CTU) for compression, the CTUs in a picture have the same size, such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTU includes three coding tree blocks (CTBs), which are one luma CTB and two chroma CTBs. Each CTU can be recursively quad-tree split into one or multiple coding units (CUs). For example, a CTU of 64×64 pixels can be split into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUs of 16×16 pixels. In an example, each CU is analyzed to determine a prediction type for the CU, such as an inter prediction type or an intra prediction type. The CU is split into one or more prediction units (PUs) depending on the temporal and/or spatial predictability. Generally, each PU includes a luma prediction block (PB), and two chroma PBs. In an embodiment, a prediction operation in coding (encoding/decoding) is performed in the unit of a prediction block. Using a luma prediction block as an example of a prediction block, the prediction block includes a matrix of values (e.g., luma values) for pixels, such as 8×8 pixels, 16×16 pixels, 8×16 pixels, 16×8 pixels, and the like. 
       FIG. 6  shows a diagram of a video encoder ( 603 ) according to another embodiment of the disclosure. The video encoder ( 603 ) is configured to receive a processing block (e.g., a prediction block) of sample values within a current video picture in a sequence of video pictures, and encode the processing block into a coded picture that is part of a coded video sequence. In an example, the video encoder ( 603 ) is used in the place of the video encoder ( 303 ) in the  FIG. 3  example. 
     In an HEVC example, the video encoder ( 603 ) receives a matrix of sample values for a processing block, such as a prediction block of 8×8 samples, and the like. The video encoder ( 603 ) determines whether the processing block is best coded using intra mode, inter mode, or bi-prediction mode using, for example, rate-distortion optimization. When the processing block is to be coded in intra mode, the video encoder ( 603 ) may use an intra prediction technique to encode the processing block into the coded picture; and when the processing block is to be coded in inter mode or bi-prediction mode, the video encoder ( 603 ) may use an inter prediction or bi-prediction technique, respectively, to encode the processing block into the coded picture. In certain video coding technologies, merge mode can be an inter picture prediction submode where the MV is derived from one or more MV predictors without the benefit of a coded MV component outside the predictors. In certain other video coding technologies, a MV component applicable to the subject block may be present. In an example, the video encoder ( 603 ) includes other components, such as a mode decision module (not shown) to determine the mode of the processing blocks. 
     In the  FIG. 6  example, the video encoder ( 603 ) includes the inter encoder ( 630 ), an intra encoder ( 622 ), a residue calculator ( 623 ), a switch ( 626 ), a residue encoder ( 624 ), a general controller ( 621 ), and an entropy encoder ( 625 ) coupled together as shown in  FIG. 6 . 
     The inter encoder ( 630 ) is configured to receive the samples of the current block (e.g., a processing block), compare the block to one or more reference blocks in reference pictures (e.g., blocks in previous pictures and later pictures), generate inter prediction information (e.g., description of redundant information according to inter encoding technique, MVs, merge mode information), and calculate inter prediction results (e.g., prediction block) based on the inter prediction information using any suitable technique. In some examples, the reference pictures are decoded reference pictures that are decoded based on the encoded video information. 
     The intra encoder ( 622 ) is configured to receive the samples of the current block (e.g., a processing block), in some cases compare the block to blocks already coded in the same picture, generate quantized coefficients after transform, and in some cases also intra prediction information (e.g., an intra prediction direction information according to one or more intra encoding techniques). In an example, the intra encoder ( 622 ) also calculates intra prediction results (e.g., prediction block) based on the intra prediction information and reference blocks in the same picture. 
     The general controller ( 621 ) is configured to determine general control data and control other components of the video encoder ( 603 ) based on the general control data. In an example, the general controller ( 621 ) determines the mode of the block, and provides a control signal to the switch ( 626 ) based on the mode. For example, when the mode is the intra mode, the general controller ( 621 ) controls the switch ( 626 ) to select the intra mode result for use by the residue calculator ( 623 ), and controls the entropy encoder ( 625 ) to select the intra prediction information and include the intra prediction information in the bitstream; and when the mode is the inter mode, the general controller ( 621 ) controls the switch ( 626 ) to select the inter prediction result for use by the residue calculator ( 623 ), and controls the entropy encoder ( 625 ) to select the inter prediction information and include the inter prediction information in the bitstream. 
     The residue calculator ( 623 ) is configured to calculate a difference (residue data) between the received block and prediction results selected from the intra encoder ( 622 ) or the inter encoder ( 630 ). The residue encoder ( 624 ) is configured to operate based on the residue data to encode the residue data to generate the transform coefficients. In an example, the residue encoder ( 624 ) is configured to convert the residue data from a spatial domain to a frequency domain, and generate the transform coefficients. The transform coefficients are then subject to quantization processing to obtain quantized transform coefficients. In various embodiments, the video encoder ( 603 ) also includes a residue decoder ( 628 ). The residue decoder ( 628 ) is configured to perform inverse-transform, and generate the decoded residue data. The decoded residue data can be suitably used by the intra encoder ( 622 ) and the inter encoder ( 630 ). For example, the inter encoder ( 630 ) can generate decoded blocks based on the decoded residue data and inter prediction information, and the intra encoder ( 622 ) can generate decoded blocks based on the decoded residue data and the intra prediction information. The decoded blocks are suitably processed to generate decoded pictures and the decoded pictures can be buffered in a memory circuit (not shown) and used as reference pictures in some examples. 
     The entropy encoder ( 625 ) is configured to format the bitstream to include the encoded block. The entropy encoder ( 625 ) is configured to include various information according to a suitable standard such as HEVC. In an example, the entropy encoder ( 625 ) is configured to include the general control data, the selected prediction information (e.g., intra prediction information or inter prediction information), the residue information, and other suitable information in the bitstream. Note that, according to the disclosed subject matter, when coding a block in the merge submode of either inter mode or bi-prediction mode, there is no residue information. 
       FIG. 7  shows a diagram of a video decoder ( 710 ) according to another embodiment of the disclosure. The video decoder ( 710 ) is configured to receive coded pictures that are part of a coded video sequence, and decode the coded pictures to generate reconstructed pictures. In an example, the video decoder ( 710 ) is used in the place of the video decoder ( 310 ) in the  FIG. 3  example. 
     In the  FIG. 7  example, the video decoder ( 710 ) includes an entropy decoder ( 771 ), an inter decoder ( 780 ), a residue decoder ( 773 ), a reconstruction module ( 774 ), and an intra decoder ( 772 ) coupled together as shown in  FIG. 7 . 
     The entropy decoder ( 771 ) can be configured to reconstruct, from the coded picture, certain symbols that represent the syntax elements of which the coded picture is made up. Such symbols can include, for example, the mode in which a block is coded (such as, for example, intra mode, inter mode, bi-predicted mode, the latter two in merge submode or another submode), prediction information (such as, for example, intra prediction information or inter prediction information) that can identify certain sample or metadata that is used for prediction by the intra decoder ( 772 ) or the inter decoder ( 780 ), respectively, residual information in the form of, for example, quantized transform coefficients, and the like. In an example, when the prediction mode is inter or bi-predicted mode, the inter prediction information is provided to the inter decoder ( 780 ); and when the prediction type is the intra prediction type, the intra prediction information is provided to the intra decoder ( 772 ). The residual information can be subject to inverse quantization and is provided to the residue decoder ( 773 ). 
     The inter decoder ( 780 ) is configured to receive the inter prediction information, and generate inter prediction results based on the inter prediction information. 
     The intra decoder ( 772 ) is configured to receive the intra prediction information, and generate prediction results based on the intra prediction information. 
     The residue decoder ( 773 ) is configured to perform inverse quantization to extract de-quantized transform coefficients, and process the de-quantized transform coefficients to convert the residual from the frequency domain to the spatial domain. The residue decoder ( 773 ) may also require certain control information (to include the Quantizer Parameter (QP)), and that information may be provided by the entropy decoder ( 771 ) (data path not depicted as this may be low volume control information only). 
     The reconstruction module ( 774 ) is configured to combine, in the spatial domain, the residual as output by the residue decoder ( 773 ) and the prediction results (as output by the inter or intra prediction modules as the case may be) to form a reconstructed block, that may be part of the reconstructed picture, which in turn may be part of the reconstructed video. It is noted that other suitable operations, such as a deblocking operation and the like, can be performed to improve the visual quality. 
     It is noted that the video encoders ( 303 ), ( 503 ), and ( 603 ), and the video decoders ( 310 ), ( 410 ), and ( 710 ) can be implemented using any suitable technique. In an embodiment, the video encoders ( 303 ), ( 503 ), and ( 603 ), and the video decoders ( 310 ), ( 410 ), and ( 710 ) can be implemented using one or more integrated circuits. In another embodiment, the video encoders ( 303 ), ( 503 ), and ( 603 ), and the video decoders ( 310 ), ( 410 ), and ( 710 ) can be implemented using one or more processors that execute software instructions. 
     II. Block Partition 
       FIG. 8  shows exemplary block partitions according to some embodiments of the disclosure. 
     In some related examples such as VP9 that is proposed by Alliance for Open Media (AOMedia), a 4-way partition tree can be used, which starts from a 64×64 level down to a 4×4 level with some additional restrictions for blocks of 8×8 and below as shown in  FIG. 8 . It is noted that partitions designated as R can be referred to as recursive partitions. That is, the same partition tree is repeated at a lower scale until the lowest 4×4 level is reached. 
     In some related examples such as AV1 that is proposed by AOMedia and based on VP9, the partition-tree can be expanded to a 10-way structure as shown in  FIG. 8 , and the largest coding block size (referred to as superblock in VP9/AV1 parlance) is increased to start from 128×128. It is noted that 4:1/1:4 rectangular partitions are included in AV1 but not included in VP9. None of the rectangular partitions can be further subdivided. In addition, more flexibility can be supported in AV1 in the use of partitions below 8×8 level, since the inter prediction can be performed on 2×2 chroma blocks in some examples. 
     In some related examples such as HEVC, a CTU can be split into CUs by using a quadtree structure denoted as a coding tree to adapt to various local characteristics. A decision on whether to code a picture area using inter-picture (temporal) or intra-picture (spatial) prediction can be made at a CU level. Each CU can be further split into one, two, or four PUs according to a PU splitting type. Inside one PU, a same prediction process can be applied and relevant information can be transmitted to a decoder on a PU basis. After obtaining a residual block by applying a prediction process based on a PU splitting type, a CU can be partitioned into TUs according to another quadtree structure like a coding tree for the CU. One key feature of the HEVC structure is that it has multiple partition conceptions including CU, PU, and TU. In HEVC, a CU or a TU can only be a square shape, while a PU may be a square or a rectangular shape for an inter predicted block. In HEVC, one coding block may be further split into four square sub-blocks, and a transform process can be performed on each sub-block, i.e., TU. Each TU can be further split recursively (e.g., using a quadtree split) into smaller TUs. The quadtree split can be referred to as a residual quadtree (RQT). 
     At a picture boundary, HEVC employs an implicit quadtree split so that a block can continue performing quadtree splitting until a size of the block fits a picture boundary. 
     In some related examples such as VVC, a quad-tree plus binary-tree (QTBT) partitioning structure can be applied. The quad-tree plus binary tree (QTBT) structure removes the concepts of multiple partition types (i.e., it removes the separation of the CU, PU, and TU concepts), and supports more flexibility for CU partition shapes. 
       FIGS. 9A and 9B  show an exemplary block partitioning using QTBT and a corresponding tree structure according to an embodiment of the disclosure. The solid lines indicate QT splitting and dotted lines indicate BT splitting. In each splitting (i.e., non-leaf) node of the BT, one flag is signaled to indicate which splitting type (i.e., horizontal or vertical) is used. In  FIG. 9B , 0 indicates a horizontal splitting and 1 indicates a vertical splitting. For the QT splitting, there is no need to indicate the splitting type since QT splitting always splits a block both horizontally and vertically to produce 4 sub-blocks with an equal size. 
     In the QTBT structure, a CU can have either a square or rectangular shape. As shown in  FIGS. 9A and 9B , a CTU is first partitioned by a QT structure. The QT leaf nodes can be further partitioned by a BT structure. There are two splitting types, symmetric horizontal splitting and symmetric vertical splitting, in the BT splitting. The BT leaf nodes are CUs, and segmentation into two CUs is used for prediction and transform processing without any further partitioning. Accordingly, a CU, PU, and TU can have the same block size in the QTBT structure. 
     A CU sometimes can include CBs of different color components, such as in JEM. For example, one CU can contain one luma CB and two chroma CBs in the case of P and B slices with the 4:2:0 chroma format. In other examples, a CU can include CBs of a single component, e.g., one CU can contain only one luma CB or just two chroma CBs in the case of I slices. 
     The following parameters are defined for the QTBT partitioning scheme: CTU size (a root node size of a QT, for example as in HEVC), MinQTSize (the minimum allowed QT leaf node size), MaxBTSize (the maximum allowed BT root node size), MaxBTDepth (the maximum allowed BT depth), and MinBTSize (the minimum allowed BT leaf node size). 
     In one example of the QTBT partitioning structure, the CTU size is set as 128×128 luma samples with two corresponding 64×64 blocks of chroma samples, the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, the MinBTSize (for both width and height) is set as 4×4, and the MaxBTDepth is set as 4. The QT partitioning is applied to the CTU first to generate QT leaf nodes. The QT leaf nodes may have a size from 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If the leaf QT node is 128×128, it will not be further split by the BT since the size exceeds the MaxBTSize (i.e., 64×64). Otherwise, the leaf QT node could be further partitioned by the BT tree. Therefore, the QT leaf node is also the root node for the BT and it has a BT depth of 0. When the BT depth reaches MaxBTDepth (i.e., 4), no further splitting is considered. When the BT node has a width equal to MinBTSize (i.e., 4), no further horizontal splitting is considered. Similarly, when the BT node has a height equal to MinBTSize, no further vertical splitting is considered. The leaf nodes of the BT are further processed by prediction and transform processing without any further partitioning. For example, the maximum CTU size is 256×256 luma samples such as in JEM. 
     III. Prediction in AV1 
     In some related examples such as VP9, 8 directional modes are supported, which correspond to angles from 45 to 207 degrees. To exploit more varieties of spatial redundancy in directional textures, in some related examples such as AV1, directional intra modes are extended to an angle set with a finer granularity. The original 8 angles are slightly changed and referred to as nominal angles, and these 8 nominal angles are named as V_PRED, H_PRED, D45_PRED, D135_PRED, D113_PRED, D157_PRED, D203_PRED, and D67_PRED. 
       FIG. 10  shows exemplary nominal angles according to an embodiment of the disclosure. Each nominal angle can be associated with 7 finer angles, so there can be 56 directional angles in total, in some related examples such as AV1. A prediction angle can be represented by a nominal intra angle plus an angle delta. The angle delta can be equal to a coefficient multiplied by a step size of 3 degrees. The coefficient can be in a range of −3 to 3. In AV1, 8 nominal modes together with 5 non-angular smooth modes are firstly signaled. Then, if a current mode is an angular mode, an index is further signaled to indicate an angle delta to a corresponding nominal angle. To implement directional prediction modes in AV1 through a generic way, all the 56 directional intra prediction angles in AV1 can be implemented with a unified directional predictor that projects each pixel to a reference sub-pixel location and interpolates the reference sub-pixel by a 2-tap bilinear filter. 
     In some related examples such as AV1, there are 5 non-directional smooth intra prediction modes, which are DC, PAETH, SMOOTH, SMOOTH_V, and SMOOTH_H. For DC prediction, an average of left and above neighboring samples is used as a predictor of a block to be predicted. For PAETH prediction, top, left, and top-left reference samples are firstly fetched, and then a value which is closest to (top+left—top-left) is set as a predictor for a pixel to be predicted. 
       FIG. 11  shows positions of top, left, and top-left samples for one pixel in a current block according to an embodiment of the disclosure. For SMOOTH, SMOOTH_V, and SMOOTH_H modes, a block is predicted using quadratic interpolation in vertical or horizontal directions, or an average of both directions. 
     In some related examples such as AV1, recursive filtering based intra prediction modes can be used. 
       FIG. 12  shows an exemplary recursive filter intra mode according to an embodiment of the disclosure. 
     To capture decaying spatial correlation with references on the edges, FILTER INTRA modes are designed for luma blocks. Five filter intra modes are defined in AV1, each represented by a set of eight 7-tap filters reflecting a correlation between pixels in a 4×2 patch and 7 neighbors adjacent to the patch. For example, weighting factors of a 7-tap filter are position dependent. As shown in  FIG. 12 , an 8×8 block is split into eight 4×2 patches, which are indicated by B0, B1, B2, B3, B4, B5, B6, and B7. For each patch, its 7 neighbors, indicated by R0˜R7, are used to predict pixels in the respective patch. For patch B0, all the neighbors are already reconstructed. But for other patches, not all the neighbors are reconstructed, then the predicted values of immediate neighbors are used as reference values. For example, all the neighbors of patch B7 are not reconstructed, so the prediction samples of the neighbors (i.e., B5 and B6) of patch B7 are used instead. 
     In some related examples such as VP9, three reference frames can be used for an inter prediction. The three reference frames include LAST (nearest past), GOLDEN (distant past), and ALTREF (temporally filtered future) frames. 
     In some related examples such as AV1, extended reference frames can be used. For example, in addition to the three reference frames used in VP9, four more types of reference frames can be used in AV1. The four more types of reference frames include LAST2, LAST3, BWDREF, and ALTREF2 frames. LAST2 and LAST3 frames are two near past frames, and BWDREF and ALTREF2 frames are two future frames. In addition, BWDREF frame is a look-ahead frame coded without temporal filtering and more useful as a backward reference in a relatively short distance. ALTREF2 frame is an intermediate filtered future reference frame between GOLDEN and ALTREF frames. 
       FIG. 13  shows an exemplary multi-layer reference frame structure according to an embodiment of the disclosure. In  FIG. 13 , an adaptive number of frames share the same GOLDEN and ALTREF frames. BWDREF frame is a look-ahead frame directly coded without applying temporal filtering, thus more applicable as a backward reference in a relatively short distance. ALTREF2 frame serves as an intermediate filtered future reference between GOLDEN and ALTREF frames. All the new references can be picked by a single prediction mode or be combined into a pair to form a compound mode. AV1 provides an abundant set of reference frame pairs, providing both bi-directional compound prediction and uni-directional compound prediction, thus can encode a variety of videos with dynamic temporal correlation characteristics in a more adaptive and optimal way. 
       FIG. 14  shows an exemplary candidate motion vector list building process according to an embodiment of the disclosure. The spatial and temporal reference motion vectors can be classified into two categories based on where they appear: the nearest spatial neighbors and the rest. In some related examples, the motion vectors from immediate above, left, and top-right neighboring blocks of a current block can have a higher correlation with the current block than the rest and, hence, are considered with higher priority. Within each category, the motion vectors are ranked in descending order of their appearance counts within the spatial and temporal search range. A motion vector candidate with a higher appearance count can be considered to be “popular” in the local region, i.e., a higher prior probability. The two categories are concatenated to form a ranked list. 
       FIG. 15  shows an exemplary motion-field estimation process according to an embodiment of the disclosure. 
     In some related examples such as AV1, dynamic spatial and temporal motion-vector references can be used. For example, motion vector reference selection schemes can be incorporated to efficiently code motion vectors. In the motion-vector reference selection schemes, a spatial neighbor can be wider than those used in VP9. In addition, a motion-field estimation process can be used to find temporal motion-vector reference candidates. The motion field estimation process can work in three stages: motion vector buffering, motion trajectory creation, and motion vector projection. First, for each coded frame, reference frame indices and associated motion vectors of the respective coded frame can be stored. The stored information can be referenced by a next coding frame to generate a motion field of the next coding frame. The motion field estimation can examine the motion trajectories, e.g., MV Ref2  in  FIG. 15  pointing from a block in one reference frame Ref2 to another reference frame Ref0 Ref2 . Then, the motion field estimation process searches through a collocated 128×128 area for all motion trajectories in an 8×8 block resolution that passes each 64×64 processing unit. Next, at the coding block level, once the reference frame(s) have been determined, motion vector candidates can be derived by linearly projecting passing motion trajectories onto the desired reference frames, e.g. converting MV Ref2  in  FIG. 15  to MV 0  or MV 1 . 
     Once all the candidate motion-vectors are found, the candidate motion-vectors can be sorted, merged, and ranked to build up to four final candidates. Then, an index of a selected reference motion vector from the list can be signaled and a motion vector difference can be optionally coded. 
     In some related examples such as AV1, in order to decrease prediction errors around block boundaries by combining predictions obtained from adjacent motion vectors, a block-based prediction can be combined with secondary predictors from top and left edges by applying 1-D filters in vertical and horizontal directions, respectively. This method can be referred to as overlapped block motion compensation (OBMC). 
       FIGS. 16A and 16B  show exemplary overlapped regions (shaded regions) to be predicted using a top-neighboring block (2) and a left-neighboring block (4), respectively. The shaded regions of predicted blocks (0) can be predicted by recursively generating blended prediction samples via a 1-D raised cosine filter. 
     In some related examples such as AV1, two affine prediction models, which are referred to as a global warped motion compensation and a local warped motion compensation, can be used. The former signals a frame-level affine model between a frame and its reference while the latter handles varying local motions implicitly with minimal overhead. Local motion parameters can be derived at a block level using 2D motion vectors from causal neighbors. This affine model is realized through a consecutive horizontal and vertical shearing operation based upon an 8-tap interpolation filter at 1/64 pixel precision. 
       FIG. 17  shows an exemplary two-step warping process in which a horizontal-shearing is followed by a vertical-shearing. In  FIG. 17 , the affine model is realized in a local warped motion compensation, which first performs the horizontal-shearing and then the vertical-shearing. 
     IV. Frame-Based Super-Resolution in AV1 
       FIG. 18  shows an overall loop-filtering pipeline including a frame-level super-resolution in AV1. At an encoder side, a source frame can be first down-scaled in a non-normative way and encoded at a lower resolution. At a decoder side, a deblocking filter and a constrained directional enhancement filter (CDEF) can be applied to remove coding artifacts while preserving edges at the lower resolution. Then, a linear up-sampling filter can be applied along a horizontal direction only to get a full-resolution reconstruction. A loop-restoration filter can be optionally applied afterwards at full-resolution to recover high-frequency details lost during the down-sampling and quantization. 
     In some related examples such as AV1, the super-resolution is a special mode signaled at a frame level. Each coded frame can use the horizontal-only super-resolution mode with an arbitrary resolution within ratio constraints. Whether to apply the linear up-sampling after decoding and the scaling-ratio to be used can be signaled. The up-sampling ratio can have 9 possible values given as d/8 with d=8, 9 . . . and 16. The corresponding down-sampling ratios before encoding can be 8/d. 
     Given the output frame dimensions W×H and the up-sampling ratio d, both encoder and decoder can calculate the low-resolution coded frame dimension as w×H, where the reduced width w=(8W+d/2)/d. The linear upscaling process takes in the reduced-resolution frame w×H and outputs a frame with the dimension W×H as specified in a frame header. The normative horizontal linear upscaler in AV1 uses a 1/16-th phase linear 8-tap filter for interpolation of each row. 
     V. CU-Based Super-Resolution Coding 
     In some related examples such as AV1, the super-resolution is performed on a frame-level. That is, the super-resolution is applied to all areas in a picture with a fixed scaling ratio. However, signal statistics in different areas within a picture can vary widely. Thus, applying the down-sampling and/or up-sampling to all areas may not always be a good rate-distortion trade-off. 
     Applying the down-sampling and/or up-sampling to a picture region in an adaptive manner can be performed using a pre-processing approach and/or a post-processing approach such as the use of a mask and/or segment information to choose the picture region for the down-sampling and/or up-sampling. However, this process cannot guarantee that the rate-distortion performance improvement is over other coding methods that do not employ the super-resolution. 
     This disclosure includes methods of block-level super-resolution coding. 
     According to aspects of the disclosure, a mixed-resolution prediction can be used to emulate a frame-level super-resolution adapted for use at the block level. 
     In one embodiment, a block-level flag can be used to indicate whether or not a reduced-resolution coding is used for a coding block. For example, if the block-level flag is set to a first pre-defined value (e.g., 1), the reduced-resolution coding is enabled for the block. A prediction-sample generation such as the prediction procedure in AV1 as described above can be performed by using reference samples or pictures in the reduced-resolution to generate a reduced-resolution prediction block for the coding block. Then, the reduced-resolution prediction block can be used to generate a reduced-resolution reconstruction block for the coding block. Finally, the reduced-resolution reconstruction block can be up-sampled back to a full-resolution reconstruction block for the coding block. 
     In one embodiment, if the block-level flag is set to a second pre-defined value (e.g., 0), the prediction-sample generation can be performed in the full-resolution (or original resolution) and followed by the full-resolution reconstruction. 
       FIG. 19  shows an example implementation using the block-level flag according to an embodiment of the disclosure. When the block-level flag is on, for example when the block-level flag is equal to the first pre-defined value, a source block ( 1901 ) at an encoder side can be down-sampled through a down-sampler module ( 1920 ) to generate a down-sampled source block ( 1902 ). The down-sampled source block ( 1902 ) can be combined with a reduced-resolution prediction block ( 1903 ) to generate a down-sampled residual block ( 1904 ). Then, the down-sampled residual block ( 1904 ) can be coded into a coded video bitstream through a module (or multiple modules, not shown in  FIG. 19 ) including the transform, quantization, and entropy-encoding processes. In order to decode the source block ( 1901 ), the coded video bitstream received at a decoder side can be processed through the entropy-decoding, inverse quantization, and inverse transform processes to generate a down-sampled residual block ( 1911 ). The down-sampled residual block ( 1911 ) can be combined with a reduced-resolution prediction block ( 1912 ) to generate a down-sampled reconstruction block ( 1913 ). The down-sampled reconstruction block ( 1913 ) can be up-sampled through an up-sampled module ( 1930 ) to generate a full-resolution reconstruction block ( 1914 ) for the source block ( 1901 ). It is noted that the reduced-resolution prediction block ( 1912 ) can be generated by down-sampling a full-resolution reconstruction block of a reference block of the source block ( 1901 ) through a down-sampler module ( 1940 ). 
     When the block-level flag is off, for example when the block-level flag is equal to the second pre-defined value, the down-sample modules ( 1920 ) and ( 1940 ) and the up-sampler module ( 1930 ) in  FIG. 19  are not be applied. The down-sampled or reduced-resolution blocks ( 1902 )-( 1904 ) and ( 1911 )-( 1913 ) can become full-resolution counterparts. 
     In one embodiment, for a coding block with a size of M×N, a reference block with a size of M×N can be down-sampled with down-sampling factors D X  and D Y  along horizontal and vertical directions, respectively, to generate a reduced-resolution prediction block with a size of (M/D X ) by (N/D Y ). Example values of M and N can include, but are not limited to, 256, 128, 64, 32, 16, and 8. The down-sampling factors D X  and D Y  are integers that include, but are not limited to, 2, 4, and 8. 
     In one embodiment, the block-level flag can be signaled or inferred adaptively per CU, superblock, prediction block, transform block, tile, coded-segment, frame, or sequence basis. 
     In one embodiment, for the up-sampler module ( 1930 ) that is used to up-sample the reduced-resolution reconstruction block, up-sampling filter coefficients can be signaled directly or an index of a set of filter coefficients from a plurality of sets of pre-defined coefficients can be signaled. 
     According to aspects of the disclosure, for being referenced by a motion vector of a current block, motion vectors of spatially and/or temporally neighboring blocks of the current block can be scaled with the same or different resolutions from the current block. 
     For example, when the current block is coded with sampling ratios (or down-sampling factors) D X  and D Y  along the horizontal and vertical directions, respectively, the motion vectors of the spatially and/or temporally neighboring blocks with sampling ratios D ref,x  and D ref,y  can be scaled by 
     
       
         
           
             
               
                 D 
                 
                   ref 
                   , 
                   x 
                 
               
               
                 D 
                 x 
               
             
             ⁢ 
             
                 
             
             ⁢ 
             and 
             ⁢ 
             
                 
             
             ⁢ 
             
               
                 D 
                 
                   ref 
                   , 
                   y 
                 
               
               
                 D 
                 y 
               
             
           
         
       
     
     for horizontal and vertical components, respectively. 
       FIG. 20  shows an exemplary spatially neighboring motion vector referencing according to an embodiment of the disclosure. 
     In an embodiment, based on a motion vector referencing method such as the spatially neighboring motion vector referencing method in AV1, a reference motion vector list building process can search a neighboring region in a unit of W×H luma samples in an order of (1) through (8) as indicated in  FIG. 20 . The motion vectors for a top W×H region T ij , a left W×H region L ij , a top-left W×H region TL, and a top-right W×H region TR are scaled by 
     
       
         
           
             
               D 
               ref 
             
             
               D 
               cur 
             
           
         
       
     
     before used as reference motion vectors in the candidate list, where D ref  is a sampling ratio of a W×H region and D cur  is a sampling ratio to be used for the current block. Example value of W×H includes, but is not limited to 8×8. 
       FIG. 21  shows an exemplary temporally neighboring motion vector referencing according to an embodiment of the disclosure. 
     In one embodiment, based on a motion vector referencing method such as the temporally neighboring motion vector referencing method in AV1, the motion vectors mf_mv_1 and mf_mv_2 for a current block located at (blk_row, blk_col) in a current frame can be obtained as follows. 
     As shown in  FIG. 21 , a motion vector ref my for a W×H region located at (ref_blk_row, ref_blk_col) within a designated search area of a reference frame (reference_frame1) is used to find a motion trajectory toward a prior frame (prior_frame). If the trajectory crosses the current block located at (blk_row, blk_col), the motion vectors mf_mv_1 and mf_mv_2 for reference_frame1 and reference_frame2 can be given as 
     
       
         
           
             
               
                 
                   
                     mf_mv 
                     ⁢ 
                     _ 
                     ⁢ 
                     2. 
                     ⁢ 
                     x 
                   
                   = 
                   
                     
                       - 
                       
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           0 
                         
                       
                     
                     · 
                     
                       ref_mv 
                       . 
                       x 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     mf_mv 
                     ⁢ 
                     _ 
                     ⁢ 
                     2. 
                     ⁢ 
                     y 
                   
                   = 
                   
                     
                       - 
                       
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         
                           d 
                           ⁢ 
                           
                               
                           
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                     · 
                     
                       ref_mv 
                       . 
                       y 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     mf_mv 
                     ⁢ 
                     _ 
                     ⁢ 
                     1 
                     ⁢ 
                     x 
                   
                   = 
                   
                     
                       - 
                       
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
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                     · 
                     
                       ref_mv 
                       . 
                       x 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     mf_mv 
                     ⁢ 
                     _ 
                     ⁢ 
                     1 
                     ⁢ 
                     y 
                   
                   = 
                   
                     
                       - 
                       
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           0 
                         
                       
                     
                     · 
                     
                       ref_mv 
                       . 
                       y 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where ‘.x’ and ‘.y’ denote horizontal and vertical components of a motion vector, respectively. 
     The derived motion vectors mf_mv_1 and mf_mv_2 are further scaled by 
     
       
         
           
             
               D 
               ref 
             
             
               D 
               cur 
             
           
         
       
     
     before being used as reference motion vectors in the candidate list, where D ref  is a sampling ratio of a W×H block and D cur  is a sampling ratio to be used for the current block located at (blk_row, blk_col) in the current frame. An example value of W×H may be, but is not limited to, 8×8. 
     According to aspects of the disclosure, motion vector candidates of a current block can be classified into two (or multiple) categories. As shown in  FIG. 14 , motion vectors of spatially neighboring blocks located at an immediate top row, an immediate left column, and a top-right corner of a current block can be classified as a first category (e.g., category 1) of motion vector candidates for the current block, while all other candidates are classified as a second category (e.g., category 2). Within each category, the motion vector candidates can be sorted in a descending order of a number of counts that each candidate appears. That is, a first motion vector candidate that appears more often than a second motion vector candidate in a candidate list is located prior to the second motion vector candidate in the candidate list. In addition, the candidate list of the first category (e.g., category 1) can be concatenated by that of the second category (e.g., category 2) to form a single candidate list. 
     In some embodiments, in addition to the category and number of counts within each category, the sampling ratios used for motion vector candidates can be incorporated in building the candidate list. 
     In one embodiment, a motion vector candidate from a neighboring block that has the same sampling factor as a current block can have a higher priority for being selected in a candidate list of each category given the same appearance counts. 
     In one embodiment, two separate candidate lists can be constructed. A first candidate list can only include candidate motion vectors from the neighboring blocks with the same sampling ratio. A second candidate list can include candidate motion vectors from neighboring blocks with different sampling ratios. Which candidate list is used can be signaled or inferred in addition to a signaling of an index of a chosen reference motion vector in the candidate list. 
     In one embodiment, the second candidate list can be scanned only when a number of candidates in the first candidate list is less than a designated number of reference motion vectors to be indexed and signaled. 
     In one embodiment, the two separate candidate lists can be merged to form a single list based on an interleaved manner. For example, when a total number of candidates from both lists is more than or equal to the designated number of reference motion vectors, one from the first candidate list and another from the second candidate list can be chosen in an ascending order of entry positions in the lists until the designated number of reference motion vectors in the combined list is reached. 
       FIG. 22  shows an exemplary spatially neighboring motion vector referencing for affine motion prediction according to an embodiment of the disclosure. 
     In one embodiment, based on a motion vector referencing method such as the affine motion prediction method in AV1, an affine model that projects a sample at (x, y) in a current block to a prediction sample in a reference block at (x′, y′) in a reference frame is given as: 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             x 
                             ′ 
                           
                         
                       
                       
                         
                           
                             y 
                             ′ 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               h 
                               
                                 1 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               h 
                               
                                 1 
                                 ⁢ 
                                 2 
                               
                             
                           
                           
                             
                               h 
                               
                                 1 
                                 ⁢ 
                                 3 
                               
                             
                           
                         
                         
                           
                             
                               h 
                               
                                 2 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               h 
                               
                                 2 
                                 ⁢ 
                                 2 
                               
                             
                           
                           
                             
                               h 
                               
                                 2 
                                 ⁢ 
                                 3 
                               
                             
                           
                         
                       
                       ] 
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           
                             x 
                           
                         
                         
                           
                             y 
                           
                         
                         
                           
                             1 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     The affine parameters {h ij :i=1,2 and j=1,2} can be obtained as follows. A sample position in the current frame is defined as (a k , b k )=(x k , y k )−(x 0 , y 0 ), where k is an index of the neighboring block with the same reference frame as the current block (k=0 corresponds to the current block). In the example of  FIG. 22 , k=2, 3, 5, and 6. 
     The corresponding sample position in the reference frame is then given as (a′ k , b′ k )=(a k , b k )+D ref,k ·(mv k .x, mv k .y)−D cur ·(mv 0 .x, mv 0 .y), where mv k .x and mv k .y are the horizontal and vertical components of the motion vector of the neighboring block k (k=0 corresponds to the current block), respectively. D ref,k  and D cur  are the sampling ratios of the neighboring block k and the current block, respectively. 
     A least-square solution can be obtained as 
     
       
         
           
             
               [ 
               
                 
                   
                     
                       h 
                       11 
                     
                   
                 
                 
                   
                     
                       h 
                       12 
                     
                   
                 
               
               ] 
             
             = 
             
               
                 
                   
                     ( 
                     
                       
                         P 
                         T 
                       
                       ⁢ 
                       P 
                     
                     ) 
                   
                   
                     - 
                     1 
                   
                 
                 ⁢ 
                 
                   P 
                   T 
                 
                 ⁢ 
                 q 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   and 
                   ⁢ 
                   
                       
                   
                   [ 
                   
                     
                       
                         
                           h 
                           21 
                         
                       
                     
                     
                       
                         
                           h 
                           22 
                         
                       
                     
                   
                   ] 
                 
               
               = 
               
                 
                   
                     ( 
                     
                       
                         P 
                         T 
                       
                       ⁢ 
                       P 
                     
                     ) 
                   
                   
                     - 
                     1 
                   
                 
                 ⁢ 
                 
                   P 
                   T 
                 
                 ⁢ 
                 r 
               
             
           
         
       
     
     where P, q, and r are given as follows in the example shown in  FIG. 22 : 
     
       
         
           
             
               
                 
                   
                     P 
                     = 
                     
                       [ 
                       
                         
                           
                             
                               
                                 a 
                                 2 
                               
                               , 
                               
                                 b 
                                 2 
                               
                             
                           
                         
                         
                           
                             
                               
                                 a 
                                 3 
                               
                               , 
                               
                                 b 
                                 3 
                               
                             
                           
                         
                         
                           
                             
                               
                                 a 
                                 5 
                               
                               , 
                               
                                 b 
                                 5 
                               
                             
                           
                         
                         
                           
                             
                               
                                 a 
                                 6 
                               
                               , 
                               
                                 b 
                                 6 
                               
                             
                           
                         
                       
                       ] 
                     
                   
                   , 
                   
                     q 
                     = 
                     
                       [ 
                       
                         
                           
                             
                               a 
                               2 
                               ′ 
                             
                           
                         
                         
                           
                             
                               a 
                               3 
                               ′ 
                             
                           
                         
                         
                           
                             
                               a 
                               5 
                               ′ 
                             
                           
                         
                         
                           
                             
                               a 
                               6 
                               ′ 
                             
                           
                         
                       
                       ] 
                     
                   
                   , 
                   
                     
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       r 
                     
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 b 
                                 2 
                                 ′ 
                               
                             
                           
                           
                             
                               
                                 b 
                                 3 
                                 ′ 
                               
                             
                           
                           
                             
                               
                                 b 
                                 5 
                                 ′ 
                               
                             
                           
                           
                             
                               
                                 b 
                                 6 
                                 ′ 
                               
                             
                           
                         
                         ] 
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     It is assumed in the description above that the affine parameter (h 13 , h 23 ) corresponds to a translational motion vector in the full-resolution. When the reduced-resolution prediction with a sampling ratio D is applied to a block, (h 13 , h 23 ) needs to be scaled accordingly as 
     
       
         
           
             
               ( 
               
                 
                   
                     h 
                     
                       1 
                       ⁢ 
                       3 
                     
                   
                   D 
                 
                 , 
                 
                   
                     h 
                     
                       2 
                       ⁢ 
                       3 
                     
                   
                   D 
                 
               
               ) 
             
             . 
           
         
       
     
     Such down-scaling operation can reduce a numerical precision of the motion vector (e.g., down-sampling the motion vector by 2 N  can cause the motion vector to discard its lower N bits). In order to maintain the same precision as that in the full-resolution domain, the original resolution of the motion vector can be increased. 
     According to aspects of the disclosure, when scaling a neighboring motion vector for coding a block using a reduced-resolution coding mode, the horizontal component and vertical component of the scaled motion vector can be derived using one of the following approaches. 
     In a first approach, when the scaling factor is a power of two (e.g., 2 N ), then the lower N bits of the horizontal component of the motion vector and the lower N bits of the vertical component of the motion vector are discarded to get the scaled motion vector. 
     In a second approach, when the scaling factor is a power of two (e.g., 2 N ), the motion vector is first added with a rounding factor (e.g., 2 N−1 ), then the lower N bits of the horizontal component of the motion vector and the lower N bits of the vertical component of the motion vector are discarded to get the scaled motion vector. 
     In a third approach, when the scaling factor is not a power of two, a look-up table can be used to derive the values of the horizontal component and vertical component of the scaled motion vector. 
     VI. Flowchart 
       FIG. 23  shows a flow chart outlining an exemplary process ( 2300 ) according to an embodiment of the disclosure. In various embodiments, the process ( 2300 ) is executed by processing circuitry, such as the processing circuitry in the terminal devices ( 210 ), ( 220 ), ( 230 ) and ( 240 ), the processing circuitry that performs functions of the video encoder ( 303 ), the processing circuitry that performs functions of the video decoder ( 310 ), the processing circuitry that performs functions of the video decoder ( 410 ), the processing circuitry that performs functions of the intra prediction module ( 452 ), the processing circuitry that performs functions of the video encoder ( 503 ), the processing circuitry that performs functions of the predictor ( 535 ), the processing circuitry that performs functions of the intra encoder ( 622 ), the processing circuitry that performs functions of the intra decoder ( 772 ), and the like. In some embodiments, the process ( 2300 ) is implemented in software instructions, thus when the processing circuitry executes the software instructions, the processing circuitry performs the process ( 2300 ). 
     The process ( 2300 ) may generally start at step (S 2310 ), where the process ( 2300 ) decodes a video bitstream to obtain a reduced-resolution residual block for a current block. Then, the process ( 2300 ) proceeds to step (S 2320 ). 
     At step (S 2320 ), the process ( 2300 ) determines that a block level flag is set to a pre-defined value. The pre-defined value indicates that the current block is coded in reduced-resolution coding. Then, the process ( 2300 ) proceeds to step (S 2330 ). 
     At step (S 2330 ), the process ( 2300 ) generates a reduced-resolution prediction block for the current block by down-sampling a full-resolution reference block of the current block. Then, the process ( 2300 ) proceeds to step (S 2340 ). 
     At step (S 2340 ), the process ( 2300 ) generates a reduced-resolution reconstruction block for the current block based on the reduced-resolution prediction block and the reduced-resolution residual block. Then, the process ( 2300 ) proceeds to step ( 2350 ). 
     At step (S 2350 ), the process ( 2300 ) generates a full-resolution reconstruction block for the current block by up-sampling the reduced-resolution reconstruction block. Then, the process ( 2300 ) terminates. 
     In an embodiment, the process ( 2300 ) determines a size of the reduced-resolution prediction block based on a size of the full-resolution reference block and down-sampling factors of the current block. 
     In an embodiment, the process ( 2300 ) decodes the block level flag for the current block from the video bitstream. The block level flag indicates that the current block is coded in the reduced-resolution coding. 
     In an embodiment, the process ( 2300 ) decodes one of a filter coefficient or an index of the filter coefficient from the video bitstream. The filter coefficient is used in the up-sampling the reduced-resolution reconstruction block. 
     In an embodiment, the process ( 2300 ) scales a motion vector of a first neighboring block of the current block based on a scaling factor that is a ratio of down-sampling factors of the current block and the first neighboring block. The process ( 2300 ) constructs a first motion vector candidate list for the current block. The first motion vector candidate list includes the scaled motion vector of the first neighboring block. 
     In an embodiment, the process ( 2300 ) determines the scaled motion vector based on a shift operation in response to the scaling factor being a power of two. In an example, when the scaling factor is 2 N , then the lower N bits of the horizontal component of the motion vector and the lower N bits of the vertical component of the motion vector are discarded to get the scaled motion vector. In another example, when the scaling factor is 2 N , the motion vector is first added with a rounding factor (e.g., 2 N−1 ), then the lower N bits of the horizontal component of the motion vector and the lower N bits of the vertical component of the motion vector are discarded to get the scaled motion vector. 
     In an embodiment, the process ( 2300 ) determines the scaled motion vector based on a look-up table in response to the scaling factor not being a power of two. 
     In an embodiment, the process ( 2300 ) determines a priority of the scaled motion vector in the first motion vector candidate list based on the down-sampling factors of the current block and the first neighboring block. 
     In an embodiment, the process ( 2300 ) constructs a second motion vector candidate list for the current block based on one or more second neighboring blocks of the current block. Each of the one or more second neighboring blocks has the same down-sampling factors as the current block. The process ( 2300 ) constructs a third motion vector candidate list for the current block based on one or more third neighboring blocks of the current block. Each of the one or more third neighboring blocks has different down-sampling factors from the current block. 
     In an embodiment, the process ( 2300 ) scans the third motion vector candidate list based on a number of motion vector candidates in the second motion vector candidate list being less than a designated number. 
     In an embodiment, the process ( 2300 ) determines a fourth motion vector candidate list for the current block by merging the second motion vector candidate list and the third motion vector candidate list in an interleaved manner. 
     In an embodiment, the process ( 2300 ) determines an affine parameter of the current block based on down-sampling factors of the current block. 
     VII. Computer System 
     The techniques described above, can be implemented as computer software using computer-readable instructions and physically stored in one or more computer-readable media. For example,  FIG. 24  shows a computer system ( 2400 ) suitable for implementing certain embodiments of the disclosed subject matter. 
     The computer software can be coded using any suitable machine code or computer language, that may be subject to assembly, compilation, linking, or like mechanisms to create code comprising instructions that can be executed directly, or through interpretation, micro-code execution, and the like, by one or more computer central processing units (CPUs), Graphics Processing Units (GPUs), and the like. 
     The instructions can be executed on various types of computers or components thereof, including, for example, personal computers, tablet computers, servers, smartphones, gaming devices, internet of things devices, and the like. 
     The components shown in  FIG. 24  for computer system ( 2400 ) are exemplary in nature and are not intended to suggest any limitation as to the scope of use or functionality of the computer software implementing embodiments of the present disclosure. Neither should the configuration of components be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary embodiment of a computer system ( 2400 ). 
     Computer system ( 2400 ) may include certain human interface input devices. Such a human interface input device may be responsive to input by one or more human users through, for example, tactile input (such as: keystrokes, swipes, data glove movements), audio input (such as: voice, clapping), visual input (such as: gestures), olfactory input (not depicted). The human interface devices can also be used to capture certain media not necessarily directly related to conscious input by a human, such as audio (such as: speech, music, ambient sound), images (such as: scanned images, photographic images obtain from a still image camera), video (such as two-dimensional video, three-dimensional video including stereoscopic video). 
     Input human interface devices may include one or more of (only one of each depicted): keyboard ( 2401 ), mouse ( 2402 ), trackpad ( 2403 ), touch screen ( 2410 ), data-glove (not shown), joystick ( 2405 ), microphone ( 2406 ), scanner ( 2407 ), and camera ( 2408 ). 
     Computer system ( 2400 ) may also include certain human interface output devices. Such human interface output devices may be stimulating the senses of one or more human users through, for example, tactile output, sound, light, and smell/taste. Such human interface output devices may include tactile output devices (for example tactile feedback by the touch-screen ( 2410 ), data-glove (not shown), or joystick ( 2405 ), but there can also be tactile feedback devices that do not serve as input devices), audio output devices (such as: speakers ( 2409 ), headphones (not depicted)), visual output devices (such as screens ( 2410 ) to include CRT screens, LCD screens, plasma screens, OLED screens, each with or without touch-screen input capability, each with or without tactile feedback capability—some of which may be capable to output two dimensional visual output or more than three dimensional output through means such as stereographic output; virtual-reality glasses (not depicted), holographic displays and smoke tanks (not depicted)), and printers (not depicted). These visual output devices (such as screens ( 2410 )) can be connected to a system bus ( 2448 ) through a graphics adapter ( 2450 ). 
     Computer system ( 2400 ) can also include human accessible storage devices and their associated media such as optical media including CD/DVD ROM/RW ( 2420 ) with CD/DVD or the like media ( 2421 ), thumb-drive ( 2422 ), removable hard drive or solid state drive ( 2423 ), legacy magnetic media such as tape and floppy disc (not depicted), specialized ROM/ASIC/PLD based devices such as security dongles (not depicted), and the like. 
     Those skilled in the art should also understand that term “computer readable media” as used in connection with the presently disclosed subject matter does not encompass transmission media, carrier waves, or other transitory signals. 
     Computer system ( 2400 ) can also include a network interface ( 2454 ) to one or more communication networks ( 2455 ). The one or more communication networks ( 2455 ) can for example be wireless, wireline, optical. The one or more communication networks ( 2455 ) can further be local, wide-area, metropolitan, vehicular and industrial, real-time, delay-tolerant, and so on. Examples of the one or more communication networks ( 2455 ) include local area networks such as Ethernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TV wireline or wireless wide area digital networks to include cable TV, satellite TV, and terrestrial broadcast TV, vehicular and industrial to include CANBus, and so forth. Certain networks commonly require external network interface adapters that attached to certain general purpose data ports or peripheral buses ( 2449 ) (such as, for example USB ports of the computer system ( 2400 )); others are commonly integrated into the core of the computer system ( 2400 ) by attachment to a system bus as described below (for example Ethernet interface into a PC computer system or cellular network interface into a smartphone computer system). Using any of these networks, computer system ( 2400 ) can communicate with other entities. Such communication can be uni-directional, receive only (for example, broadcast TV), uni-directional send-only (for example CANbus to certain CANbus devices), or bi-directional, for example to other computer systems using local or wide area digital networks. Certain protocols and protocol stacks can be used on each of those networks and network interfaces as described above. 
     Aforementioned human interface devices, human-accessible storage devices, and network interfaces can be attached to a core ( 2440 ) of the computer system ( 2400 ). 
     The core ( 2440 ) can include one or more Central Processing Units (CPU) ( 2441 ), Graphics Processing Units (GPU) ( 2442 ), specialized programmable processing units in the form of Field Programmable Gate Areas (FPGA) ( 2443 ), hardware accelerators for certain tasks ( 2444 ), graphics adapters ( 2450 ), and so forth. These devices, along with Read-only memory (ROM) ( 2445 ), Random-access memory ( 2446 ), internal mass storage ( 2447 ) such as internal non-user accessible hard drives, SSDs, and the like, may be connected through the system bus ( 2448 ). In some computer systems, the system bus ( 2448 ) can be accessible in the form of one or more physical plugs to enable extensions by additional CPUs, GPU, and the like. The peripheral devices can be attached either directly to the core&#39;s system bus ( 2448 ), or through a peripheral bus ( 2449 ). In an example, the screen ( 2410 ) can be connected to the graphics adapter ( 2450 ). Architectures for a peripheral bus include PCI, USB, and the like. 
     CPUs ( 2441 ), GPUs ( 2442 ), FPGAs ( 2443 ), and accelerators ( 2444 ) can execute certain instructions that, in combination, can make up the aforementioned computer code. That computer code can be stored in ROM ( 2445 ) or RAM ( 2446 ). Transitional data can be also be stored in RAM ( 2446 ), whereas permanent data can be stored for example, in the internal mass storage ( 2447 ). Fast storage and retrieve to any of the memory devices can be enabled through the use of cache memory, that can be closely associated with one or more CPU ( 2441 ), GPU ( 2442 ), mass storage ( 2447 ), ROM ( 2445 ), RAM ( 2446 ), and the like. 
     The computer readable media can have computer code thereon for performing various computer-implemented operations. The media and computer code can be those specially designed and constructed for the purposes of the present disclosure, or they can be of the kind well known and available to those having skill in the computer software arts. 
     As an example and not by way of limitation, the computer system having architecture ( 2400 ), and specifically the core ( 2440 ) can provide functionality as a result of processor(s) (including CPUs, GPUs, FPGA, accelerators, and the like) executing software embodied in one or more tangible, computer-readable media. Such computer-readable media can be media associated with user-accessible mass storage as introduced above, as well as certain storage of the core ( 2440 ) that are of non-transitory nature, such as core-internal mass storage ( 2447 ) or ROM ( 2445 ). The software implementing various embodiments of the present disclosure can be stored in such devices and executed by core ( 2440 ). A computer-readable medium can include one or more memory devices or chips, according to particular needs. The software can cause the core ( 2440 ) and specifically the processors therein (including CPU, GPU, FPGA, and the like) to execute particular processes or particular parts of particular processes described herein, including defining data structures stored in RAM ( 2446 ) and modifying such data structures according to the processes defined by the software. In addition or as an alternative, the computer system can provide functionality as a result of logic hardwired or otherwise embodied in a circuit (for example: accelerator ( 2444 )), which can operate in place of or together with software to execute particular processes or particular parts of particular processes described herein. Reference to software can encompass logic, and vice versa, where appropriate. Reference to a computer-readable media can encompass a circuit (such as an integrated circuit (IC)) storing software for execution, a circuit embodying logic for execution, or both, where appropriate. The present disclosure encompasses any suitable combination of hardware and software. 
     While this disclosure has described several exemplary embodiments, there are alterations, permutations, and various substitute equivalents, which fall within the scope of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described herein, embody the principles of the disclosure and are thus within the spirit and scope thereof. 
     APPENDIX A: ACRONYMS 
     ALF: Adaptive Loop Filter 
     AMVP: Advanced Motion Vector Prediction 
     APS: Adaptation Parameter Set 
     ASIC: Application-Specific Integrated Circuit 
     ATMVP: Alternative/Advanced Temporal Motion Vector Prediction 
     AV1: AOMedia Video 1 
     AV2: AOMedia Video 2 
     BMS: Benchmark Set 
     BV: Block Vector 
     CANBus: Controller Area Network Bus 
     CB: Coding Block 
     CC-ALF: Cross-Component Adaptive Loop Filter 
     CD: Compact Disc 
     CDEF: Constrained Directional Enhancement Filter 
     CPR: Current Picture Referencing 
     CPU: Central Processing Unit 
     CRT: Cathode Ray Tube 
     CTB: Coding Tree Block 
     CTU: Coding Tree Unit 
     CU: Coding Unit 
     DPB: Decoder Picture Buffer 
     DPCM: Differential Pulse-Code Modulation 
     DPS: Decoding Parameter Set 
     DVD: Digital Video Disc 
     FPGA: Field Programmable Gate Area 
     JCCR: Joint CbCr Residual Coding 
     JVET: Joint Video Exploration Team 
     GOP: Groups of Pictures 
     GPU: Graphics Processing Unit 
     GSM: Global System for Mobile communications 
     HDR: High Dynamic Range 
     HEVC: High Efficiency Video Coding 
     HRD: Hypothetical Reference Decoder 
     IBC: Intra Block Copy 
     IC: Integrated Circuit 
     ISP: Intra Sub-Partitions 
     JEM: Joint Exploration Model 
     LAN: Local Area Network 
     LCD: Liquid-Crystal Display 
     LR: Loop Restoration Filter 
     LRU: Loop Restoration Unit 
     LTE: Long-Term Evolution 
     MPM: Most Probable Mode 
     MV: Motion Vector 
     OLED: Organic Light-Emitting Diode 
     PBs: Prediction Blocks 
     PCI: Peripheral Component Interconnect 
     PDPC: Position Dependent Prediction Combination 
     PLD: Programmable Logic Device 
     PPS: Picture Parameter Set 
     PU: Prediction Unit 
     RAM: Random Access Memory 
     ROM: Read-Only Memory 
     SAO: Sample Adaptive Offset 
     SCC: Screen Content Coding 
     SDR: Standard Dynamic Range 
     SEI: Supplementary Enhancement Information 
     SNR: Signal Noise Ratio 
     SPS: Sequence Parameter Set 
     SSD: Solid-state Drive 
     TU: Transform Unit 
     USB: Universal Serial Bus 
     VPS: Video Parameter Set 
     VUI: Video Usability Information 
     VVC: Versatile Video Coding 
     WAIP: Wide-Angle Intra Prediction