Patent Publication Number: US-2022224935-A1

Title: Methods and apparatus of video coding using subblock-based temporal motion vector prediction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of U.S. patent application Ser. No. 17/187,587, filed on Feb. 26, 2021, which is a continuation application of International Application No. PCT/US2019/048864, filed Aug. 29, 2019 which claims the benefit of U.S. Provisional Application No. 62/724,506, filed Aug. 29, 2018, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present application generally relates to video data encoding and decoding, and in particular, to method and system of video coding using subblock-based temporal motion vector prediction. 
     BACKGROUND 
     Digital video is supported by a variety of electronic devices, such as digital televisions, laptop or desktop computers, tablet computers, digital cameras, digital recording devices, digital media players, video gaming consoles, smart phones, video teleconferencing devices, video streaming devices, etc. The electronic devices transmit, receive, encode, decode, and/or store digital video data by implementing video compression/decompression standards as defined by MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), High Efficiency Video Coding (HEVC), and Versatile Video Coding (VVC) standard. Video compression typically includes performing spatial (intra frame) prediction and/or temporal (inter frame) prediction to reduce or remove redundancy inherent in the video data. For block-based video coding, a video frame is partitioned into one or more slices, each slice having multiple video blocks, which may also be referred to as coding tree units (CTUs). Each CTU may contain one coding unit (CU) or recursively split into smaller CUs until the predefined minimum CU size is reached. Each CU (also named leaf CU) contains one or multiple transform units (TUs) and each CU also contains one or multiple prediction units (PUs). Each CU can be coded in either intra, inter or IBC modes. Video blocks in an intra coded (I) slice of a video frame are encoded using spatial prediction with respect to reference samples in neighbor blocks within the same video frame. Video blocks in an inter coded (P or B) slice of a video frame may use spatial prediction with respect to reference samples in neighbor blocks within the same video frame or temporal prediction with respect to reference samples in other previous and/or future reference video frames. 
     Spatial or temporal prediction based on a reference block that has been previously encoded, e.g., a neighbor block, results in a predictive block for a current video block to be coded. The process of finding the reference block may be accomplished by block matching algorithm. Residual data representing pixel differences between the current block to be coded and the predictive block is referred to as a residual block or prediction errors. An inter-coded block is encoded according to a motion vector that points to a reference block in a reference frame forming the predictive block, and the residual block. The process of determining the motion vector is typically referred to as motion estimation. An intra coded block is encoded according to an intra prediction mode and the residual block. For further compression, the residual block is transformed from the pixel domain to a transform domain, e.g., frequency domain, resulting in residual transform coefficients, which may then be quantized. The quantized transform coefficients, initially arranged in a two-dimensional array, may be scanned to produce a one-dimensional vector of transform coefficients, and then entropy encoded into a video bitstream to achieve even more compression. 
     The encoded video bitstream is then saved in a computer-readable storage medium (e.g., flash memory) to be accessed by another electronic device with digital video capability or directly transmitted to the electronic device wired or wirelessly. The electronic device then performs video decompression (which is an opposite process to the video compression described above) by, e.g., parsing the encoded video bitstream to obtain syntax elements from the bitstream and reconstructing the digital video data to its original format from the encoded video bitstream based at least in part on the syntax elements obtained from the bitstream, and renders the reconstructed digital video data on a display of the electronic device. 
     With digital video quality going from high definition, to 4K×2K or even 8K×4K, the amount of vide data to be encoded/decoded grows exponentially. It is a constant challenge in terms of how the video data can be encoded/decoded more efficiently while maintaining the image quality of the decoded video data. 
     SUMMARY 
     The present application describes implementations related to video data encoding and decoding and, more particularly, to system and method of video encoding and decoding using subblock-based temporal motion vector prediction. 
     According to a first aspect of the present application, a method of subblock-based temporal motion vector prediction is performed at a computing device having one or more processors and memory storing a plurality of programs to be executed by the one or more processors. The computing device acquires a video bitstream including data associated with multiple encoded pictures. While decoding a current picture in the video bitstream, the computing device selects, according to syntax elements signaled in the video bitstream, one reference picture as a collocated picture of the current picture, and determines a temporal vector between the collocated picture and the current picture from motion information of spatially neighboring blocks of a current code unit (CU) according to a fixed order. Next, the computing device splits the current CU into multiple sub-CUs, each sub-CU corresponding to a respective subblock of the current picture. The computing device then obtains a temporal motion vector predictor for each sub-CU of the current CU from (i) the temporal vector between the collocated picture and the current picture and (ii) motion information of a block in the collocated picture that corresponds to the respective subblock of the current picture and decodes the current CU according to the temporal motion vector predictors of the plurality of sub-CUs of the current CU. 
     According to a second aspect of the present application, a computing device includes one or more processors, memory and a plurality of programs stored in the memory. The programs, when executed by the one or more processors, cause the computing device to perform the aforementioned operations as described above. 
     According to a third aspect of the present application, a non-transitory computer readable storage medium stores a plurality of programs for execution by a computing device having one or more processors. The programs, when executed by the one or more processors, cause the computing device to perform the aforementioned operations as described above. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the implementations and are incorporated herein and constitute a part of the specification, illustrate the described implementations and together with the description serve to explain the underlying principles. Like reference numerals refer to corresponding parts. 
         FIG. 1  is a block diagram illustrating an exemplary video encoding and decoding system in accordance with some implementations of the present disclosure. 
         FIG. 2  is a block diagram illustrating an exemplary video encoder in accordance with some implementations of the present disclosure. 
         FIG. 3  is a block diagram illustrating an exemplary video decoder in accordance with some implementations of the present disclosure. 
         FIGS. 4A-4D  are block diagrams illustrating how a frame is recursively quad-tree partitioned into multiple video blocks of different sizes in accordance with some implementations of the present disclosure. 
         FIG. 5A  is a block diagram illustrating spatially neighboring and temporally collocated block positions of a current CU to be encoded in accordance with some implementations of the present disclosure. 
         FIG. 5B  is a flowchart illustrating an exemplary process by which a list of motion vector candidates are identified in accordance with some implementations of the present disclosure. 
         FIG. 5C  is a block diagram illustrating how a subblock-based temporal motion vector prediction is performed between a current picture and a collocated picture in accordance with some implementations of the present disclosure. 
         FIG. 6  is a flowchart illustrating an exemplary process by which a video decoder implements the techniques of constructing subblock-based temporal motion vector prediction for a current picture from motion information of a collocated picture in accordance with some implementations of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to specific implementations, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous non-limiting specific details are set forth in order to assist in understanding the subject matter presented herein. But it will be apparent to one of ordinary skill in the art that various alternatives may be used without departing from the scope of claims and the subject matter may be practiced without these specific details. For example, it will be apparent to one of ordinary skill in the art that the subject matter presented herein can be implemented on many types of electronic devices with digital video capabilities. 
       FIG. 1  is a block diagram illustrating an exemplary system  10  for encoding and decoding video blocks in parallel in accordance with some implementations of the present disclosure. As shown in  FIG. 1 , system  10  includes a source device  12  that generates and encodes video data to be decoded at a later time by a destination device  14 . Source device  12  and destination device  14  may comprise any of a wide variety of electronic devices, including desktop or laptop computers, tablet computers, smart phones, set-top boxes, digital televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some implementations, source device  12  and destination device  14  are equipped with wireless communication capabilities. 
     In some implementations, destination device  14  may receive the encoded video data to be decoded via a link  16 . Link  16  may comprise any type of communication medium or device capable of moving the encoded video data from source device  12  to destination device  14 . In one example, link  16  may comprise a communication medium to enable source device  12  to transmit the encoded video data directly to destination device  14  in real-time. The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to destination device  14 . The communication medium may comprise any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device  12  to destination device  14 . 
     In some other implementations, the encoded video data may be transmitted from output interface  22  to a storage device  32 . Subsequently, the encoded video data in storage device  32  may be accessed by destination device  14  via input interface  28 . Storage device  32  may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, storage device  32  may correspond to a file server or another intermediate storage device that may hold the encoded video data generated by source device  12 . Destination device  14  may access the stored video data from storage device  32  via streaming or downloading. The file server may be any type of computer capable of storing encoded video data and transmitting the encoded video data to destination device  14 . Exemplary file servers include a web server (e.g., for a website), an FTP server, network attached storage (NAS) devices, or a local disk drive. Destination device  14  may access the encoded video data through any standard data connection, including a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from storage device  32  may be a streaming transmission, a download transmission, or a combination of both. 
     As shown in  FIG. 1 , source device  12  includes a video source  18 , a video encoder  20  and an output interface  22 . Video source  18  may include a source such as a video capture device, e.g., a video camera, a video archive containing previously captured video, a video feed interface to receive video from a video content provider, and/or a computer graphics system for generating computer graphics data as the source video, or a combination of such sources. As one example, if video source  18  is a video camera of a security surveillance system, source device  12  and destination device  14  may form camera phones or video phones. However, the implementations described in the present application may be applicable to video coding in general, and may be applied to wireless and/or wired applications. 
     The captured, pre-captured, or computer-generated video may be encoded by video encoder  20 . The encoded video data may be transmitted directly to destination device  14  via output interface  22  of source device  12 . The encoded video data may also (or alternatively) be stored onto storage device  32  for later access by destination device  14  or other devices, for decoding and/or playback. Output interface  22  may further include a modem and/or a transmitter. 
     Destination device  14  includes an input interface  28 , a video decoder  30 , and a display device  34 . Input interface  28  may include a receiver and/or a modem and receive the encoded video data over link  16 . The encoded video data communicated over link  16 , or provided on storage device  32 , may include a variety of syntax elements generated by video encoder  20  for use by video decoder  30  in decoding the video data. Such syntax elements may be included within the encoded video data transmitted on a communication medium, stored on a storage medium, or stored a file server. 
     In some implementations, destination device  14  may include a display device  34 , which can be an integrated display device and an external display device that is configured to communicate with destination device  14 . Display device  34  displays the decoded video data to a user, and may comprise any of a variety of display devices such as a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device. 
     Video encoder  20  and video decoder  30  may operate according to proprietary or industry standards, such as VVC, HEVC, MPEG-4, Part 10, Advanced Video Coding (AVC), or extensions of such standards. It should be understood that the present application is not limited to a specific video coding/decoding standard and may be applicable to other video coding/decoding standards. It is generally contemplated that video encoder  20  of source device  12  may be configured to encode video data according to any of these current or future standards. Similarly, it is also generally contemplated that video decoder  30  of destination device  14  may be configured to decode video data according to any of these current or future standards. 
     Video encoder  20  and video decoder  30  each may be implemented as any of a variety of suitable encoder circuitry, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof. When implemented partially in software, an electronic device may store instructions for the software in a suitable, non-transitory computer-readable medium and execute the instructions in hardware using one or more processors to perform the video coding/decoding operations disclosed in the present disclosure. Each of video encoder  20  and video decoder  30  may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device. 
       FIG. 2  is a block diagram illustrating an exemplary video encoder  20  in accordance with some implementations described in the present application. Video encoder  20  may perform intra and inter predictive coding of video blocks within video frames. Intra predictive coding relies on spatial prediction to reduce or remove spatial redundancy in video data within a given video frame or picture. Inter predictive coding relies on temporal prediction to reduce or remove temporal redundancy in video data within adjacent video frames or pictures of a video sequence. 
     As shown in  FIG. 2 , video encoder  20  includes video data memory  40 , prediction processing unit  41 , decoded picture buffer (DPB)  64 , summer  50 , transform processing unit  52 , quantization unit  54 , and entropy encoding unit  56 . Prediction processing unit  41  further includes motion estimation unit  42 , motion compensation unit  44 , partition unit  45 , intra prediction processing unit  46 , and intra block copy (BC) unit  48 . In some implementations, video encoder  20  also includes inverse quantization unit  58 , inverse transform processing unit  60 , and summer  62  for video block reconstruction. A deblocking filter (not shown) may be positioned between summer  62  and DPB  64  to filter block boundaries to remove blockiness artifacts from reconstructed video. An in loop filter (not shown) may also be used in addition to the deblocking filter to filter the output of summer  62 . Video encoder  20  may take the form of a fixed or programmable hardware unit or may be divided among one or more of the illustrated fixed or programmable hardware units. 
     Video data memory  40  may store video data to be encoded by the components of video encoder  20 . The video data in video data memory  40  may be obtained, for example, from video source  18 . DPB  64  is a buffer that stores reference video data for use in encoding video data by video encoder  20  (e.g., in intra or inter predictive coding modes). Video data memory  40  and DPB  64  may be formed by any of a variety of memory devices. In various examples, video data memory  40  may be on-chip with other components of video encoder  20 , or off-chip relative to those components. 
     As shown in  FIG. 2 , after receiving video data, partition unit  45  within prediction processing unit  41  partitions the video data into video blocks. This partitioning may also include partitioning a video frame into slices, tiles, or other larger coding units (CUs) according to a predefined splitting structures such as quad-tree structure associated with the video data. The video frame may be divided into multiple video blocks (or sets of video blocks referred to as tiles). Prediction processing unit  41  may select one of a plurality of possible predictive coding modes, such as one of a plurality of intra predictive coding modes or one of a plurality of inter predictive coding modes, for the current video block based on error results (e.g., coding rate and the level of distortion). Prediction processing unit  41  may provide the resulting intra or inter prediction coded block to summer  50  to generate a residual block and to summer  62  to reconstruct the encoded block for use as part of a reference frame subsequently. Prediction processing unit  41  also provides syntax elements, such as motion vectors, intra-mode indicators, partition information, and other such syntax information, to entropy encoding unit  56 . 
     In order to select an appropriate intra predictive coding mode for the current video block, intra prediction processing unit  46  within prediction processing unit  41  may perform intra predictive coding of the current video block relative to one or more neighbor blocks in the same frame as the current block to be coded to provide spatial prediction. Motion estimation unit  42  and motion compensation unit  44  within prediction processing unit  41  perform inter predictive coding of the current video block relative to one or more predictive blocks in one or more reference frames to provide temporal prediction. Video encoder  20  may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data. 
     In some implementations, motion estimation unit  42  determines the inter prediction mode for a current video frame by generating a motion vector, which indicates the displacement of a prediction unit (PU) of a video block within the current video frame relative to a predictive block within a reference video frame, according to a predetermined pattern within a sequence of video frames. Motion estimation, performed by motion estimation unit  42 , is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within a reference frame (or other coded unit) relative to the current block being coded within the current frame (or other coded unit). The predetermined pattern may designate video frames in the sequence as P frames or B frames. Intra BC unit  48  may determine vectors, e.g., block vectors, for intra BC coding in a manner similar to the determination of motion vectors by motion estimation unit  42  for inter prediction, or may utilize motion estimation unit  42  to determine the block vector. 
     A predictive block is a block of a reference frame that is deemed as closely matching the PU of the video block to be coded in terms of pixel difference, which may be determined by sum of absolute difference (SAD), sum of square difference (SSD), or other difference metrics. In some implementations, video encoder  20  may calculate values for sub-integer pixel positions of reference frames stored in DPB  64 . For example, video encoder  20  may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference frame. Therefore, motion estimation unit  42  may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision. 
     Motion estimation unit  42  calculates a motion vector for a PU of a video block in an inter prediction coded frame by comparing the position of the PU to the position of a predictive block of a reference frame selected from a first reference frame list (e.g., List 0 ) or a second reference frame list (e.g., List 1 ), each of which identifies one or more reference frames stored in DPB  64 . Motion estimation unit  42  sends the calculated motion vector to motion compensation unit  44  and then to entropy encoding unit  56 . 
     Motion compensation, performed by motion compensation unit  44 , may involve fetching or generating the predictive block based on the motion vector determined by motion estimation unit  42 . Upon receiving the motion vector for the PU of the current video block, motion compensation unit  44  may locate a predictive block to which the motion vector points in one of the reference frame lists, retrieve the predictive block from DPB  64 , and forward the predictive block to summer  50 . Summer  50  then forms a residual video block of pixel difference values by subtracting pixel values of the predictive block provided by motion compensation unit  44  from the pixel values of the current video block being coded. The pixel difference values forming the residual vide block may include luma or chroma difference components or both. Motion compensation unit  44  may also generate syntax elements associated with the video blocks of a video frame for use by video decoder  30  in decoding the video blocks of the video frame. The syntax elements may include, for example, syntax elements defining the motion vector used to identify the predictive block, any flags indicating the prediction mode, or any other syntax information described herein. Note that motion estimation unit  42  and motion compensation unit  44  may be highly integrated, but are illustrated separately for conceptual purposes. 
     In some implementations, intra BC unit  48  may generate vectors and fetch predictive blocks in a manner similar to that described above in connection with motion estimation unit  42  and motion compensation unit  44 , but with the predictive blocks being in the same frame as the current block being coded and with the vectors being referred to as block vectors as opposed to motion vectors. In particular, intra BC unit  48  may determine an intra-prediction mode to use to encode a current block. In some examples, intra BC unit  48  may encode a current block using various intra-prediction modes, e.g., during separate encoding passes, and test their performance through rate-distortion analysis. Next, intra BC unit  48  may select, among the various tested intra-prediction modes, an appropriate intra-prediction mode to use and generate an intra-mode indicator accordingly. For example, intra BC unit  48  may calculate rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and select the intra-prediction mode having the best rate-distortion characteristics among the tested modes as the appropriate intra-prediction mode to use. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original, unencoded block that was encoded to produce the encoded block, as well as a bitrate (i.e., a number of bits) used to produce the encoded block. Intra BC unit  48  may calculate ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block. 
     In other examples, intra BC unit  48  may use motion estimation unit  42  and motion compensation unit  44 , in whole or in part, to perform such functions for Intra BC prediction according to the implementations described herein. In either case, for Intra block copy, a predictive block may be a block that is deemed as closely matching the block to be coded, in terms of pixel difference, which may be determined by sum of absolute difference (SAD), sum of squared difference (SSD), or other difference metrics, and identification of the predictive block may include calculation of values for sub-integer pixel positions. 
     Whether the predictive block is from the same frame according to intra prediction, or a different frame according to inter prediction, video encoder  20  may form a residual video block by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. The pixel difference values forming the residual video block may include both luma and chroma component differences. 
     Intra prediction processing unit  46  may intra-predict a current video block, as an alternative to the inter-prediction performed by motion estimation unit  42  and motion compensation unit  44 , or the intra block copy prediction performed by intra BC unit  48 , as described above. In particular, intra prediction processing unit  46  may determine an intra prediction mode to use to encode a current block. To do so, intra prediction processing unit  46  may encode a current block using various intra prediction modes, e.g., during separate encoding passes, and intra prediction processing unit  46  (or a mode select unit, in some examples) may select an appropriate intra prediction mode to use from the tested intra prediction modes. Intra prediction processing unit  46  may provide information indicative of the selected intra-prediction mode for the block to entropy encoding unit  56 . Entropy encoding unit  56  may encode the information indicating the selected intra-prediction mode in the bitstream. 
     After prediction processing unit  41  determines the predictive block for the current video block via either inter prediction or intra prediction, summer  50  forms a residual video block by subtracting the predictive block from the current video block. The residual video data in the residual block may be included in one or more transform units (TUs) and is provided to transform processing unit  52 . Transform processing unit  52  transforms the residual video data into residual transform coefficients using a transform, such as a discrete cosine transform (DCT) or a conceptually similar transform. 
     Transform processing unit  52  may send the resulting transform coefficients to quantization unit  54 . Quantization unit  54  quantizes the transform coefficients to further reduce bit rate. The quantization process may also reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, quantization unit  54  may then perform a scan of a matrix including the quantized transform coefficients. Alternatively, entropy encoding unit  56  may perform the scan. 
     Following quantization, entropy encoding unit  56  entropy encodes the quantized transform coefficients into a video bitstream using, e.g., context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding or another entropy encoding methodology or technique. The encoded bitstream may then be transmitted to video decoder  30 , or archived in storage device  32  for later transmission to or retrieval by video decoder  30 . Entropy encoding unit  56  may also entropy encode the motion vectors and the other syntax elements for the current video frame being coded. 
     Inverse quantization unit  58  and inverse transform processing unit  60  apply inverse quantization and inverse transformation, respectively, to reconstruct the residual video block in the pixel domain for generating a reference block for prediction of other video blocks. As noted above, motion compensation unit  44  may generate a motion compensated predictive block from one or more reference blocks of the frames stored in DPB  64 . Motion compensation unit  44  may also apply one or more interpolation filters to the predictive block to calculate sub-integer pixel values for use in motion estimation. 
     Summer  62  adds the reconstructed residual block to the motion compensated predictive block produced by motion compensation unit  44  to produce a reference block for storage in DPB  64 . The reference block may then be used by intra BC unit  48 , motion estimation unit  42  and motion compensation unit  44  as a predictive block to inter predict another video block in a subsequent video frame. 
       FIG. 3  is a block diagram illustrating an exemplary video decoder  30  in accordance with some implementations of the present application. Video decoder  30  includes video data memory  79 , entropy decoding unit  80 , prediction processing unit  81 , inverse quantization unit  86 , inverse transform processing unit  88 , summer  90 , and DPB  92 . Prediction processing unit  81  further includes motion compensation unit  82 , intra prediction processing unit  84 , and intra BC unit  85 . Video decoder  30  may perform a decoding process generally reciprocal to the encoding process described above with respect to video encoder  20  in connection with  FIG. 2 . For example, motion compensation unit  82  may generate prediction data based on motion vectors received from entropy decoding unit  80 , while intra-prediction unit  84  may generate prediction data based on intra-prediction mode indicators received from entropy decoding unit  80 . 
     In some examples, a unit of video decoder  30  may be tasked to perform the implementations of the present application. Also, in some examples, the implementations of the present disclosure may be divided among one or more of the units of video decoder  30 . For example, intra BC unit  85  may perform the implementations of the present application, alone, or in combination with other units of video decoder  30 , such as motion compensation unit  82 , intra prediction processing unit  84 , and entropy decoding unit  80 . In some examples, video decoder  30  may not include intra BC unit  85  and the functionality of intra BC unit  85  may be performed by other components of prediction processing unit  81 , such as motion compensation unit  82 . 
     Video data memory  79  may store video data, such as an encoded video bitstream, to be decoded by the other components of video decoder  30 . The video data stored in video data memory  79  may be obtained, for example, from storage device  32 , from a local video source, such as a camera, via wired or wireless network communication of video data, or by accessing physical data storage media (e.g., a flash drive or hard disk). Video data memory  79  may include a coded picture buffer (CPB) that stores encoded video data from an encoded video bitstream. Decoded picture buffer (DPB)  92  of video decoder  30  stores reference video data for use in decoding video data by video decoder  30  (e.g., in intra or inter predictive coding modes). Video data memory  79  and DPB  92  may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magneto-resistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. For illustrative purpose, video data memory  79  and DPB  92  are depicted as two distinct components of video decoder  30  in  FIG. 3 . But it will be apparent to one skilled in the art that video data memory  79  and DPB  92  may be provided by the same memory device or separate memory devices. In some examples, video data memory  79  may be on-chip with other components of video decoder  30 , or off-chip relative to those components. 
     During the decoding process, video decoder  30  receives an encoded video bitstream that represents video blocks of an encoded video frame and associated syntax elements. Video decoder  30  may receive the syntax elements at the video frame level and/or the video block level. Entropy decoding unit  80  of video decoder  30  entropy decodes the bitstream to generate quantized coefficients, motion vectors or intra-prediction mode indicators, and other syntax elements. Entropy decoding unit  80  then forwards the motion vectors and other syntax elements to prediction processing unit  81 . 
     When the video frame is coded as an intra predictive coded (I) frame or for intra coded predictive blocks in other types of frames, intra prediction processing unit  84  of prediction processing unit  81  may generate prediction data for a video block of the current video frame based on a signaled intra prediction mode and reference data from previously decoded blocks of the current frame. 
     When the video frame is coded as an inter-predictive coded (i.e., B or P) frame, motion compensation unit  82  of prediction processing unit  81  produces one or more predictive blocks for a video block of the current video frame based on the motion vectors and other syntax elements received from entropy decoding unit  80 . Each of the predictive blocks may be produced from a reference frame within one of the reference frame lists. Video decoder  30  may construct the reference frame lists, e.g., List 0  and List 1 , using default construction techniques based on reference frames stored in DPB  92 . 
     In some examples, when the video block is coded according to the intra BC mode described herein, intra BC unit  85  of prediction processing unit  81  produces predictive blocks for the current video block based on block vectors and other syntax elements received from entropy decoding unit  80 . The predictive blocks may be within a reconstructed region of the same picture as the current video block defined by video encoder  20 . 
     Motion compensation unit  82  and/or intra BC unit  85  determines prediction information for a video block of the current video frame by parsing the motion vectors and other syntax elements, and then uses the prediction information to produce the predictive blocks for the current video block being decoded. For example, motion compensation unit  82  uses some of the received syntax elements to determine a prediction mode (e.g., intra or inter prediction) used to code video blocks of the video frame, an inter prediction frame type (e.g., B or P), construction information for one or more of the reference frame lists for the frame, motion vectors for each inter predictive encoded video block of the frame, inter prediction status for each inter predictive coded video block of the frame, and other information to decode the video blocks in the current video frame. 
     Similarly, intra BC unit  85  may use some of the received syntax elements, e.g., a flag, to determine that the current video block was predicted using the intra BC mode, construction information of which video blocks of the frame are within the reconstructed region and should be stored in DPB  92 , block vectors for each intra BC predicted video block of the frame, intra BC prediction status for each intra BC predicted video block of the frame, and other information to decode the video blocks in the current video frame. 
     Motion compensation unit  82  may also perform interpolation using the interpolation filters as used by video encoder  20  during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks. In this case, motion compensation unit  82  may determine the interpolation filters used by video encoder  20  from the received syntax elements and use the interpolation filters to produce predictive blocks. 
     Inverse quantization unit  86  inverse quantizes the quantized transform coefficients provided in the bitstream and entropy decoded by entropy decoding unit  80  using the same quantization parameter calculated by video encoder  20  for each video block in the video frame to determine a degree of quantization. Inverse transform processing unit  88  applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to reconstruct the residual blocks in the pixel domain. 
     After motion compensation unit  82  or intra BC unit  85  generates the predictive block for the current video block based on the vectors and other syntax elements, summer  90  reconstructs decoded video block for the current video block by summing the residual block from inverse transform processing unit  88  and a corresponding predictive block generated by motion compensation unit  82  and intra BC unit  85 . An in-loop filter (not pictured) may be positioned between summer  90  and DPB  92  to further process the decoded video block. The decoded video blocks in a given frame are then stored in DPB  92 , which stores reference frames used for subsequent motion compensation of next video blocks. DPB  92 , or a memory device separate from DPB  92 , may also store decoded video for later presentation on a display device, such as display device  34  of  FIG. 1 . 
     In a typical video coding process, a video sequence typically includes an ordered set of frames or pictures. Each frame may include three sample arrays, denoted SL, SCb, and SCr. SL is a two-dimensional array of luma samples. SCb is a two-dimensional array of Cb chroma samples. SCr is a two-dimensional array of Cr chroma samples. In other instances, a frame may be monochrome and therefore includes only one two-dimensional array of luma samples. 
     As shown in  FIG. 4A , video encoder  20  (or more specifically partition unit  45 ) generates an encoded representation of a frame by first partitioning the frame into a set of coding tree units (CTUs). A video frame may include an integer number of CTUs ordered consecutively in a raster scan order from left to right and from top to bottom. Each CTU is a largest logical coding unit and the width and height of the CTU are signaled by the video encoder  20  in a sequence parameter set, such that all the CTUs in a video sequence have the same size being one of 128×128, 64×64, 32×32, and 16×16. But it should be noted that the present application is not necessarily limited to a particular size. As shown in  FIG. 4B , each CTU may comprise one coding tree block (CTB) of luma samples, two corresponding coding tree blocks of chroma samples, and syntax elements used to code the samples of the coding tree blocks. The syntax elements describe properties of different types of units of a coded block of pixels and how the video sequence can be reconstructed at the video decoder  30 , including inter or intra prediction, intra prediction mode, motion vectors, and other parameters. In monochrome pictures or pictures having three separate color planes, a CTU may comprise a single coding tree block and syntax elements used to code the samples of the coding tree block. A coding tree block may be an N×N block of samples. 
     To achieve a better performance, video encoder  20  may recursively perform tree partitioning such as binary-tree partitioning, quad-tree partitioning or a combination of both on the coding tree blocks of the CTU and divide the CTU into smaller coding units (CUs). As depicted in  FIG. 4C , the 64×64 CTU  400  is first divided into four smaller CU, each having a block size of 32×32. Among the four smaller CUs, CU  410  and CU  420  are each divided into four CUs of 16×16 by block size. The two 16×16 CUs  430  and  440  are each further divided into four CUs of 8×8 by block size.  FIG. 4D  depicts a quad-tree data structure illustrating the end result of the partition process of the CTU  400  as depicted in  FIG. 4C , each leaf node of the quad-tree corresponding to one CU of a respective size ranging from 32×32 to 8×8. Like the CTU depicted in  FIG. 4B , each CU may comprise a coding block (CB) of luma samples and two corresponding coding blocks of chroma samples of a frame of the same size, and syntax elements used to code the samples of the coding blocks. In monochrome pictures or pictures having three separate color planes, a CU may comprise a single coding block and syntax structures used to code the samples of the coding block. 
     In some implementations, video encoder  20  may further partition a coding block of a CU into one or more M×N prediction blocks (PB). A prediction block is a rectangular (square or non-square) block of samples on which the same prediction, inter or intra, is applied. A prediction unit (PU) of a CU may comprise a prediction block of luma samples, two corresponding prediction blocks of chroma samples, and syntax elements used to predict the prediction blocks. In monochrome pictures or pictures having three separate color planes, a PU may comprise a single prediction block and syntax structures used to predict the prediction block. Video encoder  20  may generate predictive luma, Cb, and Cr blocks for luma, Cb, and Cr prediction blocks of each PU of the CU. 
     Video encoder  20  may use intra prediction or inter prediction to generate the predictive blocks for a PU. If video encoder  20  uses intra prediction to generate the predictive blocks of a PU, video encoder  20  may generate the predictive blocks of the PU based on decoded samples of the frame associated with the PU. If video encoder  20  uses inter prediction to generate the predictive blocks of a PU, video encoder  20  may generate the predictive blocks of the PU based on decoded samples of one or more frames other than the frame associated with the PU. 
     After video encoder  20  generates predictive luma, Cb, and Cr blocks for one or more PUs of a CU, video encoder  20  may generate a luma residual block for the CU by subtracting the CU&#39;s predictive luma blocks from its original luma coding block such that each sample in the CU&#39;s luma residual block indicates a difference between a luma sample in one of the CU&#39;s predictive luma blocks and a corresponding sample in the CU&#39;s original luma coding block. Similarly, video encoder  20  may generate a Cb residual block and a Cr residual block for the CU, respectively, such that each sample in the CU&#39;s Cb residual block indicates a difference between a Cb sample in one of the CU&#39;s predictive Cb blocks and a corresponding sample in the CU&#39;s original Cb coding block and each sample in the CU&#39;s Cr residual block may indicate a difference between a Cr sample in one of the CU&#39;s predictive Cr blocks and a corresponding sample in the CU&#39;s original Cr coding block. 
     Furthermore, as illustrated in  FIG. 4C , video encoder  20  may use quad-tree partitioning to decompose the luma, Cb, and Cr residual blocks of a CU into one or more luma, Cb, and Cr transform blocks. A transform block is a rectangular (square or non-square) block of samples on which the same transform is applied. A transform unit (TU) of a CU may comprise a transform block of luma samples, two corresponding transform blocks of chroma samples, and syntax elements used to transform the transform block samples. Thus, each TU of a CU may be associated with a luma transform block, a Cb transform block, and a Cr transform block. In some examples, the luma transform block associated with the TU may be a sub-block of the CU&#39;s luma residual block. The Cb transform block may be a sub-block of the CU&#39;s Cb residual block. The Cr transform block may be a sub-block of the CU&#39;s Cr residual block. In monochrome pictures or pictures having three separate color planes, a TU may comprise a single transform block and syntax structures used to transform the samples of the transform block. 
     Video encoder  20  may apply one or more transforms to a luma transform block of a TU to generate a luma coefficient block for the TU. A coefficient block may be a two-dimensional array of transform coefficients. A transform coefficient may be a scalar quantity. Video encoder  20  may apply one or more transforms to a Cb transform block of a TU to generate a Cb coefficient block for the TU. Video encoder  20  may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU. 
     After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), video encoder  20  may quantize the coefficient block. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. After video encoder  20  quantizes a coefficient block, video encoder  20  may entropy encode syntax elements indicating the quantized transform coefficients. For example, video encoder  20  may perform Context-Adaptive Binary Arithmetic Coding (CABAC) on the syntax elements indicating the quantized transform coefficients. Finally, video encoder  20  may output a bitstream that includes a sequence of bits that forms a representation of coded frames and associated data, which is either saved in storage device  32  or transmitted to destination device  14 . 
     After receiving a bitstream generated by video encoder  20 , video decoder  30  may parse the bitstream to obtain syntax elements from the bitstream. Video decoder  30  may reconstruct the frames of the video data based at least in part on the syntax elements obtained from the bitstream. The process of reconstructing the video data is generally reciprocal to the encoding process performed by video encoder  20 . For example, video decoder  30  may perform inverse transforms on the coefficient blocks associated with TUs of a current CU to reconstruct residual blocks associated with the TUs of the current CU. Video decoder  30  also reconstructs the coding blocks of the current CU by adding the samples of the predictive blocks for PUs of the current CU to corresponding samples of the transform blocks of the TUs of the current CU. After reconstructing the coding blocks for each CU of a frame, video decoder  30  may reconstruct the frame. 
     As noted above, video coding achieves video compression using primarily two modes, i.e., intra-frame prediction (or intra-prediction) and inter-frame prediction (or inter-prediction). It is noted that IBC could be regarded as either intra-frame prediction or a third mode. Between the two modes, inter-frame prediction contributes more to the coding efficiency than intra-frame prediction because of the use of motion vectors for predicting a current video block from a reference video block. 
     But with the ever improving video data capturing technology and more refined video block size for preserving details in the video data, the amount of data required for representing motion vectors for a current frame also increases substantially. One way of overcoming this challenge is to benefit from the fact that not only a group of neighboring CUs in both the spatial and temporal domains have similar video data for predicting purpose but the motion vectors between these neighboring CUs are also similar. Therefore, it is possible to use the motion information of spatially neighboring CUs and/or temporally collocated CUs as an approximation of the motion information (e.g., motion vector) of a current CU by exploring their spatial and temporal correlation, which is also referred to as “motion vector predictor” (MVP) of the current CU. 
     Instead of encoding, into the video bitstream, an actual motion vector of the current CU determined by motion estimation unit  42  as described above in connection with  FIG. 2 , the motion vector predictor of the current CU is subtracted from the actual motion vector of the current CU to produce a motion vector difference (MVD) for the current CU. By doing so, there is no need to encode the actual motion vector determined by motion estimation unit  42  for each CU of a frame into the video bitstream and the amount of data used for representing motion information in the video bitstream can be significantly decreased. 
     Like the process of choosing a predictive block in a reference frame during inter-frame prediction of a code block, a set of rules need to be adopted by both video encoder  20  and video decoder  30  for constructing a motion vector candidate list for a current CU using those potential candidate motion vectors associated with spatially neighboring CUs and/or temporally collocated CUs of the current CU and then selecting one member from the motion vector candidate list as a motion vector predictor for the current CU. By doing so, there is no need to transmit the motion vector candidate list itself between video encoder  20  and video decoder  30  and an index of the selected motion vector predictor within the motion vector candidate list is sufficient for video encoder  20  and video decoder  30  to use the same motion vector predictor within the motion vector candidate list for encoding and decoding the current CU. 
     In some implementations, each inter-prediction CU has three motion vector prediction modes including inter (which is also referred to as “advanced motion vector prediction” (AMVP)), skip, and merge for constructing the motion vector candidate list. Under each mode, one or more motion vector candidates may be added to the motion vector candidate list according to the algorithms described below. Ultimately one of them in the candidate list is used as the best motion vector predictor of the inter-prediction CU to be encoded into the video bitstream by video encoder  20  or decoded from the video bitstream by video decoder  30 . To find the best motion vector predictor from the candidate list, a motion vector competition (MVC) scheme is introduced to select a motion vector from a given candidate set of motion vectors, i.e., the motion vector candidate list, that includes spatial and temporal motion vector candidates. 
     After one MVP candidate is selected within the given candidate set of motion vectors for a current CU, video encoder  20  may generate one or more syntax elements for the corresponding MVP candidate and encode them into the video bitstream such that video decoder  30  can retrieve the MVP candidate from the video bitstream using the syntax elements. Depending on the specific mode used for constructing the motion vectors candidate set, different modes (e.g., AMVP, merge, skip, etc.) have different sets of syntax elements. For the AMVP mode, the syntax elements include inter prediction indicators (e.g., List 0 , List 1 , or bi-directional prediction), reference indices, motion vector candidate indices, motion vector differences and prediction residual signal, etc. For the skip mode and the merge mode, only merge indices are encoded into the bitstream because the current CU inherits the other syntax elements including the inter prediction indicators, reference indices, and motion vectors from a neighboring CU referred by the coded merge index. In the case of a skip coded CU, the motion vector prediction residual signal is also omitted. 
       FIG. 5A  is a block diagram illustrating spatially neighboring and temporally collocated block positions of a current CU to be encoded/decoded in accordance with some implementations of the present disclosure. For a given mode (e.g., AMVP, merge, or skip), a motion vector prediction (MVP) candidate list is constructed by first checking the availability of motion vectors associated with the spatially left (A 0 , A 1 ) and above (B 0 , B 1 , B 2 ) neighboring block positions, and the availability of motion vectors associated with temporally collocated block positions. During the process of constructing the MVP candidate list, redundant MVP candidates are removed from the candidate list and, if necessary, zero-valued motion vector is added to make the candidate list to have a fixed length (note that different modes may have different fixed lengths). After the construction of the MVP candidate list, video encoder  20  can select the best motion vector predictor from the candidate list and encode the corresponding index indicating the chosen candidate into the video bitstream. 
     Using  FIG. 5A  as an example and assuming that the candidate list has a fixed length of two,  FIG. 5B  is a flowchart illustrating an exemplary process by which a list of motion vector candidates are identified in accordance with some implementations of the present disclosure. In particular, the motion vector predictor (MVP) candidate list for the current CU may be constructed by performing the following steps under the AMVP mode as depicted in  FIG. 5B :
         1) Step  505 : Selection of two MVP candidates from five spatially neighboring CUs
           a) Derive up to one non-scaled MVP candidate from one of the two left spatial neighbour CUs starting with A 0  and ending with A 1 ;   b) If no non-scaled MVP candidate from left is available in the previous step, derive up to one scaled MVP candidate from one of the two left spatial neighbour CUs starting with A 0  and ending with A 1 ;   c) Derive up to one non-scaled MVP candidate from one of the three above spatial neighbour CUs starting with B 0 , then B 1 , and ending with B 2 ;   d) If neither A 0  nor A 1  is available or if they are coded in intra modes, derive up to one scaled MVP candidate from one of the three above spatial neighbour CUs starting with B 0 , then B 1 , and ending with B 2 ;   
           2) Step  510 : Selection of one MVP candidate from two temporally collocated CUs;   3) Step  515 : Removal of duplicate MVP candidates found in the previous steps from the MVP candidate list;   4) Step  520 : Add up to two zero-valued MVPs to the MVP candidate list.   5) Step  525 : Removal of MVP candidates whose index is larger than 1 from the MVP candidate list.   6) Step  530 : Finalization of the two MVP candidates in the MVP candidate list for the current CU.       

     Since there are only two candidates in the AMVP-mode MVP candidate list constructed above, an associated syntax element like a binary flag is encoded into the bitstream to indicate that which of the two MVP candidates within the candidate list is used for decoding the current CU. 
     In some implementations, the process of selecting a temporal motion vector predictor for encoding/decoding the current CU is performed at a sub-CU level so as to improve the accuracy of a decoded picture. This process first identifies a collocated picture for the current picture including the current CU and then determines a temporal vector (also known as “motion shift” in the present application). Next, the process splits the current CU into multiple sub-CUs and derives motion information for each of the sub-CUs from a corresponding block in the collocated picture identified by the temporal vector according to a predefined algorithm, which is also referred to as “subblock-based temporal motion vector prediction” (SbTMVP). 
       FIG. 5C  is a block diagram illustrating how a subblock-based temporal motion vector prediction is performed between a current picture and a collocated picture in accordance with some implementations of the present disclosure. In this example, the current CU  535 - 1  is a 64×64 code block and it is divided into 8×8 sub-CUs, each sub-CU being an 8×8 code block. In order to derive the motion information for each sub-CU, the SbTMVP process is divided into two main steps:
         Step One: Identify the corresponding block  540 - 1  in the collocated picture  540  using a so-called “temporal vector” between the current picture  535  and the collocated picture  540 .   Step Two: Split the current CU  535 - 1  into multiple sub-CUs and obtain motion information for the sub-CUs including the motion vector and the reference index (which is zero by default) of each sub-CU  535 - 3  from a corresponding block in the collocated picture  540 .       

     As noted above, the collocated picture  540  is assumed to be known for the current picture  535  before the execution of the SbTMVP process. For example, the collocated picture is typically one reference picture of the current picture, which is selected from one of the two reference picture lists of the current picture, e.g., List 0  and List 1 . In some implementations, the corresponding block is the one at the same relative position in the collocated picture as the current CU in the current picture. In some other implementations (e.g.,  FIG. 5C ), the corresponding block is not necessarily the one at the same relative position in the collocated picture as the current CU  535 - 1  in the current picture. Instead, there is a temporal vector linking the center of the current CU  535 - 1  in the current picture to the center of the corresponding block  540 - 1  in the collocated picture  540 . 
     In some implementations, the SbTMVP process is implemented as part of the process of constructing a motion vector candidate list described above for a current CU during video encoding. In other words, if it is determined to that the current CU is to be processed using the SbTMVP, a parameter corresponding to the SbTMVP is added to the motion vector candidate list accordingly. In some other implementations, the SbTMVP process is implemented independently from the process of constructing a motion vector candidate list described above for a current CU during video encoding. In other words, the SbTMVP is treated as a separate inter-prediction mode like the aforementioned inter-prediction modes. Due to the symmetric nature between the encoding process and the decoding process, the rest of the present application uses the decoding of a current CU to illustrate how the SbTMVP process is employed for predicting the temporal motion vector of the current CU. 
       FIG. 6  is a flowchart illustrating an exemplary decoding process by which a video decoder implements the techniques of constructing subblock-based temporal motion vector prediction for a current picture from motion information of a collocated picture in accordance with some implementations of the present disclosure. 
     First, video decoder  30  acquires ( 610 ) an encoded video bitstream including data associated with multiple encoded pictures. As depicted in  FIGS. 4A and 4C , each picture includes multiple rows of coding tree units (CTUs) and each CTU includes one or more coding units (CUs). Video decoder  30  extracts different pieces of information from the video bitstream, such as syntax elements and pixel values, to reconstruct the picture row by row. 
     In this example, it is assumed that video decoder  30  is decoding the current picture  535  depicted in  FIG. 5C  ( 630 ) or more specifically, a current CU  535 - 1  in the current picture  535 . As noted above, the current picture  535  has multiple reference pictures, e.g., List 0  and/or List. For the purpose of temporal motion vector prediction, one of the reference pictures is the so-called “collocated picture”  540  of the current picture  535  as depicted in  FIG. 5C . Therefore, video decoder  30  first determine one of the reference pictures as a collocated picture of the current picture according to the syntax elements signaled in the video bitstream in a predefined order ( 630 - 1 ). For example, in the derivation of the temporal motion vector candidates, an explicit flag in the slice header (collocated_from_l0_flag) is firstly sent to video decoder  30  to indicate whether the collocated picture is selected from, e.g., List 0  or List 1 . A collocated reference index (collocated_ref_idx) is further sent to video decoder  30  indicating which reference picture in that list is selected as the collocated picture for deriving the temporal motion vector candidate. 
     In either case, after a reference picture of the current picture is identified as being the collocated picture, video decoder  30  determines a motion shift (also known as “temporal vector”) between the collocated picture and the current picture (see, e.g., temporal vector  537  depicted in  FIG. 5C ) from the motion information of the spatially neighbouring blocks of the current CU  535 - 1  according to a fixed order ( 630 - 3 ). As noted above, each CTU (including one or more CUs) has multiple spatially neighbouring blocks like A 0 , A 1 , B 0 , B 1 , B 2 , etc. Each of the spatially neighbouring blocks may have a motion vector pointing to a corresponding block within a respective reference picture of the current CU. In some implementations, video decoder  30  checks each reference picture associated with a corresponding spatially neighbouring block until one of them is the same as the collocated picture. The checking order is adaptive. In some implementations, the checking order starts with one of List 0  and List 1  according to the low delay condition (LDC) and the syntax element “collocated_from_l0_flag”. LDC is a Boolean variable to indicate whether all reference pictures have smaller Picture Order Count (POC) then the current picture. For example, List 0  may include at least one reference picture preceding the current picture in time and optionally, one or more reference pictures following the current picture in time. List 1  may include at least one reference picture following the current picture in time or only reference pictures preceding the current picture in time. After identifying the reference picture associated with a particular spatially neighbouring block of the current CU, video decoder  30  determines the motion information of the spatially neighbouring block and uses it as the motion shift between the collocated picture and the current picture. Using the motion shift, a block within the collocated picture can be identified as corresponding to the current CU. After establishing a mapping relationship between the current CU  535 - 1  and the corresponding block  540 - 1  as depicted in  FIG. 5C , video decoder  30  can start constructing temporal motion vector prediction for each subblock of the current CU  535 - 1 . 
     Note that it is assumed that video encoder  20  has split the current CU  535 - 1  into multiple sub-CUs during the generation of the video bitstream received by video decoder  30 . Therefore, video decoder  30  can use, from the video bitstream, the same set of syntax elements for splitting the current CU  535 - 1  into multiple sub-CUs ( 630 - 5 ). As depicted in  FIG. 5C , each sub-CU  535 - 3  in the current picture  535  has a corresponding subblock at the same relative position in the collocated picture  540 . Video decoder  30  is responsible for reconstructing the same temporal motion vector prediction for each sub-CU as video encoder  20  does when encoding the sub-CU into the video bitstream according to the SbTMVP process. 
     In some implementations, video decoder  30  obtains a temporal motion vector predictor for each sub-CU of the current CU based on two pieces of information, i.e., the motion shift between the collocated picture and the current picture and motion information of a block in the collocated picture that corresponds to the respective subblock of the current picture ( 630 - 7 ). For example, for each sub-CU in the current picture  535 , video decoder  30  identifies a block in the collocated picture at a same relative location as the subblock of the sub-CU in the current picture according to the motion shift between the collocated picture and the current picture. Next, video decoder  30  determines motion information of the identified block in the collocated picture and selects a motion vector as well as a reference index from the determined motion information of the identified block to derive the temporal motion vector predictor of the sub-CU according to the ratio of the POC differences between the current picture and its reference picture and the POC differences between the collocated picture and the reference picture of the corresponding block. 
     On the other hand, the corresponding block  540 - 1  in the collocated picture  540  may belong to different CUs, CTUs or even different slices or tiles. It is possible that different subblocks in the corresponding block  540 - 1  may have different prediction modes such that some of the subblocks in the corresponding block  540 - 1  may not have motion vectors at all. In some implementations, video decoder  30  deals with this situation by checking a particular subblock within the corresponding block  540 - 1  and determining whether the particular subblock has motion information. If the particular subblock has motion information, video decoder  30  may use the particular subblock&#39;s as a default temporal motion vector predictor for another sub-CU of the current CU when the sub-CU&#39;s corresponding block in the collocated picture does not have motion information. For example, before processing any sub-CU in the current CU, video decoder  30  may first examine the corresponding block of a block (or a sample) at or near the center of the corresponding block  540 - 1  (e.g., the one at the right side immediately below the center of the corresponding block  540 - 1 ) and check whether the block has motion information or not. If the block has no motion information, video decoder  30  then assumes that the SbTMVP process does not apply to the current CU and proceeds to process another merge candidates in the current picture. But if the block has motion information, video decoder  30  assumes that the SbTMVP process applies to the current CU and uses the motion information as the default temporal motion vector predictor for any sub-CU of the current CU when its corresponding block at the same relative location in the collocated picture does not have motion information for constructing the sub-CU&#39;s temporal motion vector predictor. Assuming that the SbTMVP process applies to the current CU, video decoder  30  uses the temporal motion vector predictors obtained for the plurality of sub-CUs of the current CU to decode the current CU accordingly ( 630 - 9 ). 
     As noted above, intra block copy (IBC) can significantly improve the coding efficiency of screen content materials. Since IBC mode is implemented as a block-level coding mode, block matching (BM) is performed at video encoder  20  to find an optimal block vector for each CU. Here, a block vector is used to indicate the displacement from the current block to a reference block, which has already been reconstructed within the current picture. An IBC-coded CU is treated as the third prediction mode other than the intra or inter prediction modes. 
     At the CU level, the IBC mode can be signaled as IBC AMVP mode or IBC skip/merge mode as follows:
         IBC AMVP mode: a block vector difference (BVD) between the actual block vector of a CU and a block vector predictor of the CU selected from block vector candidates of the CU is encoded in the same way as a motion vector difference is encoded under the AMVP mode described above. The block vector prediction method uses two block vector candidates as predictors, one from left neighbor and the other one from above neighbor (if IBC coded). When either neighbor is not available, a default block vector will be used as a block vector predictor. A binary flag is signaled to indicate the block vector predictor index.   IBC skip/merge mode: a merge candidate index is used to indicate which of the block vector candidates in the merge candidate list from neighboring IBC coded blocks is used to predict the block vector for the current block.       

     The video coder may initialize a context for a current wavefront for performing context adaptive binary arithmetic coding (CABAC) of the current wavefront based on data of the first two blocks of the above wavefront, as well as one or more elements of a slice header for a slice including the first code block of the current wavefront. The video coder may perform CABAC initialization of a subsequent wavefront (or CTU row) using the context states after coding two CTUs of a CTU row above the subsequent CTU row. In other words, before beginning coding of a current wavefront, a video coder may code at least two blocks of a wavefront above the current wavefront, assuming the current wavefront is not the top row of CTUs of a picture. The video coder may then initialize a CABAC context for the current wavefront after coding at least two blocks of a wavefront above the current wavefront. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the implementations described in the present application. A computer program product may include a computer-readable medium. 
     The terminology used in the description of the implementations herein is for the purpose of describing particular implementations only and is not intended to limit the scope of claims. As used in the description of the implementations and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, elements, and/or components, but do not preclude the presence or addition of one or more other features, elements, components, and/or groups thereof. 
     It will also be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first electrode could be termed a second electrode, and, similarly, a second electrode could be termed a first electrode, without departing from the scope of the implementations. The first electrode and the second electrode are both electrodes, but they are not the same electrode. 
     The description of the present application has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, and alternative implementations will be apparent to those of ordinary skill in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others skilled in the art to understand the invention for various implementations and to best utilize the underlying principles and various implementations with various modifications as are suited to the particular use contemplated. Therefore, it is to be understood that the scope of claims is not to be limited to the specific examples of the implementations disclosed and that modifications and other implementations are intended to be included within the scope of the appended claims.