Patent Publication Number: US-2021195205-A1

Title: Overlapped block motion compensation using temporal neighbors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/IB2019/057140 filed on Aug. 26, 2019, which claims the priority to and benefits of International Patent Application No. PCT/CN2018/102163, filed on Aug. 24, 2018. All the aforementioned patent applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     This patent document relates to video coding and decoding techniques, devices and systems. 
     BACKGROUND 
     In spite of the advances in video compression, digital video still accounts for the largest bandwidth use on the internet and other digital communication networks. As the number of connected user devices capable of receiving and displaying video increases, it is expected that the bandwidth demand for digital video usage will continue to grow. 
     SUMMARY 
     Devices, systems and methods related to digital video coding, and specifically, to an overlapped block motion compensation (OBMC) process based on temporal neighbors are described. The described methods may be applied to both the existing video coding standards (e.g., High Efficiency Video Coding (HEVC)) and future video coding standards or video codecs. 
     In one representative aspect, the disclosed technology may be used to provide a method for video processing. This method includes generating, based on a weighted sum of at least two temporary prediction blocks, a prediction block for a current video block, a first of the at least two temporary prediction blocks being based on a first motion information associated with the current video block, and a second of the at least two temporary prediction blocks being based on a second motion information associated with at least one neighboring block of the current video block; and performing, based on the prediction block, a conversion between the current video block and a bitstream representation of the current video block. 
     In some embodiments, the method preferably include the at least one neighboring block comprising a temporally neighboring block. 
     In some embodiments, the method preferably include a weighting factor of the second temporary prediction block being based on a location or coding mode of the at least one neighboring block. 
     In some embodiments, the method preferably include the current video block being coded with a sub-block based coding tool, and a final prediction of a current sub-block of the current video block being based on at least a motion information of temporally neighboring blocks of the current sub-block. 
     In some embodiments, the method preferably include the current video block being coded without a sub-block based coding tool. 
     In some embodiments, the method preferably include a final prediction for each of a subset of sub-blocks of the current video block being based on the second motion information, and the subset excluding at least one sub-block of the current video block. 
     In some embodiments, the method preferably include performing the conversion being further based, upon a determination of an availability of motion information associated with at least one spatially neighboring block of the current video block, on a third motion information associated with the at least one spatially neighboring block. 
     In some embodiments, the method preferably include the first motion information and the second motion information not being derived from a same prediction process. 
     In some embodiments, the method preferably include the temporally neighboring block being located in a collocated picture that is signaled in a sequence parameter set (SPS), a picture parameter set (PPS), a video parameter set (VPS) or a slice header. 
     In some embodiments, the method preferably include the temporally neighboring block being located in a predetermined reference picture. 
     In some embodiments, the method preferably include the predetermined reference picture being in list 0 or list 1. 
     In some embodiments, the method preferably include the temporally neighboring block being located in one of a plurality of reference pictures that are signaled in a sequence parameter set (SPS), a picture parameter set (PPS), a video parameter set (VPS), a slice header or a tile header. 
     In some embodiments, the method preferably include the temporally neighboring block being a collocated block in a selected reference picture. 
     In some embodiments, the method preferably include a current prediction unit (PU) or coding unit (CU) comprising the current video block, and a motion vector of the current PU or CU comprising an identification of the temporally neighboring block. 
     In some embodiments, the method preferably include the motion vector being a scaled motion vector. 
     In some embodiments, the method preferably include a current prediction unit (PU) or coding unit (CU) comprising the current video block, a motion vector of the current PU or CU being scaled to a first reference picture of the current PU or CU, and a motion vector of the temporally neighboring block being scaled to the first reference picture. 
     In some embodiments, the method preferably include a motion vector of the at least neighboring block being scaled to a predetermined reference picture. 
     In some embodiments, the method preferably include the predetermined reference picture being a first reference picture in list 0 or list 1. 
     In some embodiments, the method preferably include a motion vector of the temporally neighboring block being scaled to one of a plurality of reference pictures that are signaled in a sequence parameter set (SPS), a picture parameter set (PPS), a video parameter set (VPS) or a slice header. 
     In some embodiments, the method preferably include the weighting factor being a first weighting factor upon a determination that the at least one neighboring block comprises a spatially neighboring block of the current video block. 
     In some embodiments, the method preferably include the weighting factor being a second weighting factor upon a determination that the at least one neighboring block comprises a temporally neighboring block of the current video block. 
     In some embodiments, the method preferably include the weighting factor being a third weighting factor upon a determination that the current video block is coded using an intra prediction mode. 
     In some embodiments, the method preferably include the second and third weighting factors being signaled in a sequence parameter set (SPS), a picture parameter set (PPS), a video parameter set (VPS) or a slice header. 
     In some embodiments, the method preferably include the first weighting factor being greater than the second weighting factor, and the second weighting factor being greater than the third weighting factor. 
     In some embodiments, the method preferably include the second weighting factor being equal to the third weighting factor. 
     In some embodiments, the method preferably include the first, second or third weighting factors being further based on dimensions of the current video block. 
     In some embodiments, the method preferably include performing the conversion being based on a coding mode of the current video block, a size or a shape of the current video block, or a size of a sub-block of the current video block. 
     In some embodiments, the method preferably include the coding mode of the current video block comprising a conventional translation motion with an affine mode being disabled. 
     In some embodiments, the method preferably include a product of a height of the current video block and a width of a current video block being greater than or equal to a threshold. 
     In some embodiments, the method preferably include a height of the current video block being greater than or equal to a first threshold, and a width of the current video block being greater than or equal to a second threshold. 
     In some embodiments, the method preferably include performing the conversion being further based on a slice type of a slice comprising the current video block, a low-delay check flag or a temporal layer. 
     In some embodiments, the method preferably include the performing the conversion comprising applying the motion compensation process on a luma component of the current video block. 
     In some embodiments, the method preferably include the performing the conversion comprising applying the motion compensation process on one or more of a plurality of chroma components of the current video block. 
     In some embodiments, the method preferably include performing the conversion being further based on signaling in a sequence parameter set (SPS), a picture parameter set (PPS), a video parameter set (VPS), a slice header, a coding tree unit (CTU), a coding unit (CU), a group of CTUs or a group of CUs. 
     In some embodiments, the method preferably include one or more weights of the weighted sum being based on a coordinate of a sample within the current video block. 
     In some embodiments, the method preferably include one or more weights of the weighted sum being based on a distance of a sample within the current video block to a boundary of the current video block. 
     In some embodiments, the method preferably include generating the prediction block being part of an overlapped block motion compensation (OBMC) process. 
     In some embodiments, the method preferably include performing the conversion comprising generating the bitstream representation from the current video block. 
     In some embodiments, the method preferably include performing the conversion comprising generating the current video block from the bitstream representation. 
     In another representative aspect, the above-described method is embodied in the form of processor-executable code and stored in a computer-readable program medium. 
     In yet another representative aspect, a device that is configured or operable to perform the above-described method is disclosed. The device may include a processor that is programmed to implement this method. 
     In yet another representative aspect, a video decoder apparatus may implement a method as described herein. 
     The above and other aspects and features of the disclosed technology are described in greater detail in the drawings, the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of constructing a merge candidate list. 
         FIG. 2  shows an example of positions of spatial candidates. 
         FIG. 3  shows an example of candidate pairs subject to a redundancy check of spatial merge candidates. 
         FIGS. 4A and 4B  show examples of the position of a second prediction unit (PU) based on the size and shape of the current block. 
         FIG. 5  shows an example of motion vector scaling for temporal merge candidates. 
         FIG. 6  shows an example of candidate positions for temporal merge candidates. 
         FIG. 7  shows an example of generating a combined bi-predictive merge candidate. 
         FIG. 8  shows an example of constructing motion vector prediction candidates. 
         FIG. 9  shows an example of motion vector scaling for spatial motion vector candidates. 
         FIG. 10  shows an example of motion prediction using the alternative temporal motion vector prediction (ATMVP) algorithm for a coding unit (CU). 
         FIG. 11  shows an example of a coding unit (CU) with sub-blocks and neighboring blocks used by the spatial-temporal motion vector prediction (STMVP) algorithm. 
         FIGS. 12A and 12B  show example snapshots of sub-block when using the overlapped block motion compensation (OBMC) algorithm. 
         FIG. 13  shows an example of a simplified affine motion model. 
         FIG. 14  shows an example of an affine motion vector field (MVF) per sub-block. 
         FIG. 15  shows an example of motion vector prediction (MVP) for the AF_INTER affine motion mode. 
         FIGS. 16A and 16B  show examples of the 4-parameter and 6-parameter affine models, respectively. 
         FIGS. 17A and 17B  show example candidates for the AF_MERGE affine motion mode. 
         FIG. 18  shows an example of temporally neighboring blocks of a current block. 
         FIGS. 19A and 19B  examples of motion information from spatially and temporally neighboring blocks used for OBMC of a prediction unit (PU) or coding unit (CU). 
         FIG. 20  shows a flowchart of an example method for video coding. 
         FIG. 21  is a block diagram of an example of a hardware platform for implementing a visual media decoding or a visual media encoding technique described in the present document. 
         FIG. 22  is a block diagram of an example video processing system in which disclosed techniques may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     Due to the increasing demand of higher resolution video, video coding methods and techniques are ubiquitous in modern technology. Video codecs typically include an electronic circuit or software that compresses or decompresses digital video, and are continually being improved to provide higher coding efficiency. A video codec converts uncompressed video to a compressed format or vice versa. There are complex relationships between the video quality, the amount of data used to represent the video (determined by the bit rate), the complexity of the encoding and decoding algorithms, sensitivity to data losses and errors, ease of editing, random access, and end-to-end delay (latency). The compressed format usually conforms to a standard video compression specification, e.g., the High Efficiency Video Coding (HEVC) standard (also known as H.265 or MPEG-H Part  2 ), the Versatile Video Coding standard to be finalized, or other current and/or future video coding standards. 
     Embodiments of the disclosed technology may be applied to existing video coding standards (e.g., HEVC, H.265) and future standards to improve compression performance. Section headings are used in the present document to improve readability of the description and do not in any way limit the discussion or the embodiments (and/or implementations) to the respective sections only. 
     1. Examples of Inter-Prediction in HEVC/H.265 
     Video coding standards have significantly improved over the years, and now provide, in part, high coding efficiency and support for higher resolutions. Recent standards such as HEVC and H.265 are based on the hybrid video coding structure wherein temporal prediction plus transform coding are utilized. 
     1.1 Examples of Prediction Modes 
     Each inter-predicted PU (prediction unit) has motion parameters for one or two reference picture lists. In some embodiments, motion parameters include a motion vector and a reference picture index. In other embodiments, the usage of one of the two reference picture lists may also be signaled using inter_pred_idc. In yet other embodiments, motion vectors may be explicitly coded as deltas relative to predictors. 
     When a CU is coded with skip mode, one PU is associated with the CU, and there are no significant residual coefficients, no coded motion vector delta or reference picture index. A merge mode is specified whereby the motion parameters for the current PU are obtained from neighboring PUs, including spatial and temporal candidates. The merge mode can be applied to any inter-predicted PU, not only for skip mode. The alternative to merge mode is the explicit transmission of motion parameters, where motion vector, corresponding reference picture index for each reference picture list and reference picture list usage are signaled explicitly per each PU. 
     When signaling indicates that one of the two reference picture lists is to be used, the PU is produced from one block of samples. This is referred to as ‘uni-prediction’. Uni-prediction is available both for P-slices and B-slices. 
     When signaling indicates that both of the reference picture lists are to be used, the PU is produced from two blocks of samples. This is referred to as ‘bi-prediction’. Bi-prediction is available for B-slices only. 
     1.1.1 Embodiments of Constructing Candidates for Merge Mode 
     When a PU is predicted using merge mode, an index pointing to an entry in the merge candidates list is parsed from the bitstream and used to retrieve the motion information. The construction of this list can be summarized according to the following sequence of steps: 
     Step 1: Initial candidates derivation
         Step 1.1: Spatial candidates derivation   Step 1.2: Redundancy check for spatial candidates   Step 1.3: Temporal candidates derivation       

     Step 2: Additional candidates insertion
         Step 2.1: Creation of bi-predictive candidates   Step 2.2: Insertion of zero motion candidates       

       FIG. 1  shows an example of constructing a merge candidate list based on the sequence of steps summarized above. For spatial merge candidate derivation, a maximum of four merge candidates are selected among candidates that are located in five different positions. For temporal merge candidate derivation, a maximum of one merge candidate is selected among two candidates. Since constant number of candidates for each PU is assumed at decoder, additional candidates are generated when the number of candidates does not reach to maximum number of merge candidate (MaxNumMergeCand) which is signalled in slice header. Since the number of candidates is constant, index of best merge candidate is encoded using truncated unary binarization (TU). If the size of CU is equal to 8, all the PUs of the current CU share a single merge candidate list, which is identical to the merge candidate list of the 2N×2N prediction unit. 
     1.1.2 Constructing Spatial Merge Candidates 
     In the derivation of spatial merge candidates, a maximum of four merge candidates are selected among candidates located in the positions depicted in  FIG. 2 . The order of derivation is A 1 , B 1 , B 0 , A 0  and B 2 . Position B 2  is considered only when any PU of position A 1 , B 1 , B 0 , A 0  is not available (e.g. because it belongs to another slice or tile) or is intra coded. After candidate at position A 1  is added, the addition of the remaining candidates is subject to a redundancy check which ensures that candidates with same motion information are excluded from the list so that coding efficiency is improved. 
     To reduce computational complexity, not all possible candidate pairs are considered in the mentioned redundancy check. Instead only the pairs linked with an arrow in  FIG. 3  are considered and a candidate is only added to the list if the corresponding candidate used for redundancy check has not the same motion information. Another source of duplicate motion information is the “second PU” associated with partitions different from 2N×2N. As an example,  FIGS. 4A and 4B  depict the second PU for the case of N×2N and 2N×N, respectively. When the current PU is partitioned as N×2N, candidate at position A 1  is not considered for list construction. In some embodiments, adding this candidate may lead to two prediction units having the same motion information, which is redundant to just have one PU in a coding unit. Similarly, position B 1  is not considered when the current PU is partitioned as 2N×N. 
     1.1.3 Constructing Temporal Merge Candidates 
     In this step, only one candidate is added to the list. Particularly, in the derivation of this temporal merge candidate, a scaled motion vector is derived based on co-located PU belonging to the picture which has the smallest POC difference with current picture within the given reference picture list. The reference picture list to be used for derivation of the co-located PU is explicitly signaled in the slice header. 
       FIG. 5  shows an example of the derivation of the scaled motion vector for a temporal merge candidate (as the dotted line), which is scaled from the motion vector of the co-located PU using the POC distances, tb and td, where tb is defined to be the POC difference between the reference picture of the current picture and the current picture and td is defined to be the POC difference between the reference picture of the co-located picture and the co-located picture. The reference picture index of temporal merge candidate is set equal to zero. For a B-slice, two motion vectors, one is for reference picture list 0 and the other is for reference picture list 1, are obtained and combined to make the bi-predictive merge candidate. 
     In the co-located PU (Y) belonging to the reference frame, the position for the temporal candidate is selected between candidates C 0  and C 1 , as depicted in  FIG. 6 . If PU at position C 0  is not available, is intra coded, or is outside of the current CTU, position C 1  is used. Otherwise, position C 0  is used in the derivation of the temporal merge candidate. 
     1.1.4 Constructing Additional Types of Merge Candidates 
     Besides spatio-temporal merge candidates, there are two additional types of merge candidates: combined bi-predictive merge candidate and zero merge candidate. Combined bi-predictive merge candidates are generated by utilizing spatio-temporal merge candidates. Combined bi-predictive merge candidate is used for B-Slice only. The combined bi-predictive candidates are generated by combining the first reference picture list motion parameters of an initial candidate with the second reference picture list motion parameters of another. If these two tuples provide different motion hypotheses, they will form a new bi-predictive candidate. 
       FIG. 7  shows an example of this process, wherein two candidates in the original list ( 710 , on the left), which have mvL0 and refIdxL0 or mvL1 and refIdxL1, are used to create a combined bi-predictive merge candidate added to the final list ( 720 , on the right). 
     Zero motion candidates are inserted to fill the remaining entries in the merge candidates list and therefore hit the MaxNumMergeCand capacity. These candidates have zero spatial displacement and a reference picture index which starts from zero and increases every time a new zero motion candidate is added to the list. The number of reference frames used by these candidates is one and two for uni- and bi-directional prediction, respectively. In some embodiments, no redundancy check is performed on these candidates. 
     1.1.5 Examples of Motion Estimation Regions for Parallel Processing 
     To speed up the encoding process, motion estimation can be performed in parallel whereby the motion vectors for all prediction units inside a given region are derived simultaneously. The derivation of merge candidates from spatial neighborhood may interfere with parallel processing as one prediction unit cannot derive the motion parameters from an adjacent PU until its associated motion estimation is completed. To mitigate the trade-off between coding efficiency and processing latency, a motion estimation region (MER) may be defined. The size of the MER may be signaled in the picture parameter set (PPS) using the “log2_parallel_merge_level_minus2” syntax element. When a MER is defined, merge candidates falling in the same region are marked as unavailable and therefore not considered in the list construction. 
     1.2 Embodiments of Advanced Motion Vector Prediction (AMVP) 
     AMVP exploits spatio-temporal correlation of motion vector with neighboring PUs, which is used for explicit transmission of motion parameters. It constructs a motion vector candidate list by firstly checking availability of left, above temporally neighboring PU positions, removing redundant candidates and adding zero vector to make the candidate list to be constant length. Then, the encoder can select the best predictor from the candidate list and transmit the corresponding index indicating the chosen candidate. Similarly with merge index signaling, the index of the best motion vector candidate is encoded using truncated unary. The maximum value to be encoded in this case is 2 (see  FIG. 8 ). In the following sections, details about derivation process of motion vector prediction candidate are provided. 
     1.2.1 Examples of Constructing Motion Vector Prediction Candidates 
       FIG. 8  summarizes derivation process for motion vector prediction candidate, and may be implemented for each reference picture list with refidx as an input. 
     In motion vector prediction, two types of motion vector candidates are considered: spatial motion vector candidate and temporal motion vector candidate. For spatial motion vector candidate derivation, two motion vector candidates are eventually derived based on motion vectors of each PU located in five different positions as previously shown in  FIG. 2 . 
     For temporal motion vector candidate derivation, one motion vector candidate is selected from two candidates, which are derived based on two different co-located positions. After the first list of spatio-temporal candidates is made, duplicated motion vector candidates in the list are removed. If the number of potential candidates is larger than two, motion vector candidates whose reference picture index within the associated reference picture list is larger than 1 are removed from the list. If the number of spatio-temporal motion vector candidates is smaller than two, additional zero motion vector candidates is added to the list. 
     1.2.2 Constructing Spatial Motion Vector Candidates 
     In the derivation of spatial motion vector candidates, a maximum of two candidates are considered among five potential candidates, which are derived from PUs located in positions as previously shown in  FIG. 2 , those positions being the same as those of motion merge. The order of derivation for the left side of the current PU is defined as A 0 , A 1 , and scaled A 0 , scaled A 1 . The order of derivation for the above side of the current PU is defined as B 0 , B 1 , B 2 , scaled B 0 , scaled B 1 , scaled B 2 . For each side there are therefore four cases that can be used as motion vector candidate, with two cases not required to use spatial scaling, and two cases where spatial scaling is used. The four different cases are summarized as follows:
         No spatial scaling
           (1) Same reference picture list, and same reference picture index (same POC)   (2) Different reference picture list, but same reference picture (same POC)   
           Spatial scaling
           (3) Same reference picture list, but different reference picture (different POC)   (4) Different reference picture list, and different reference picture (different POC)   
               

     The no-spatial-scaling cases are checked first followed by the cases that allow spatial scaling. Spatial scaling is considered when the POC is different between the reference picture of the neighbouring PU and that of the current PU regardless of reference picture list. If all PUs of left candidates are not available or are intra coded, scaling for the above motion vector is allowed to help parallel derivation of left and above MV candidates. Otherwise, spatial scaling is not allowed for the above motion vector. 
     As shown in the example in  FIG. 9 , for the spatial scaling case, the motion vector of the neighbouring PU is scaled in a similar manner as for temporal scaling. One difference is that the reference picture list and index of current PU is given as input; the actual scaling process is the same as that of temporal scaling. 
     1.2.3 Constructing Temporal Motion Vector Candidates 
     Apart from the reference picture index derivation, all processes for the derivation of temporal merge candidates are the same as for the derivation of spatial motion vector candidates (as shown in the example in  FIG. 6 ). In some embodiments, the reference picture index is signaled to the decoder. 
     2. Example of Inter Prediction Methods in Joint Exploration Model (JEM) 
     In some embodiments, future video coding technologies are explored using a reference software known as the Joint Exploration Model (JEM). In JEM, sub-block based prediction is adopted in several coding tools, such as affine prediction, alternative temporal motion vector prediction (ATMVP), spatial-temporal motion vector prediction (STMVP), bi-directional optical flow (BIO), Frame-Rate Up Conversion (FRUC), Locally Adaptive Motion Vector Resolution (LAMVR), Overlapped Block Motion Compensation (OBMC), Local Illumination Compensation (LIC), and Decoder-side Motion Vector Refinement (DMVR). 
     2.1 Examples of Sub-CU Based Motion Vector Prediction 
     In the JEM with quadtrees plus binary trees (QTBT), each CU can have at most one set of motion parameters for each prediction direction. In some embodiments, two sub-CU level motion vector prediction methods are considered in the encoder by splitting a large CU into sub-CUs and deriving motion information for all the sub-CUs of the large CU. Alternative temporal motion vector prediction (ATMVP) method allows each CU to fetch multiple sets of motion information from multiple blocks smaller than the current CU in the collocated reference picture. In spatial-temporal motion vector prediction (STMVP) method motion vectors of the sub-CUs are derived recursively by using the temporal motion vector predictor and spatial neighbouring motion vector. In some embodiments, and to preserve more accurate motion field for sub-CU motion prediction, the motion compression for the reference frames may be disabled. 
     2.1.1 Examples of Alternative Temporal Motion Vector Prediction (ATMVP) 
     In the ATMVP method, the temporal motion vector prediction (TMVP) method is modified by fetching multiple sets of motion information (including motion vectors and reference indices) from blocks smaller than the current CU. 
       FIG. 10  shows an example of ATMVP motion prediction process for a CU  1000 . The ATMVP method predicts the motion vectors of the sub-CUs  1001  within a CU  1000  in two steps. The first step is to identify the corresponding block  1051  in a reference picture  1050  with a temporal vector. The reference picture  1050  is also referred to as the motion source picture. The second step is to split the current CU  1000  into sub-CUs  1001  and obtain the motion vectors as well as the reference indices of each sub-CU from the block corresponding to each sub-CU. 
     In the first step, a reference picture  1050  and the corresponding block is determined by the motion information of the spatial neighboring blocks of the current CU  1000 . To avoid the repetitive scanning process of neighboring blocks, the first merge candidate in the merge candidate list of the current CU  1000  is used. The first available motion vector as well as its associated reference index are set to be the temporal vector and the index to the motion source picture. This way, the corresponding block may be more accurately identified, compared with TMVP, wherein the corresponding block (sometimes called collocated block) is always in a bottom-right or center position relative to the current CU. 
     In the second step, a corresponding block of the sub-CU  1051  is identified by the temporal vector in the motion source picture  1050 , by adding to the coordinate of the current CU the temporal vector. For each sub-CU, the motion information of its corresponding block (e.g., the smallest motion grid that covers the center sample) is used to derive the motion information for the sub-CU. After the motion information of a corresponding N×N block is identified, it is converted to the motion vectors and reference indices of the current sub-CU, in the same way as TMVP of HEVC, wherein motion scaling and other procedures apply. For example, the decoder checks whether the low-delay condition (e.g. the POCs of all reference pictures of the current picture are smaller than the POC of the current picture) is fulfilled and possibly uses motion vector MVx (e.g., the motion vector corresponding to reference picture list X) to predict motion vector MVy (e.g., with X being equal to 0 or 1 and Y being equal to 1−X) for each sub-CU. 
     2.1.2 Examples of Spatial-Temporal Motion Vector Prediction (STMVP) 
     In the STMVP method, the motion vectors of the sub-CUs are derived recursively, following raster scan order.  FIG. 11  shows an example of one CU with four sub-blocks and neighboring blocks. Consider an 8×8 CU  1100  that includes four 4×4 sub-CUs A ( 1101 ), B ( 1102 ), C ( 1103 ), and D ( 1104 ). The neighboring 4×4 blocks in the current frame are labelled as a ( 1111 ), b ( 1112 ), c ( 1113 ), and d ( 1114 ). 
     The motion derivation for sub-CU A starts by identifying its two spatial neighbors. The first neighbor is the N×N block above sub-CU A  1101  (block c  1113 ). If this block c ( 1113 ) is not available or is intra coded the other N×N blocks above sub-CU A ( 1101 ) are checked (from left to right, starting at block c  1113 ). The second neighbor is a block to the left of the sub-CU A  1101  (block b  1112 ). If block b ( 1112 ) is not available or is intra coded other blocks to the left of sub-CU A  1101  are checked (from top to bottom, staring at block b  1112 ). The motion information obtained from the neighboring blocks for each list is scaled to the first reference frame for a given list. Next, temporal motion vector predictor (TMVP) of sub-block A  1101  is derived by following the same procedure of TMVP derivation as specified in HEVC. The motion information of the collocated block at block D  1104  is fetched and scaled accordingly. Finally, after retrieving and scaling the motion information, all available motion vectors are averaged separately for each reference list. The averaged motion vector is assigned as the motion vector of the current sub-CU. 
     2.1.3 Examples of Sub-CU Motion Prediction Mode Signaling 
     In some embodiments, the sub-CU modes are enabled as additional merge candidates and there is no additional syntax element required to signal the modes. Two additional merge candidates are added to merge candidates list of each CU to represent the ATMVP mode and STMVP mode. In other embodiments, up to seven merge candidates may be used, if the sequence parameter set indicates that ATMVP and STMVP are enabled. The encoding logic of the additional merge candidates is the same as for the merge candidates in the HM, which means, for each CU in P or B slice, two more RD checks may be needed for the two additional merge candidates. In some embodiments, e.g., JEM, all bins of the merge index are context coded by CABAC (Context-based Adaptive Binary Arithmetic Coding). In other embodiments, e.g., HEVC, only the first bin is context coded and the remaining bins are context by-pass coded. 
     2.2 Examples of Adaptive Motion Vector Difference Resolution 
     In some embodiments, motion vector differences (MVDs) (between the motion vector and predicted motion vector of a PU) are signalled in units of quarter luma samples when use_integer_mv_flag is equal to 0 in the slice header. In the JEM, a locally adaptive motion vector resolution (LAMVR) is introduced. In the JEM, MVD can be coded in units of quarter luma samples, integer luma samples or four luma samples. The MVD resolution is controlled at the coding unit (CU) level, and MVD resolution flags are conditionally signalled for each CU that has at least one non-zero MVD components. 
     For a CU that has at least one non-zero MVD components, a first flag is signalled to indicate whether quarter luma sample MV precision is used in the CU. When the first flag (equal to 1) indicates that quarter luma sample MV precision is not used, another flag is signalled to indicate whether integer luma sample MV precision or four luma sample MV precision is used. 
     When the first MVD resolution flag of a CU is zero, or not coded for a CU (meaning all MVDs in the CU are zero), the quarter luma sample MV resolution is used for the CU. When a CU uses integer-luma sample MV precision or four-luma-sample MV precision, the MVPs in the AMVP candidate list for the CU are rounded to the corresponding precision. 
     In the encoder, CU-level RD checks are used to determine which MVD resolution is to be used for a CU. That is, the CU-level RD check is performed three times for each MVD resolution. To accelerate encoder speed, the following encoding schemes are applied in the JEM:
         During RD check of a CU with normal quarter luma sample MVD resolution, the motion information of the current CU (integer luma sample accuracy) is stored. The stored motion information (after rounding) is used as the starting point for further small range motion vector refinement during the RD check for the same CU with integer luma sample and 4 luma sample MVD resolution so that the time-consuming motion estimation process is not duplicated three times.   RD check of a CU with 4 luma sample MVD resolution is conditionally invoked. For a CU, when RD cost integer luma sample MVD resolution is much larger than that of quarter luma sample MVD resolution, the RD check of 4 luma sample MVD resolution for the CU is skipped.       

     2.3 Examples of Higher Motion Vector Storage Accuracy 
     In HEVC, motion vector accuracy is one-quarter pel (one-quarter luma sample and one-eighth chroma sample for 4:2:0 video). In the JEM, the accuracy for the internal motion vector storage and the merge candidate increases to 1/16 pel. The higher motion vector accuracy ( 1/16 pel) is used in motion compensation inter prediction for the CU coded with skip/merge mode. For the CU coded with normal AMVP mode, either the integer-pel or quarter-pel motion is used. 
     SHVC upsampling interpolation filters, which have same filter length and normalization factor as HEVC motion compensation interpolation filters, are used as motion compensation interpolation filters for the additional fractional pel positions. The chroma component motion vector accuracy is 1/32 sample in the JEM, the additional interpolation filters of 1/32 pel fractional positions are derived by using the average of the filters of the two neighbouring 1/16 pel fractional positions. 
     2.4 Examples of Overlapped Block Motion Compensation (OBMC) 
     In the JEM, OBMC can be switched on and off using syntax at the CU level. When OBMC is used in the JEM, the OBMC is performed for all motion compensation (MC) block boundaries except the right and bottom boundaries of a CU. Moreover, it is applied for both the luma and chroma components. In the JEM, an MC block corresponds to a coding block. When a CU is coded with sub-CU mode (includes sub-CU merge, affine and FRUC mode), each sub-block of the CU is a MC block. To process CU boundaries in a uniform fashion, OBMC is performed at sub-block level for all MC block boundaries, where sub-block size is set equal to 4×4, as shown in  FIGS. 12A and 12B . 
       FIG. 12A  shows sub-blocks at the CU/PU boundary, and the hatched sub-blocks are where OBMC applies. Similarly,  FIG. 12B  shows the sub-Pus in ATMVP mode. 
     When OBMC applies to the current sub-block, besides current motion vectors, motion vectors of four connected neighboring sub-blocks, if available and are not identical to the current motion vector, are also used to derive prediction block for the current sub-block. These multiple prediction blocks based on multiple motion vectors are combined to generate the final prediction signal of the current sub-block. 
     Prediction block based on motion vectors of a neighboring sub-block is denoted as PN, with N indicating an index for the neighboring above, below, left and right sub-blocks and prediction block based on motion vectors of the current sub-block is denoted as PC. When PN is based on the motion information of a neighboring sub-block that contains the same motion information to the current sub-block, the OBMC is not performed from PN. Otherwise, every sample of PN is added to the same sample in PC, i.e., four rows/columns of PN are added to PC. The weighting factors {¼, ⅛, 1/16, 1/32} are used for PN and the weighting factors {¾, ⅞, 15/16, 31/32} are used for PC. The exception are small MC blocks, (i.e., when height or width of the coding block is equal to 4 or a CU is coded with sub-CU mode), for which only two rows/columns of PN are added to PC. In this case weighting factors {¼, ⅛} are used for PN and weighting factors {¾, ⅞} are used for PC. For PN generated based on motion vectors of vertically (horizontally) neighboring sub-block, samples in the same row (column) of PN are added to PC with a same weighting factor. 
     In the JEM, for a CU with size less than or equal to 256 luma samples, a CU level flag is signaled to indicate whether OBMC is applied or not for the current CU. For the CUs with size larger than 256 luma samples or not coded with AMVP mode, OBMC is applied by default. At the encoder, when OBMC is applied for a CU, its impact is taken into account during the motion estimation stage. The prediction signal formed by OBMC using motion information of the top neighboring block and the left neighboring block is used to compensate the top and left boundaries of the original signal of the current CU, and then the normal motion estimation process is applied. 
     2.5 Examples of Affine Motion Compensation Prediction 
     In HEVC, only a translation motion model is applied for motion compensation prediction (MCP). However, the camera and objects may have many kinds of motion, e.g. zoom in/out, rotation, perspective motions, and/or other irregular motions. JEM, on the other hand, applies a simplified affine transform motion compensation prediction.  FIG. 13  shows an example of an affine motion field of a block  1300  described by two control point motion vectors Vo and Vi. The motion vector field (MVF) of the block  1300  can be described by the following equation: 
     
       
         
           
             
               
                 
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     Here, MvPre is the motion vector fraction accuracy (e.g., 1/16 in JEM). (v 2x , v 2y ) is motion vector of the bottom-left control point, calculated according to Eq. (1). M and N can be adjusted downward if necessary to make it a divisor of w and h, respectively. 
       FIG. 14  shows an example of affine MVF per sub-block for a block  1400 . To derive motion vector of each M×N sub-block, the motion vector of the center sample of each sub-block can be calculated according to Eq. (1), and rounded to the motion vector fraction accuracy (e.g., 1/16 in JEM). Then the motion compensation interpolation filters can be applied to generate the prediction of each sub-block with derived motion vector. After the MCP, the high accuracy motion vector of each sub-block is rounded and saved as the same accuracy as the normal motion vector. 
     In the JEM, there are two affine motion modes: AF_INTER mode and AF_MERGE mode. For CUs with both width and height larger than 8, AF_INTER mode can be applied. An affine flag in CU level is signaled in the bitstream to indicate whether AF_INTER mode is used. In the AF_INTER mode, a candidate list with motion vector pair {(v 0 , v 1 )|v 0 ={V A , V B , V C }, v 1 ={v D ,v E }} is constructed using the neighboring blocks. 
       FIG. 15  shows an example of motion vector prediction (MVP) for a block  1500  in the AF_INTER mode. As shown in  FIG. 15 , v 0  is selected from the motion vectors of the sub-block A, B, or C. The motion vectors from the neighboring blocks can be scaled according to the reference list. The motion vectors can also be scaled according to the relationship among the Picture Order Count (POC) of the reference for the neighboring block, the POC of the reference for the current CU, and the POC of the current CU. The approach to select v 1  from the neighboring sub-block D and E is similar. If the number of candidate list is smaller than 2, the list is padded by the motion vector pair composed by duplicating each of the AMVP candidates. When the candidate list is larger than 2, the candidates can be firstly sorted according to the neighboring motion vectors (e.g., based on the similarity of the two motion vectors in a pair candidate). In some implementations, the first two candidates are kept. In some embodiments, a Rate Distortion (RD) cost check is used to determine which motion vector pair candidate is selected as the control point motion vector prediction (CPMVP) of the current CU. An index indicating the position of the CPMVP in the candidate list can be signaled in the bitstream. After the CPMVP of the current affine CU is determined, affine motion estimation is applied and the control point motion vector (CPMV) is found. Then the difference of the CPMV and the CPMVP is signaled in the bitstream. 
     In AF_INTER mode, when 4/6 parameter affine mode is used, ⅔ control points are required, and therefore ⅔ MVD needs to be coded for these control points, as shown in  FIGS. 16A and 16B , respectively. In an existing implementation, the MV may be derived as follows, e.g., it predicts mvd 1  and mvd 2  from mvd 0 . 
     
       
      
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     In some embodiments, and at the encoder, MVD of AF_INTER are derived iteratively. If it is assumed that the MVD derivation process is iterated n times, then the final MVD is calculated as follows, wherein a i  and b i  are the estimated affine parameters, and mvd[k] h  and mvd[k] v  are the derived horizontal and vertical component of mvd k  (k=0, 1) in the ith iteration. 
         mvd [1] h =Σ i=0   n−1   mvd [1] i   h =Σ i+1   n−1 ( a   i   *w+mvd [0] i   h )=Σ i+0   n−1   a   i   *w+Σ   i=0   n−1   mvd [0] i   h   =w*Σ   i+0   n−1   a   i   +mvd [0] h   Eq. (3)
 
         mvd [1] v =Σ i=0   n−1   mvd [1] i   v =Σ i=0   n−1 (− b   i   *w+mvd [0] i   v )=−Σ i+0   n−1   b   i   *w+Σ   i+0   n−1   mvd [0] i   v   =−w*Σ   i+0   n−1   b   i   +mvd [0] v   Eq. (4)
 
     Thus, in this implementation, which predicts mvd 1  from mvd 0 , only (w*Σ i+0   n−1 a i , −w*Σ i+0   n−1 b i ) is encoded for mvd 1 . 
     When a CU is applied in AF_MERGE mode, it gets the first block coded with an affine mode from the valid neighboring reconstructed blocks.  FIG. 17A  shows an example of the selection order of candidate blocks for a current CU  1700 . As shown in  FIG. 17A , the selection order can be from left ( 1701 ), above ( 1702 ), above right ( 1703 ), left bottom ( 1704 ) to above left ( 1705 ) of the current CU  1700 .  FIG. 17B  shows another example of candidate blocks for a current CU  1700  in the AF_MERGE mode. If the neighboring left bottom block  1701  is coded in affine mode, as shown in  FIG. 17B , the motion vectors v 2 , v 3  and v 4  of the top left corner, above right corner, and left bottom corner of the CU containing the sub-block  1701  are derived. The motion vector v 0  of the top left corner on the current CU  1700  is calculated based on v2, v3 and v4. The motion vector v1 of the above right of the current CU can be calculated accordingly. 
     After the CPMV of the current CU v0 and v1 are computed according to the affine motion model in Eq. (1), the MVF of the current CU can be generated. In order to identify whether the current CU is coded with AF_MERGE mode, an affine flag can be signaled in the bitstream when there is at least one neighboring block is coded in affine mode. 
     3. Drawbacks of Existing Implementations 
     In one existing implementation of OBMC, fixed weighting factors are used for generating the prediction sample P N  (prediction generated by using neighboring MV) and P C  (prediction generated by using current MV) when generating the final prediction. This may be problematic if P N  and P C  are dissimilar (e.g., in screen content coding), since a large difference may cause artifacts. 
     In another existing implementation, and for a PU/CU that is not coded with a sub-block mode, e.g., all sub-blocks within the PU/CU have identical motion information, OBMC cannot be performed for sub-blocks that are not at the left or above the PU/CU boundary. When neighboring blocks are coded in intra mode, even sub-blocks at the left or above the PU/CU boundary cannot perform OBMC. 
     4. Example Methods for OBMC Based on Temporal Neighbors 
     Embodiments of the presently disclosed technology overcome the drawbacks of existing implementations, thereby providing video coding with higher coding efficiencies. The OBMC process based on temporally neighboring blocks, based on the disclosed technology, may enhance both existing and future video coding standards, is elucidated in the following examples described for various implementations. The examples of the disclosed technology provided below explain general concepts, and are not meant to be interpreted as limiting. In an example, unless explicitly indicated to the contrary, the various features described in these examples may be combined. 
     Example 1. In one example, the generation of prediction block of one block depends on motion information of temporal neighboring blocks in addition to the motion information associated with the current block. 
     Example Usage of Proposed Methods
         (a) In one example, motion information of temporal neighboring blocks (named temporal motion information for short) is used in the OBMC process to generate P N .   (b) In one example, for a block coded with sub-block based coding tools (e.g., ATMVP), the generation of final prediction block for a sub-block may depend on the motion information of temporal neighboring blocks in addition to its own motion information, motion information from its surrounding sub-blocks.   (c) Alternatively, the usage of temporal motion information could only be applied to blocks coded without sub-block coding tools, e.g., all sub-blocks within the PU/CU have identical motion information.   (d) In one example, motion information of temporal neighboring blocks (as shown in  FIG. 18 ) may be used to generate the final prediction blocks for partial of the current block. In another example, for the down-right area of the PU/CU as shown in  FIGS. 19A and 19B , temporal motion information may be utilized.   (e) In one example, usage of the temporal motion information in OBMC process may further depend on the availability of motion information of spatial neighboring blocks. In one example, if left and/or above neighboring blocks/sub-blocks of the PU/CU is intra coded, motion information of temporal neighboring blocks may be used to generate P N  for the left/above boundary of the PU.   (f) In one example, the proposed method may be automatically disabled if the current block&#39;s motion information is derived from the same temporal neighboring block, e.g., the current block is coded with merge mode, its motion information is from the TMVP process, and the temporal neighboring blocks defined in the proposed method are the col-located temporal neighboring block in the collocated picture used for TMVP process.       

     Example Embodiments of Temporally Neighboring Blocks
         (g) In one example, temporal neighboring blocks are located in the collocated picture signaled in SPS/PPS/VPS or slice header.
           (i) Alternatively, temporal neighboring blocks are located in a predefined reference picture. For example, the first reference picture in list 0 or list 1.   (ii) Alternatively, temporal neighboring blocks are located in one or multiple reference pictures and indications of these pictures are signaled in SPS/PPS/VPS or slice header.   
           (h) In one example, temporal neighboring blocks are the collocated blocks in the selected reference pictures. Alternatively, temporal neighboring blocks are identified by MV or scaled MV of the current PU/CU.
           (i) In one example, if the selected reference picture for identifying temporal neighboring blocks is in reference picture list X of the current picture, then MV (scaled if necessary) of list X is used to identify the temporal neighboring blocks. If MV of list X is unavailable, then MV (scaled if necessary) of list 1−X is used to identify the temporal neighboring blocks.   (ii) In one example, if the selected reference picture for identifying temporal neighboring blocks is in both reference picture list 0 and list 1 of the current picture, then MV of list 0 (1) is first checked and then MV of list 1 (0) is checked. The first available MV (scaled if necessary) is used to identify the temporal neighboring blocks.   (iii) In one example, if the selected reference picture for identifying temporal neighboring blocks is the same with a reference picture of current PU, then MV pointing to that reference picture is used to identify the temporal neighboring blocks.   
           (i) In one example, motion vectors of temporal neighboring blocks are scaled to the same reference pictures of the current PU/CU, and then are used for OBMC.
           (i) Alternatively, motion vectors of temporal neighboring blocks are scaled to some predefined reference pictures, e.g., the first reference picture in list 0 or list 1.   (ii) Alternatively, motion vectors of temporal neighboring blocks are scaled to one or multiple reference pictures signaled in SPS/PPS/VPS or slice header.   
               

     Example 2. In one example, the generation of prediction blocks of one block may rely on motion information of current block and intra prediction modes of a neighboring block.
         (a) In one example, in the OBMC process, if the current PU/CU is coded with inter mode, and its neighboring block/sub-block is coded with intra mode, the reconstructed samples and the intra mode of the neighboring block/sub-block is used to generate PN for the corresponding above/left PU boundary block/sub-block (or the entire PU). Then, OBMC is performed.       

     Example 3. In one example, the generation of prediction blocks of one block may rely on motion information of neighboring blocks and intra prediction modes of the current block.
         (a) Alternatively, if the current PU/CU is coded with intra mode, and its neighboring block/sub-block is coded with inter mode, motion information of neighboring block/sub-block is used to generate PN for the corresponding above/left PU/CU boundary block/sub-block (or the entire PU).
           (i) Alternatively, in addition, motion information of spatial neighboring block/sub-block is used to generate PN for the above/left PU/CU boundary sub-blocks, while motion information of temporal neighboring blocks/sub-blocks are used to generate PN for other sub-blocks.   
           (b) Alternatively, if the current PU/CU is coded with intra mode, and its neighboring block/sub-block is also coded with intra mode, the reconstructed samples and the intra mode of the neighboring block/sub-block is used to generate PN for the corresponding above/left PU/CU boundary block/sub-block (or the entire PU).
           (i) Alternatively, in addition, motion information of temporal neighboring block/sub-block is used to generate PN for all sub-blocks that are not at the above/left PU/CU boundary.   (ii) Alternatively, motion information of temporal neighboring blocks is used to generate PN for the entire PU/CU.   
               

     Example 4. It is proposed that the weighting factor of P N  in OBMC is different when is generated by MVs of spatial neighboring blocks (the weighting factor is denoted by W1), temporal MVs as claimed in item 1 (the weighting factor is denoted by W2), or intra-prediction as claimed in item ⅔ (the weighting factor is denoted by W3).
         (a) In one example, W1&gt;W2&gt;W3.   (b) Alternatively, W2=W3.   (c) Alternatively, weights may further depend on other information, such as the distance of a row/column to the block boundary, block size/block shape/coded modes, etc.   (d) Weights used to prediction blocks generated by temporal motion information or intra modes may be signaled in VPS/SPS/PPS/Slice header or pre-defined.       

     Example 5. It is proposed that the weighting factor of P N  in OBMC is dependent on the difference (denoted by P diff ) between P N  and P C .
         (a) In one example, weighting factors may be adaptively selected from a predefined weighting factor set (like { 1/32, 1/16, ⅛, ¼, ½}).   (b) In one example, individual weight is assigned to each pixel, larger weight (i.e., closer to ½) W N  is assigned to P N  for smaller |P diff | and vice versa (the weight of P C  denoted by W C  is equal to 1−W N ).   (c) In one example, one same weight is assigned to a group of pixels.
           (i) In one example, one column/line is a group.   (ii) In one example, several columns/lines are a group.   (iii) In one example, a sub-block with size M×N is a group, wherein M and N are positive integers.   (iv) In one example, pixels with similar values are grouped together. For example, pixels with P C  (or P N ) value in the range of [V max   i V min   i ] forms the ith group.   (v) In one example, pixels with similar P diff  are grouped together. For example, pixels with |P diff | value in the range of [V max   i V min   i ] forms the ith group.   (vi) In one example, the weight depends on the average |P diff | of all pixels within the group, and a larger weight is assigned to P N  for a smaller average |P diff|.      (vii) In one example, OBMC is disabled when P N  is quite different from P C , for example, average of |P diff | is larger than a threshold T, wherein T&gt;0.   (viii) Alternatively, the weighting factor is not selected from a predefined set, but is calculated as a function of the pixel position and |P diff |.   
           (d) In one example, the weight is the same for all P N  predicted from one same neighboring motion information, and it depends on the difference between the neighboring motion information and the current motion information.
           (i) In one example, if the neighboring motion and the current motion uses different reference pictures, a larger/smaller weight is assigned to P N .   (ii) In one example, if the neighboring motion and the current motion uses same reference pictures, but the motion vectors are quite different, a larger/smaller weight is assigned to P N .   
               

     Example 6. The proposed methods may be applied to certain modes, block sizes/shapes, and/or certain sub-block sizes.
         (a) The proposed methods may be applied to certain modes, such as conventional translational motion (e.g., affine mode is disabled).   (b) The proposed methods may be applied to certain block sizes.
           (i) In one example, it is only applied to a block with w×h≥T, where w and h are the width and height of the current block.   (ii) In another example, it is only applied to a block with w≥T &amp;&amp; h≥T.   
           (c) Usage of the proposed method may be invoked under further conditions, e.g., based on block sizes/block shapes/coded modes/slice types/low delay check flags/temporal layers, etc.       

     Example 7. In one example, the proposed methods may be applied on all color (or chroma) components. Alternatively, they may be applied only to some color components. For example, they may be only applied on the luma component. 
     Example 8. In one example, whether to and how to apply the proposed methods can be signaled from the encoder to the decoder in VPS/SPS/PPS/slice header/CTU/CU/group of CTUs/group of CUs. 
     The examples described above may be incorporated in the context of the method described below, e.g., method  2000 , which may be implemented at a video decoder or a video encoder. 
       FIG. 20  shows a flowchart of an exemplary method for video decoding. The method  2000  includes, at step  2010 , generating, based on a weighted sum of at least two temporary prediction blocks, a prediction block for a current video block. In some embodiments, a first of the at least two temporary prediction blocks being based on a first motion information associated with the current video block, and a second of the at least two temporary prediction blocks being based on a second motion information associated with at least one neighboring block of the current video block. 
     The method  2000  includes, at step  2020 , performing, based on the prediction block, a conversion between the current video block and a bitstream representation of the current video block. 
     In some embodiments, the at least one neighboring block comprises a temporally neighboring block. 
     In some embodiments, a weighting factor of the second temporary prediction block being based on a location or coding mode of the at least one neighboring block. 
     In the methods described herein, in some embodiments, the conversion may include encoding the video block and video to generate a coded representation or a bitstream. In some embodiments, the conversion may include decoding a coded representation or bitstream to generate pixel values of the video block. In some embodiments, the conversion may be a transcoding operation in which bitrate or format of video representation is changed. 
     5. Example Implementations of the Disclosed Technology 
       FIG. 21  is a block diagram of a video processing apparatus  2100 . The apparatus  2100  may be used to implement one or more of the methods described herein. The apparatus  2100  may be embodied in a smartphone, tablet, computer, Internet of Things (IoT) receiver, and so on. The apparatus  2100  may include one or more processors  2102 , one or more memories  2104  and video processing hardware  2106 . The processor(s)  2102  may be configured to implement one or more methods (including, but not limited to, method  2000 ) described in the present document. The memory (memories)  2104  may be used for storing data and code used for implementing the methods and techniques described herein. The video processing hardware  2106  may be used to implement, in hardware circuitry, some techniques described in the present document. 
       FIG. 22  is a block diagram showing an example video processing system  2200  in which various techniques disclosed herein may be implemented. Various implementations may include some or all of the components of the system  2200 . The system  2200  may include input  2202  for receiving video content. The video content may be received in a raw or uncompressed format, e.g., 8 or 10 bit multi-component pixel values, or may be in a compressed or encoded format. The input  2202  may represent a network interface, a peripheral bus interface, or a storage interface. Examples of network interface include wired interfaces such as Ethernet, passive optical network (PON), etc. and wireless interfaces such as Wi-Fi or cellular interfaces. 
     The system  2200  may include a coding component  2204  that may implement the various coding or encoding methods described in the present document. The coding component  2204  may reduce the average bitrate of video from the input  2202  to the output of the coding component  2204  to produce a coded representation of the video. The coding techniques are therefore sometimes called video compression or video transcoding techniques. The output of the coding component  2204  may be either stored, or transmitted via a communication connected, as represented by the component  2206 . The stored or communicated bitstream (or coded) representation of the video received at the input  2202  may be used by the component  2208  for generating pixel values or displayable video that is sent to a display interface  2210 . The process of generating user-viewable video from the bitstream representation is sometimes called video decompression. Furthermore, while certain video processing operations are referred to as “coding” operations or tools, it will be appreciated that the coding tools or operations are used at an encoder and corresponding decoding tools or operations that reverse the results of the coding will be performed by a decoder. 
     Examples of a peripheral bus interface or a display interface may include universal serial bus (USB) or high definition multimedia interface (HDMI) or Displayport, and so on. Examples of storage interfaces include SATA (serial advanced technology attachment), PCI, IDE interface, and the like. The techniques described in the present document may be embodied in various electronic devices such as mobile phones, laptops, smartphones or other devices that are capable of performing digital data processing and/or video display. 
     In some embodiments, the video coding methods may be implemented using an apparatus that is implemented on a hardware platform as described with respect to  FIG. 21 . 
     From the foregoing, it will be appreciated that specific embodiments of the presently disclosed technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the presently disclosed technology is not limited except as by the appended claims. 
     Implementations of the subject matter and the functional operations described in this patent document can be implemented in various systems, digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Implementations of the subject matter described in this specification can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a tangible and non-transitory computer readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter effecting a machine-readable propagated signal, or a combination of one or more of them. The term “data processing unit” or “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). 
     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Computer readable media suitable for storing computer program instructions and data include all forms of nonvolatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     It is intended that the specification, together with the drawings, be considered exemplary only, where exemplary means an example. As used herein, the use of “or” is intended to include “and/or”, unless the context clearly indicates otherwise. 
     While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments. 
     Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.