WO2020070729A1 - Size restriction based on motion information - Google Patents

Abstract

Devices, systems and methods for digital video coding, which includes memory bandwidth reductions for sub-block coding tools, are described. An exemplary method for video coding includes determining, for a conversion between a coding unit of a video and a bitstream representation of the video, a sub-block size for sub-blocks of the coding unit based on a motion information associated with the coding unit and performing the conversion based on the determined sub-block size..

Description

SIZE RESTRICTION BASED ON MOTION INFORMATION

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Under the applicable patent law and/or rules pursuant to the Paris Convention, this application is made to timely claim the priority to and benefit of International Patent Application No. PCT/CN2018/109255, filed on October 6, 2018. For all purposes under the U.S. law, the entire contents of the before-mentioned patent application are incorporated by reference as part of the disclosure of this patent document.

TECHNICAL FIELD

[0002] This patent document relates to video coding techniques, devices and systems.

BACKGROUND

[0003] 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

[0004] Devices, systems and methods related to digital video coding, and specifically, to memory bandwidth reductions for sub-block coding tools in video coding 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.

[0005] In one representative aspect, the disclosed technology may be used to provide a method for video processing. This method includes determining, for a conversion between a coding unit of a video and a bitstream representation of the video, a sub-block size for sub blocks of the coding unit based on a motion information associated with the coding unit and performing the conversion based on the determined sub-block size.

[0006] In another representative aspect, the disclosed technology may be used to provide a method for video processing. This method includes determining, for a conversion between a coding unit of a video comprising sub-blocks and a bitstream representation of the video, whether to use a memory bandwidth reduction tool for the conversion based on a size of the coding unit and performing the conversion based on the determining.

[0007] In another representative aspect, the disclosed technology may be used to provide a method for video processing. This method includes determining, for a conversion in an affine mode between a coding unit of a video and a bitstream representation of the video, a sub-block size based on information associated with the coding unit. The method also includes performing the conversion in the affine mode based on the sub-block size.

[0008] In another representative aspect, the disclosed technology may be used to provide a method for video coding. This method includes selecting, for a bitstream representation of a current block, a sub-block size based on a motion information associated with the current block, and performing, based on the sub-block size, a conversion between the bitstream representation and the current block.

[0009] 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.

[0010] 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.

[0011] In yet another representative aspect, a video decoder apparatus may implement a method as described herein.

[0012] 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

[0013] FIG. 1 shows an example of constructing a merge candidate list.

[0014] FIG. 2 shows an example of positions of spatial candidates.

[0015] FIG. 3 shows an example of candidate pairs subject to a redundancy check of spatial merge candidates.

[0016] 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.

[0017] FIG. 5 shows an example of motion vector scaling for temporal merge candidates.

[0018] FIG. 6 shows an example of candidate positions for temporal merge candidates.

[0019] FIG. 7 shows an example of generating a combined bi-predictive merge candidate. [0020] FIG. 8 shows an example of constructing motion vector prediction candidates.

[0021] FIG. 9 shows an example of motion vector scaling for spatial motion vector candidates.

[0022] FIG. 10 shows an example of motion prediction using the alternative temporal motion vector prediction (ATMVP) algorithm for a coding unit (CU).

[0023] 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.

[0024] FIG. 12 shows an example flowchart for encoding with different MV precisions.

[0025] FIGS. 13A and 13B show example snapshots of sub-block when using the overlapped block motion compensation (OBMC) algorithm.

[0026] FIG. 14 shows an example of neighboring samples used to derive parameters for the local illumination compensation (LIC) algorithm.

[0027] FIG. 15 shows an example of a simplified affine motion model.

[0028] FIG. 16 shows an example of an affine motion vector field (MVF) per sub-block.

[0029] FIG. 17 shows an example of motion vector prediction (MVP) for the AF INTER affine motion mode.

[0030] FIGS. 18A and 18B show examples of the 4-parameter and 6-parameter affine models, respectively.

[0031] FIGS. 19A and 19B show example candidates for the AF MERGE affine motion mode.

[0032] FIG. 20 shows an example of bilateral matching in pattern matched motion vector derivation (PMMVD) mode, which is a special merge mode based on the frame-rate up conversion (FRUC) algorithm.

[0033] FIG. 21 shows an example of template matching in the FRUC algorithm.

[0034] FIG. 22 shows an example of unilateral motion estimation in the FRUC algorithm.

[0035] FIG. 23 shows an example of an optical flow trajectory used by the bi-directional optical flow (BIO) algorithm.

[0036] FIGS. 24A and 24B show example snapshots of using of the bi-directional optical flow (BIO) algorithm without block extensions.

[0037] FIG. 25 shows an example of the decoder-side motion vector refinement (DMVR) algorithm based on bilateral template matching. [0038] FIG. 26 shows an example of a reference rectangular area of a coding unit.

[0039] FIG. 27 shows an example of a motion vector constraint in affine mode.

[0040] FIG. 28 shows a flowchart of an example method for video coding.

[0041] FIG. 29 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.

[0042] FIG. 30 is a block diagram of an example video processing system in which disclosed techniques may be implemented.

[0043] FIG. 31 is a flowchart representation of a method for video processing in accordance with the present technology.

[0044] FIG. 32 is a flowchart representation of another method for video processing in accordance with the present technology.

[0045] FIG. 33 is a flowchart representation of yet another method for video processing in accordance with the present technology.

DETAILED DESCRIPTION

[0046] 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.

[0047] 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

[0048] 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

[0049] 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 _predjdc. In yet other embodiments, motion vectors may be explicitly coded as deltas relative to predictors.

[0050] 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.

[0051] 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.

[0052] 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

[0053] 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:

[0054] Step 1 : Initial candidates derivation

[0055] Step 1.1 : Spatial candidates derivation

[0056] Step 1.2: Redundancy check for spatial candidates [0057] Step 1.3: Temporal candidates derivation

[0058] Step 2: Additional candidates insertion

[0059] Step 2.1 : Creation of bi-predictive candidates

[0060] Step 2.2: Insertion of zero motion candidates

[0061] 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 2Nx2N prediction unit. 1.1.2 Constructing spatial merge candidates

[0062] 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 Ai_, Bi_, Bo_, Ao and B₂. Position B₂ is considered only when any PU of position Ai, Bi, B₀, A₀ is not available (e.g. because it belongs to another slice or tile) or is intra coded. After candidate at position Ai 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.

[0063] 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 2Nx2N. As an example, FIG. 4A and 4B depict the second PU for the case of Nx2N and 2NxN, respectively. When the current PU is partitioned as Nx2N, candidate at position Ai 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 Bi is not considered when the current PU is partitioned as 2NxN.

1.1.3 Constructing temporal merge candidates

[0064] 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.

[0065] 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.

[0066] In the co-located PU (Y) belonging to the reference frame, the position for the temporal candidate is selected between candidates Co and Ci, as depicted in FIG. 6. If PU at position Co is not available, is intra coded, or is outside of the current CTU, position Ci is used. Otherwise, position Co is used in the derivation of the temporal merge candidate.

1.1.4 Constructing additional types of merge candidates

[0067] 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.

[0068] FIG. 7 shows an example of this process, wherein two candidates in the original list (710, on the left), which have mvLO and refldxLO or mvLl and refldxLl, are used to create a combined bi-predictive merge candidate added to the final list (720, on the right).

[0069] 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

[0070] 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)

[0071] 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

[0072] FIG. 8 summarizes derivation process for motion vector prediction candidate, and may be implemented for each reference picture list with refidx as an input.

[0073] 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.

[0074] 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

[0075] 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 Ao, Ai,and scaled Ao, scaled Ai. The order of derivation for the above side of the current PU is defined as Bo, Bi, B₂, scaled Bo, scaled Bi, scaled B₂. 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:

[0076] — No spatial scaling

[0077] (1) Same reference picture list, and same reference picture index (same POC)

[0078] (2) Different reference picture list, but same reference picture (same POC)

[0079] — Spatial scaling

[0080] (3) Same reference picture list, but different reference picture (different POC)

[0081] (4) Different reference picture list, and different reference picture (different POC)

[0082] 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.

[0083] 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

[0084] 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)

[0085] 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

[0086] 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)

[0087] 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. [0088] 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.

[0089] 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.

[0090] 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 NxN 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 l-X) for each sub-CU.

2.1.2 Examples of spatial-temporal motion vector prediction (STMVP)

[0091] 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 8x8 CU 1100 that includes four 4x4 sub-CUs A (1101), B (1102), C (1103), and D (1104). The neighboring 4x4 blocks in the current frame are labelled as a (1111), b (1112), c (1113), and d (1114). [0092] The motion derivation for sub-CU A starts by identifying its two spatial neighbors. The first neighbor is the NxN block above sub-CU A 1101 (block c 1113). If this block c (1113) is not available or is intra coded the other NxN 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

[0093] 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

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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:

[0098] — 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.

[0099] — 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.

[00100] The encoding process is shown in FIG. 12. First, 1/4 pel MV is tested and the RD cost is calculated and denoted as RDCostO, then integer MV is tested and the RD cost is denoted as RDCostl. If RDCostl < th * RDCostO (wherein th is a positive valued threshold), then 4-pel MV is tested; otherwise, 4-pel MV is skipped. Basically, motion information and RD cost etc. are already known for 1/4 pel MV when checking integer or 4-pel MV, which can be reused to speed up the encoding process of integer or 4-pel MV.

2.3 Examples of higher motion vector storage accuracy

[00101] 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.

[00102] 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)

[00103] 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 4x4, as shown in FIGS. 13A and 13B.

[00104] FIG. 13A shows sub-blocks at the CU/PU boundary, and the hatched sub-blocks are where OBMC applies. Similarly, FIG. 13B shows the sub-Pus in ATMVP mode.

[00105] 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.

[00106] 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 {1/4, 1/8, 1/16, 1/32} are used for PN and the weighting factors {3/4, 7/8, 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 {1/4, 1/8} are used for PN and weighting factors {3/4, 7/8} 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.

[00107] 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 local illumination compensation (LIC)

[00108] LIC is based on a linear model for illumination changes, using a scaling factor a and an offset b. And it is enabled or disabled adaptively for each inter-mode coded coding unit (CU).

[00109] When LIC applies for a CU, a least square error method is employed to derive the parameters a and b by using the neighboring samples of the current CU and their corresponding reference samples. FIG. 14 shows an example of neighboring samples used to derive parameters of the IC algorithm. Specifically, and as shown in FIG. 14, the subsampled (2: 1 subsampling) neighbouring samples of the CU and the corresponding samples (identified by motion information of the current CU or sub-CU) in the reference picture are used. The IC parameters are derived and applied for each prediction direction separately.

[00110] When a CU is coded with merge mode, the LIC flag is copied from neighboring blocks, in a way similar to motion information copy in merge mode; otherwise, an LIC flag is signaled for the CU to indicate whether LIC applies or not.

[00111] When LIC is enabled for a picture, an additional CU level RD check is needed to determine whether LIC is applied or not for a CU. When LIC is enabled for a CU, the mean- removed sum of absolute difference (MR-SAD) and mean-removed sum of absolute Hadamard- transformed difference (MR-SATD) are used, instead of SAD and SATD, for integer pel motion search and fractional pel motion search, respectively.

[00112] To reduce the encoding complexity, the following encoding scheme is applied in the JEM:

[00113] — LIC is disabled for the entire picture when there is no obvious illumination change between a current picture and its reference pictures. To identify this situation, histograms of a current picture and every reference picture of the current picture are calculated at the encoder. If the histogram difference between the current picture and every reference picture of the current picture is smaller than a given threshold, LIC is disabled for the current picture; otherwise, LIC is enabled for the current picture.

2.6 Examples of affine motion compensation prediction

[00114] 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. 15 shows an example of an affine motion field of a block 1500 described by two control point motion vectors Vo and Vi. The motion vector field (MVF) of the block 1500 can be described by the following equation:

[00115]

[00116] As shown in FIG. 15, (v_0x, vo_y) is motion vector of the top-left corner control point, and (vi_x, vi_y) is motion vector of the top-right corner control point. To simplify the motion compensation prediction, sub-block based affine transform prediction can be applied. The sub block size MxN is derived as follows:

[00117]

[00118] 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.

[00119] FIG. 16 shows an example of affine MVT per sub-block for a block 1600. 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.

2.6.1 Examples of AF INTER mode

[00120] 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 {(v₀, v_x) |v₀ =

i^s constructed using the neighboring blocks.

[00121] FIG. 17 shows an example of motion vector prediction (MVP) for a block 1700 in the AF INTER mode. As shown in FIG. 17, vo 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 vi 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.

[00122] In AF INTER mode, when 4/6 parameter affine mode is used, 2/3 control points are required, and therefore 2/3 MVD needs to be coded for these control points, as shown in FIGS. 18A and 18B. In an existing implementation, the MV may be derived as follows, e.g., it predicts mvdi and mvd₂ from mvdo.

[00123]

[00124]

[00125]

[00126] Herein, mvdi and mvi are the predicted motion vector, motion vector difference

and motion vector of the top-left pixel (i = 0), top-right pixel (i = 1) or left-bottom pixel (i = 2) respectively, as shown in FIG. 18B. In some embodiments, the addition of two motion vectors (e.g., mvA(xA, yA) and mvB(xB, yB)) is equal to summation of two components separately. For example, newMV = mvA + mvB implies that the two components of newMV are set to (xA + xB) and (yA + yB), respectively.

2.6.2 Examples of fast affine ME algorithms in AF INTER mode

[00127] In some embodiments of the affine mode, MV of 2 or 3 control points needs to be determined jointly. Directly searching the multiple MVs jointly is computationally complex. In an example, a fast affine ME algorithm is proposed and is adopted into VTM/BMS.

[00128] For example, the fast affine ME algorithm is described for the 4-parameter affine model, and the idea can be extended to 6-parameter affine model:

[00129]

[00130]

[00131] Replacing (a-l) with a’ enables the motion vectors to be rewritten as:

[00132]

[00133] If it is assumed that the motion vectors of the two controls points (0, 0) and (0, w) are known, from Equation (5) the affine parameters may be derived as: [00134]

[00135] The motion vectors can be rewritten in vector form as:

[00136]

[00137] Herein, P = (x, y) is the pixel position,

[00138]

[00139]

[00140] In some embodiments, and at the encoder, the MVD of AF INTER may be derived iteratively. Denote MV^P) as the MV derived in the ith iteration for position P and denote dMVc¹ as the delta updated for MVc in the ith iteration. Then in the (i+l)th iteration,

[00141]

[00142]

[00143]

[00144] Denote Picref as the reference picture and denote Pic_cur as the current picture and denote Q = P + MV^l(P). If the MSE is used as the matching criterion, then the function that needs to be minimized may be written as:

[00145]

[00146]

[00147] If it is assumed that is small enough, may be

rewritten, as an approximation based on a l-st order Taylor expansion, as:

[00148]

[00149]

is adopted, then:

[00150]

[00151]

[00152] The term may be derived by setting the derivative of the error function to zero,

and then computing delta MV of the control points (0, 0) and (0, w) according to A(P) *

as follows:

[00153]

[00154]

[00155]

[00156]

[00157] In some embodiments, this MVD derivation process may be iterated n times, and the final MVD may be calculated as follows:

[00158]

[00159]

[00160]

[00161]

[00162] In the aforementioned implementation, predicting delta MV of control point (0, w), denoted by mvdi from delta MV of control point (0, 0), denoted by mvdo, results in only

being encoded for mvdi.

2.6.3 Examples of AF MERGE mode

[00163] 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. 19A shows an example of the selection order of candidate blocks for a current CU 1900. As shown in FIG. 19A, the selection order can be from left (1901), above (1902), above right (1903), left bottom (1904) to above left (1905) of the current CU 1900. FIG. 19B shows another example of candidate blocks for a current CU 1900 in the AF MERGE mode. If the neighboring left bottom block 1901 is coded in affine mode, as shown in FIG. 19B, the motion vectors v₂, v₃ and v₄ of the top left corner, above right corner, and left bottom corner of the CU containing the sub-block 1901 are derived. The motion vector vo of the top left corner on the current CU 1900 is calculated based on v2, v3 and v4. The motion vector vl of the above right of the current CU can be calculated accordingly.

[00164] After the CPMV of the current CU vO and vl 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.

2.7 Examples of pattern matched motion vector derivation (PMMVD)

[00165] The PMMVD mode is a special merge mode based on the Frame-Rate Up Conversion (FRUC) method. With this mode, motion information of a block is not signaled but derived at decoder side.

[00166] A FRUC flag can be signaled for a CU when its merge flag is true. When the FRUC flag is false, a merge index can be signaled and the regular merge mode is used. When the FRUC flag is true, an additional FRUC mode flag can be signaled to indicate which method (e.g., bilateral matching or template matching) is to be used to derive motion information for the block.

[00167] At the encoder side, the decision on whether using FRUC merge mode for a CU is based on RD cost selection as done for normal merge candidate. For example, multiple matching modes (e.g., bilateral matching and template matching) are checked for a CU by using RD cost selection. The one leading to the minimal cost is further compared to other CU modes. If a FRUC matching mode is the most efficient one, FRUC flag is set to true for the CU and the related matching mode is used.

[00168] Typically, motion derivation process in FRUC merge mode has two steps: a CU-level motion search is first performed, then followed by a Sub-CU level motion refinement. At CU level, an initial motion vector is derived for the whole CU based on bilateral matching or template matching. First, a list of MV candidates is generated and the candidate that leads to the minimum matching cost is selected as the starting point for further CU level refinement. Then a local search based on bilateral matching or template matching around the starting point is performed. The MV results in the minimum matching cost is taken as the MV for the whole CU. Subsequently, the motion information is further refined at sub-CU level with the derived CU motion vectors as the starting points.

[00169] For example, the following derivation process is performed for a W X H CU motion information derivation. At the first stage, MV for the whole W x H CU is derived. At the second stage, the CU is further split into M x M sub-CUs. The value of M is calculated as in Eq. (3), D is a predefined splitting depth which is set to 3 by default in the JEM. Then the MV for each sub- CU is derived.

[00171] FIG. 20 shows an example of bilateral matching used in the Frame-Rate Lip

Conversion (FRUC) method. The bilateral matching is used to derive motion information of the current CFT by finding the closest match between two blocks along the motion trajectory of the current CFT (2000) in two different reference pictures (2010, 2011). Finder the assumption of continuous motion trajectory, the motion vectors MV0 (2001) and MV1 (2002) pointing to the two reference blocks are proportional to the temporal distances, e.g., TD0 (2003) and TD1 (2004), between the current picture and the two reference pictures. In some embodiments, when the current picture 2000 is temporally between the two reference pictures (2010, 2011) and the temporal distance from the current picture to the two reference pictures is the same, the bilateral matching becomes mirror based bi-directional MV.

[00172] FIG. 21 shows an example of template matching used in the Frame-Rate FTp

Conversion (FRFTC) method. Template matching can be used to derive motion information of the current CFT 2100 by finding the closest match between a template (e.g., top and/or left neighboring blocks of the current CFT) in the current picture and a block (e.g., same size to the template) in a reference picture 2110. Except the aforementioned FRFTC merge mode, the template matching can also be applied to AMVP mode. In both JEM and HEVC, AMVP has two candidates. With the template matching method, a new candidate can be derived. If the newly derived candidate by template matching is different to the first existing AMVP candidate, it is inserted at the very beginning of the AMVP candidate list and then the list size is set to two (e.g., by removing the second existing AMVP candidate). When applied to AMVP mode, only CFT level search is applied.

[00173] The MV candidate set at CFT level can include the following: (1) original AMVP candidates if the current CFT is in AMVP mode, (2) all merge candidates, (3) several MVs in the interpolated MV field (described later), and (4) top and left neighboring motion vectors.

[00174] When using bilateral matching, each valid MV of a merge candidate can be used as an input to generate a MV pair with the assumption of bilateral matching. For example, one valid MV of a merge candidate is (MVa, ref_a) at reference list A. Then the reference picture reft of its paired bilateral MV is found in the other reference list B so that reft and reft are temporally at different sides of the current picture. If such a reft is not available in reference list B, reft is determined as a reference which is different from ref_a and its temporal distance to the current picture is the minimal one in list B. After reft is determined, MVb is derived by scaling MVa based on the temporal distance between the current picture and reft, reft.

[00175] In some implementations, four MVs from the interpolated MV field can also be added to the CU level candidate list. More specifically, the interpolated MVs at the position (0, 0),

(W/2, 0), (0, H/2) and (W/2, H/2) of the current CU are added. When FRUC is applied in AMVP mode, the original AMVP candidates are also added to CU level MV candidate set. In some implementations, at the CU level, 15 MVs for AMVP CUs and 13 MVs for merge CUs can be added to the candidate list.

[00176] The MV candidate set at sub-CU level includes:

[00177] (1) an MV determined from a CU-level search,

[00178] (2) top, left, top-left and top-right neighboring MVs,

[00179] (3) scaled versions of collocated MVs from reference pictures,

[00180] (4) one or more ATMVP candidates (e.g., up to four), and

[00181] (5) one or more STMVP candidates (e.g., up to four).

[00182] The scaled MVs from reference pictures are derived as follows. The reference pictures in both lists are traversed. The MVs at a collocated position of the sub-CU in a reference picture are scaled to the reference of the starting CU-level MV. ATMVP and STMVP candidates can be the four first ones. At the sub-CU level, one or more MVs (e.g., up to 17) are added to the candidate list.

[00183] Generation of an interpolated MV field. Before coding a frame, interpolated motion field is generated for the whole picture based on unilateral ME. Then the motion field may be used later as CU level or sub-CU level MV candidates.

[00184] In some embodiments, the motion field of each reference pictures in both reference lists is traversed at 4x4 block level. FIG. 22 shows an example of unilateral Motion Estimation (ME) 2200 in the FRUC method. For each 4x4 block, if the motion associated to the block passing through a 4x4 block in the current picture and the block has not been assigned any interpolated motion, the motion of the reference block is scaled to the current picture according to the temporal distance TD0 and TD1 (the same way as that of MV scaling of TMVP in HEVC) and the scaled motion is assigned to the block in the current frame. If no scaled MV is assigned to a 4x4 block, the block’s motion is marked as unavailable in the interpolated motion field. [00185] Interpolation and matching cost. When a motion vector points to a fractional sample position, motion compensated interpolation is needed. To reduce complexity, bi-linear interpolation instead of regular 8-tap HEVC interpolation can be used for both bilateral matching and template matching.

[00186] The calculation of matching cost is a bit different at different steps. When selecting the candidate from the candidate set at the CU level, the matching cost can be the absolute sum difference (SAD) of bilateral matching or template matching. After the starting MV is determined, the matching cost C of bilateral matching at sub-CU level search is calculated as follows:

[00187]

[00188] Here, w is a weighting factor. In some embodiments, w can be empirically set to 4.

MV and MV^S indicate the current MV and the starting MV, respectively. S D may still be used as the matching cost of template matching at sub-CU level search.

[00189] In FRUC mode, MV is derived by using luma samples only. The derived motion will be used for both luma and chroma for MC inter prediction. After MV is decided, final MC is performed using 8-taps interpolation filter for luma and 4-taps interpolation filter for chroma.

[00190] MV refinement is a pattern based MV search with the criterion of bilateral matching cost or template matching cost. In the JEM, two search patterns are supported - an unrestricted center-biased diamond search (UCBDS) and an adaptive cross search for MV refinement at the CU level and sub-CU level, respectively. For both CU and sub-CU level MV refinement, the MV is directly searched at quarter luma sample MV accuracy, and this is followed by one-eighth luma sample MV refinement. The search range of MV refinement for the CU and sub-CU step are set equal to 8 luma samples.

[00191] In the bilateral matching merge mode, bi-prediction is applied because the motion information of a CU is derived based on the closest match between two blocks along the motion trajectory of the current CU in two different reference pictures. In the template matching merge mode, the encoder can choose among uni-prediction from listO, uni-prediction from listl, or bi prediction for a CU. The selection ca be based on a template matching cost as follows:

[00192] If costBi <= factor * min (costO, costl)

[00193] bi-prediction is used;

[00194] Otherwise, if costO <= costl [00195] uni-prediction from listO is used;

[00196] Otherwise,

[00197] uni-prediction from listl is used;

[00198] Here, costO is the SAD of listO template matching, costl is the SAD of listl template matching and costBi is the SAD of bi-prediction template matching. For example, when the value of factor is equal to 1.25, it means that the selection process is biased toward bi-prediction. The inter prediction direction selection can be applied to the CU-level template matching process.

2.8 Examples of bi-directional optical flow (BIO)

[00199] The bi-directional optical flow (BIO) method is a sample-wise motion refinement performed on top of block- wise motion compensation for bi-prediction. In some

implementations, the sample-level motion refinement does not use signaling.

[00200] Let be the luma value from reference k (k= 0, 1) after block motion compensation, and denote as the horizontal and vertical components of the gradient,

respectively. Assuming the optical flow is valid, the motion vector field ( v_x , v_y) is given by:

[00201]

[00202] Combining this optical flow equation with Hermite interpolation for the motion trajectory of each sample results in a unique third-order polynomial that matches both the function values and derivatives at the ends. The value of this

polynomial at t= 0 is the BIO prediction:

[00204] FIG. 23 shows an example optical flow trajectory in the Bi-directional Optical flow (BIO) method. Here, t₀ and t_c denote the distances to the reference frames. Distances t₀ and t_c are calculated based on POC for Refo and Refi: To=POC(current) - POC(Refo), ti= POC(Refi) - POC(current). If both predictions come from the same time direction (either both from the past or both from the future) then the signs are differen

In this case, BIO is applied if the prediction is not from the same time moment Both referenced regions have

non-zero motion (e.g.

and the block motion vectors are proportional to the time distance (e.g.,

[00205] The motion vector field (v_x, v_y) is determined by minimizing the difference D between values in points A and B. FIG. 9 show an example of intersection of motion trajectory and reference frame planes. Model uses only first linear term of a local Taylor expansion for D:

[00206]

[00207] All values in the above equation depend on the sample location, denoted as

Assuming the motion is consistent in the local surrounding area, D can be minimized inside the (2M+l)x(2M+l) square window W centered on the currently predicted point (i,y), where M is equal to 2:

[00208]

[00209] For this optimization problem, the JEM uses a simplified approach making first a minimization in the vertical direction and then in the horizontal direction. This results in the following:

[00210]

[00211]

[00212] where,

[00213]

[00214] In order to avoid division by zero or a very small value, regularization parameters r and m can be introduced in Eq. (28) and Eq. (29), where:

[00215]

[00216]

[00217] Here, d is bit depth of the video samples.

[00218] In order to keep the memory access for BIO the same as for regular bi-predictive motion compensation, all prediction and gradients values, are

calculated for positions inside the current block. FIG. 24A shows an example of access positions outside of a block 2400. As shown in FIG. 24A, in Eq. (30), (2M+l)x(2M+l) square window W centered in currently predicted point on a boundary of predicted block needs to accesses positions outside of the block. In the JEM, values of 7®, dl^ / dx , d\^(k) / dy outside of the block are set to be equal to the nearest available value inside the block. For example, this can be implemented as a padding area 2401, as shown in FIG. 24B.

[00219] With BIO, it is possible that the motion field can be refined for each sample. To reduce the computational complexity, a block-based design of BIO is used in the JEM. The motion refinement can be calculated based on a 4x4 block. In the block-based BIO, the values of S_n in Eq. (30) of all samples in a 4x4 block can be aggregated, and then the aggregated values of S_n in are used to derived BIO motion vectors offset for the 4x4 block. More specifically, the following formula can used for block-based BIO derivation:

^[00220]

[00221] Here, b_k denotes the set of samples belonging to the k-th 4x4 block of the predicted block. s_n in Eq (28) and Eq (29) are replaced by (( s_n,bk ) » 4 ) to derive the associated motion vector offsets.

[00222] In some scenarios, MV regiment of BIO may be unreliable due to noise or irregular motion. Therefore, in BIO, the magnitude of MV regiment is clipped to a threshold value. The threshold value is determined based on whether the reference pictures of the current picture are all from one direction. For example, if all the reference pictures of the current picture are from one direction, the value of the threshold is set to 12 x 2^14-d; otherwise, it is set to 12 x 2^13-d.

[00223] Gradients for BIO can be calculated at the same time with motion compensation interpolation using operations consistent with HEVC motion compensation process (e.g., 2D separable Finite Impulse Response (FIR)). In some embodiments, the input for the 2D separable FIR is the same reference frame sample as for motion compensation process and fractional position (fracX,fracY) according to the fractional part of block motion vector. For horizontal gradient dl/dx, a signal is first interpolated vertically using BlOfilterS corresponding to the fractional position fracY with de-scaling shift d- 8. Gradient filter BIOfilterG is then applied in horizontal direction corresponding to the fractional position fracX with de-scaling shift by 18—d. For vertical gradient dl/dy, a gradient filter is applied vertically using BIOfilterG corresponding to the fractional position fracY with de-scaling shift d- 8 The signal displacement is then performed using BlOfilterS in horizontal direction corresponding to the fractional position fracX with de-scaling shift by 18—d. The length of interpolation filter for gradients calculation

BIOfilterG and signal displacement BIOfilterF can be shorter (e.g., 6-tap) in order to maintain reasonable complexity. Table 1 shows example filters that can be used for gradients calculation of different fractional positions of block motion vector in BIO. Table 2 shows example interpolation filters that can be used for prediction signal generation in BIO.

[00224] In the JEM, BIO can be applied to all bi-predicted blocks when the two predictions are from different reference pictures. When Local Illumination Compensation (LIC) is enabled for a CU, BIO can be disabled. [00225] In some embodiments, OBMC is applied for a block after normal MC process. To reduce the computational complexity, BIO may not be applied during the OBMC process. This means that BIO is applied in the MC process for a block when using its own MV and is not applied in the MC process when the MV of a neighboring block is used during the OBMC process.

2.9 Examples of decoder-side motion vector refinement (DMVR)

[00226] In a bi-prediction operation, for the prediction of one block region, two prediction blocks, formed using a motion vector (MV) of listO and a MV of listl, respectively, are combined to form a single prediction signal. In the decoder-side motion vector refinement (DMVR) method, the two motion vectors of the bi-prediction are further refined by a bilateral template matching process. The bilateral template matching applied in the decoder to perform a distortion-based search between a bilateral template and the reconstruction samples in the reference pictures in order to obtain a refined MV without transmission of additional motion information.

[00227] In DMVR, a bilateral template is generated as the weighted combination (i.e.

average) of the two prediction blocks, from the initial MV0 of listO and MV1 of listl, respectively, as shown in FIG. 25. The template matching operation consists of calculating cost measures between the generated template and the sample region (around the initial prediction block) in the reference picture. For each of the two reference pictures, the MV that yields the minimum template cost is considered as the updated MV of that list to replace the original one. In the JEM, nine MV candidates are searched for each list. The nine MV candidates include the original MV and 8 surrounding MVs with one luma sample offset to the original MV in either the horizontal or vertical direction, or both. Finally, the two new MVs, i.e., MV0' and MV1' as shown in FIG. 25, are used for generating the final bi-prediction results. A sum of absolute differences (SAD) is used as the cost measure.

[00228] DMVR is applied for the merge mode of bi-prediction with one MV from a reference picture in the past and another from a reference picture in the future, without the transmission of additional syntax elements. In the JEM, when LIC, affine motion, FRUC, or sub-CU merge candidate is enabled for a CU, DMVR is not applied.

3. Drawbacks of existing implementations

[00229] In sub-block based coding tools like ATMVP, STMVP and affine mode, smaller sub- block usually brings higher coding gain, albeit at the expense of higher memory bandwidth. On the other hand, if larger sub-blocks are directly used to reduce the memory bandwidth, it may result in nontrivial coding performance loss.

[00230] In some implementations of the affine mode, motion vectors are derived for each 4x4 block and motion compensation is also performed at 4x4 block level, which may increase the memory bandwidth compared with 4x8, 8x4 or 8x8 block.

[00231] In an existing implementation, restrictions to the affine mode are proposed for the worst-case bandwidth reduction. To ensure the worst-case bandwidth of the affine block is not worse than an 4x8 or 8x4 block, the motion vector differences between affine control points are used to decide whether the sub-block size of the affine block is 4x4 or 8x8.

[00232] The memory bandwidth reduction for the affine mode is controlled by restricting the motion vector difference between the affine control points (also named as the control points difference). In general, if the control points differences satisfy the restriction below, as in (34), the affine motion is using 4x4 sub-blocks (namely 4x4 affine mode). Otherwise, it is using 8x8 sub-block s (8x8 affine mode). The restrictions for the 6-parameters and 4-parameters model are given as follows.

[00233] Restriction for the 6-parameter affine model:

[00234]

[00235] In the above equation, the left-hand side represents the shrink of span level of sub affine blocks, the (7/2) factor indicates a 3.5 pixel shift, and the NormQ function is a

normalization of the motion vector difference based on (WxH). In some embodiments, the NormQ is defined as:

1002361

[00237] Restriction for the 4-parameter affine model:

[00238] In the 4-parameters affine model, are set as the follows:

[00239]

[00240]

[00241]

[00242] By applying (36) to (34), a restriction of the 4-parameters affine model is established.

[00243] In another existing implementation, each 8x8 block within the block is regarded as the basic unit. The MVs of all four 4x4 sub-blocks inside the 8x8 block are constrained such that the max difference between integer parts of the four 4x4 sub-block MVs is no more than 1 pixel.

So that the bandwidth is (8+7+l)*(8+7+l)/(8*8)=4 sample/pixel.

[00244] Both implementations are able to reduce the worst-case bandwidth to be smaller than 4x8 or 8x4, but results in a nontrivial coding performance loss.

4. Exemplary embodiments for the affine inter mode

[00245] In some embodiments, the memory bandwidth problem in affine mode may be resolved by analyzing the involved area in reference picture for affine model. As shown in FIG. 26, reference pixels of the four corners of the CU defines a smallest rectangular area that can cover all reference pixels of the CU. Suppose size of the CU and its rectangular reference area are WxH and NUN respectively, if (M + 7) * (N + 7) / (W * H), e.g., average memory bandwidth, is smaller than a given value, the pixels in the outer rectangle 2603 shown in FIG. 26 that are required to interpolate the reference area in one time may be loaded, but may interpolate different sub-block of the CU with different MVs.

[00246] In affine mode, motion vectors of the corner pixel are actually derived for the sub block including the corner pixel, details are described as follows.

[00247] For 6-parameter model as shown in Equation (38), motion vector of the four corner pixels are derived by substitute (x, y) with (SW/2, SH/2), (W - SW/2, SH/2), (SW/2, H - SH/2), (W - SW/2, H - SH/2) into Equation (38), wherein WxH and SWxSH are the CU size and the sub-block size, respectively. Motion vectors of the four corner pixels are shown in Equations (39) to (42), and 0, 1, 2 and 3 refer to the top-left, top-right, bottom-left and bottom-right corner respectively.

[00248]

[00249]

[00250]

[00251]

[00252]

[00253] With the motion vectors, positions of the reference pixels (relative to the top-left corner of current CU) can be calculated as follows:

[00254]

[00255]

[00256]

[00257]

[00258] The horizonal and vertical distance between any two reference pixels of the four corner pixels are listed in Table 3. As seen therein, horizonal/vertical distance between P0 and Pl is same with that of P2 and P3, and horizonal/vertical distance between P0 and P2 is same with that of Pl and P3. Denote the maximum horizonal and vertical distance by horMax and verMax, then the rectangular reference area is horMax* verMax. For any given affine model parameters, the rectangular reference area of the CU can be calculated, and the required memory bandwidth determined.

Table 3: Exemplary distances between different reference pixels

[00259] Similarly, for 4-parameter affine model in Equation (47), replace e and /in Equations

(39) to (46) and Table 3 with -b and a.

[00260]

5. Example methods for memory bandwidth reduction in sub-block coding tools

[00261] Embodiments of the presently disclosed technology overcome the drawbacks of existing implementations, thereby providing video coding with higher coding efficiencies. The memory bandwidth reductions in sub-block coding tools, 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.

[00262] Although the examples are described in the context of a coding unit (CU), they may be also applied to a block, a prediction unit (PDU), and so on. Furthermore, denote the size of the reference rectangular area of the CU as M0/ NO and Ml xNl for prediction direction 0 and 1 respectively and denote size of the CU as WxH.

[00263] Example 1. It is proposed that sub-block size used in ATMVP/STMVP may depend on the motion information (like prediction direction, motion vector, reference pictures, etc.) of the CU (or current block).

[00264] (a) In one example, sub-blocks with a smaller size may be used for uni-prediction and with a larger size may be used for bi-prediction. For example, if the current CU is uni- predicted, an 4x4, 4x8 or 8x4 sub-block is used. Alternatively, if the current CU is bi-predicted, an 8x8 sub-block is used.

[00265] (b) In one example, if the current CU is uni-predicted, MxN sub-block is used when CU height is larger than CU width and NxM sub-block is used in other cases. N is larger or equal than M, such as M=4 and N = 8.

[00266] (i) Alternatively, if the current CU is uni-predicted, MxN sub-block is used when CU height is larger than CU width and NxM sub-block is used in other cases. M is larger or equal than N, such as M=8 and N = 4.

[00267] (c) In one example, ATMVP/STMVP is disabled for a CU with size WxH if W

<=N and H<=M, for example, N =M=8.

[00268] (i) Alternatively, ATMVP/STMVP is disabled for a CU with size WxH if

W <=N or H<=M, for example, N =M=8.

[00269] (ii) Alternatively, ATMVP/STMVP is disabled for a CU with size WxH if

W >=N and H>=M, for example, N =M=64.

[00270] (iii) Alternatively, ATMVP/STMVP is disabled for a CU with size WxH if W >=N or H>=M, for example, N =M=64.

[00271] (d) In one example, MVs of different sub-blocks are constrained to be within a given range around one or more selected MVs.

[00272] (i) In one example, the selected MV is the MV of the center sub-block.

[00273] (ii) In one example, the selected MV is the MV of the top-left corner sub block.

[00274] Example 2 It is proposed that the sub-block size used in affine mode may depend on the affine mode parameters, prediction direction, reference pictures and shape of the CU.

[00275] (a) In one example, the number of padding pixels for each dimension of the reference rectangular area of the CU is FLEN1 and FLEN2. For bi-predicted CU, if (M0 + FFEN1) * (NO + FFEN2) + (Ml + FFEN1) * (Nl + FFEN2) is smaller than or equal to TH1 *

W * H, sub-block SWlxSHl is used; otherwise sub-block SW2xSH2 is used.

[00276] (i) In some embodiments, for uni-predicted CU, if (M0 + FFEN1) * (NO +

FFEN2) is smaller than or equal to TH2 * W * H, sub-block SWlxSHl is used; otherwise sub block SW2xSH2 is used.

[00277] (ii) In some embodiments, for bi-predicted CU, if (M0 + FFEN1) * (NO +

FFEN2) is larger than TH3 * W * H, or if (Ml + FFEN1) * (Nl + FFEN2) is larger than TH3 * W * H, sub-block SW2xSH2 is used; otherwise sub-block SWlxSHl is used.

[00278] (iii) In some embodiments, for bi-predicted CU, different sub-block size may be used for different prediction directions. If (M0 + FFEN1) * (NO + FFEN2) is larger than TH4 * W * H, sub-block SW2xSH2 is used for prediction direction 0, otherwise sub-block SWlxSHl is used. If (Ml + FLEN1) * (Nl + FLEN2) is larger than TH4 * W * H, sub-block SW2xSH2 is used for prediction direction 1, otherwise sub-block SWlxSHl is used.

[00279] (iv) In one example, the size of sub-block may depend on the position of the current block. For example, the size of sub-block may be different when the current block is at the Coding Tree Unit (CTU) boundary or not.

[00280] (v) In one example, SW1 and SH1 are equal to 4, and SW2 and SH2 are equal to 8. Alternatively, SWlxSHl is equal to 4x8 or 8x4. Alternatively, SW1 and SH1 may depend on shape of the CU. For example, SWlxSHl is set equal to 8x4 for CU whose width is larger than or equal to height, and SWlxSHl is set equal to 8x4 for other CUs.

[00281] (vi) FLEN1 and FLEN2 may be set equal to length of the interpolate filter, or alternatively, it is set equal to length of the interpolate filter plus/minus N, for example, N is equal to 1. Alternatively, FLEN1 is set equal to zero if the horizonal MV component is of integer MV precision, and FLEN2 is set equal to zero if the vertical MV component is of integer MV precision.

[00282] (vii) TH1, TH2 and TH3 are positive float numbers.

[00283] (1) In one example, TH2 is set equal to 10.3125 (165/16), and

TH1, TH3 and TH4 are set equal to 5.15625 (165/32).

[00284] (2) In one example, TH2 is set equal to 7.03125 (225/32), and

TH1, TH3 and TH4 are set equal to 3.515625 (225/64).

[00285] (b) In one example, sub-block SWlxSHl is always used, however, the MV of each sub-block may be constrained. As shown in FIG. 27, the CU is first split into SW2xSH2 (larger than SWlxSHl) sub-blocks, then the center position is used to derive motion vector (denoted as MVc) of the SW2xSH2 sub-block, finally, MV of each SWlxSHl sub-block within the SW2xSH2 block is derived, and the MV is restricted to be in a given range around the MVc. Such process is performed in each prediction direction for bi-predicted CU.

[00286] (i) In one example, SWlxSHl is equal to 4x4, and SW2xSH2 is equal to

8x8. Motion vector of the top-left, top-right, bottom-left and bottom-right 4x4 sub-blocks are denoted as MVo, MVi, MV₂, MV₃ respectively. For each prediction direction X (X = 0 or 1), abs(MViLX[0] - MVcLX[0]) is restricted to be smaller than Rhon, and abs(MVLX[l] - MVcLX[l]) is restricted to be smaller than Rven, wherein Rhon and Rven are positive float numbers and they can be different for different i (i = 0, 1, 2 or 3), and MViLX[0] and MViLX[0] are the horizonal and vertical component of MViLX respectively.

[00287] (ii) In one example, SWlxSHl is equal to 4x4, and SW2xSH2 is equal to

4x8. Motion vector of the top and bottom 4x4 sub-blocks are denoted as MVo and MVi respectively. For each prediction direction X (X = 0 or 1), abs(MViLX[0] - MVcLX[0]) is restricted to be smaller than Rhon, and abs(MViLX[l] - MVcLX[l]) is restricted to be smaller than Rven, wherein Rhon and Rven are positive float numbers and they can be different for different i (i = 0 or 1), and MViLX[0] and MViLX[l] are the horizonal and vertical component of MViLX respectively.

[00288] (iii) In one example, SWlxSHl is equal to 4x4, and SW2xSH2 is equal to

8x4. Motion vector of the left and right 4x4 sub-blocks are denoted as MVo and MVi respectively. For each prediction direction X (X = 0 or 1), abs(MViLX[0] - MVcLX[0]) is restricted to be smaller than Rhon, and abs(MViLX[l] - MVcLX[l]) is restricted to be smaller than Rven, wherein Rhon and Rven are positive float numbers and they can be different for different i (i = 0 or 1), and MViLX[0] and MViLX[l] are the horizonal and vertical component of MViLX respectively.

[00289] (iv) In one example, MVc is set equal to MV of one of the SWlxSHl sub block, for example, MVc is set equal to MV of the top-left SWlxSHl sub-block.

[00290] (v) Rhon and Rven may depend on prediction direction of the CU and size of SW2xSH2, for example, they can be larger for uni-predicted CU and smaller for bi-predicted CU, and they can be larger for larger SW2xSH2 and vice versa.

[00291] (vi) In one example, when SW2xSH2 is equal to 8x8/8x4/4x8 and

SWlxSHl is equal to 4x4, Rhon and Rven are all set equal to l/2-pel.

[00292] (vii) In one example, when SW2xSH2 is equal to 8x8/8x4/4x8 and

SWlxSHl is equal to 4x4, Rhon and Rven are all set equal to l/4-pel.

[00293] (viii) In one example, when SW2xSH2 is equal to 8x8/8x4/4x8 and

SWlxSHl is equal to 4x4, Rhon are set equal to l/2-pel and Rven are set equal to l-pel.

[00294] (ix) In one example, when SW2xSH2 is equal to 8x8/8x4/4x8 and

SWlxSHl is equal to 4x4, Rhon are set equal to l/2-pel and Rven are set equal to l/4-pel.

[00295] Example 3 When CU size is larger than W_maxxH_max, it may be split into several smaller regions and each region is processed based on the techniques described in Example 2. [00296] (a) In one example, when CU size is 128x128, it is split into 4 64x64 regions.

[00297] (b) In one example, when CU size is 128x64 or 64x128, it is split into 2 64x64 regions.

[00298] Example 4 Whether to apply or how to apply memory bandwidth reduction on sub block coded CU may depend on the CU size.

[00299] (a) In one example, if CU size is larger than WxH, the memory bandwidth reduction method is not performed.

[00300] (b) In one example, if CU size is smaller than WxH, the memory bandwidth reduction method is not performed.

[00301] The examples described above may be incorporated in the context of the method described below, e.g., method 2800, which may be implemented at a video decoder or a video encoder.

[00302] FIG. 28 shows a flowchart of an exemplary method for video coding. The method 2800 includes, at step 2810, selecting, for a bitstream representation of a current block, a sub block size based on a motion information associated with the current block.

[00303] In some embodiments, and in the context of Example 1, selecting the sub-block size comprises selecting a first size if performing the conversion comprises uni-prediction and selecting a second size if performing the conversion comprises bi-prediction, and wherein the first size is smaller than the second size. In one example, a height of the current block is greater than a width of the current block, and the sub-block size is 4x8. In another example, a width of the current block is greater than a height of the current block, and the sub-block size is 8x4.

[00304] In some embodiments, wherein the sub-block size is MxN, wherein a size of the current block is WxH, and wherein performing the conversion excludes alternative temporal motion vector prediction (ATMVP) and spatial-temporal motion vector prediction (STMVP). In one example, W < N and H < M, and N = M = 8. In another example, W < N or H < M, and N = M = 8. In yet another example, W > N and H > M, and N = M = 64. In yet another example, W > N or H > M, and N = M = 64.

[00305] In some embodiments, selecting the sub-block size is further based on one or more dimensions of the current block, and the motion information comprises at least one of a prediction direction, a motion vector, or a reference picture. In other embodiments, performing the conversion is based on a motion vector of a center sub-block of the current block. In yet other embodiments, performing the conversion is based on a motion vector of a top-left corner sub block of the current block.

[00306] The method 2800 includes, at step 2820, performing, based on the sub-block size, a conversion between the bitstream representation and the current block. In some embodiments, the conversion generates the current block from the bitstream representation (e.g., as might be implemented in a video decoder). In other embodiments, the conversion generates the bitstream representation from the current block (e.g., as might be implemented in a video encoder).

[00307] In some embodiments, and in the context of Example 2, performing the conversion is based on an affine mode, and wherein the selecting the sub-block size is further based on one or more affine model parameters.

[00308] In some embodiments, and in the context of Example 2, performing the conversion is based on an affine mode, wherein a size of the current block is WxH, wherein a size of a reference rectangular area for the current block is M0/N0 for a first prediction direction, wherein the size of the reference rectangular area is Ml xNl for a second prediction direction, and wherein a number of padding pixels for each dimension of the reference rectangular area is FLEN1 and FLEN2, respectively.

[00309] In one scenario, performing the conversion comprises bi-prediction, wherein selecting the sub-block size comprises selecting a first size if (M0 + FLEN1) x (NO + FLEN2) + (Ml + FLEN1) x (Nl + FLEN2) is less than or equal to TH1 x W ^c H and selecting a second size otherwise, and wherein THl is a positive fraction. For example, TH1 is 165/32 or 225/64.

[00310] In another scenario, performing the conversion comprises uni-prediction, wherein selecting the sub-block size comprises selecting a first size if (M0 + FFEN1) x (NO + FFEN2) is less than or equal to TH2 x W x H and selecting a second size otherwise, and wherein TH2 is a positive fraction. For example, TH2 is 165/16 or 225/32.

[00311] In yet another scenario, performing the conversion comprises bi-prediction, wherein selecting the sub-block size comprises selecting a first size if (M0 + FFEN1) x (NO + FFEN2) or (Ml + FFEN1) x (Nl + FFEN2) is greater than TH3 x W x H and selecting a second size otherwise, and wherein TH3 is a positive fraction. For example, TH3 is 165/32 or 225/64.

[00312] In yet another scenario, performing the conversion comprises bi-prediction, and different sub-block sizes are used for different prediction directions. For example, selecting the sub-block size comprises selecting a first size for a first prediction direction if (M0 + FFEN1) x (NO + FLEN2) is less than or equal to TH4 x W ^c H and selecting a second size otherwise, and where TH4 is a positive fraction. Furthermore, selecting the sub-block size further comprises selecting the first size for a second prediction direction if (Ml + FLEN1) x (Nl + FLEN2) is less than or equal to TH4 x W x H and selecting the second size otherwise. In an example, TH4 is 165/32 or 225/64.

[00313] In the exemplary scenarios described above, the first size may be 4x4 or 4x8 or 8x4, and the second size may be 8x8.

[00314] In some embodiments, performing the conversion is based on a center position of a sub-block of size SW2xSH2 and comprises bi-prediction, and wherein the sub-block of size SW2xSH2 comprises a plurality of sub-blocks of size SW1 xSHl .

[00315] In some embodiments, an absolute difference between a first motion vector for the sub-block of size SW2xSH2 and a second motion vector of one of the plurality of sub-blocks of size SW1 xSHl is less than a predetermined threshold. In one example, SW2xSH2 is 8x8 or 8x4 or 4x8, SW1 xSHl is 4x4, and the predetermined threshold is 1 /2-pel. In another example, SW2xSH2 is 8x8 or 8x4 or 4x8, SW1 xSHl is 4x4, and the predetermined threshold is l/4-pel.

[00316] In some embodiments, and in the context of Example 3, a size of the current block is 128x128, and performing the conversion comprises splitting the current block into four 64x64 blocks and performing the conversion for each of the four 64x46 blocks.

[00317] In some embodiments, a size of the current block is 128x64 or 64x 128, and performing the conversion comprises splitting the current block into two 64x64 blocks and performing the conversion for each of the two 64x64 blocks.

[00318] In some embodiments, and for the embodiments described above, the current block may be a coding unit or a prediction unit.

6. Example implementations of the disclosed technology

[00319] FIG. 29 is a block diagram of a video processing apparatus 2900. The apparatus 2900 may be used to implement one or more of the methods described herein. The apparatus 2900 may be embodied in a smartphone, tablet, computer, Internet of Things (IoT) receiver, and so on. The apparatus 2900 may include one or more processors 2902, one or more memories 2904 and video processing hardware 2906. The processor(s) 2902 may be configured to implement one or more methods (including, but not limited to, method 2800) described in the present document. The memory (memories) 2904 may be used for storing data and code used for implementing the methods and techniques described herein. The video processing hardware 2906 may be used to implement, in hardware circuitry, some techniques described in the present document.

[00320] 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. 29.

[00321] FIG. 30 is a block diagram showing an example video processing system 3000 in which various techniques disclosed herein may be implemented. Various implementations may include some or all of the components of the system 3000. The system 3000 may include input 3002 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 3002 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.

[00322] The system 3000 may include a coding component 3004 that may implement the various coding or encoding methods described in the present document. The coding component 3004 may reduce the average bitrate of video from the input 3002 to the output of the coding component 3004 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 3004 may be either stored, or transmitted via a communication connected, as represented by the component 3006. The stored or communicated bitstream (or coded) representation of the video received at the input 3002 may be used by the component 3008 for generating pixel values or displayable video that is sent to a display interface 3010. 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.

[00323] 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.

[00324] FIG. 31 is a flowchart representation of a method 3100 for video processing in accordance with the present technology. The method 3100 includes, at operation 3110,

determining, for a conversion between a coding unit of a video and a bitstream representation of the video, a sub-block size for sub-blocks of the coding unit based on a motion information associated with the coding unit. The method 3100 includes, at operation 3120, performing the conversion based on the determined sub-block size. In some embodiments, the conversion generates the coding unit from the bitstream representation. In some embodiments, the conversion generates the bitstream representation from the coding unit.

[00325] In some embodiments, selecting the sub-block size comprises selecting a first size as the sub-block size in case the conversion comprises a uni-predicted coding unit and selecting a second size in case the conversion comprises a bi-predicted coding unit. The first size can be smaller than the second size. In some embodiments, in case the conversion comprises a uni- predicted coding unit, the first size includes 4x4, 4x8, or 8x4 as the sub-block size. In some embodiments, in case the conversion comprises a bi-predicted coding unit, the second size includes 8x8 as the sub-block size. In some embodiments, in case the conversion comprises a uni-predicted coding unit, the coding unit comprises MxN subblocks upon determining that a height of the coding unit is greater than a width of the coding unit. In some embodiments, in case the conversion comprises a uni-predicted coding unit, the coding unit comprises NxM subblocks upon determining that a height of the coding unit is smaller than or equal to a width of the coding unit. In some embodiments, N is larger than or equal to M. For example, M = 4 and N = 8. In some embodiments, M is larger than or equal to N. For example, M = 8 and N = 4.

[00326] In some embodiments, the current block has a dimension of WxH, W and H being positive integers. The conversion excludes performing alternative temporal motion vector prediction (ATMVP) or spatial-temporal motion vector prediction (STMVP) to the coding unit.

In some embodiments, the conversion excludes performing ATMVP or STMVP to the coding unit in case W < N and H < M, N and M being positive integers. For example, N = M = 8. In some embodiments, the conversion excludes performing ATMVP or STMVP to the coding unit in case W < N or H < M. For example, N = M = 8. In some embodiments, the conversion excludes performing ATMVP or STMVP to the coding unit in case W > N and H > M. For example, N = M = 64. In some embodiments, the conversion excludes performing ATMVP or STMVP to the coding unit in case W > N or H > M. For example, N = M = 64.

[00327] In some embodiments, the motion information comprises at least one of a prediction direction, a motion vector, or a reference picture of the coding unit. In some embodiments, a motion vector of a sub-block of the coding unit is constrained within a range around a selected motion vector. In some embodiments, the selected motion vector is a motion vector of a center sub-block of the coding unit. In some embodiments, the selected motion vector is a motion vector of a top-left corner sub-block of the coding unit.

[00328] In some embodiments, the sub-block size is selectively adjusted based on a size of the coding unit. In some embodiments, the sub-block size is not adjusted in case the coding unit is larger than WmaxxHmax, Wmax and Hmax being positive integers. In some embodiments, the sub-block size is not adjusted in case the coding unit is smaller than Wmin/Hmin, Wmin and Hmin being positive integers.

[00329] FIG. 32 is a flowchart representation of a method 3200 for video processing in accordance with the present technology. The method 3200 includes, at operation 3210, determining, for a conversion between a coding unit of a video comprising sub-blocks and a bitstream representation of the video, whether to use a memory bandwidth reduction tool for the conversion based on a size of the coding unit. The method 3200 includes, at operation 3220, performing the conversion based on the determining. In some embodiments, the memory bandwidth reduction tool is not used in case the coding unit is larger than WmaxxHmax, Wmax and Hmax being positive integers. In some embodiments, the memory bandwidth reduction tool is not used in case the coding unit is smaller than WminxHmin, Wmin and Hmin being positive integers. In some embodiments, the conversion generates the coding unit from the bitstream representation. In some embodiments, the conversion generates the bitstream representation from the coding unit.

[00330] FIG. 33 is a flowchart representation of a method 3300 for video processing in accordance with the present technology. The method 3300 includes, at operation 3310, determining, for a conversion in an affine mode between a coding unit of a video and a bitstream representation of the video, a sub-block size based on information associated with the coding unit. The method 3300 includes, at operation 3320, performing the conversion in the affine mode based on the sub-block size. The conversion can generate the coding unit from the bitstream representation. The conversion can generate the bitstream representation from the coding unit. In some embodiments, the information comprises one or more parameters of the affine mode, a prediction direction of the coding unit, one or more reference pictures of the coding unit, or a shape of the coding unit.

[00331] In some embodiments, a size of the coding unit is WxH, wherein a size of a first reference rectangular area for the current block is M0/N0 for a first prediction direction, wherein a size of a second reference rectangular area is Ml xNl for a second prediction direction, and wherein each dimension of the first and second reference rectangular areas includes FLEN1 and FLEN2 additional pixels for motion compensation, respectively. In some embodiments, the conversion comprises bi-prediction, and wherein the sub-block size is a first size in case (M0 + FLEN1) x (NO + FLEN2) + (Ml + FLEN1) x (Nl + FLEN2) is less than or equal to TH1 x W x H and the sub-block size is a second size otherwise, TH1 being a positive fraction. TH1 can be 165/32 or 225/64. In some embodiments, the conversion comprises uni-prediction, and wherein the sub-block size is a first size in case (M0 + FLEN1) x (NO + FLEN2) is less than or equal to TH2 x W x H and the sub-block size is a second size otherwise, TH2 being a positive fraction. TH2 can be 165/16 or 225/32. In some embodiments, the conversion comprises bi-prediction, and wherein the sub-block size is a first size in case (M0 + FLEN1) x (NO + FLEN2) or (Ml + FLEN1) x (Nl + FLEN2) is greater than TH3 x W x H and the sub-block size is a second size otherwise, TH3 being a positive fraction. TH3 can be 165/32 or 225/64.

[00332] In some embodiments, the conversion comprises bi-prediction, and wherein different sub-block sizes are used for different prediction directions. In some embodiments, the sub-block size is a first size for a first prediction direction in case (M0 + FLEN1) x (NO + FLEN2) is less than or equal to TH4 x W x H and the sub-block size is a second size otherwise, TH4 being a positive fraction. In some embodiments, the sub-block size is the first size for a second prediction direction in case (Ml + FLEN1) x (Nl + FLEN2) is less than or equal to TH4 x W x H and the sub-block size is the second size otherwise. TH4 can be 165/32 or 225/64.

[00333] In some embodiments, the first size is 4x4, 4x8, or 8x4, and wherein the second size is 8x8. In some embodiments, the first size is determined based on a shape of the coding unit. In some embodiments, FLEN1 and FLEN2 are determined based on a length of an interpolate filter. In some embodiments, the length of the interpolate filter is L, and wherein FLEN1 = L ± N and FLEN2 = L ± N, N being a positive integer. For example, N = 1. In some embodiments, FLEN1 and FLEN2 are determined according to a precision of a motion vector of the coding unit. In some embodiments, FLEN1 = 0 in case a horizontal component of the motion vector is of an integer precision. In some embodiments, FLEN2 = 0 in case a vertical component of the motion vector is of an integer precision.

[00334] In some embodiments, the sub-block size is determined further based on a position of the coding unit. In some embodiments, the sub-block size is determined based on whether the coding unit positioned at a boundary of a coding tree unit. In some embodiments, the conversion comprises bi-prediction and the sub-block size is fixed to SW1 xSHl. The coding unit is partitioned into at least one area having a size of SW2/SH2 that comprises a plurality of sub blocks each having the sub-block size of SW1 xSHl, a motion vector of the area MVc is derived according to a center position of the area, and a motion vector of each of the plurality of sub blocks is confined within a range around the motion vector of the area MVc. In some

embodiments, for one or more prediction directions of each of the plurality of sub-blocks, an absolute difference between the motion vector of each of the plurality of sub-blocks and the motion vector of the area MVc is less than one or more thresholds associated with prediction directions of the motion vector. In some embodiments, the one or more thresholds are different for different prediction directions of each of the plurality of sub-blocks. In some embodiments, the one or more thresholds are determined based on a prediction direction of the coding unit or a size of the area. In some embodiments, the motion vector of the area MVc is same as a first motion vector of one of the plurality of sub-blocks. In some embodiments, the first motion vector corresponds to a top-left sub-block of the plurality of sub-blocks.

[00335] In some embodiments, SW2xSH2 is 8x8, 8x4, or 4x8, SW1 xSHl is 4x4, and the one or more predetermined thresholds are all equal to l/2-pel. In some embodiments, SW2xSH2 is 8x8, 8x4, or 4x8, SW1 xSHl is 4x4, and the one or more predetermined thresholds are all equal to l/4-pel. In some embodiments, SW2xSH2 is 8x8, 8x4, or 4x8, SWl xSHl is 4x4, a first threshold of the one or more predetermined thresholds is equal to l/2-pel, and a second threshold of the one or more predetermined thresholds is equal to l-pel. In some embodiments, SW2xSH2 is 8x8, 8x4, or 4x8, SW1 xSHl is 4x4, a first threshold of the one or more predetermined thresholds is equal to 1 /2-pel, and a second threshold of the one or more predetermined thresholds is equal to 1 /4-pel.

[00336] In some embodiments, the coding unit is partitioned into at least one area in case a size of the coding unit is larger than Wmax xHmax. In some embodiments, the size of the coding unit is 128x128, and the coding unit is partitioned into four areas, each having a size of

SW2xSH2 = 64x64. In some embodiments, the size of the coding is 128x64 or 64x128, and the coding unit is partitioned into two areas, each having a size of SW2xSH2 = 64x64. In some embodiments, the sub-block size is fixed in case the coding unit is larger than WmaxxHmax or smaller than WminxHmin, Wmax, Hmax, Wmin, and Hmin being positive integers. In some embodiments, the motion vector of each of the plurality of sub-blocks is not confined within the range around the motion vector of the area MVc in case the coding unit is larger than

WmaxxHmax or smaller than WminxHmin, Wmax, Hmax, Wmin, and Hmin being positive integers.

[00337] 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.

[00338] 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.

[00339] 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.

[00340] 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).

[00341] 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.

[00342] 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. [00343] 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.

[00344] 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.

[00345] 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.

Claims

CLAIMS What is claimed is:

1. A method for video processing, comprising:

determining, for a conversion between a coding unit of a video and a bitstream representation of the video, a sub-block size for sub-blocks of the coding unit based on a motion information associated with the coding unit; and

performing the conversion based on the determined sub-block size.

2. The method of claim 1, wherein the conversion generates the coding unit from the bitstream representation.

3. The method of claim 1, wherein the conversion generates the bitstream representation from the coding unit.

4. The method of any one or more of claims 1 to 3, wherein selecting the sub-block size comprises:

selecting a first size as the sub-block size in case the conversion comprises a uni- predicted coding unit; and

selecting a second size in case the conversion comprises a bi-predicted coding unit, wherein the first size is smaller than the second size.

5. The method of claim 4, wherein, in case the conversion comprises a uni-predicted coding unit, the first size includes 4x4, 4x8, or 8x4 as the sub-block size.

6. The method of claim 4, wherein, in case the conversion comprises bi-predicted coding unit, the second size includes 8x8 as the sub-block size.

7. The method of any one or more of claims 1 to 6, wherein, in case the conversion comprises a uni-predicted coding unit, the coding unit comprises M*N subblocks upon determining that a height of the coding unit is greater than a width of the coding unit.

8. The method of any of one or more claims 1 to 6, wherein, in case the conversion comprises a uni-predicted coding unit, the coding unit comprises NxM subblocks upon determining that a height of the coding unit is smaller than or equal to a width of the coding unit.

9. The method of claim 7 or 8, wherein N is larger than or equal to M.

10. The method of claim 9, wherein M = 4 and N = 8.

11. The method of claim 7 or 8, wherein M is larger than or equal to N.

12. The method of claim 11, wherein M = 8 and N = 4.

13. The method of any one or more of claims 1 to 12, wherein the current block has a dimension of WxH, W and H being positive integers, and wherein the conversion excludes performing alternative temporal motion vector prediction (ATMVP) or spatial-temporal motion vector prediction (STMVP) to the coding unit.

14. The method of claim 13, wherein the conversion excludes performing ATMVP or STMVP to the coding unit in case W < N and H < M, N and M being positive integers.

15. The method of claim 14, wherein N = M = 8.

16. The method of claim 13, wherein the conversion excludes performing ATMVP or STMVP to the coding unit in case W < N or H < M.

17 The method of claim 16, wherein N = M = 8.

18. The method of claim 13, wherein the conversion excludes performing ATMVP or STMVP to the coding unit in case W > N and H > M.

19. The method of claim 18, wherein N = M = 64.

20. The method of claim 13, wherein the conversion excludes performing ATMVP or STMVP to the coding unit in case W > N or H > M.

21. The method of claim 20, wherein N = M = 64.

22. The method of any one or more of claims 1 to 21, wherein the motion information comprises at least one of a prediction direction, a motion vector, or a reference picture of the coding unit.

23. The method of any one or more of claims 1 to 22, wherein a motion vector of a sub-block of the coding unit is constrained within a range around a selected motion vector.

24. The method of claim 23, wherein the selected motion vector is a motion vector of a center sub-block of the coding unit.

25. The method of claim 23, wherein the selected motion vector is a motion vector of a top- left corner sub-block of the coding unit.

26. The method of any one or more of claims 1 to 25, wherein the sub-block size is selectively adjusted based on a size of the coding unit.

27. The method of claim 26, wherein the sub-block size is not adjusted in case the coding unit is larger than W_maxxH_max, Wmax and Hmax being positive integers.

28. The method of claim 26, wherein the sub-block size is not adjusted in case the coding unit is smaller than W_minxH_min, Wmin and H_min being positive integers.

29. A method of video processing, comprising:

determining, for a conversion between a coding unit of a video comprising sub-blocks and a bitstream representation of the video, whether to use a memory bandwidth reduction tool for the conversion based on a size of the coding unit; and

performing the conversion based on the determining.

30. The method of claim 29, wherein the memory bandwidth reduction tool is not used in case the coding unit is larger than WmaxxHmax, Wmax and Hmax being positive integers.

31. The method of claim 29, wherein the memory bandwidth reduction tool is not used in case the coding unit is smaller than WminxH_min, Wmin and H_min being positive integers.

32. The method of any one or more of claims 29 to 31, wherein the conversion generates the coding unit from the bitstream representation.

33. The method of any one or more of claims 29 to 31, wherein the conversion generates the bitstream representation from the coding unit.

34. An apparatus in a video system comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to implement the method recited in one or more of claims 1 to 33.

35. A computer program product stored on a non-transitory computer readable media, the computer program product including program code for a processor to carry out the method recited in one or more of claims 1 to 33.