WO2025189031A1

WO2025189031A1 - Method, apparatus, and medium for video processing

Info

Publication number: WO2025189031A1
Application number: PCT/US2025/018784
Authority: WO
Inventors: Jizheng Xu; Li Zhang
Original assignee: ByteDance Inc
Current assignee: ByteDance Inc
Priority date: 2024-03-07
Filing date: 2025-03-06
Publication date: 2025-09-12
Anticipated expiration: 2026-09-07
Also published as: WO2025189031A8

Abstract

Embodiments of the present disclosure provide a solution for video processing. A method for video processing is proposed. The method comprises: generating, for a conversion between a current block of a video and a bitstream of the video, a prediction block of the current block or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and performing the conversion based on the prediction block or the at least one picture.

Description

METHOD, APPARATUS, AND MEDIUM FOR VIDEO PROCESSING FIELDS [0001] Embodiments of the present disclosure relates generally to video processing techniques, and more particularly, to video coding. BACKGROUND [0002] In nowadays, digital video capabilities are being applied in various aspects of peoples’ lives. Multiple types of video compression technologies, such as motion picture expert group (MPEG)-2, MPEG-4, international telecommunication union - telecommunication standardization sector (ITU-T) H.263, ITU-T H.264/MPEG-4 Part 10 advanced video coding (AVC), ITU-T H.265 high efficiency video coding (HEVC) standard, versatile video coding (VVC) standard, have been proposed for video encoding/decoding. However, coding efficiency and coding quality of video coding techniques is generally expected to be further improved. SUMMARY [0003] Embodiments of the present disclosure provide a solution for video processing. [0004] In a first aspect, a method for video processing is proposed. The method comprises: generating, for a conversion between a current block of a video and a bitstream of the video, a prediction block of the current block or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and performing the conversion based on the prediction block or the at least one picture. [0005] Based on the method in accordance with the first aspect of the present disclosure, the auxiliary information (such as motion flow information, depth information, or the like) is used for coding the video. Compared with the conventional solution where such auxiliary information is not considered, the proposed solution can advantageously reduce coding complexity and improve the quality of the reference picture and/or the prediction block for the current block. Thereby, the coding efficiency and the coding quality can be improved. [0006] In a second aspect, an apparatus for video processing is proposed. The apparatus comprises a processor and a non-transitory memory with instructions thereon. The instructions upon execution by the processor, cause the processor to perform a method in accordance with the first aspect of the present disclosure. [0007] In a third aspect, a non-transitory computer-readable storage medium is proposed. The non-transitory computer-readable storage medium stores instructions that cause a processor to perform a method in accordance with the first aspect of the present disclosure. [0008] In a fourth aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and generating the bitstream based on the prediction block or the at least one picture. [0009] In a fifth aspect, a method for storing a bitstream of a video is proposed. The method comprises: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; generating the bitstream based on the prediction block or the at least one picture; and storing the bitstream in a non-transitory computer- readable recording medium. [0010] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. BRIEF DESCRIPTION OF THE DRAWINGS [0011] Through the following detailed description with reference to the accompanying drawings, the above and other objectives, features, and advantages of example embodiments of the present disclosure will become more apparent. In the example embodiments of the present disclosure, the same reference numerals usually refer to the same components. [0012] Fig. 1 illustrates a block diagram of an example video coding system in accordance with some embodiments of the present disclosure; [0013] Fig. 2 illustrates a block diagram of an example video encoder in accordance with some embodiments of the present disclosure; [0014] Fig. 3 illustrates a block diagram of an example video decoder in accordance with some embodiments of the present disclosure; [0015] Fig. 4 illustrates positions of spatial merge candidate; [0016] Fig. 5 illustrates candidate pairs considered for redundancy check of spatial merge candidates; [0017] Fig. 6 illustrates motion vector scaling for temporal merge candidate; [0018] Fig. 7 illustrates candidate positions for temporal merge candidate, C0 and C1; [0019] Fig. 8 illustrates MMVD Search Point; [0020] Fig. 9 illustrates symmetrical MVD mode; [0021] Fig.10 illustrates control point based affine motion model; [0022] Fig.11 illustrates affine MVF per subblock; [0023] Fig. 12 illustrates locations of inherited affine motion predictors; [0024] Fig.13 illustrates control point motion vector inheritance; [0025] Fig.14 illustrates locations of candidates position for constructed affine merge mode; [0026] Fig. 15 illustrates motion vector usage for proposed combined method; _{[0027] Fig. 16 illustrates subblock MV VSB and pixel Δv(i, j) (gray arrow);} [0028] Fig. 17A illustrates spatial neighboring blocks used by ATVMP; [0029] Fig. 17B illustrates deriving sub-CU motion field by applying a motion shift from spatial neighbor and scaling the motion information from the corresponding collocated sub- CUs; [0030] Fig. 18 illustrates extended CU region used in BDOF; [0031] Fig. 19 illustrates decoding side motion vector refinement; [0032] Fig.20 illustrates examples of the GPM splits grouped by identical angles; [0033] Fig.21 illustrates a uni-prediction MV selection for geometric partitioning mode; [0034] Fig. 22 illustrates exemplified generation of a bending weight ^^_^ using geometric partitioning mode; [0035] Fig.23 illustrates top and left neighboring blocks used in CIIP weight derivation; [0036] Fig. 24 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure; and [0037] Fig. 25 illustrates a block diagram of a computing device in which various embodiments of the present disclosure can be implemented. [0038] Throughout the drawings, the same or similar reference numerals usually refer to the same or similar elements. DETAILED DESCRIPTION [0039] Principle of the present disclosure will now be described with reference to some embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. The disclosure described herein can be implemented in various manners other than the ones described below. [0040] In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs. [0041] References in the present disclosure to “one embodiment,” “an embodiment,” “an example embodiment,” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an example embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. [0042] It shall be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the listed terms. [0043] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and/or “including”, when used herein, specify the presence of stated features, elements, and/or components etc., but do not preclude the presence or addition of one or more other features, elements, components and/ or combinations thereof. Example Environment [0044] Fig. 1 is a block diagram that illustrates an example video coding system 100 that may utilize the techniques of this disclosure. As shown, the video coding system 100 may include a source device 110 and a destination device 120. The source device 110 can be also referred to as a video encoding device, and the destination device 120 can be also referred to as a video decoding device. In operation, the source device 110 can be configured to generate encoded video data and the destination device 120 can be configured to decode the encoded video data generated by the source device 110. The source device 110 may include a video source 112, a video encoder 114, and an input/output (I/O) interface 116. [0045] The video source 112 may include a source such as a video capture device. Examples of the video capture device include, but are not limited to, an interface to receive video data from a video content provider, a computer graphics system for generating video data, and/or a combination thereof. [0046] The video data may comprise one or more pictures. The video encoder 114 encodes the video data from the video source 112 to generate a bitstream. The bitstream may include a sequence of bits that form a coded representation of the video data. The bitstream may include coded pictures and associated data. The coded picture is a coded representation of a picture. The associated data may include sequence parameter sets, picture parameter sets, and other syntax structures. The I/O interface 116 may include a modulator/demodulator and/or a transmitter. The encoded video data may be transmitted directly to destination device 120 via the I/O interface 116 through the network 130A. The encoded video data may also be stored onto a storage medium/server 130B for access by destination device 120. [0047] The destination device 120 may include an I/O interface 126, a video decoder 124, and a display device 122. The I/O interface 126 may include a receiver and/or a modem. The I/O interface 126 may acquire encoded video data from the source device 110 or the storage medium/server 130B. The video decoder 124 may decode the encoded video data. The display device 122 may display the decoded video data to a user. The display device 122 may be integrated with the destination device 120, or may be external to the destination device 120 which is configured to interface with an external display device. [0048] The video encoder 114 and the video decoder 124 may operate according to a video compression standard, such as the High Efficiency Video Coding (HEVC) standard, Versatile Video Coding (VVC) standard and other current and/or further standards. [0049] Fig. 2 is a block diagram illustrating an example of a video encoder 200, which may be an example of the video encoder 114 in the system 100 illustrated in Fig.1, in accordance with some embodiments of the present disclosure. [0050] The video encoder 200 may be configured to implement any or all of the techniques of this disclosure. In the example of Fig. 2, the video encoder 200 includes a plurality of functional components. The techniques described in this disclosure may be shared among the various components of the video encoder 200. In some examples, a processor may be configured to perform any or all of the techniques described in this disclosure. [0051] In some embodiments, the video encoder 200 may include a partition unit 201, a prediction unit 202 which may include a mode select unit 203, a motion estimation unit 204, a motion compensation unit 205 and an intra-prediction unit 206, a residual generation unit 207, a transform unit 208, a quantization unit 209, an inverse quantization unit 210, an inverse transform unit 211, a reconstruction unit 212, a buffer 213, and an entropy encoding unit 214. [0052] In other examples, the video encoder 200 may include more, fewer, or different functional components. In an example, the prediction unit 202 may include an intra block copy (IBC) unit. The IBC unit may perform prediction in an IBC mode in which at least one reference picture is a picture where the current video block is located. [0053] Furthermore, although some components, such as the motion estimation unit 204 and the motion compensation unit 205, may be integrated, but are represented in the example of Fig.2 separately for purposes of explanation. [0054] The partition unit 201 may partition a picture into one or more video blocks. The video encoder 200 and the video decoder 300 may support various video block sizes. [0055] The mode select unit 203 may select one of the coding modes, intra or inter, e.g., based on error results, and provide the resulting intra-coded or inter-coded block to a residual generation unit 207 to generate residual block data and to a reconstruction unit 212 to reconstruct the encoded block for use as a reference picture. In some examples, the mode select unit 203 may select a combined inter and intra prediction (CIIP) mode in which the prediction is based on an inter prediction signal and an intra prediction signal. The mode select unit 203 may also select a resolution for a motion vector (e.g., a sub-pixel or integer pixel precision) for the block in the case of inter-prediction. [0056] To perform inter prediction on a current video block, the motion estimation unit 204 may generate motion information for the current video block by comparing one or more reference frames from buffer 213 to the current video block. The motion compensation unit 205 may determine a predicted video block for the current video block based on the motion information and decoded samples of pictures from the buffer 213 other than the picture associated with the current video block. [0057] The motion estimation unit 204 and the motion compensation unit 205 may perform different operations for a current video block, for example, depending on whether the current video block is in an I-slice, a P-slice, or a B-slice. As used herein, an “I-slice” may refer to a portion of a picture composed of macroblocks, all of which are based upon macroblocks within the same picture. Further, as used herein, in some aspects, “P-slices” and “B-slices” may refer to portions of a picture composed of macroblocks that are not dependent on macroblocks in the same picture. [0058] In some examples, the motion estimation unit 204 may perform uni-directional prediction for the current video block, and the motion estimation unit 204 may search reference pictures of list 0 or list 1 for a reference video block for the current video block. The motion estimation unit 204 may then generate a reference index that indicates the reference picture in list 0 or list 1 that contains the reference video block and a motion vector that indicates a spatial displacement between the current video block and the reference video block. The motion estimation unit 204 may output the reference index, a prediction direction indicator, and the motion vector as the motion information of the current video block. The motion compensation unit 205 may generate the predicted video block of the current video block based on the reference video block indicated by the motion information of the current video block. [0059] Alternatively, in other examples, the motion estimation unit 204 may perform bi- directional prediction for the current video block. The motion estimation unit 204 may search the reference pictures in list 0 for a reference video block for the current video block and may also search the reference pictures in list 1 for another reference video block for the current video block. The motion estimation unit 204 may then generate reference indexes that indicate the reference pictures in list 0 and list 1 containing the reference video blocks and motion vectors that indicate spatial displacements between the reference video blocks and the current video block. The motion estimation unit 204 may output the reference indexes and the motion vectors of the current video block as the motion information of the current video block. The motion compensation unit 205 may generate the predicted video block of the current video block based on the reference video blocks indicated by the motion information of the current video block. [0060] In some examples, the motion estimation unit 204 may output a full set of motion information for decoding processing of a decoder. Alternatively, in some embodiments, the motion estimation unit 204 may signal the motion information of the current video block with reference to the motion information of another video block. For example, the motion estimation unit 204 may determine that the motion information of the current video block is sufficiently similar to the motion information of a neighboring video block. [0061] In one example, the motion estimation unit 204 may indicate, in a syntax structure associated with the current video block, a value that indicates to the video decoder 300 that the current video block has the same motion information as the another video block. [0062] In another example, the motion estimation unit 204 may identify, in a syntax structure associated with the current video block, another video block and a motion vector difference (MVD). The motion vector difference indicates a difference between the motion vector of the current video block and the motion vector of the indicated video block. The video decoder 300 may use the motion vector of the indicated video block and the motion vector difference to determine the motion vector of the current video block. [0063] As discussed above, video encoder 200 may predictively signal the motion vector. Two examples of predictive signaling techniques that may be implemented by video encoder 200 include advanced motion vector prediction (AMVP) and merge mode signaling. [0064] The intra prediction unit 206 may perform intra prediction on the current video block. When the intra prediction unit 206 performs intra prediction on the current video block, the intra prediction unit 206 may generate prediction data for the current video block based on decoded samples of other video blocks in the same picture. The prediction data for the current video block may include a predicted video block and various syntax elements. [0065] The residual generation unit 207 may generate residual data for the current video block by subtracting (e.g., indicated by the minus sign) the predicted video block (s) of the current video block from the current video block. The residual data of the current video block may include residual video blocks that correspond to different sample components of the samples in the current video block. [0066] In other examples, there may be no residual data for the current video block, for example in a skip mode, and the residual generation unit 207 may not perform the subtracting operation. [0067] The transform unit 208 may generate one or more transform coefficient video blocks for the current video block by applying one or more transforms to a residual video block associated with the current video block. [0068] After the transform unit 208 generates a transform coefficient video block associated with the current video block, the quantization unit 209 may quantize the transform coefficient video block associated with the current video block based on one or more quantization parameter (QP) values associated with the current video block. [0069] The inverse quantization unit 210 and the inverse transform unit 211 may apply inverse quantization and inverse transforms to the transform coefficient video block, respectively, to reconstruct a residual video block from the transform coefficient video block. The reconstruction unit 212 may add the reconstructed residual video block to corresponding samples from one or more predicted video blocks generated by the prediction unit 202 to produce a reconstructed video block associated with the current video block for storage in the buffer 213. [0070] After the reconstruction unit 212 reconstructs the video block, loop filtering operation may be performed to reduce video blocking artifacts in the video block. [0071] The entropy encoding unit 214 may receive data from other functional components of the video encoder 200. When the entropy encoding unit 214 receives the data, the entropy encoding unit 214 may perform one or more entropy encoding operations to generate entropy encoded data and output a bitstream that includes the entropy encoded data. [0072] Fig. 3 is a block diagram illustrating an example of a video decoder 300, which may be an example of the video decoder 124 in the system 100 illustrated in Fig.1, in accordance with some embodiments of the present disclosure. [0073] The video decoder 300 may be configured to perform any or all of the techniques of this disclosure. In the example of Fig. 3, the video decoder 300 includes a plurality of functional components. The techniques described in this disclosure may be shared among the various components of the video decoder 300. In some examples, a processor may be configured to perform any or all of the techniques described in this disclosure. [0074] In the example of Fig. 3, the video decoder 300 includes an entropy decoding unit 301, a motion compensation unit 302, an intra prediction unit 303, an inverse quantization unit 304, an inverse transform unit 305, a reconstruction unit 306 and a buffer 307. The video decoder 300 may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder 200. [0075] The entropy decoding unit 301 may retrieve an encoded bitstream. The encoded bitstream may include entropy coded video data (e.g., encoded blocks of video data). The entropy decoding unit 301 may decode the entropy coded video data, and from the entropy decoded video data, the motion compensation unit 302 may determine motion information including motion vectors, motion vector precision, reference picture list indexes, and other motion information. The motion compensation unit 302 may, for example, determine such information by performing the AMVP and merge mode. AMVP is used, including derivation of several most probable candidates based on data from adjacent PBs and the reference picture. Motion information typically includes the horizontal and vertical motion vector displacement values, one or two reference picture indices, and, in the case of prediction regions in B slices, an identification of which reference picture list is associated with each index. As used herein, in some aspects, a “merge mode” may refer to deriving the motion information from spatially or temporally neighboring blocks. [0076] The motion compensation unit 302 may produce motion compensated blocks, possibly performing interpolation based on interpolation filters. Identifiers for interpolation filters to be used with sub-pixel precision may be included in the syntax elements. [0077] The motion compensation unit 302 may use the interpolation filters as used by the video encoder 200 during encoding of the video block to calculate interpolated values for sub-integer pixels of a reference block. The motion compensation unit 302 may determine the interpolation filters used by the video encoder 200 according to the received syntax information and use the interpolation filters to produce predictive blocks. [0078] The motion compensation unit 302 may use at least part of the syntax information to determine sizes of blocks used to encode frame(s) and/or slice(s) of the encoded video sequence, partition information that describes how each macroblock of a picture of the encoded video sequence is partitioned, modes indicating how each partition is encoded, one or more reference frames (and reference frame lists) for each inter-encoded block, and other information to decode the encoded video sequence. As used herein, in some aspects, a “slice” may refer to a data structure that can be decoded independently from other slices of the same picture, in terms of entropy coding, signal prediction, and residual signal reconstruction. A slice can either be an entire picture or a region of a picture. [0079] The intra prediction unit 303 may use intra prediction modes for example received in the bitstream to form a prediction block from spatially adjacent blocks. The inverse quantization unit 304 inverse quantizes, i.e., de-quantizes, the quantized video block coefficients provided in the bitstream and decoded by entropy decoding unit 301. The inverse transform unit 305 applies an inverse transform. [0080] The reconstruction unit 306 may obtain the decoded blocks, e.g., by summing the residual blocks with the corresponding prediction blocks generated by the motion compensation unit 302 or intra-prediction unit 303. If desired, a deblocking filter may also be applied to filter the decoded blocks in order to remove blockiness artifacts. The decoded video blocks are then stored in the buffer 307, which provides reference blocks for subsequent motion compensation/intra prediction and also produces decoded video for presentation on a display device. [0081] Some exemplary embodiments of the present disclosure will be described in detailed hereinafter. It should be understood that section headings are used in the present document to facilitate ease of understanding and do not limit the embodiments disclosed in a section to only that section. Furthermore, while certain embodiments are described with reference to Versatile Video Coding or other specific video codecs, the disclosed techniques are applicable to other video coding technologies also. Furthermore, while some embodiments describe video coding steps in detail, it will be understood that corresponding steps decoding that undo the coding will be implemented by a decoder. Furthermore, the term video processing encompasses video coding or compression, video decoding or decompression and video transcoding in which video pixels are represented from one compressed format into another compressed format or at a different compressed bitrate. 1. Brief Summary This disclosure is related to image/video coding technologies. Specifically, it is related to the inter picture coding with auxiliary information, including optical flow, depth, etc. The ideas may be applied individually or in various combinations, to any codec, e.g., the versatile video coding (VVC) standard. 2. Introduction 2.1 Video coding standards Video coding standards have evolved primarily through the development of the well-known ITU- T and ISO/IEC standards. The ITU-T produced H.261 and H.263, ISO/IEC produced MPEG-1 and MPEG-4 Visual, and the two organizations jointly produced the H.262/MPEG-2 Video and H.264/MPEG-4 Advanced Video Coding (AVC) and H.265/HEVC standards. Since H.262, the video coding standards are based on the hybrid video coding structure wherein temporal prediction plus transform coding are utilized. To explore the future video coding technologies beyond HEVC, the Joint Video Exploration Team (JVET) was founded by VCEG and MPEG jointly in 2015. Since then, many new methods have been adopted by JVET and put into the reference software named Joint Exploration Model (JEM). The JVET was later renamed to be the Joint Video Experts Team (JVET) when the Versatile Video Coding (VVC) project officially started. VVC is the new coding standard, targeting at 50% bitrate reduction as compared to HEVC, that has been finalized by the JVET at its 19th meeting ended on July 1, 2020. The Versatile Video Coding (VVC) standard (ITU-T H.266 | ISO/IEC 23090-3) and the associated Versatile Supplemental Enhancement Information for coded video bitstreams (VSEI) standard (ITU-T H.274 | ISO/IEC 23002-7) have been designed for use in a maximally broad range of applications, including both the traditional uses such as television broadcast, video conferencing, or playback from storage media, and also newer and more advanced use cases such as adaptive bit rate streaming, video region extraction, composition and merging of content from multiple coded video bitstreams, multiview video, scalable layered coding, and viewport-adaptive 360° immersive media. The Essential Video Coding (EVC) standard (ISO/IEC 23094-1) is another video coding standard that has recently been developed by MPEG. 2.2 Inter prediction For each inter-predicted CU, motion parameters consisting of motion vectors, reference picture indices and reference picture list usage index, and additional information needed for the new coding feature of VVC to be used for inter-predicted sample generation. The motion parameter can be signalled in an explicit or implicit manner. When a CU is coded with skip mode, the CU is associated with one PU and has 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 CU are obtained from neighbouring CUs, including spatial and temporal candidates, and additional schedules introduced in VVC. The merge mode can be applied to any inter-predicted CU, 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 flag and other needed information are signalled explicitly per each CU. Beyond the inter coding features in HEVC, VVC includes a number of new and refined inter prediction coding tools listed as follows: – Extended merge prediction, – High precision (1/16 pel) motion compensation and motion vector storage, – Merge mode with MVD (MMVD), – Symmetric MVD (SMVD) signalling, – Affine motion compensated prediction, – Subblock-based temporal motion vector prediction (SbTMVP), – Adaptive motion vector resolution (AMVR), – Bi-prediction with CU-level weight (BCW), – Bi-directional optical flow (BDOF), – Decoder side motion vector refinement (DMVR), – Geometric partitioning mode (GPM), – Combined inter and intra prediction (CIIP), – Reference picture resampling. The following text provides the details on those inter prediction methods specified in VVC. 2.2.1 Extended merge prediction In VVC, the merge candidate list is constructed by including the following five types of candidates in order: 1) Spatial MVP from spatial neighbour CUs, 2) Temporal MVP from collocated CUs, 3) History-based MVP from an FIFO table, 4) Pairwise average MVP, 5) Zero MVs. The size of merge list is signalled in sequence parameter set header and the maximum allowed size of merge list is 6. For each CU code in merge mode, an index of best merge candidate is encoded using truncated unary binarization (TU). The first bin of the merge index is coded with context and bypass coding is used for other bins. The derivation process of each category of merge candidates is provided in this session. As done in HEVC, VVC also supports parallel derivation of the merging candidate lists for all CUs within a certain size of area. 2.2.1.1 Spatial candidates derivation The derivation of spatial merge candidates in VVC is same to that in HEVC except the positions of first two merge candidates are swapped. A maximum of four merge candidates are selected among candidates located in the positions depicted in Fig.4. The order of derivation is B_0, A_0, B_1, A1 and B2. Position B2 is considered only when one or more than one CUs of position B0, A0, B1, A1 are not available (e.g. because it belongs to another slice or tile) or is intra coded. After candidate at position A₁ is added, the addition of the remaining candidates is subject to a redundancy check which ensures that candidates with same motion information are excluded from the list so that coding efficiency is improved. To reduce computational complexity, not all possible candidate pairs are considered in the mentioned redundancy check. Instead only the pairs linked with an arrow in Fig.5 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. 2.2.1.2 Temporal candidates derivation 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 CU belonging to the collocated reference picture. The reference picture list and the reference index to be used for derivation of the co-located CU is explicitly signalled in the slice header. The scaled motion vector for temporal merge candidate is obtained as illustrated by the dotted line in Fig.6, which is scaled from the motion vector of the co-located CU 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. The position for the temporal candidate is selected between candidates C₀ and C₁, as depicted in Fig.7. If CU at position C₀ is not available, is intra coded, or is outside of the current row of CTUs, position C1 is used. Otherwise, position C0 is used in the derivation of the temporal merge candidate. 2.2.1.3 History-based merge candidates derivation The history-based MVP (HMVP) merge candidates are added to merge list after the spatial MVP and TMVP. In this method, the motion information of a previously coded block is stored in a table and used as MVP for the current CU. The table with multiple HMVP candidates is maintained during the encoding/decoding process. The table is reset (emptied) when a new CTU row is encountered. Whenever there is a non-subblock inter-coded CU, the associated motion information is added to the last entry of the table as a new HMVP candidate. The HMVP table size S is set to be 6, which indicates up to 5 History-based MVP (HMVP) candidates may be added to the table. When inserting a new motion candidate to the table, a constrained first-in-first-out (FIFO) rule is utilized wherein redundancy check is firstly applied to find whether there is an identical HMVP in the table. If found, the identical HMVP is removed from the table and all the HMVP candidates afterwards are moved forward, and the identical HMVP is inserted to the last entry of the table. HMVP candidates could be used in the merge candidate list construction process. The latest several HMVP candidates in the table are checked in order and inserted to the candidate list after the TMVP candidate. Redundancy check is applied on the HMVP candidates to the spatial or temporal merge candidate. To reduce the number of redundancy check operations, the following simplifications are introduced: 1. The last two entries in the table are redundancy checked to A1 and B1 spatial candidates, respectively. 2. Once the total number of available merge candidates reaches the maximally allowed merge candidates minus 1, the merge candidate list construction process from HMVP is terminated. 2.2.1.4 Pair-wise average merge candidate derivation Pairwise average candidates are generated by averaging predefined pairs of candidates in the existing merge candidate list, using the first two merge candidates. The first merge candidate is defined as p0Cand and the second merge candidate can be defined as p1Cand, respectively. The averaged motion vectors are calculated according to the availability of the motion vector of p0Cand and p1Cand separately for each reference list. If both motion vectors are available in one list, these two motion vectors are averaged even when they point to different reference pictures, and its reference picture is set to the one of p0Cand; if only one motion vector is available, use the one directly; if no motion vector is available, keep this list invalid. Also, if the half-pel interpolation filter indices of p0Cand and p1Cand are different, it is set to 0. When the merge list is not full after pair-wise average merge candidates are added, the zero MVPs are inserted in the end until the maximum merge candidate number is encountered. 2.2.1.5 Merge estimation region Merge estimation region (MER) allows independent derivation of merge candidate list for the CUs in the same merge estimation region (MER). A candidate block that is within the same MER to the current CU is not included for the generation of the merge candidate list of the current CU. In addition, the updating process for the history-based motion vector predictor candidate list is updated only if ( xCb + cbWidth ) >> Log2ParMrgLevel is greater than xCb >> Log2ParMrgLevel and ( yCb + cbHeight ) >> Log2ParMrgLevel is great than ( yCb >> Log2ParMrgLevel ) and where ( xCb, yCb ) is the top-left luma sample position of the current CU in the picture and ( cbWidth, cbHeight ) is the CU size. The MER size is selected at encoder side and signalled as log2_parallel_merge_level_minus2 in the sequence parameter set. 2.2.2 High precision (1/16 pel) motion compensation and motion vector storage VVC increases the MV precision to 1/16 luma sample, to improve the prediction efficiency of slow motion video. This higher motion accuracy is particularly helpful for video contents with locally varying and non-translational motion such as in case of affine mode. For fractional position samples generation of higher MV accuracy, HEVC’s 8-tap luma interpolation filters and 4-tap chroma interpolation filters are extended to 16 phases for luma and 32 phases for chroma. This extended filter set is applied in MC process of inter coded CUs except the CUs in affine mode. For affine mode, a set of 6-tap luma interpolation filter with 16 phases is used for lower computational complexity as well as memory bandwidth saving. In VVC, the highest precision of explicitly signalled motion vectors for non-affine CU is quarter- luma-sample. In some inter prediction modes such as the affine mode, motion vectors can be signalled at 1/16-luma-sample precision. In all inter coded CU with implicitly inferred MVs, the MVs are derived at 1/16-luma-sample precision and motion compensated prediction is performed at 1/16-sample-precision. In terms of internal motion field storage, all motion vectors are stored at 1/16-luma-sample precision. For temporal motion field storage used by TMVP and SbTVMP, motion field compression is performed at 8x8 size granularity in contrast to the 16x16 size granularity in HEVC. 2.2.3 Merge mode with MVD (MMVD) In addition to merge mode, where the implicitly derived motion information is directly used for prediction samples generation of the current CU, the merge mode with motion vector differences (MMVD) is introduced in VVC. A MMVD flag is signalled right after sending a regular merge flag to specify whether MMVD mode is used for a CU. In MMVD, after a merge candidate is selected, it is further refined by the signalled MVDs information. The further information includes a merge candidate flag, an index to specify motion magnitude, and an index for indication of motion direction. In MMVD mode, one for the first two candidates in the merge list is selected to be used as MV basis. The mmvd candidate flag is signalled to specify which one is used between the first and second merge candidates. Distance index specifies motion magnitude information and indicate the pre-defined offset from the starting point. As shown in Fig.8, an offset is added to either horizontal component or vertical component of starting MV. The relation of distance index and pre-defined offset is specified in Table 2-1. Table 2-1 – The relation of distance index and pre-defined offset Direction index represents the direction of the MVD relative to the starting point. The direction index can represent of the four directions as shown in Table 2-2. It’s noted that the meaning of MVD sign could be variant according to the information of starting MVs. When the starting MVs is an un-prediction MV or bi-prediction MVs with both lists point to the same side of the current picture (i.e. POCs of two references are both larger than the POC of the current picture, or are both smaller than the POC of the current picture), the sign in Table 2-2 specifies the sign of MV offset added to the starting MV. When the starting MVs is bi-prediction MVs with the two MVs point to the different sides of the current picture (i.e. the POC of one reference is larger than the POC of the current picture, and the POC of the other reference is smaller than the POC of the current picture), and the difference of POC in list 0 is greater than the one in list 1, the sign in Table 2-2 specifies the sign of MV offset added to the list0 MV component of starting MV and the sign for the list1 MV has opposite value. Otherwise, if the difference of POC in list 1 is greater than list 0, the sign in Table 2-2 specifies the sign of MV offset added to the list1 MV component of starting MV and the sign for the list0 MV has opposite value. The MVD is scaled according to the difference of POCs in each direction. If the differences of POCs in both lists are the same, no scaling is needed. Otherwise, if the difference of POC in list 0 is larger than the one of list 1, the MVD for list 1 is scaled, by defining the POC difference of L0 as td and POC difference of L1 as tb, described in Fig.6. If the POC difference of L1 is greater than L0, the MVD for list 0 is scaled in the same way. If the starting MV is uni-predicted, the MVD is added to the available MV. Table 2-2 – Sign of MV offset specified by direction index 2.2.4 Symmetric MVD coding In VVC, besides the normal unidirectional prediction and bi-directional prediction mode MVD signalling, symmetric MVD mode for bi-predictional MVD signalling is applied. In the symmetric MVD mode, motion information including reference picture indices of both list-0 and list-1 and MVD of list-1 are not signaled but derived. The decoding process of the symmetric MVD mode is as follows: 1. At slice level, variables BiDirPredFlag, RefIdxSymL0 and RefIdxSymL1 are derived as follows: – If mvd_l1_zero_flag is 1, BiDirPredFlag is set equal to 0. – Otherwise, if the nearest reference picture in list-0 and the nearest reference picture in list- 1 form a forward and backward pair of reference pictures or a backward and forward pair of reference pictures, BiDirPredFlag is set to 1, and both list-0 and list-1 reference pictures are short-term reference pictures. Otherwise BiDirPredFlag is set to 0. 2. At CU level, a symmetrical mode flag indicating whether symmetrical mode is used or not is explicitly signaled if the CU is bi-prediction coded and BiDirPredFlag is equal to 1. When the symmetrical mode flag is true, only mvp_l0_flag, mvp_l1_flag and MVD0 are explicitly signaled. The reference indices for list-0 and list-1 are set equal to the pair of reference pictures, respectively. MVD1 is set equal to ( − MVD0 ). The final motion vectors are shown in below _formula. In the encoder, symmetric MVD motion estimation starts with initial MV evaluation. A set of initial MV candidates comprising of the MV obtained from uni-prediction search, the MV obtained from bi-prediction search and the MVs from the AMVP list. The one with the lowest rate-distortion cost is chosen to be the initial MV for the symmetric MVD motion search. 2.2.5 Affine motion compensated prediction In HEVC, only translation motion model is applied for motion compensation prediction (MCP). While in the real world, there are many kinds of motion, e.g. zoom in/out, rotation, perspective motions and the other irregular motions. In VVC, a block-based affine transform motion compensation prediction is applied. As shown Fig. 10, the affine motion field of the block is described by motion information of two control point (4-parameter, shown in the subdiagram 1000) or three control point motion vectors (6-parameter, shown in the subdiagram 1002). For 4-parameter affine motion model, motion vector at sample location (x, y) in a block is derived as: For 6-parameter affine motion model, motion vector at sample location (x, y) in a block is derived as: where (mv_0x, mv_0y) is motion vector of the top-left corner control point, (mv_1x, mv_1y) is motion vector of the top-right corner control point, and (mv2x, mv2y) is motion vector of the bottom-left corner control point. In order to simplify the motion compensation prediction, block based affine transform prediction is applied. To derive motion vector of each 4×4 luma subblock, the motion vector of the center sample of each subblock, as shown in Fig. 11, is calculated according to above equations, and rounded to 1/16 fraction accuracy. Then the motion compensation interpolation filters are applied to generate the prediction of each subblock with derived motion vector. The subblock size of chroma-components is also set to be 4×4. The MV of a 4×4 chroma subblock is calculated as the average of the MVs of the top-left and bottom-right luma subblocks in the collocated 8x8 luma region. As done for translational motion inter prediction, there are also two affine motion inter prediction modes: affine merge mode and affine AMVP mode. 2.2.5.1 Affine merge prediction AF_MERGE mode can be applied for CUs with both width and height larger than or equal to 8. In this mode the CPMVs of the current CU is generated based on the motion information of the spatial neighboring CUs. There can be up to five CPMVP candidates and an index is signalled to indicate the one to be used for the current CU. The following three types of CPVM candidate are used to form the affine merge candidate list: – Inherited affine merge candidates that extrapolated from the CPMVs of the neighbour CUs; – Constructed affine merge candidates CPMVPs that are derived using the translational MVs of the neighbour CUs; – Zero MVs. In VVC, there are maximum two inherited affine candidates, which are derived from affine motion model of the neighboring blocks, one from left neighboring CUs and one from above neighboring CUs. The candidate blocks are shown in Fig.12. For the left predictor, the scan order is A0->A1, and for the above predictor, the scan order is B0->B1->B2. Only the first inherited candidate from each side is selected. No pruning check is performed between two inherited candidates. When a neighboring affine CU is identified, its control point motion vectors are used to derive the CPMVP candidate in the affine merge list of the current CU. As shown in Fig. 13, if the neighbour left bottom block A is coded in affine mode, the motion vectors v_ଶ , v_ଷ and v_ସ of the top left corner, above right corner and left bottom corner of the CU which contains the block A are attained. When block A is coded with 4-parameter affine model, the two CPMVs of the current CU are calculated according to v_ଶ, and v_ଷ. In case that block A is coded with 6-parameter affine model, the three CPMVs of the current CU are calculated according to v_ଶ , v_ଷ and v_ସ. Constructed affine candidate means the candidate is constructed by combining the neighbor translational motion information of each control point. The motion information for the control points is derived from the specified spatial neighbors and temporal neighbor shown in Fig. 14. CPMV_k (k=1, 2, 3, 4) represents the k-th control point. For CPMV₁, the B2->B3->A2 blocks are checked and the MV of the first available block is used. For CPMV2, the B1->B0 blocks are checked and for CPMV3, the A1->A0 blocks are checked. For TMVP is used as CPMV4 if it’s available. After MVs of four control points are attained, affine merge candidates are constructed based on those motion information. The following combinations of control point MVs are used to construct in order: {CPMV₁, CPMV₂, CPMV₃}, {CPMV₁, CPMV₂, CPMV₄}, {CPMV₁, CPMV₃, CPMV₄}, {CPMV2, CPMV3, CPMV4}, { CPMV1, CPMV2}, { CPMV1, CPMV3}. The combination of 3 CPMVs constructs a 6-parameter affine merge candidate and the combination of 2 CPMVs constructs a 4-parameter affine merge candidate. To avoid motion scaling process, if the reference indices of control points are different, the related combination of control point MVs is discarded. After inherited affine merge candidates and constructed affine merge candidate are checked, if the list is still not full, zero MVs are inserted to the end of the list. 2.2.5.2 Affine AMVP prediction Affine AMVP mode can be applied for CUs with both width and height larger than or equal to 16. An affine flag in CU level is signalled in the bitstream to indicate whether affine AMVP mode is used and then another flag is signalled to indicate whether 4-parameter affine or 6-parameter affine. In this mode, the difference of the CPMVs of current CU and their predictors CPMVPs is signalled in the bitstream. The affine AVMP candidate list size is 2 and it is generated by using the following four types of CPVM candidate in order: – Inherited affine AMVP candidates that extrapolated from the CPMVs of the neighbour CUs; – Constructed affine AMVP candidates CPMVPs that are derived using the translational MVs of the neighbour CUs; – Translational MVs from neighboring CUs; – Zero MVs. The checking order of inherited affine AMVP candidates is same to the checking order of inherited affine merge candidates. The only difference is that, for AVMP candidate, only the affine CU that has the same reference picture as in current block is considered. No pruning process is applied when inserting an inherited affine motion predictor into the candidate list. Constructed AMVP candidate is derived from the specified spatial neighbors shown in Fig. 14. The same checking order is used as done in affine merge candidate construction. In addition, reference picture index of the neighboring block is also checked. The first block in the checking order that is inter coded and has the same reference picture as in current CUs is used. There is only one When the current CU is coded with 4-parameter affine mode, and ^^^^_^ and ^^^^_^ are both availlalbe, they are added as one candidate in the affine AMVP list. When the current CU is coded with 6-parameter affine mode, and all three CPMVs are available, they are added as one candidate in the affine AMVP list. Otherwise, constructed AMVP candidate is set as unavailable. If affine AMVP list candidates is still less than 2 after valid inherited affine AMVP candidates and constructed AMVP candidate are inserted, ^^^^_^ , ^^^^_^ and ^^^^_ଶ will be added, in order, as the translational MVs to predict all control point MVs of the current CU, when available. Finally, zero MVs are used to fill the affine AMVP list if it is still not full. 2.2.5.3 Affine motion information storage In VVC, the CPMVs of affine CUs are stored in a separate buffer. The stored CPMVs are only used to generate the inherited CPMVPs in affine merge mode and affine AMVP mode for the lately coded CUs. The subblock MVs derived from CPMVs are used for motion compensation, MV derivation of merge/AMVP list of translational MVs and deblocking. To avoid the picture line buffer for the additional CPMVs, affine motion data inheritance from the CUs from above CTU is treated differently to the inheritance from the normal neighboring CUs. If the candidate CU for affine motion data inheritance is in the above CTU line, the bottom-left and bottom-right subblock MVs in the line buffer instead of the CPMVs are used for the affine MVP derivation. In this way, the CPMVs are only stored in local buffer. If the candidate CU is 6- parameter affine coded, the affine model is degraded to 4-parameter model. As shown in Fig.15, along the top CTU boundary, the bottom-left and bottom right subblock motion vectors of a CU are used for affine inheritance of the CUs in bottom CTUs. 2.2.5.4 Prediction refinement with optical flow for affine mode Subblock based affine motion compensation can save memory access bandwidth and reduce computation complexity compared to pixel based motion compensation, at the cost of prediction accuracy penalty. To achieve a finer granularity of motion compensation, prediction refinement with optical flow (PROF) is used to refine the subblock based affine motion compensated prediction without increasing the memory access bandwidth for motion compensation. In VVC, after the subblock based affine motion compensation is performed, luma prediction sample is refined by adding a difference derived by the optical flow equation. The PROF is described as following four steps: Step 1) The subblock-based affine motion compensation is performed to generate subblock _prediction _{Step2) The spatial gradients ^^௫(^^, ^^) and ^^௬(^^, ^^) of the subblock prediction are calculated at} each sample location using a 3-tap filter [−1, 0, 1]. The gradient calculation is exactly the same as _{gradient calculation in BDOF.} _{^^௫(^^, ^^) = (^^(^^ + 1, ^^) ≫ ^^ℎ^^^^^^1) − (^^(^^ − 1, ^^) ≫ ^^ℎ^^^^^^1) (4)} _{^^ℎ^^^^^^1 is used to control the gradient’s precision. The subblock (i.e. 4x4) prediction is extended} by one sample on each side for the gradient calculation. To avoid additional memory bandwidth and additional interpolation computation, those extended samples on the extended borders are copied from the nearest integer pixel position in the reference picture. _{Step 3) The luma prediction refinement is calculated by the following optical flow equation.} _{Δ^^(^^, ^^) = ^^௫(^^, ^^) ∗ Δ^^௫(^^, ^^) + ^^௬(^^, ^^) ∗ Δ^^௬(^^, ^^) (6)} _{where the Δ^^(^^, ^^) is the difference between sample MV computed for sample location (^^, ^^),} _{denoted by ^^(^^, ^^), and the subblock MV of the subblock to which sample (^^, ^^) belongs, as shown} _{in Fig. 16. The Δ^^(^^, ^^) is quantized in the unit of 1/32 luam sample precision.} Since the affine model parameters and the sample location relative to the subblock center are not _{changed from subblock to subblock,} _{can be calculated for the first subblock, and reused} _{for other subblocks in the same CU. Let ^^^^(^^, ^^) and ^^^^(^^, ^^) be the horizontal and vertical} _{offset from the sample location (^^, ^^) to the center of the subblock (^^ௌ^ , ^^ௌ^), Δ^^(^^, ^^) can be} _{derived by the following equation,} ₍₇₎ ₍₈₎ _{In order to keep accuracy, the enter of the subblock (^^ௌ^ , ^^ௌ^) is calculated as} ( ( W_SB − 1 )/2, ( H_SB − 1 ) / 2 ), where W_SB and H_SB are the subblock width and height, respectively. For 4-parameter affine model, For 6-parameter affine model, _{where (^^^௫, ^^^௬), (^^^௫, ^^^௬), (^^ଶ௫, ^^ଶ௬) are the top-left, top-right and bottom-left control point} motion vectors, ^^ and ℎ are the width and height of the CU. _{Step 4) Finally, the luma prediction refinement} _{is added to the subblock prediction ^^(^^, ^^).} _{The final prediction I’ is generated as the following equation.} PROF is not applied in two cases for an affine coded CU: 1) all control point MVs are the same, which indicates the CU only has translational motion; 2) the affine motion parameters are greater than a specified limit because the subblock based affine MC is degraded to CU based MC to avoid large memory access bandwidth requirement. A fast encoding method is applied to reduce the encoding complexity of affine motion estimation with PROF. PROF is not applied at affine motion estimation stage in following two situations: a) if this CU is not the root block and its parent block does not select the affine mode as its best mode, PROF is not applied since the possibility for current CU to select the affine mode as best mode is low; b) if the magnitude of four affine parameters (C, D, E, F) are all smaller than a predefined threshold and the current picture is not a low delay picture, PROF is not applied because the improvement introduced by PROF is small for this case. In this way, the affine motion estimation with PROF can be accelerated. 2.2.5.5 Adaptive bypass of affine ME If enabled using the VTM encoder parameter AdaptBypassAffineMe, adaptive bypass of affine ME is used as an encoder only operation used to speed up encoding. Before performing affine ME for a CU, the coding modes of its five spatial neighbours (above, left, above-right, bottom-left, above-left) are checked. If the number of available neighbours is greater than or equal to 4 and none of them are coded as affine or SbTMVP mode, affine ME is bypassed. In addition following two conditions are considered. 1) If a CU is no larger than 16x16, affine ME is not bypassed. 2) If the best mode for a CU is affine merge so far, and the current picture does not have symmetric reference pair (SMVD condition) or the absolute temporal distance between the current picture and SMVD reference is larger than 1, affine ME is not bypassed. 2.2.6 Subblock-based temporal motion vector prediction (SbTMVP) VVC supports the subblock-based temporal motion vector prediction (SbTMVP) method. Similar to the temporal motion vector prediction (TMVP) in HEVC, SbTMVP uses the motion field in the collocated picture to improve motion vector prediction and merge mode for CUs in the current picture. The same collocated picture used by TMVP is used for SbTVMP. SbTMVP differs from TMVP in the following two main aspects: – TMVP predicts motion at CU level but SbTMVP predicts motion at sub-CU level; – Whereas TMVP fetches the temporal motion vectors from the collocated block in the collocated picture (the collocated block is the bottom-right or center block relative to the current CU), SbTMVP applies a motion shift before fetching the temporal motion information from the collocated picture, where the motion shift is obtained from the motion vector from one of the spatial neighboring blocks of the current CU. The SbTVMP process is illustrated in Fig.17A and Fig.17B. SbTMVP predicts the motion vectors of the sub-CUs within the current CU in two steps. In the first step, the spatial neighbor A1 in Fig. 17A is examined. If A1 has a motion vector that uses the collocated picture as its reference picture, this motion vector is selected to be the motion shift to be applied. If no such motion is identified, then the motion shift is set to (0, 0). In the second step, the motion shift identified in Step 1 is applied (i.e. added to the current block’s coordinates) to obtain sub-CU level motion information (motion vectors and reference indices) from the collocated picture as shown in Fig.17B. The example in Fig.17B assumes the motion shift is set to block A1’s motion. Then, for each sub-CU, the motion information of its corresponding block (the smallest motion grid that covers the center sample) in the collocated picture is used to derive the motion information for the sub-CU. After the motion information of the collocated sub-CU is identified, it is converted to the motion vectors and reference indices of the current sub-CU in a similar way as the TMVP process of HEVC, where temporal motion scaling is applied to align the reference pictures of the temporal motion vectors to those of the current CU. In VVC, a combined subblock based merge list which contains both SbTVMP candidate and affine merge candidates is used for the signalling of subblock based merge mode. The SbTVMP mode is enabled/disabled by a sequence parameter set (SPS) flag. If the SbTMVP mode is enabled, the SbTMVP predictor is added as the first entry of the list of subblock based merge candidates, and followed by the affine merge candidates. The size of subblock based merge list is signalled in SPS and the maximum allowed size of the subblock based merge list is 5 in VVC. The sub-CU size used in SbTMVP is fixed to be 8x8, and as done for affine merge mode, SbTMVP mode is only applicable to the CU with both width and height are larger than or equal to 8. The encoding logic of the additional SbTMVP merge candidate is the same as for the other merge candidates, that is, for each CU in P or B slice, an additional RD check is performed to decide whether to use the SbTMVP candidate. 2.2.7 Adaptive motion vector resolution (AMVR) In HEVC, motion vector differences (MVDs) (between the motion vector and predicted motion vector of a CU) are signalled in units of quarter-luma-sample when use_integer_mv_flag is equal to 0 in the slice header. In VVC, a CU-level adaptive motion vector resolution (AMVR) scheme is introduced. AMVR allows MVD of the CU to be coded in different precision. Dependent on the mode (normal AMVP mode or affine AVMP mode) for the current CU, the MVDs of the current CU can be adaptively selected as follows: – Normal AMVP mode: quarter-luma-sample, half-luma-sample, integer-luma-sample or four- luma-sample. – Affine AMVP mode: quarter-luma-sample, integer-luma-sample or 1/16 luma-sample. The CU-level MVD resolution indication is conditionally signalled if the current CU has at least one non-zero MVD component. If all MVD components (that is, both horizontal and vertical MVDs for reference list L0 and reference list L1) are zero, quarter-luma-sample MVD resolution is inferred. For a CU that has at least one non-zero MVD component, a first flag is signalled to indicate whether quarter-luma-sample MVD precision is used for the CU. If the first flag is 0, no further signaling is needed and quarter-luma-sample MVD precision is used for the current CU. Otherwise, a second flag is signalled to indicate half-luma-sample or other MVD precisions (interger or four- luma sample) is used for normal AMVP CU. In the case of half-luma-sample, a 6-tap interpolation filter instead of the default 8-tap interpolation filter is used for the half-luma sample position. Otherwise, a third flag is signalled to indicate whether integer-luma-sample or four-luma-sample MVD precision is used for normal AMVP CU. In the case of affine AMVP CU, the second flag is used to indicate whether integer-luma-sample or 1/16 luma-sample MVD precision is used. In order to ensure the reconstructed MV has the intended precision (quarter-luma-sample, half-luma- sample, integer-luma-sample or four-luma-sample), the motion vector predictors for the CU will be rounded to the same precision as that of the MVD before being added together with the MVD. The motion vector predictors are rounded toward zero (that is, a negative motion vector predictor is rounded toward positive infinity and a positive motion vector predictor is rounded toward negative infinity). The encoder determines the motion vector resolution for the current CU using RD check. To avoid always performing CU-level RD check four times for each MVD resolution, in VTM, the RD check of MVD precisions other than quarter-luma-sample is only invoked conditionally. For normal AVMP mode, the RD cost of quarter-luma-sample MVD precision and integer-luma sample MV precision is computed first. Then, the RD cost of integer-luma-sample MVD precision is compared to that of quarter-luma-sample MVD precision to decide whether it is necessary to further check the RD cost of four-luma-sample MVD precision. When the RD cost for quarter- luma-sample MVD precision is much smaller than that of the integer-luma-sample MVD precision, the RD check of four-luma-sample MVD precision is skipped. Then, the check of half-luma- sample MVD precision is skipped if the RD cost of integer-luma-sample MVD precision is significantly larger than the best RD cost of previously tested MVD precisions. For affine AMVP mode, if affine inter mode is not selected after checking rate-distortion costs of affine merge/skip mode, merge/skip mode, quarter-luma-sample MVD precision normal AMVP mode and quarter- luma-sample MVD precision affine AMVP mode, then 1/16 luma-sample MV precision and 1-pel MV precision affine inter modes are not checked. Furthermore affine parameters obtained in quarter-luma-sample MV precision affine inter mode is used as starting search point in 1/16 luma- sample and quarter-luma-sample MV precision affine inter modes. 2.2.8 Bi-prediction with CU-level weight (BCW) In HEVC, the bi-prediction signal is generated by averaging two prediction signals obtained from two different reference pictures and/or using two different motion vectors. In VVC, the bi- prediction mode is extended beyond simple averaging to allow weighted averaging of the two _{prediction signals.} _{^^bi-pred = ൫(8 − ^^) ∗ ^^^ + ^^ ∗ ^^^ + 4൯ ≫ 3 (11)} _{Five weights are allowed in the weighted averaging bi-prediction, ^^ ∈ ^−2, 3, 4, 5, 10^. For each} bi-predicted CU, the weight w is determined in one of two ways: 1) for a non-merge CU, the weight index is signalled after the motion vector difference; 2) for a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. BCW is only applied to CUs with 256 or more luma samples (i.e., CU width times CU height is greater than or equal to 256). For low-delay pictures, all 5 weights are used. For non-low-delay pictures, only 3 weights (w∈{3,4,5}) are used. – At the encoder, fast search algorithms are applied to find the weight index without significantly increasing the encoder complexity. These algorithms are summarized as follows. When combined with AMVR, unequal weights are only conditionally checked for 1-pel and 4-pel motion vector precisions if the current picture is a low-delay picture. – When combined with affine, affine ME will be performed for unequal weights if and only if the affine mode is selected as the current best mode. – When the two reference pictures in bi-prediction are the same, unequal weights are only conditionally checked. – Unequal weights are not searched when certain conditions are met, depending on the POC distance between current picture and its reference pictures, the coding QP, and the temporal level. The BCW weight index is coded using one context coded bin followed by bypass coded bins. The first context coded bin indicates if equal weight is used; and if unequal weight is used, additional bins are signalled using bypass coding to indicate which unequal weight is used. Weighted prediction (WP) is a coding tool supported by the H.264/AVC and HEVC standards to efficiently code video content with fading. Support for WP was also added into the VVC standard. WP allows weighting parameters (weight and offset) to be signalled for each reference picture in each of the reference picture lists L0 and L1. Then, during motion compensation, the weight(s) and offset(s) of the corresponding reference picture(s) are applied. WP and BCW are designed for different types of video content. In order to avoid interactions between WP and BCW, which will complicate VVC decoder design, if a CU uses WP, then the BCW weight index is not signalled, and w is inferred to be 4 (i.e. equal weight is applied). For a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. This can be applied to both normal merge mode and inherited affine merge mode. For constructed affine merge mode, the affine motion information is constructed based on the motion information of up to 3 blocks. The BCW index for a CU using the constructed affine merge mode is simply set equal to the BCW index of the first control point MV. In VVC, CIIP and BCW cannot be jointly applied for a CU. When a CU is coded with CIIP mode, the BCW index of the current CU is set to 2, e.g. equal weight. 2.2.9 Bi-directional optical flow (BDOF) The bi-directional optical flow (BDOF) tool is included in VVC. BDOF, previously referred to as BIO, was included in the JEM. Compared to the JEM version, the BDOF in VVC is a simpler version that requires much less computation, especially in terms of number of multiplications and the size of the multiplier. BDOF is used to refine the bi-prediction signal of a CU at the 4×4 subblock level. BDOF is applied to a CU if it satisfies all the following conditions: – The CU is coded using “true” bi-prediction mode, i.e., one of the two reference pictures is prior to the current picture in display order and the other is after the current picture in display order; – The distances (i.e. POC difference) from two reference pictures to the current picture are same; – Both reference pictures are short-term reference pictures; – The CU is not coded using affine mode or the SbTMVP merge mode; – CU has more than 64 luma samples; – Both CU height and CU width are larger than or equal to 8 luma samples; – BCW weight index indicates equal weight; – WP is not enabled for the current CU; – CIIP mode is not used for the current CU. BDOF is only applied to the luma component. As its name indicates, the BDOF mode is based on the optical flow concept, which assumes that the motion of an object is smooth. For each 4×4 _{subblock, a motion refinement ൫^^௫, ^^௬൯ is calculated by minimizing the difference between the} L0 and L1 prediction samples. The motion refinement is then used to adjust the bi-predicted sample values in the 4x4 subblock. The following steps are applied in the BDOF process. _{First, the horizontal and vertical gradients,} _{(^^, ^^) , ^^ = 0,1 , of the two} prediction signals are computed by directly calculating the difference between two neighboring samples, i.e., _{where ^^} ^{(^)} _{(^^, ^^) are the sample value at coordinate (^^, ^^) of the prediction signal in list ^^, ^^ =} _{0,1, and shift1 is calculated based on the luma bit depth, bitDepth, as shift1 = max( 6, bitDepth-} 6). _{Then, the auto- and cross-correlation of the gradients, ^^^, ^^ଶ, ^^ଷ, ^^ହ and ^^^, are calculated as} where where Ω is a 6×6 window around the 4×4 subblock, and the values of ^^_^ and ^^_^ are set equal to min( 1, bitDepth − 11 ) and min( 4, bitDepth − 8 ), respectively. _{The motion refinement ൫^^௫, ^^௬൯ is then derived using the cross- and auto-correlation terms using} _{the following:} _{^^௫ = ^^^ > 0? ^^^^^^^^3 ^−^^ℎᇱ ᇱ} _{^ூை , ^^ℎ^ூை ,−൫(^^ ^} _{ଷ ∙ 2 ್ି^ೌ) ≫ ⌊logଶ ^^^⌋൯^ : 0} _{^^ ᇱ ᇱ ^} _{௬ = ^^ହ > 0? ^^^^^^^^3^−^^ℎ^ூை , ^^ℎ^ூை ,−൬^^^^ ∙ 2 ್ି^ೌ} _{is the floor function,} _{and ^^ௌమ = 12.} Based on the motion refinement and the gradients, the following adjustment is calculated for each sample in the 4×4 subblock: Finally, the BDOF samples of the CU are calculated by adjusting the bi-prediction samples as _follows: These values are selected such that the multipliers in the BDOF process do not exceed 15-bit, and the maximum bit-width of the intermediate parameters in the BDOF process is kept within 32-bit. _{In order to derive the gradient values, some prediction samples ^^(^)(^^, ^^) in list ^^ (^^ = 0,1)} outside of the current CU boundaries need to be generated. As depicted in Fig.18, the BDOF in VVC uses one extended row/column around the CU’s boundaries. In order to control the computational complexity of generating the out-of-boundary prediction samples, prediction samples in the extended area (white positions) are generated by taking the reference samples at the nearby integer positions (using floor() operation on the coordinates) directly without interpolation, and the normal 8-tap motion compensation interpolation filter is used to generate prediction samples within the CU (gray positions). These extended sample values are used in gradient calculation only. For the remaining steps in the BDOF process, if any sample and gradient values outside of the CU boundaries are needed, they are padded (i.e. repeated) from their nearest neighbors. When the width and/or height of a CU are larger than 16 luma samples, it will be split into subblocks with width and/or height equal to 16 luma samples, and the subblock boundaries are treated as the CU boundaries in the BDOF process. The maximum unit size for BDOF process is limited to 16x16. For each subblock, the BDOF process could skipped. When the SAD of between the initial L0 and L1 prediction samples is smaller than a threshold, the BDOF process is not applied to the subblock. The threshold is set equal to (8 * W*( H >> 1 ), where W indicates the subblock width, and H indicates subblock height. To avoid the additional complexity of SAD calculation, the SAD between the initial L0 and L1 prediction samples calculated in DVMR process is re-used here. If BCW is enabled for the current block, i.e., the BCW weight index indicates unequal weight, then bi-directional optical flow is disabled. Similarly, if WP is enabled for the current block, i.e., the luma_weight_lx_flag is 1 for either of the two reference pictures, then BDOF is also disabled. When a CU is coded with symmetric MVD mode or CIIP mode, BDOF is also disabled. 2.2.10 Decoder side motion vector refinement (DMVR) In order to increase the accuracy of the MVs of the merge mode, a bilateral-matching (BM) based decoder side motion vector refinement is applied in VVC. In bi-prediction operation, a refined MV is searched around the initial MVs in the reference picture list L0 and reference picture list L1. The BM method calculates the distortion between the two candidate blocks in the reference picture list L0 and list L1. As illustrated in Fig.19, the SAD between the slashed blocks based on each MV candidate around the initial MV is calculated. The MV candidate with the lowest SAD becomes the refined MV and used to generate the bi-predicted signal. In VVC, the application of DMVR is restricted and is only applied for the CUs which are coded with following modes and features: – CU level merge mode with bi-prediction MV; – One reference picture is in the past and another reference picture is in the future with respect to the current picture; – The distances (i.e. POC difference) from two reference pictures to the current picture are same; – Both reference pictures are short-term reference pictures; – CU has more than 64 luma samples; – Both CU height and CU width are larger than or equal to 8 luma samples; – BCW weight index indicates equal weight; – WP is not enabled for the current block; – CIIP mode is not used for the current block. The refined MV derived by DMVR process is used to generate the inter prediction samples and also used in temporal motion vector prediction for future pictures coding. While the original MV is used in deblocking process and also used in spatial motion vector prediction for future CU coding. The additional features of DMVR are mentioned in the following sub-clauses. 2.2.10.1 Searching scheme In DVMR, the search points are surrounding the initial MV and the MV offset obey the MV difference mirroring rule. In other words, any points that are checked by DMVR, denoted by _{candidate MV pair (MV0, MV1) obey the following two equations:} _{^^^^0ᇱ = ^^^^0 + ^^^^_^^^^^^^^^^^^ (18)} _{^^^^1ᇱ = ^^^^1 − ^^^^_^^^^^^^^^^^^ (19)} where ^^^^_^^^^^^^^^^^^ represents the refinement offset between the initial MV and the refined MV in one of the reference pictures. The refinement search range is two integer luma samples from the initial MV. The searching includes the integer sample offset search stage and fractional sample refinement stage. 25 points full search is applied for integer sample offset searching. The SAD of the initial MV pair is first calculated. If the SAD of the initial MV pair is smaller than a threshold, the integer sample stage of DMVR is terminated. Otherwise SADs of the remaining 24 points are calculated and checked in raster scanning order. The point with the smallest SAD is selected as the output of integer sample offset searching stage. To reduce the penalty of the uncertainty of DMVR refinement, it is proposed to favor the original MV during the DMVR process. The SAD between the reference blocks referred by the initial MV candidates is decreased by 1/4 of the SAD value. The integer sample search is followed by fractional sample refinement. To save the calculational complexity, the fractional sample refinement is derived by using parametric error surface equation, instead of additional search with SAD comparison. The fractional sample refinement is conditionally invoked based on the output of the integer sample search stage. When the integer sample search stage is terminated with center having the smallest SAD in either the first iteration or the second iteration search, the fractional sample refinement is further applied. In parametric error surface based sub-pixel offsets estimation, the center position cost and the costs at four neighboring positions from the center are used to fit a 2-D parabolic error surface equation _{of the following form:} _{^^(^^, ^^) = ^^(^^ − ^^ )ଶ + ( )ଶ} _{^^^ ^^ ^^ − ^^^^^ + ^^ (20)} where (^^_^^^,^^_^^^) corresponds to the fractional position with the least cost and C corresponds to the minimum cost value. By solving the above equations by using the cost value of the five search _{points, the (^^^^^, ^^^^^) is computed as:} _{^^^^^ = (^^(−1,0) − ^^(1,0))/(2(^^(−1,0) + ^^(1,0) − 2^^(0,0))) (21)} _{^^^^^ = (^^(0,−1) − ^^(0,1))/(2(൫^^(0,−1) + ^^(0,1) − 2^^(0,0)൯) (22)} The value of ^^_^^^ and ^^_^^^ are automatically constrained to be between − 8 and 8 since all cost values are positive and the smallest value is ^^(0,0). This corresponds to half peal offset with _{1/16th-pel MV accuracy in VVC. The computed fractional (^^^^^, ^^^^^) are added to the integer} distance refinement MV to get the sub-pixel accurate refinement delta MV. 2.2.10.2 Bilinear-interpolation and sample padding In VVC, the resolution of the MVs is 1/16 luma samples. The samples at the fractional position are interpolated using an 8-tap interpolation filter. In DMVR, the search points are surrounding the initial fractional-pel MV with integer sample offset, therefore the samples of those fractional position need to be interpolated for DMVR search process. To reduce the calculation complexity, the bi-linear interpolation filter is used to generate the fractional samples for the searching process in DMVR. Another important effect is that by using bi-linear filter is that with 2-sample search range, the DVMR does not access more reference samples compared to the normal motion compensation process. After the refined MV is attained with DMVR search process, the normal 8-tap interpolation filter is applied to generate the final prediction. In order to not access more reference samples to normal MC process, the samples, which is not needed for the interpolation process based on the original MV but is needed for the interpolation process based on the refined MV, will be padded from those available samples. 2.2.10.3 Maximum DMVR processing unit When the width and/or height of a CU are larger than 16 luma samples, it will be further split into subblocks with width and/or height equal to 16 luma samples. The maximum unit size for DMVR searching process is limit to 16x16. 2.2.11 Geometric partitioning mode (GPM) In VVC, a geometric partitioning mode is supported for inter prediction. The geometric partitioning mode is signalled using a CU-level flag as one kind of merge mode, with other merge modes including the regular merge mode, the MMVD mode, the CIIP mode and the subblock merge mode. In total 64 partitions are supported by geometric partitioning mode for each possible _{CU size ^^ × ℎ = 2^ × 2^ with ^^,^^ ∈ ^3⋯6^ excluding 8x64 and 64x8.} When this mode is used, a CU is split into two parts by a geometrically located straight line (as shown in Fig.20). The location of the splitting line is mathematically derived from the angle and offset parameters of a specific partition. Each part of a geometric partition in the CU is inter- predicted using its own motion; only uni-prediction is allowed for each partition, that is, each part has one motion vector and one reference index. The uni-prediction motion constraint is applied to ensure that same as the conventional bi-prediction, only two motion compensated prediction are needed for each CU. The uni-prediction motion for each partition is derived using the process described in 2.2.11.1. If geometric partitioning mode is used for the current CU, then a geometric partition index indicating the partition mode of the geometric partition (angle and offset), and two merge indices (one for each partition) are further signalled. The number of maximum GPM candidate size is signalled explicitly in SPS and specifies syntax binarization for GPM merge indices. After predicting each of part of the geometric partition, the sample values along the geometric partition edge are adjusted using a blending processing with adaptive weights as in 2.2.11.2. This is the prediction signal for the whole CU, and transform and quantization process will be applied to the whole CU as in other prediction modes. Finally, the motion field of a CU predicted using the geometric partition modes is stored as in 2.2.11.3. 2.2.11.1 Uni-prediction candidate list construction The uni-prediction candidate list is derived directly from the merge candidate list constructed according to the extended merge prediction process in 2.2.1. Denote n as the index of the uni- prediction motion in the geometric uni-prediction candidate list. The LX motion vector of the n- th extended merge candidate, with X equal to the parity of n, is used as the n-th uni-prediction motion vector for geometric partitioning mode. These motion vectors are marked with “x” in Fig. 21. In case a corresponding LX motion vector of the n-the extended merge candidate does not exist, the L(1 − X) motion vector of the same candidate is used instead as the uni-prediction motion vector for geometric partitioning mode. 2.2.11.2 Blending along the geometric partitioning edge After predicting each part of a geometric partition using its own motion, blending is applied to the two prediction signals to derive samples around geometric partition edge. The blending weight for each position of the CU are derived based on the distance between individual position and the partition edge. _{The distance for a position (^^, ^^) to the partition edge are derived as:} _{^^(^^, ^^) = (2^^ + 1 − ^^) cos(^^^) + (2^^ + 1 − ℎ) sin(^^^) − ^^^ (23)} _{where ^^, ^^ are the indices for angle and offset of a geometric partition, which depend on the} signaled geometric partition index. The sign of ^^_௫,^ and ^^_௬,^ depend on angle index ^^. _{The weights for each part of a geometric partition are derived as following:} _{^^^^^^^^^^(^^,^^) = ^^^^^^^^^^^^^^ ? 32 + ^^(^^, ^^): 32 − ^^(^^, ^^) (27)} ^_{^^(^^, ^^) = 1 − ^^^(^^,^^) (29)} The partIdx depends on the angle index ^^. One example of weigh ^^_^ is illustrated in Fig.22. 2.2.11.3 Motion field storage for geometric partitioning mode Mv1 from the first part of the geometric partition, Mv2 from the second part of the geometric partition and a combined Mv of Mv1 and Mv2 are stored in the motion filed of a geometric partitioning mode coded CU. _{The stored motion vector type for each individual position in the motion filed are determined as:} _{^^^^^^^^^^ = ^^^^^^(^^^^^^^^^^^^^^^^^^) < 32 ? 2 ∶ ( ^^^^^^^^^^^^^^^^^^ ≤ 0 ? ( 1 − ^^^^^^^^^^^^^^ ): ^^^^^^^^^^^^^^ )} (30) _{where motionIdx is equal to ^^(4^^ + 2, 4^^ + 2), which is recalculated from equation (23). The} partIdx depends on the angle index ^^. If sType is equal to 0 or 1, Mv0 or Mv1 are stored in the corresponding motion field, otherwise if sType is equal to 2, a combined Mv from Mv0 and Mv2 are stored. The combined Mv are generated using the following process: 1) If Mv1 and Mv2 are from different reference picture lists (one from L0 and the other from L1), then Mv1 and Mv2 are simply combined to form the bi-prediction motion vectors. 2) Otherwise, if Mv1 and Mv2 are from the same list, only uni-prediction motion Mv2 is stored. 2.2.12 Combined inter and intra prediction (CIIP) In VVC, when a CU is coded in merge mode, if the CU contains at least 64 luma samples (that is, CU width times CU height is equal to or larger than 64), and if both CU width and CU height are less than 128 luma samples, an additional flag is signalled to indicate if the combined inter/intra prediction (CIIP) mode is applied to the current CU. As its name indicates, the CIIP prediction combines an inter prediction signal with an intra prediction signal. The inter prediction signal in the CIIP mode ^^_^^௧^^ is derived using the same inter prediction process applied to regular merge mode; and the intra prediction signal ^^_^^௧^^ is derived following the regular intra prediction process with the planar mode. Then, the intra and inter prediction signals are combined using weighted averaging, where the weight value is calculated depending on the coding modes of the top and left neighbouring blocks (depicted in Fig.23) as follows: – If the top neighbor is available and intra coded, then set isIntraTop to 1, otherwise set isIntraTop to 0; – If the left neighbor is available and intra coded, then set isIntraLeft to 1, otherwise set isIntraLeft to 0; – If (isIntraLeft + isIntraTop) is equal to 2, then wt is set to 3; – Otherwise, if (isIntraLeft + isIntraTop) is equal to 1, then wt is set to 2; – Otherwise, set wt to 1. _{The CIIP prediction is formed as follows:} 2.2.13 Reference picture resampling (RPR) In HEVC, the spatial resolution of pictures cannot change unless a new sequence using a new SPS starts, with an IRAP picture. VVC enables picture resolution change within a sequence at a position without encoding an IRAP picture, which is always intra-coded. This feature is sometimes referred to as reference picture resampling (RPR), as the feature needs resampling of a reference picture used for inter prediction when that reference picture has a different resolution than the current picture being decoded. In order to avoid additional processing steps, the RPR process in VVC is designed to be embedded in the motion compensation process and performed at the block level. In the motion compensation stage, the scaling ratio is used together with motion information to locate the reference samples in the reference picture to be used in the interpolation process. In VVC, the scaling ratio is restricted to be larger than or equal to 1/2 (2 times downsampling from the reference picture to the current picture), and less than or equal to 8 (8 times upsampling). Three sets of resampling filters with different frequency cutoffs are specified to handle various scaling ratios between a reference picture and the current picture. The three sets of resampling filters are applied respectively for the scaling ratio ranging from 1/2 to 1/1.75, from 1/1.75 to 1/1.25, and from 1/1.25 to 8. Each set of resampling filters has 16 phases for luma and 32 phases for chroma which is same to the case of motion compensation interpolation filters. It is worthy noted that the filter set of normal MC interpolation is used in the case of scaling ratio ranging from 1/1.25 to 8. Actually the normal MC interpolation process is a special case of the resampling process with scaling ratio ranging from 1/1.25 to 8. In addition to conventional translational block motion, the affine mode has three sets of 6-tap interpolation filters that are used for the luma component to cover the different scaling ratios in RPR. The horizontal and vertical scaling ratios are derived based on picture width and height, and the left, right, top and bottom scaling offsets specified for the reference picture and the current picture. 2.2.13.1 Signalling of resolution and cropping window In VVC, the maximum picture resolution and the corresponding conformance cropping window are signalled in the SPS, while in the PPS the picture resolution of each current picture is signalled. Such signalling arrangement can be used to support RPR. When the picture resolution of the current picture (from PPS) is smaller than the maximum picture resolution (from SPS), it is possible to signal conformance cropping offsets in the PPS that are different from the conformance cropping offsets signalled in the SPS. 2.2.14 Miscellaneous inter prediction aspects To reduce memory bandwidth, the inter-coded 4x4 size CU is not allowed in VVC. For inter-coded 4x8/8x4 CU, only uni-directional mode is allowed. When the motion information from merge mode is bi-directional, it is converted to uni-directional by keeping only the list 0 motion information. 3. Problems Existing video coding standards and their test models do not consider any auxiliary information that may be produced at the content producer. However, some auxiliary information can be obtained accurately, which can help improving video coding efficiency and/or reducing encoding/decoding complexity. 4. Detailed Solutions To solve the above problems, methods as summarized below are disclosed. The solutions should be considered as examples to explain the general concepts and should not be interpreted in a narrow way. Furthermore, these solutions can be applied individually or combined in any manner. 1) One or more reference pictures may be generated from coded pictures and corresponding auxiliary information available at the encoder to predict the current picture. a. In one example, the corresponding auxiliary information may be lossy coded and sent to the decoder and both the encoder and decoder may generate reference pictures based on the same reconstructed auxiliary information. b. In one example, those reference pictures generated may be inserted into the decoded picture buffer (DPB) for inter picture prediction. i. In one example, those reference pictures may be inserted to some fixed positions in the DPB. 1. In one example, those reference pictures may be inserted before all other reference pictures. 2. In one example, those reference pictures may be inserted after all other reference pictures. ii. In one example, when one coded picture is removed from the DPB, its corresponding generated reference pictures are also removed from the DPB. iii. In one example, those generated reference pictures are all marked as reference pictures. 1. In one example, those generated reference pictures are all marked as long-term reference pictures. iv. In one example, all motion vectors associated to those generated reference pictures are with values of (0, 0), i.e. zero motion vector. v. In one example, whether and/or how to use generated reference pictures when coding a block may be indicated by one or more syntax elements in the bitstream. 1. In one example, when a generated reference picture is used when coding a block, the reference index for the coded picture from which the generated reference picture is from; and a flag or index indicating whether or which generated reference picture is used may be signalled in the bitstream. c. In one example, one reference picture may be generated from a coded picture and its associated depth information and/or camera parameters. i. In one example, the depth information may be coded in the bitstream. ii. In one example, a depth-image- based-rendering method may be used to generate the reference picture. d. In one example, one reference picture may be generated from a coded picture and its associated motion flow information. i. In one example, the motion flow information may be coded in the bitstream. 2) A prediction area may be generated from coded pictures and corresponding auxiliary information to predict the current coding area. a. In one example, the prediction area and the current coding area may be both of rectangular. i. In one example, the prediction area and the current coding area are blocks with a size of 2^M, 2^N. b. In one example, the prediction area and the current coding area may be both of triangle. c. In one example, the final prediction may be a fusion among the prediction area generated and other predictions, for example, intra prediction, generalized bi- directional prediction. d. In one example, whether and how to use the prediction area for the current coding area may be indicated by one or more syntax elements in the bitstream. e. In one example, the prediction area is generated from a coded picture and its associated depth information and/or camera parameters. i. In one example, the depth information may be coded in the bitstream. ii. In one example, a depth-image- based-rendering method may be used to generate the prediction area. f. In one example, the prediction area is generated from a coded picture and its associated motion flow information. i. In one example, the motion flow information may be coded in the bitstream. 3) Auxiliary information, such as motion flow, depth, may be conveyed with an SEI message. [0082] More details of the embodiments of the present disclosure will be described below which are related to inter prediction with auxiliary information. The embodiments of the present disclosure should be considered as examples to explain the general concepts and should not be interpreted in a narrow way. Furthermore, these embodiments can be applied individually or combined in any manner. [0083] As used herein, the term “block” may represent a coding tree block (CTB), a coding tree unit (CTU), a coding block (CB), a coding unit (CU), a prediction unit (PU), a transform unit (TU), a prediction block (PB), a transform block (TB), a subblock, a tile, a slice, a subpicture, a video processing unit comprising multiple samples/pixels, and/or the like. A block may be rectangular or non-rectangular. [0084] Fig. 24 illustrates a flowchart of a method 2400 for video processing in accordance with some embodiments of the present disclosure. The method 2400 may be implemented during a conversion between a current block of a video and a bitstream of the video. As shown in Fig. 24, the method 2400 starts at 2402 where a prediction block of the current block or at least one picture for the current block is generated based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures. The one or more coded pictures are coded before the current block, that is, the one or more coded pictures are coded before a picture comprising the current block. [0085] In addition, the auxiliary information comprises at least one of motion flow information or depth information. As used herein, the motion flow may quantify the motion of an object between consecutive frames captured by a video capture device, such as a camera or the like. The motion flow may also be referred to as an optical flow. Moreover, the term “depth” refers to the distance information from an object in a scene to a video capture device or an observer. Generally, depth represents a third dimension in a 3D scene, complementing the two spatial dimensions of an image (usually the x- and y-axes). In a depth map, each pixel corresponds to a specific depth value, which indicates the distance of the object or point in the real world from the reference plane, typically the plane of the camera sensor. In one example embodiment, each sample in a coded picture may possess such motion flow information and/or depth information. In other words, the auxiliary information is a sample-level or pixel level information. Alternatively, a plurality of samples in a coded picture may share the same motion flow information and/or depth information. It should be understood that the possible implementations of the auxiliary information described above are merely illustrative and the auxiliary information may comprise any other suitable information. The scope of the present disclosure is not limited in this respect. [0086] In some embodiments, the auxiliary information may be determined at a producer of the video, a.k.a., a content producer. By way of example, the producer may comprise a camera capturing a video, a website or an application that processes raw data and generates a video, and/or the like. In one example embodiment, the auxiliary information may be lossy encoded and indicated in the bitstream. At the encoder side, the auxiliary information may be reconstructed from the encoded data, and at the decoder side, the auxiliary information may be reconstructed from the bitstream. Moreover, both the encoder and decoder may use the same reconstructed auxiliary information to generate the prediction block of the current block or at least one picture for the current block. Alternatively, the auxiliary information may be losslessly encoded and indicated in the bitstream. By way of example rather than limitation, the auxiliary information may be comprised in a supplemental enhancement information (SEI) message in the bitstream. It should be noted that the auxiliary information may also be comprised in any other suitable syntax structure in the bitstream. The scope of the present disclosure is not limited in this respect. [0087] At 2404, the conversion is performed based on the prediction block or the at least one picture. In some embodiments, the conversion may include encoding the current block into the bitstream. Alternatively or additionally, the conversion may include decoding the current block from the bitstream. It should be understood that the above illustrations and/or examples are described merely for purpose of description. The scope of the present disclosure is not limited in this respect. [0088] In view of the above, the auxiliary information (such as motion flow information, depth information, or the like) is used for coding the video. Compared with the conventional solution where such auxiliary information is not considered, the proposed solution can advantageously reduce coding complexity and improve the quality of the reference picture and/or the prediction block for the current block. Thereby, the coding efficiency and the coding quality can be improved. [0089] In some embodiments, at 2402, at least one picture for the current block may be generated based on the one or more coded pictures and the auxiliary information corresponding to the one or more coded pictures. In this case, the at least one generated picture may be different from any picture in the video. That is, each of the at least one generated picture may be a new picture. In addition, the conversion may be performed based on the at least one picture at 2404. [0090] In some embodiments, the at least one picture may be inserted into a decoded picture buffer (DPB) for the conversion. For example, the DPB may be used for inter prediction. The at least one picture may be inserted to a fixed position in the DPB. In one example embodiment, the fixed position may precede all reference pictures in the DPB. That is, the at least one picture may be inserted before all reference pictures existing in the DPB. In another example embodiment, the fixed position may follow all reference pictures in the DPB. That is, the at least one picture may be inserted after all reference pictures existing in the DPB. It should be noted that the at least one picture may also be inserted into any other suitable position(s) in the DPB, and the scope of the present disclosure is not limited in this respect. [0091] In some embodiments, each of the at least one picture may be marked as a reference picture for the current block. In this case, the at least one generated picture may be used for generating a prediction of the current block. In one example embodiment, each of the at least one picture may be marked as a long-term reference picture for the current block. Alternatively, each of the at least one picture may be marked as a short-term reference picture for the current block. The scope of the present disclosure is not limited in this respect. [0092] In some embodiments, the at least one picture may comprise a first picture generated based on a first coded picture among the one or more coded pictures. In this case, if the first coded picture is removed from the DPB, the first picture may also be removed from the DPB. In some embodiments, a motion vector associated with the at least one picture may be a zero motion vector. That is, a value of the motion vector is (0, 0). Moreover, the motion vector may be not indicated in the bitstream. [0093] In some embodiments, the bitstream may comprise one or more indications indicating at least one of the following: whether to use the at least one picture for coding the current block, or how to use the at least one picture for coding the current block. In some embodiments, the at least one picture may comprise a second picture generated based on a second coded picture among the one or more coded pictures. In this case, if the second picture is used for coding the current block, the bitstream may comprise at least one of the following: an indication indicating a reference index of the second coded picture, an indication indicating whether to use the at least one picture for coding the current block, or an indication indicating which one of the at least one picture may be used for coding the current block. [0094] In some embodiments, the auxiliary information may comprise depth information. A third picture of the at least one picture may be generated based on the depth information corresponding to the one or more coded pictures and/or at least one parameter of a device for capturing the video. For example, the at least one parameter of the video capture device may comprise a physical position, a focal length, a field of view (FOV), and/or the like. The depth information may be indicated in the bitstream. By way of example, the third picture may be generated by using a depth-image-based-rendering scheme. [0095] As used herein, the depth-image-based-rendering scheme may refer to a process of synthesizing virtual views of the scene from captured color images or videos with associated depth information. For example, the points on an original image plane may be warped to a three-dimensional (3D) world coordinates. Furthermore, the real 3D points may be back- projected onto the virtual image plane which is located at the required viewing position defined by user at the receiver side. [0096] In some additional or alternative embodiments, the auxiliary information may comprise motion flow information, and one of the at least one picture may be generated based on the motion flow information corresponding to the one or more coded pictures. For example, the motion flow information may be indicated in the bitstream. [0097] In some embodiments, at 2402, a prediction block of the current block may be generated based on the one or more coded pictures and the auxiliary information corresponding to the one or more coded pictures. For example, the prediction block and the current block may be of a same shape, such as rectangular, triangle, or the like. Additionally or alternatively, the prediction block and the current block may be of a same size. For example, a width of the current block may be 2^M, a height of the current block may be 2^N, and each of N and M may be an integer. It should be noted that the prediction block and the current block may also be of different shapes and/or sizes, and the scope of the present disclosure is not limited in this respect. [0098] In addition, at 2404, the conversion may be performed based on the prediction block. For example, a target prediction block of the current block may be determined based on the generated prediction block and at least one further prediction block of the current block, and the conversion is performed based on the target prediction block. By way of example rather than limitation, the at least one further prediction block may be determined based on intra prediction, generalized bi-directional prediction, and/or the like. In the generalized bi- directional prediction, the weighting factors for two predictions for the current block are allowed to be different. [0099] In some embodiments, the bitstream may comprise one or more indications indicating at least one of the following: whether to use the generated prediction block for coding the current block, or how to use the generated prediction block for coding the current block. [0100] In some embodiments, the auxiliary information may comprise depth information. In this case, the prediction block of the current block may be generated based on the depth information corresponding to the one or more coded pictures and/or at least one parameter of a device for capturing the video. For example, the at least one parameter of the video capture device may comprise a physical position, a focal length, FOV, and/or the like. The depth information may be indicated in the bitstream. By way of example, the prediction block of the current block may be generated by using the above-mentioned depth-image- based-rendering scheme. [0101] In some additional or alternative embodiments, the auxiliary information may comprise motion flow information, and the prediction block of the current block may be generated based on the motion flow information corresponding to the one or more coded pictures. For example, the motion flow information may be indicated in the bitstream. [0102] In view of the above, the solutions in accordance with some embodiments of the present disclosure can advantageously improve coding efficiency and coding quality. [0103] According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and generating the bitstream based on the prediction block or the at least one picture. [0104] According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. The method comprises: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; generating the bitstream based on the prediction block or the at least one picture; and storing the bitstream in a non-transitory computer-readable recording medium. [0105] Implementations of the present disclosure can be described in view of the following clauses, the features of which can be combined in any reasonable manner. [0106] Clause 1. A method for video processing, comprising: generating, for a conversion between a current block of a video and a bitstream of the video, a prediction block of the current block or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and performing the conversion based on the prediction block or the at least one picture. [0107] Clause 2. The method of clause 1, wherein the auxiliary information is determined at a producer of the video. [0108] Clause 3. The method of any of clauses 1-2, wherein the auxiliary information is lossy encoded and indicated in the bitstream. [0109] Clause 4. The method of clause 3, wherein the auxiliary information is reconstructed from the bitstream. [0110] Clause 5. The method of any of clauses 1-4, wherein the at least one picture is inserted into a decoded picture buffer (DPB) for the conversion. [0111] Clause 6. The method of clause 5, wherein the at least one picture is inserted to a fixed position in the DPB. [0112] Clause 7. The method of clause 6, wherein the fixed position precedes all reference pictures in the DPB. [0113] Clause 8. The method of clause 6, wherein the fixed position follows all reference pictures in the DPB. [0114] Clause 9. The method of any of clauses 5-8, wherein the at least one picture comprises a first picture generated based on a first coded picture among the one or more coded pictures, and in accordance with a determination that the first coded picture is removed from the DPB, the first picture is removed from the DPB. [0115] Clause 10. The method of any of clauses 1-9, wherein each of the at least one picture is marked as a reference picture for the current block. [0116] Clause 11. The method of any of clauses 1-10, wherein each of the at least one picture is marked as a long-term reference picture for the current block. [0117] Clause 12. The method of any of clauses 1-11, wherein a motion vector associated with the at least one picture is a zero motion vector. [0118] Clause 13. The method of any of clauses 1-12, wherein the bitstream comprises one or more indications indicating at least one of the following: whether to use the at least one picture for coding the current block, or how to use the at least one picture for coding the current block. [0119] Clause 14. The method of any of clauses 1-13, wherein the at least one picture comprises a second picture generated based on a second coded picture among the one or more coded pictures, and in accordance with a determination that the second picture is used for coding the current block, the bitstream comprises at least one of the following: an indication indicating a reference index of the second coded picture, an indication indicating whether to use the at least one picture for coding the current block, or an indication indicating which one of the at least one picture is used for coding the current block. [0120] Clause 15. The method of any of clauses 1-14, wherein the auxiliary information comprises depth information, and a third picture of the at least one picture is generated based on the depth information corresponding to the one or more coded pictures and/or at least one parameter of a device for capturing the video. [0121] Clause 16. The method of clause 15, wherein the depth information is indicated in the bitstream. [0122] Clause 17. The method of any of clauses 15-16, wherein the third picture is generated by using a depth-image-based-rendering scheme. [0123] Clause 18. The method of any of clauses 1-17, wherein the auxiliary information comprises motion flow information, and one of the at least one picture is generated based on the motion flow information corresponding to the one or more coded pictures. [0124] Clause 19. The method of clause 18, wherein the motion flow information is indicated in the bitstream. [0125] Clause 20. The method of any of clauses 1-19, wherein the prediction block and the current block are of a same shape. [0126] Clause 21. The method of clause 20, wherein the shape is rectangular or triangle. [0127] Clause 22. The method of any of clauses 1-21, wherein the prediction block and the current block are of a same size. [0128] Clause 23. The method of clause 22, wherein a width of the current block is 2^M, a height of the current block is 2^N, and each of N and M is an integer. [0129] Clause 24. The method of any of clauses 1-23, wherein performing the conversion comprises: determining a target prediction block of the current block based on the generated prediction block and at least one further prediction block of the current block; and performing the conversion based on the target prediction block. [0130] Clause 25. The method of clause 24, wherein the at least one further prediction block is determined based on at least one of intra prediction or generalized bi-directional prediction. [0131] Clause 26. The method of any of clauses 1-25, wherein the bitstream comprises one or more indications indicating at least one of the following: whether to use the generated prediction block for coding the current block, or how to use the generated prediction block for coding the current block. [0132] Clause 27. The method of any of clauses 1-26, wherein the auxiliary information comprises depth information, and the prediction block of the current block is generated based on the depth information corresponding to the one or more coded pictures and/or at least one parameter of a device for capturing the video. [0133] Clause 28. The method of clause 27, wherein the depth information is indicated in the bitstream. [0134] Clause 29. The method of any of clauses 27-28, wherein the prediction block is generated by using a depth-image-based-rendering scheme. [0135] Clause 30. The method of any of clauses 1-29, wherein the auxiliary information comprises motion flow information, and the prediction block of the current block is generated based on the motion flow information corresponding to the one or more coded pictures. [0136] Clause 31. The method of clause 30, wherein the motion flow information is indicated in the bitstream. [0137] Clause 32. The method of any of clauses 1-31, wherein the auxiliary information is comprised in a supplemental enhancement information (SEI) message in the bitstream. [0138] Clause 33. The method of any of clauses 1-32, wherein the conversion includes encoding the current block into the bitstream. [0139] Clause 34. The method of any of clauses 1-32, wherein the conversion includes decoding the current block from the bitstream. [0140] Clause 35. An apparatus for video processing comprising a processor and a non- transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to perform a method in accordance with any of clauses 1-34. [0141] Clause 36. A non-transitory computer-readable storage medium storing instructions that cause a processor to perform a method in accordance with any of clauses 1-34. [0142] Clause 37. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and generating the bitstream based on the prediction block or the at least one picture. [0143] Clause 38. A method for storing a bitstream of a video, comprising: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; generating the bitstream based on the prediction block or the at least one picture; and storing the bitstream in a non-transitory computer-readable recording medium. Example Device [0144] Fig. 25 illustrates a block diagram of a computing device 2500 in which various embodiments of the present disclosure can be implemented. The computing device 2500 may be implemented as or included in the source device 110 (or the video encoder 114 or 200) or the destination device 120 (or the video decoder 124 or 300). [0145] It would be appreciated that the computing device 2500 shown in Fig. 25 is merely for purpose of illustration, without suggesting any limitation to the functions and scopes of the embodiments of the present disclosure in any manner. [0146] As shown in Fig. 25, the computing device 2500 includes a general-purpose computing device 2500. The computing device 2500 may at least comprise one or more processors or processing units 2510, a memory 2520, a storage unit 2530, one or more communication units 2540, one or more input devices 2550, and one or more output devices 2560. [0147] In some embodiments, the computing device 2500 may be implemented as any user terminal or server terminal having the computing capability. The server terminal may be a server, a large-scale computing device or the like that is provided by a service provider. The user terminal may for example be any type of mobile terminal, fixed terminal, or portable terminal, including a mobile phone, station, unit, device, multimedia computer, multimedia tablet, Internet node, communicator, desktop computer, laptop computer, notebook computer, netbook computer, tablet computer, personal communication system (PCS) device, personal navigation device, personal digital assistant (PDA), audio/video player, digital camera/video camera, positioning device, television receiver, radio broadcast receiver, E-book device, gaming device, or any combination thereof, including the accessories and peripherals of these devices, or any combination thereof. It would be contemplated that the computing device 2500 can support any type of interface to a user (such as “wearable” circuitry and the like). [0148] The processing unit 2510 may be a physical or virtual processor and can implement various processes based on programs stored in the memory 2520. In a multi-processor system, multiple processing units execute computer executable instructions in parallel so as to improve the parallel processing capability of the computing device 2500. The processing unit 2510 may also be referred to as a central processing unit (CPU), a microprocessor, a controller or a microcontroller. [0149] The computing device 2500 typically includes various computer storage medium. Such medium can be any medium accessible by the computing device 2500, including, but not limited to, volatile and non-volatile medium, or detachable and non-detachable medium. The memory 2520 can be a volatile memory (for example, a register, cache, Random Access Memory (RAM)), a non-volatile memory (such as a Read-Only Memory (ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), or a flash memory), or any combination thereof. The storage unit 2530 may be any detachable or non-detachable medium and may include a machine-readable medium such as a memory, flash memory drive, magnetic disk or another other media, which can be used for storing information and/or data and can be accessed in the computing device 2500. [0150] The computing device 2500 may further include additional detachable/non- detachable, volatile/non-volatile memory medium. Although not shown in Fig. 25, it is possible to provide a magnetic disk drive for reading from and/or writing into a detachable and non-volatile magnetic disk and an optical disk drive for reading from and/or writing into a detachable non-volatile optical disk. In such cases, each drive may be connected to a bus (not shown) via one or more data medium interfaces. [0151] The communication unit 2540 communicates with a further computing device via the communication medium. In addition, the functions of the components in the computing device 2500 can be implemented by a single computing cluster or multiple computing machines that can communicate via communication connections. Therefore, the computing device 2500 can operate in a networked environment using a logical connection with one or more other servers, networked personal computers (PCs) or further general network nodes. [0152] The input device 2550 may be one or more of a variety of input devices, such as a mouse, keyboard, tracking ball, voice-input device, and the like. The output device 2560 may be one or more of a variety of output devices, such as a display, loudspeaker, printer, and the like. By means of the communication unit 2540, the computing device 2500 can further communicate with one or more external devices (not shown) such as the storage devices and display device, with one or more devices enabling the user to interact with the computing device 2500, or any devices (such as a network card, a modem and the like) enabling the computing device 2500 to communicate with one or more other computing devices, if required. Such communication can be performed via input/output (I/O) interfaces (not shown). [0153] In some embodiments, instead of being integrated in a single device, some or all components of the computing device 2500 may also be arranged in cloud computing architecture. In the cloud computing architecture, the components may be provided remotely and work together to implement the functionalities described in the present disclosure. In some embodiments, cloud computing provides computing, software, data access and storage service, which will not require end users to be aware of the physical locations or configurations of the systems or hardware providing these services. In various embodiments, the cloud computing provides the services via a wide area network (such as Internet) using suitable protocols. For example, a cloud computing provider provides applications over the wide area network, which can be accessed through a web browser or any other computing components. The software or components of the cloud computing architecture and corresponding data may be stored on a server at a remote position. The computing resources in the cloud computing environment may be merged or distributed at locations in a remote data center. Cloud computing infrastructures may provide the services through a shared data center, though they behave as a single access point for the users. Therefore, the cloud computing architectures may be used to provide the components and functionalities described herein from a service provider at a remote location. Alternatively, they may be provided from a conventional server or installed directly or otherwise on a client device. [0154] The computing device 2500 may be used to implement video encoding/decoding in embodiments of the present disclosure. The memory 2520 may include one or more video coding modules 2525 having one or more program instructions. These modules are accessible and executable by the processing unit 2510 to perform the functionalities of the various embodiments described herein. [0155] In the example embodiments of performing video encoding, the input device 2550 may receive video data as an input 2570 to be encoded. The video data may be processed, for example, by the video coding module 2525, to generate an encoded bitstream. The encoded bitstream may be provided via the output device 2560 as an output 2580. [0156] In the example embodiments of performing video decoding, the input device 2550 may receive an encoded bitstream as the input 2570. The encoded bitstream may be processed, for example, by the video coding module 2525, to generate decoded video data. The decoded video data may be provided via the output device 2560 as the output 2580. [0157] While this disclosure has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present application as defined by the appended claims. Such variations are intended to be covered by the scope of this present application. As such, the foregoing description of embodiments of the present application is not intended to be limiting.

Claims

I/We Claim: 1. A method for video processing, comprising: generating, for a conversion between a current block of a video and a bitstream of the video, a prediction block of the current block or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and performing the conversion based on the prediction block or the at least one picture.

2. The method of claim 1, wherein the auxiliary information is determined at a producer of the video.

3. The method of any of claims 1-2, wherein the auxiliary information is lossy encoded and indicated in the bitstream.

4. The method of claim 3, wherein the auxiliary information is reconstructed from the bitstream.

5. The method of any of claims 1-4, wherein the at least one picture is inserted into a decoded picture buffer (DPB) for the conversion.

6. The method of claim 5, wherein the at least one picture is inserted to a fixed position in the DPB.

7. The method of claim 6, wherein the fixed position precedes all reference pictures in the DPB.

8. The method of claim 6, wherein the fixed position follows all reference pictures in the DPB.

9. The method of any of claims 5-8, wherein the at least one picture comprises a first picture generated based on a first coded picture among the one or more coded pictures, and in accordance with a determination that the first coded picture is removed from the DPB, the first picture is removed from the DPB.

10. The method of any of claims 1-9, wherein each of the at least one picture is marked as a reference picture for the current block.

11. The method of any of claims 1-10, wherein each of the at least one picture is marked as a long-term reference picture for the current block.

12. The method of any of claims 1-11, wherein a motion vector associated with the at least one picture is a zero motion vector.

13. The method of any of claims 1-12, wherein the bitstream comprises one or more indications indicating at least one of the following: whether to use the at least one picture for coding the current block, or how to use the at least one picture for coding the current block.

14. The method of any of claims 1-13, wherein the at least one picture comprises a second picture generated based on a second coded picture among the one or more coded pictures, and in accordance with a determination that the second picture is used for coding the current block, the bitstream comprises at least one of the following: an indication indicating a reference index of the second coded picture, an indication indicating whether to use the at least one picture for coding the current block, or an indication indicating which one of the at least one picture is used for coding the current block.

15. The method of any of claims 1-14, wherein the auxiliary information comprises depth information, and a third picture of the at least one picture is generated based on the depth information corresponding to the one or more coded pictures and/or at least one parameter of a device for capturing the video.

16. The method of claim 15, wherein the depth information is indicated in the bitstream.

17. The method of any of claims 15-16, wherein the third picture is generated by using a depth-image-based-rendering scheme.

18. The method of any of claims 1-17, wherein the auxiliary information comprises motion flow information, and one of the at least one picture is generated based on the motion flow information corresponding to the one or more coded pictures.

19. The method of claim 18, wherein the motion flow information is indicated in the bitstream.

20. The method of any of claims 1-19, wherein the prediction block and the current block are of a same shape.

21. The method of claim 20, wherein the shape is rectangular or triangle.

22. The method of any of claims 1-21, wherein the prediction block and the current block are of a same size.

23. The method of claim 22, wherein a width of the current block is 2^M, a height of the current block is 2^N, and each of N and M is an integer.

24. The method of any of claims 1-23, wherein performing the conversion comprises: determining a target prediction block of the current block based on the generated prediction block and at least one further prediction block of the current block; and performing the conversion based on the target prediction block.

25. The method of claim 24, wherein the at least one further prediction block is determined based on at least one of intra prediction or generalized bi-directional prediction.

26. The method of any of claims 1-25, wherein the bitstream comprises one or more indications indicating at least one of the following: whether to use the generated prediction block for coding the current block, or how to use the generated prediction block for coding the current block.

27. The method of any of claims 1-26, wherein the auxiliary information comprises depth information, and the prediction block of the current block is generated based on the depth information corresponding to the one or more coded pictures and/or at least one parameter of a device for capturing the video.

28. The method of claim 27, wherein the depth information is indicated in the bitstream.

29. The method of any of claims 27-28, wherein the prediction block is generated by using a depth-image-based-rendering scheme.

30. The method of any of claims 1-29, wherein the auxiliary information comprises motion flow information, and the prediction block of the current block is generated based on the motion flow information corresponding to the one or more coded pictures.

31. The method of claim 30, wherein the motion flow information is indicated in the bitstream.

32. The method of any of claims 1-31, wherein the auxiliary information is comprised in a supplemental enhancement information (SEI) message in the bitstream.

33. The method of any of claims 1-32, wherein the conversion includes encoding the current block into the bitstream.

34. The method of any of claims 1-32, wherein the conversion includes decoding the current block from the bitstream.

35. An apparatus for video processing comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to perform a method in accordance with any of claims 1-34.

36. A non-transitory computer-readable storage medium storing instructions that cause a processor to perform a method in accordance with any of claims 1-34.

37. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; and generating the bitstream based on the prediction block or the at least one picture.

38. A method for storing a bitstream of a video, comprising: generating a prediction block of a current block of the video or at least one picture for the current block based on one or more coded pictures and auxiliary information corresponding to the one or more coded pictures, the one or more coded pictures being coded before the current block, and the auxiliary information comprising at least one of motion flow information or depth information; generating the bitstream based on the prediction block or the at least one picture; and storing the bitstream in a non-transitory computer-readable recording medium.