Patent Description:
This patent document relates to video coding techniques, devices and systems.

In spite of the advances in video compression, digital video still accounts for the largest bandwidth use on the internet and other digital communication networks. As the number of connected user devices capable of receiving and displaying video increases, it is expected that the bandwidth demand for digital video usage will continue to grow. Lee H et al: "non-CE4: Simplification of decoding process for SMVD reference indices", <NUM>. MPEG Meeting; <NUM> - <NUM>; Geneva, (Motion Picture Expert Group of ISO/IEC JTC1/SC29/WG11), no. m47133 <NUM> March <NUM> (<NUM>-<NUM>-<NUM>), proposes that the first L0 and L1 reference picture in each reference picture list are only considered as reference pictures of SMVD to simplify the decoding process of SMVD reference indices. Chen (Huawei) H et al: "CE4: Symmetrical MVD mode (Test <NUM>. <NUM>)", <NUM>. MPEG Meeting; <NUM> - <NUM>; Macao; (Motion Picture Expert Group of ISO/IEC JTC1/SC29/WG11), no. JVET-L0370-v1; m44396 <NUM> September <NUM> (<NUM>-<NUM>-<NUM>), pages <NUM>-<NUM>; reports the results of integrating symmetrical mode for bi-prediction over VTM-<NUM>. Lee J et al: "CE3: Simplification of MPM derivation (CE3-<NUM>)", <NUM>. JVET Meeting; <NUM> - <NUM>; Geneva, (The Joint Video Exploration Team of ISO/IEC JTC1/SC29/WG11 and ITU-T SG. <NUM>), no. JVET-N0134 <NUM> March <NUM> (<NUM>-<NUM>-<NUM>), concerns a simplification of MPM derivation.

It is therefore the object of the present invention to provide an improved method of processing video data, a corresponding apparatus for processing video data and a computer-readable storage medium with instructions for causing a corresponding method, as well as a method for storing a corresponding bitstream of a video and a computer-readable storage medium storing a bitstream generated by a corresponding method.

The present document describes various embodiments and techniques in which video coding or decoding is performed using motion vector difference coding that uses a symmetric coding or decoding technique.

In one example aspect, a method of video processing is disclosed. The method includes determining to use, for a conversion between a current video block and a bitstream representation of the current video block, a symmetric motion vector difference (SMVD) mode in which two additional motion candidates generated for the conversion based on symmetric displacement of reference blocks in two target reference pictures from a reference list <NUM> and a reference list <NUM> of reference pictures and performing the conversion using the SMVD mode. The bitstream representation indicates to disable use of a motion vector difference (MVD) mode for one of the reference list <NUM> or the reference list <NUM>.

In another example aspect, another method of video processing is disclosed. The method includes determining to use, for a conversion between a current video block and a bitstream representation of the current video block, a symmetric motion vector difference (SMVD) mode in which two additional motion candidates generated for the conversion based on symmetric displacement of reference blocks in pictures from two target reference pictures from a reference list <NUM> and a reference list <NUM> of reference pictures and performing the conversion using the SMVD mode. The bitstream representation includes a motion vector difference (MVD) information for the reference list <NUM>.

In another example aspect, another method of video processing is disclosed. The method includes determining to use, for a conversion between a current video block and a bitstream representation of the current video block, a symmetric motion vector difference (SMVD) mode in which two additional motion candidates generated for the conversion based on symmetric displacement of reference blocks in pictures from a reference list <NUM> and a reference list <NUM> of reference pictures and performing the conversion using the SMVD mode. The SMVD generates the two additional motion candidates using a first step in which each reference picture list is parsed to identify target reference pictures.

In another example aspect, another method of video processing is disclosed. The method includes determining, based on a condition to use a symmetric motion vector difference (SMVD) mode in which two additional motion candidates generated for the conversion based on symmetric displacement of reference blocks in two target reference pictures from a reference list <NUM> and a reference list <NUM> of reference pictures, and a decoder side motion vector difference (DMVD) coding mode for a conversion between a current video block and a bitstream representation of the current video block and performing the conversion based on the determining.

In another example aspect, another method of video processing is disclosed. The method includes determining, based on a condition, whether or not to use an all-zero motion vector difference for a symmetric motion vector difference (SMVD) mode in which two additional motion candidates generated for the conversion based on symmetric displacement of reference blocks in two target reference pictures from a reference list <NUM> and a reference list <NUM> of reference pictures, and a decoder side motion vector difference (DMVD) coding mode for a conversion between a current video block and a bitstream representation of the current video block and performing the conversion based on the determining.

In another example aspect, another method of video processing is disclosed. The method includes selecting a first reference picture and a second reference picture from a plurality of reference pictures associated with a current video block, and performing, based on the selected first and second reference pictures, a conversion between the current video block and a bitstream representation of the current video block, wherein a first distance between the first reference picture and a current picture comprising the current video block and a second distance between the second reference picture and the current picture are equal.

In another example aspect, another method of video processing is disclosed. The method includes determining that a first distance between a first reference picture and a current picture comprising a current video block and a second distance between a second reference picture and the current picture are unequal, and disabling, based on the determining and for a conversion between the current video block and a bitstream representation of the current video block, a symmetric motion vector difference (SMVD) mode for the current video block.

In another example aspect, another method of video processing is disclosed. The method includes deriving, for a current video block, a first motion vector difference (MVD) of a first reference picture list based on a second MVD of a second reference picture list, and performing, based on the deriving, a conversion between the current video block and a bitstream representation of the current video block, wherein the a range of the first MVD is restricted.

In one example aspect, a method of video processing is disclosed. The method includes determining, for a conversion between a block of current picture of video and a bitstream representation of the first block, whether a symmetric motion vector difference (SMVD) mode is allowable for the block based on coding information of the block; and performing the conversion based on the determination.

In one example aspect, a method of video processing is disclosed. The method includes determining, for a conversion between a block of video and a bitstream representation of the first block, whether a symmetric motion vector (SMV) mode is allowable for the block according to the method of previous examples; and performing the conversion based on the determination.

In one example aspect, a method of video processing is disclosed. The method includes determining, for a conversion between a block of a current picture of video and a bitstream representation of the block, whether a symmetric motion vector difference (SMVD) mode is allowable for the block, wherein whether the SMVD mode is allowable depends on a derivation process of two target reference pictures of the current picture, which includes a first step for searching a forward target reference picture in the reference picture list <NUM> and searching a backward target reference picture in the reference picture list <NUM>, and a second step for searching a backward target reference picture in the reference picture list <NUM> and searching a forward target reference picture in the reference picture list <NUM>, outputs of the first step and the second step being independent of each other; and performing the conversion based on the determination.

In one example aspect, a method of video processing is disclosed. The method includes deriving, for a conversion between a block of a current picture of video and a bitstream representation of the block, a second motion vector difference (MVD) of one reference picture list from a first MVD associated with the other reference picture list, wherein an MVD includes both a horizontal component and a vertical component; applying a range restriction on at least one of the first MVD and the second MVD; and performing the conversion based on at least one of the restricted first MVD and the restricted second MVD.

In yet another example aspect, a video processing apparatus is disclosed. The apparatus includes a processor configured to perform an-above disclosed method. The apparatus may further perform video encoding or video decoding.

In yet another example aspect, a computer readable medium is disclosed. The medium has code for processor-implementation of the above-described methods stored on it.

These, and other, aspects are described in the present document.

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.

This patent document is related to video coding technologies. Specifically, it is related to symmetric motion vector difference coding. It may be applied to the existing video coding standard like HEVC, or the standard (Versatile Video Coding) to be finalized. It may be also applicable to future video coding standards or video codec.

Video coding standards have evolved primarily through the development of the well-known ITU-T and ISO/IEC standards. The ITU-T produced H. <NUM> and H. <NUM>, ISO/IEC produced MPEG-<NUM> and MPEG-<NUM> Visual, and the two organizations jointly produced the H. <NUM>/MPEG-<NUM> Video and H. <NUM>/MPEG-<NUM> Advanced Video Coding (AVC) and H. <NUM>/HEVC standards. <NUM>, 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, Joint Video Exploration Team (JVET) was founded by VCEG and MPEG jointly in <NUM>. Since then, many new methods have been adopted by JVET and put into the reference software named Joint Exploration Model (JEM). In April <NUM>, the Joint Video Expert Team (JVET) between VCEG (Q6/<NUM>) and ISO/IEC JTC1 SC29/WG11 (MPEG) was created to work on the VVC standard targeting at <NUM>% bitrate reduction compared to HEVC.

Each inter-predicted PU has motion parameters for one or two reference picture lists. Motion parameters include a motion vector and a reference picture index. Usage of one of the two reference picture lists may also be signalled using inter_pred_idc. Motion vectors may be explicitly coded as deltas relative to predictors.

When a CU is coded with skip mode, one PU is associated with the CU, and there are no significant residual coefficients, no coded motion vector delta or reference picture index. A merge mode is specified whereby the motion parameters for the current PU are obtained from neighbouring PUs, including spatial and temporal candidates. The merge mode can be applied to any inter-predicted PU, not only for skip mode. The alternative to merge mode is the explicit transmission of motion parameters, where motion vector (to be more precise, motion vector differences (MVD) compared to a motion vector predictor), corresponding reference picture index for each reference picture list and reference picture list usage are signalled explicitly per each PU. Such a mode is named Advanced motion vector prediction (AMVP) in this disclosure.

When signalling indicates that one of the two reference picture lists is to be used, the PU is produced from one block of samples. This is referred to as 'uni-prediction'. Uni-prediction is available both for P-slices and B-slices.

When signalling indicates that both of the reference picture lists are to be used, the PU is produced from two blocks of samples. This is referred to as 'bi-prediction'. Bi-prediction is available for B-slices only.

The following text provides the details on the inter prediction modes specified in HEVC. The description will start with the merge mode.

In HEVC, the term inter prediction is used to denote prediction derived from data elements (e.g., sample values or motion vectors) of reference pictures other than the current decoded picture. <NUM>/AVC, a picture can be predicted from multiple reference pictures. The reference pictures that are used for inter prediction are organized in one or more reference picture lists. The reference index identifies which of the reference pictures in the list should be used for creating the prediction signal.

A single reference picture list, List <NUM>, is used for a P slice and two reference picture lists, List <NUM> and List <NUM> are used for B slices. It should be noted reference pictures included in List <NUM>/<NUM> could be from past and future pictures in terms of capturing/display order.

When a PU is predicted using merge mode, an index pointing to an entry in the merge candidates list is parsed from the bitstream and used to retrieve the motion information. The construction of this list is specified in the HEVC standard and can be summarized according to the following sequence of steps:.

These steps are also schematically depicted in <FIG>. For spatial merge candidate derivation, a maximum of four merge candidates are selected among candidates that are located in five different positions. For temporal merge candidate derivation, a maximum of one merge candidate is selected among two candidates. Since constant number of candidates for each PU is assumed at decoder, additional candidates are generated when the number of candidates obtained from step <NUM> does not reach the maximum number of merge candidate (MaxNumMergeCand) which is signalled in slice header. Since the number of candidates is constant, index of best merge candidate is encoded using truncated unary binarization (TU). If the size of CU is equal to <NUM>, all the PUs of the current CU share a single merge candidate list, which is identical to the merge candidate list of the 2N×2N prediction unit.

In the following, the operations associated with the aforementioned steps are detailed.

<FIG> depicts an example derivation process for merge candidates list construction.

In the derivation of spatial merge candidates, a maximum of four merge candidates are selected among candidates located in the positions depicted in <FIG>. The order of derivation is A<NUM>, B<NUM>, B<NUM>, A<NUM> and B<NUM>. Position B<NUM> is considered only when any PU of position A<NUM>, B<NUM>, B<NUM>, A<NUM> is not available (e.g. because it belongs to another slice or tile) or is intra coded. After candidate at position A<NUM> 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> are considered and a candidate is only added to the list if the corresponding candidate used for redundancy check has not the same motion information. Another source of duplicate motion information is the "second PU" associated with partitions different from 2Nx2N. As an example, <FIG> depict the second PU for the case of N×2N and 2N×N, respectively. When the current PU is partitioned as N×2N, candidate at position A1 is not considered for list construction. In fact, by adding this candidate will lead to two prediction units having the same motion information, which is redundant to just have one PU in a coding unit. Similarly, position B1 is not considered when the current PU is partitioned as 2N×N.

In this step, only one candidate is added to the list. Particularly, in the derivation of this temporal merge candidate, a scaled motion vector is derived based on co-located PU belonging to the picture which has the smallest POC difference with current picture within the given reference picture list. The reference picture list to be used for derivation of the co-located PU is explicitly signalled in the slice header. The scaled motion vector for temporal merge candidate is obtained as illustrated by the dotted line in <FIG> which is scaled from the motion vector of the co-located PU using the POC distances, tb and td, where tb is defined to be the POC difference between the reference picture of the current picture and the current picture and td is defined to be the POC difference between the reference picture of the co-located picture and the co-located picture. The reference picture index of temporal merge candidate is set equal to zero. A practical realization of the scaling process is described in the HEVC specification. For a B-slice, two motion vectors, one is for reference picture list <NUM> and the other is for reference picture list <NUM>, are obtained and combined to make the bi-predictive merge candidate.

In the co-located PU (Y) belonging to the reference frame, the position for the temporal candidate is selected between candidates C0 and C1, as depicted in <FIG>. If PU at position C0 is not available, is intra coded, or is outside of the current coding tree unit (CTU aka. LCU, largest coding unit) row, position C1 is used. Otherwise, position C0 is used in the derivation of the temporal merge candidate.

Besides spatial and temporal merge candidates, there are two additional types of merge candidates: combined bi-predictive merge candidate and zero merge candidate. Combined bi-predictive merge candidates are generated by utilizing spatial and temporal merge candidates. Combined bi-predictive merge candidate is used for B-Slice only. The combined bi-predictive candidates are generated by combining the first reference picture list motion parameters of an initial candidate with the second reference picture list motion parameters of another. If these two tuples provide different motion hypotheses, they will form a new bi-predictive candidate. As an example, <FIG> depicts the case when two candidates in the original list (on the left), which have mvL0 and refIdxL0 or mvL1 and refIdxL1, are used to create a combined bi-predictive merge candidate added to the final list (on the right). There are numerous rules regarding the combinations which are considered to generate these additional merge candidates.

Zero motion candidates are inserted to fill the remaining entries in the merge candidates list and therefore hit the MaxNumMergeCand capacity. These candidates have zero spatial displacement and a reference picture index which starts from zero and increases every time a new zero motion candidate is added to the list. Finally, no redundancy check is performed on these candidates.

AMVP exploits spatio-temporal correlation of motion vector with neighbouring PUs, which is used for explicit transmission of motion parameters. For each reference picture list, a motion vector candidate list is constructed by firstly checking availability of left, above temporally neighbouring PU positions, removing redundant candidates and adding zero vector to make the candidate list to be constant length. Then, the encoder can select the best predictor from the candidate list and transmit the corresponding index indicating the chosen candidate. Similarly, with merge index signaling, the index of the best motion vector candidate is encoded using truncated unary. The maximum value to be encoded in this case is <NUM> (see <FIG>). In the following sections, details about derivation process of motion vector prediction candidate are provided.

<FIG> summarizes derivation process for motion vector prediction candidate.

In motion vector prediction, two types of motion vector candidates are considered: spatial motion vector candidate and temporal motion vector candidate. For spatial motion vector candidate derivation, two motion vector candidates are eventually derived based on motion vectors of each PU located in five different positions as depicted in <FIG>.

For temporal motion vector candidate derivation, one motion vector candidate is selected from two candidates, which are derived based on two different co-located positions. After the first list of spatio-temporal candidates is made, duplicated motion vector candidates in the list are removed. If the number of potential candidates is larger than two, motion vector candidates whose reference picture index within the associated reference picture list is larger than <NUM> are removed from the list. If the number of spatio-temporal motion vector candidates is smaller than two, additional zero motion vector candidates is added to the list.

In the derivation of spatial motion vector candidates, a maximum of two candidates are considered among five potential candidates, which are derived from PUs located in positions as depicted in <FIG>, those positions being the same as those of motion merge. The order of derivation for the left side of the current PU is defined as A<NUM>, A<NUM>,and scaled A<NUM>,scaled A<NUM>. The order of derivation for the above side of the current PU is defined as B<NUM>, B<NUM>, B<NUM>, scaled B<NUM>, scaled B<NUM>, scaled B<NUM>. For each side there are therefore four cases that can be used as motion vector candidate, with two cases not required to use spatial scaling, and two cases where spatial scaling is used. The four different cases are summarized as follows.

The no-spatial-scaling cases are checked first followed by the spatial scaling. Spatial scaling is considered when the POC is different between the reference picture of the neighbouring PU and that of the current PU regardless of reference picture list. If all PUs of left candidates are not available or are intra coded, scaling for the above motion vector is allowed to help parallel derivation of left and above MV candidates. Otherwise, spatial scaling is not allowed for the above motion vector.

In a spatial scaling process, the motion vector of the neighbouring PU is scaled in a similar manner as for temporal scaling, as depicted as <FIG>. The main difference is that the reference picture list and index of current PU is given as input; the actual scaling process is the same as that of temporal scaling.

Apart for the reference picture index derivation, all processes for the derivation of temporal merge candidates are the same as for the derivation of spatial motion vector candidates (see <FIG>). The reference picture index is signalled to the decoder.

There are several new coding tools for inter prediction improvement, such as Adaptive motion vector difference resolution (AMVR) for signaling MVD, affine prediction mode, Triangular prediction mode (TPM), Advanced TMVP (ATMVP, aka SbTMVP), Generalized Bi-Prediction (GBI), Bi-directional Optical flow (BIO).

In VVC, a QuadTree/Binary Tree/MulitpleTree (QT/BT/TT) structure is adopted to divide a picture into square or rectangle blocks.

Besides QTBT/TT, separate tree (a. Dual coding tree) is also adopted in VVC for I-frames. With separate tree, the coding block structure are signaled separately for the luma and chroma components.

In bi-prediction operation, for the prediction of one block region, two prediction blocks, formed using a motion vector (MV) of list0 and a MV of list1, respectively, are combined to form a single prediction signal. In the decoder-side motion vector refinement (DMVR) method, the two motion vectors of the bi-prediction are further refined.

In JEM design, the motion vectors are refined by a bilateral template matching process. The bilateral template matching applied in the decoder to perform a distortion-based search between a bilateral template and the reconstruction samples in the reference pictures in order to obtain a refined MV without transmission of additional motion information. An example is depicted in <FIG>. The bilateral template is generated as the weighted combination (i.e. average) of the two prediction blocks, from the initial MV0 of list0 and MV1 of list1, respectively, as shown in <FIG>. The template matching operation consists of calculating cost measures between the generated template and the sample region (around the initial prediction block) in the reference picture. For each of the two reference pictures, the MV that yields the minimum template cost is considered as the updated MV of that list to replace the original one. In the JEM, nine MV candidates are searched for each list. The nine MV candidates include the original MV and <NUM> surrounding MVs with one luma sample offset to the original MV in either the horizontal or vertical direction, or both. Finally, the two new MVs, i.e., MV0' and MV1' as shown in <FIG>, are used for generating the final bi-prediction results. A sum of absolute differences (SAD) is used as the cost measure. Please note that when calculating the cost of a prediction block generated by one surrounding MV, the rounded MV (to integer pel) is actually used to obtain the prediction block instead of the real MV.

For DMVR in VVC, MVD mirroring between list <NUM> and list <NUM> is assumed as shown in <FIG> and bilateral matching is performed to refine the MVs, i.e., to find the best MVD among several MVD candidates. Denote the MVs for two reference picture lists by MVL0(L0X, L0Y), and MVL1(L1X, L1Y). The MVD denoted by (MvdX, MvdY) for list <NUM> that could minimize the cost function (e.g., SAD) is defined as the best MVD. For the SAD function, it is defined as the SAD between the reference block of list <NUM> derived with a motion vector (L0X+MvdX, L0Y+MvdY) in the list <NUM> reference picture and the reference block of list <NUM> derived with a motion vector (L1X-MvdX, L1Y-MvdY) in the list <NUM> reference picture.

The motion vector refinement process may iterate twice. In each iteration, at most <NUM> MVDs (with integer-pel precision) may be checked in two steps, as shown in <FIG>. In the first step, MVD (<NUM>, <NUM>), (-<NUM>, <NUM>), (<NUM>, <NUM>), (<NUM>, -<NUM>), (<NUM>, <NUM>) are checked. In the second step, one of the MVD (-<NUM>, -<NUM>), (-<NUM>, <NUM>), (<NUM>, -<NUM>) or (<NUM>, <NUM>) may be selected and further checked. Suppose function Sad(x, y) returns SAD value of the MVD (x, y). The MVD, denoted by (MvdX, MvdY), checked in the second step is decided as follows:
<IMG>.

In the first iteration, the starting point is the signaled MV, and in the second iteration, the starting point is the signaled MV plus the selected best MVD in the first iteration. DMVR applies only when one reference picture is a preceding picture and the other reference picture is a following picture, and the two reference pictures are with same picture order count distance from the current picture.

To further simplify the process of DMVR, it proposed several changes to the design in JEM. More specifically, the adopted DMVR design to VTM-<NUM> (to be released soon) has the following main features:.

When the following conditions are all true, DMVR may be enabled:.

Symmetric motion vector difference (SMVD) is applied for motion information coding in bi-prediction.

Firstly, in slice level, variables RefIdxSymL0 and RefIdxSymL1 to indicate the reference picture index of list <NUM>/<NUM> used in SMVD mode, respectively, are derived with the following steps. When at least one of the two variables are equal to -<NUM>, SMVD mode shall be disabled.

Output of this process are RefIdxSymL0 and RefIdxSymL0 specifying the list <NUM> and list <NUM> reference picture indices for symmetric motion vector differences, i.e., when sym_mvd_flag is equal to <NUM> for a coding unit.

The variable RefIdxSymLX with X being <NUM> and <NUM> is derived as follows:.

Secondly, in CU level, a symmetrical mode flag indicating whether symmetrical mode is used or not is explicitly signaled if the prediction direction for the CU is bi-prediction and BiDirPredFlag is equal to <NUM>.

When the flag is true, only mvp_l0_flag, mvp_l1_flag and MVD0 are explicitly signaled. The reference indices are set equal to RefIdxSymL0, RefIdxSymL1 for list <NUM> and list <NUM>, respectively. MVD1 is just set equal to -MVD0. The final motion vectors are shown in below formula.

<FIG> is an example illustration for symmetrical mode.

The current SMVD may have the following problems:.

It is noted that for the following cases, the second step (i.e., step b) needs to be invoked for all of the following cases:.

For cases <NUM>) and <NUM>), enabling SMVD is not reasonable since there is no symmetric motion among two references in the same direction (forward or backward).

The listing below should be considered as examples to explain general concepts. These embodiments should not be interpreted in a narrow way. Furthermore, these inventions can be combined in any manner.

In this document, decoder side motion vector derivation (DMVD) includes methods like DMVR and FRUC which perform motion estimation to derive or refine the block/sub-block motion information, and BIO which performs sample-wise motion refinement.

The unequal weights applied to prediction blocks may refer to that used in the GBI process, LIC process, weighted prediction process or other encoding/decoding process of a coding tool that need to apply additional operations to prediction blocks instead of average of two prediction blocks etc. al.

Suppose the reference picture in list <NUM> and list <NUM> are Ref0 and Ref1 respectively, the POC distance between the current picture and Ref0 is PocDist0 (e.g., absolute value of (POC of current picture minus POC of Ref0)), and the POC distance between Ref1 and the current picture is PocDist1 (e.g., absolute value of (POC of Ref1 minus POC of current picture)). Denote width and height of the block as W and H respectively. Suppose function abs(x) returns the absolute value of x.

All-zero MVD represents the case that all MVDs (including both x and y components) to be signaled are equal to zero.

For the SMVD mode, the two target reference pictures are denoted by RefIdxLO and RefIdxL1, respectively. Two variables, currPocDiffL0 and currPocDiffL1 are defined to denote the POC distances between reference pictures and current picture. For example, currPocDiffL0 = POC(current picture) - POC(RefIdxL0), currPocDiffL1 = POC(current picture) - POC(RefIdxL1), Abs(x) = x > <NUM>? x : -x.

The changes on top of VVC working draft version <NUM> are highlighted in bold face italics. Deleted texts are marked with.

This subsection gives one example corresponding to bullet <NUM> of section <NUM>.

Output of this process are RefIdxSymL0 and RefIdxSymL0 specifying the list <NUM> and list <NUM> reference picture indices for symmetric motion vector differences, i.e., when sym_mvd_flag is equal to <NUM> for a coding unit. The variable RefIdxSymLX with X being <NUM> and <NUM> is derived as follows:.

Output of this process are RefldxSymLO and RefldxSymLO specifying the list <NUM> and list <NUM> reference picture indices for symmetric motion vector differences, i.e., when sym_mvd_flag is equal to <NUM> for a coding unit. The variable RefIdxSymLX with X being <NUM> and <NUM> is derived as follows:.

Alternatively, the above conditions may be replaced by:.

The working draft changes are marked as bold face italics.

<FIG> is a block diagram of a video processing apparatus <NUM>. The apparatus <NUM> may be used to implement one or more of the methods described herein. The apparatus <NUM> may be embodied in a smartphone, tablet, computer, Internet of Things (IoT) receiver, and so on. The apparatus <NUM> may include one or more processors <NUM>, one or more memories <NUM> and video processing hardware <NUM>. The processor(s) <NUM> may be configured to implement one or more methods described in the present document. The memory (memories) <NUM> may be used for storing data and code used for implementing the methods and techniques described herein. The video processing hardware <NUM> may be used to implement, in hardware circuitry, some techniques described in the present document.

<FIG> is a flowchart for an example method <NUM> of video processing. The method <NUM> includes determining (<NUM>) to use, for a conversion between a current video block and a bitstream representation of the current video block, a symmetric motion vector difference (SMVD) mode in which two additional motion candidates generated for the conversion based on symmetric displacement of reference blocks in two target reference pictures from a reference list <NUM> and a reference list <NUM> of reference pictures. The method <NUM> includes performing (<NUM>) the conversion using the SMVD mode. The bitstream representation indicates to disable use of a motion vector difference (MVD) mode for one of the reference list <NUM> or the reference list <NUM>.

The following listing of examples provide embodiments that can addressed the technical problems described in the present document, among other problems.

<FIG> is a flowchart for an example method <NUM> of video processing. The method <NUM> includes, at <NUM>, determining, for a conversion between a block of current picture of video and a bitstream representation of the first block, whether a symmetric motion vector difference (SMVD) mode is allowable for the block based on coding information of the block; and at <NUM>, performing the conversion based on the determination.

In some examples, in the SMVD mode, motion vector difference (MVD) is only signaled for a reference list Y of two reference lists including a reference list X and the reference list Y, where Y = <NUM> - X, and the signaled MVD in the reference list Y is used for deriving the MVD in the reference list X, where X=<NUM> or <NUM>.

In some examples, the coding information includes a flag indicating whether MVD in reference list X is forced to be zero, X=<NUM> or <NUM>.

In some examples, even when the flag indicates the MVD in the reference list X is forced to be zero, the SMVD mode is allowed for the block.

In some examples, when the flag indicates the MVD in the reference list <NUM> is forced to be zero, the MVD in the reference list <NUM> is forced to be zero, and the SMVD mode is allowed for the block.

In some examples, when the flag indicates the MVD in the reference list <NUM> is forced to be zero, the MVD in the reference list <NUM> is not forced to be zero, but is derived from associated MVD in reference list <NUM>, and the SMVD mode is allowed for the block.

In some examples, when the flag indicates the MVD in the reference list X is forced to be zero, the SMVD mode is disallowed for the block.

In some examples, when the SMVD mode is disallowed, all related syntax elements for the SMVD mode are implicitly derived instead of being signaled.

In some examples, the reference list Y for which the MVD need to be signaled is signaled by an indication of in the bitstream.

In some examples, the indication is signaled in at least one of the sequence, picture, slice, tile, video unit level.

In some examples, the indication is signaled in at least one of sequence parameter set (SPS) and picture parameter set (PPS), picture header, slice header, and tile group header.

In some examples, the reference list Y for which the MVD need to be signaled is derived according to reference pictures in the reference lists.

In some examples, the reference list Y for which the MVD need to be signaled is derived according to two reference pictures in the two reference lists and corresponding two parameters of Abs (currPocDiffL0) and Abs (currPocDiffL1), where Abs (currPocDiffL0) indicates an absolute value of POC (picture order count) distance from the current picture to the reference picture in the reference list <NUM>, and Abs (currPocDiffL1) indicates an absolute value of POC distance from the current picture to the reference picture in the reference list <NUM>.

In some examples, if Abs (currPocDiffL0) is greater than Abs(currPocDiffL1), MVD for the reference list <NUM> is signaled, otherwise, MVD for the reference list <NUM> is signaled.

In some examples, if Abs (currPocDiffL0) is greater than or equal to Abs(currPocDiffL1), MVD for the reference list <NUM> is signaled, otherwise, MVD for the reference list <NUM> is signaled.

In some examples, if Abs (currPocDiffL0) is equal to Abs(currPocDiffL1), MVD for the reference list <NUM> is signaled, otherwise, MVD for the reference list <NUM> is signaled.

In some examples, if Abs (currPocDiffLL0) is equal to Abs(currPocDiffL1), MVD for the reference list <NUM> is signaled, otherwise, MVD for the reference list <NUM> is signaled.

In some examples, the reference list Y for which the MVD need to be signaled is changed from one video unit to another video unit.

In some examples, the video unit includes at least one of slice, tile group, tile, brick and coding unit (CU).

In some examples, selection of the reference list Y depends on reference pictures information of neighboring blocks of the block.

In some examples, selection of the reference list Y depends on two reference pictures in the two reference lists and corresponding two parameters of PocDist0 and PocDist1, where PocDist0 is absolute value of picture order count (POC) distance from the reference picture in reference list <NUM> to the current picture, and PocDist1 is absolute value of POC distance from the reference picture in reference list <NUM> to the current picture.

In some examples, the reference list with a smaller absolute value of POC distances between PocDist0 and PocDist1 is selected as the reference list Y.

In some examples, the reference list with a larger absolute value of POC distances between PocDist0 and PocDist1 is selected as the reference list Y.

In some examples, if PocDist0 and PocDist1 is equal, either the reference list <NUM> or the reference list <NUM> is selected as the reference list Y.

In some examples, in the SMVD mode, two reference pictures both are selected from one same reference list.

In some examples, one of the two reference pictures is forward reference picture whose POC is smaller than the current POC and the other is backward reference picture whose POC is larger than the current POC.

In some examples, all reference pictures in the reference list <NUM> and the reference list <NUM> are checked in order to find a first reference picture whose POC is closest to the current POC among all the POCs of reference pictures which are smaller than the current POC, and a second reference picture whose POC is closest to the current POC among all the POCs of reference pictures which are larger than the current POC.

In some examples, the SMVD mode includes a derivation process of target reference pictures, which includes a first step for searching a forward target reference picture in the reference list <NUM>, and searching a backward target reference picture in the reference list <NUM>, and a second step for searching a backward target reference picture in the reference list <NUM>, and searching a forward target reference picture in the reference list <NUM>.

In some examples, when at least one of the target reference pictures is found from the first step, the second step of the SMVD mode is skipped.

In some examples, if one target reference picture is found in the reference list <NUM> and no target reference picture is found in the reference list <NUM>, the second step of the SMVD mode is skipped.

In some examples, if no target reference picture is found in the reference list <NUM> and one target reference picture is found in the reference list <NUM>, the second step of the SMVD mode is skipped.

In some examples, when at least one of the target reference pictures is not found from the first step, the SMVD mode is disallowed, and signaling of all SMVD related syntaxes are skipped.

In some examples, the two target reference pictures found in the derivation process of target reference pictures include a forward reference picture whose POC is smaller than the current picture and a backward reference picture whose POC is larger than the current picture.

In some examples, after the derivation process of the two target reference pictures, POC of the two target reference pictures are checked, and the target reference pictures are reset to be unavailable if the two target reference pictures both have smaller POCs than the current picture or larger POCs than the current picture.

In some examples, the target reference pictures can be reset to be unavailable by resetting RefIdxSymL0 and RefIdxSymL1 to -<NUM>, where RefIdxSymL0 is index of the target reference picture in the reference list <NUM>, and RefIdxSymL1 is index of the target reference picture in the reference list <NUM>.

In some examples, when (RefIdxSymL1 !=-<NUM> && RefIdxSymL0!=-<NUM> && DiffPicOrderCnt( currPic, RefPicList[ <NUM> ][ RefIdxSymL1 ] ) * DiffPicOrderCnt( currPic, RefPicList[ <NUM> ][ RefIdxSymL1 ] ) ><NUM>) is true, the target reference pictures are reset to be unavailable, wherein DiffPicOrderCnt(pic0, pic1) returns the POC distance from the picture pic0 to the picture pic1.

In some examples, the SMVD mode includes a derivation process of target reference pictures, which removes the restriction of finding forward reference picture from reference list <NUM> and backward reference picture from reference list <NUM>, or finding forward reference picture from reference list <NUM> and backward reference picture from reference list <NUM>.

In some examples, the target reference pictures used in SMVD mode are derived with two dependent checks including a first check and a second check, each check being used for one of the reference list <NUM> and the reference list <NUM>.

In some examples, the first check is to find one reference picture in the reference list X, where X being <NUM> or <NUM>, and the second check is to find one reference picture in the reference list Y according to the output of the first check and reference pictures in the reference list Y, Y=<NUM>-X.

In some examples, for the reference list X, POC distances between each reference pictures and the current picture are calculated regardless it is forward or backward according to a given order.

In some examples, the given order is the ascending or descending order of reference picture index.

In some examples, the reference picture which has the smallest POC distance is selected as the target reference picture for the reference list X and denoted by RefInLX.

In some examples, the reference picture which has the greatest POC distance is selected as the target reference picture for the reference list X and denoted by RefInLX.

In some examples, one reference picture is selected from the reference list Y according to the reference picture RefInLX as the target reference picture for the reference list Y.

In some examples, if the reference picture RefInLX has a smaller POC distance compared to the current picture, the target reference picture for the reference list Y has a larger POC distance compared to the current picture.

In some examples, if the reference picture RefInLX has a larger POC distance compared to the current picture, the target reference picture for the reference list Y has a smaller POC distance compared to the current picture.

In some examples, if the reference picture RefInLX is a forward reference picture, the target reference picture for the reference list Y is the one with the smallest POC distance compared to the current picture among all backward reference pictures.

In some examples, if the reference picture RefInLX is a backward reference picture, the target reference picture for the reference list Y is the one with the smallest POC distance compared to the current picture among all forward reference pictures.

In some examples, the coding information includes at least one of motion vector (MV) information, MVD precision information for the block and weights of prediction samples in bi-prediction.

In some examples, the SMVD mode is disallowed when unequal weights are selected for the bi-predicted blocks.

In some examples, when the SMVD mode is disallowed, all related syntax elements related to the SMVD mode are not signaled and/or implicitly derived.

In some examples, unequal weights are disallowed in the SMVD mode.

In some examples, when the unequal weights are disallowed, all syntax elements related to the unequal weights are not signaled and/or implicitly derived.

In some examples, the SMVD mode is disallowed for certain MVD precision and/or MV precision.

In some examples, the SMVD mode is disallowed for N-pel MVD precision and/or MV precision, where N=<NUM> or <NUM>.

In some examples, only MVD precision and/or MV precision is allowed in the SMVD mode, N=<NUM>/<NUM>.

In some examples, when the SMVD mode is disallowed, all syntax elements related to the SMVD mode are not signaled and implicitly derived.

In some examples, Adaptive motion vector difference resolution (AMVR) is disallowed in the SMVD mode.

In some examples, when AMVR is disallowed, all syntax elements related to AMVR are not signaled and implicitly derived.

In some examples, decoder-side motion vector refinement (DMVR) or/and other decoder-side motion vector derivation (DMVD) methods are enabled in the SMVD mode.

In some examples, in the SMVD mode, if the MV and/or MVD precision is N-pel, DMVR or/and other DMVD methods are used to refine the MVD by a parameter mvdDmvr, mvdDmvr is with M-pel precision, where N, M = <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>.

In some examples, M is smaller than or equal to N.

In some examples, in the SMVD mode, the MVD is derived by DMVR or/and other DMVD methods instead of being signaled.

In some examples, in the SMVD mode, AMVR information is not signaled, and the MV/MVD precision is derived to be with a predefined value, where the predefined value is <NUM>/<NUM>-pel precision.

In some examples, in the SMVD mode, indication of whether DMVR or/and other DMVD methods are applied or not is signaled.

In some examples, if DMVR or/and other DMVD methods are applied, MVD is not signaled.

In some examples, the indication is signaled for certain MV and/or MVD precisions.

In some examples, the indication is signaled for <NUM>-pel or/and <NUM>-pel MV and/or MVD precisions.

In some examples, the indication is signaled only when PocDist0 is equal to PocDist1, wherein PocDist0 is the POC distance between the current picture and Ref0 and PocDist1 is the POC distance between the current picture and Ref1, and Ref0 is a preceding picture and Ref1 is a following picture of the current picture in display order.

In some examples, the indication is signaled only when PocDist0 is equal to PocDist1, wherein PocDist0 is the POC distance between the current picture and Ref0 and PocDist1 is the POC distance between the current picture and Ref1, and Ref0 is a following picture and Ref1 is a preceding picture of the current picture in display order.

In some examples, in the SMVD mode, whether all-zero MVD is allowed or not depends on the coding information of the block including SMVD mode information and motion information of the block.

In some examples, all-zero MVD is disallowed in the SMVD mode.

In some examples, if horizontal MVD is zero in the SMVD mode, vertical MVD is decreased by <NUM> if vertical MVD is positive or increased by <NUM> if vertical MVD is negative before encoding, and is increased by <NUM> if vertical MVD is positive or decreased by <NUM> if vertical MVD is negative after decoding.

In some examples, if vertical MVD is zero in the SMVD mode, horizontal MVD is decreased by <NUM> if horizontal MVD is positive or increased by <NUM> if horizontal MVD is negative before encoding, and is increased by <NUM> if horizontal MVD is positive or decreased by <NUM> if horizontal MVD is negative after decoding.

In some examples, in the SMVD mode, a higher priority is given to find two target reference pictures with the equal distance to the current picture instead of finding the closest reference pictures.

In some examples, when there are N pairs of reference pictures in the reference list <NUM> and the reference list <NUM>, denoted by (RefL01, RefL1i) with i being [<NUM>, N-<NUM>], one of the N pairs is selected as the target two reference pictures used in the SMVD mode, where N is an integer.

In some examples, for a pair of reference pictures, one reference picture is from the reference list <NUM> and the other is from the reference list <NUM>.

In some examples, for a pair of reference pictures, one reference picture is a forward reference picture from the reference list <NUM> and the other is a backward reference picture from the reference list <NUM>.

In some examples, for a pair of reference pictures, one reference picture is a backward reference picture from the reference list <NUM> and the other is a forward reference picture from the reference list <NUM>.

In some examples, for a pair of reference pictures, one reference picture is a backward reference picture and the other is a forward reference picture, with both from the same reference picture list.

In some examples, when there are N pairs of reference pictures in the reference list <NUM> and the reference list <NUM>, denoted by (RefL0i, RefL1i) with i being [<NUM>, N-<NUM>], one of the N pairs with the smallest or largest absolute value of POC distances to the current picture is selected as the target two reference pictures used in the SMVD mode, where N is an integer.

In some examples, when there is no pair of reference pictures with the equal distance to the current picture in the reference list <NUM> and the reference list <NUM>, target reference pictures are selected according to the POC distances to the current picture.

In some examples, when two target reference pictures are with unequal distance to the current picture, the SMVD mode is disallowed.

In some examples, signaling of SMVD related syntax elements including sym_mvd_flag is skipped and the SMVD mode is inferred not be used.

In some examples, the MVD of the reference picture list X is derived from the signaled MVD of reference picture list Y according to POC distances of two reference pictures and the current picture.

In some examples, when the signaled MVD for the reference list Y is MvdLY with its horizontal and vertical component denoted by MvdLY[<NUM>] and MvdLY[<NUM>], the MVD for the reference list X denoted by ( MvdLX[<NUM>], MvdLX[<NUM>]) is derived as following: <MAT> <MAT> where POC(targetRef in List X) is a POC of the reference picture in the reference list X, POC(targetRef in List Y) is a POC of the reference picture in the reference list X, and POC(current picture) is a POC of the current picture.

In some examples, in the SMVD mode, the MVD of the reference list Y is scaled according to POC distances when deriving the MVD of the reference list X, instead of using the opposite MVD of the reference list Y as the MVD for the reference list X, where X=<NUM>.

<FIG> is a flowchart for an example method <NUM> of video processing. The method <NUM> includes, at <NUM>, determining, determining, for a conversion between a block of video and a bitstream representation of the first block, whether a symmetric motion vector (SMV) mode is allowable for the block according to the method of previous examples; and at <NUM>, performing the conversion based on the determination.

<FIG> is a flowchart for an example method <NUM> of video processing. The method <NUM> includes, at <NUM>, determining, for a conversion between a block of a current picture of video and a bitstream representation of the block, whether a symmetric motion vector difference (SMVD) mode is allowable for the block, wherein whether the SMVD mode is allowable depends on a derivation process of two target reference pictures of the current picture, which includes a first step for searching a forward target reference picture in the reference picture list <NUM> and searching a backward target reference picture in the reference picture list <NUM>, and a second step for searching a backward target reference picture in the reference picture list <NUM> and searching a forward target reference picture in the reference picture list <NUM>, outputs of the first step and the second step being independent of each other; and at <NUM>, performing the conversion based on the determination.

In some examples, whether the SMVD mode is allowable is depends on whether both the target reference pictures are found in one step of the first step and the second step.

In some examples, when both the target reference pictures are found in the first step, the SMVD mode is allowed.

In some examples, when both the target reference pictures are found in the second step, the SMVD mode is allowed.

In some examples, when both the target reference pictures are not found in the first step and both the target reference pictures are not found in the second step, the SMVD mode is disallowed.

In some examples, when at least one of the two target reference pictures is not found in the first step, the second step is invoked.

In some examples, before invoking the second step, both the target reference picture indices are reset to be unavailable.

In some examples, when the indices of the two target reference pictures include variables of RefIdxSymL0 and RefIdxSymL1, with RefIdxSymL0 indicating the target reference picture index in the reference picture list <NUM> and RefIdxSymL1 indicating the target reference picture index in the reference picture list <NUM>.

In some examples, resetting both the target reference picture indices to be unavailable comprises resetting both RefIdxSymL0 and RefIdxSymL1 to -<NUM>.

<FIG> is a flowchart for an example method <NUM> of video processing. The method <NUM> includes, at <NUM>, deriving, for a conversion between a block of a current picture of video and a bitstream representation of the block, a second motion vector difference (MVD) of one reference picture list from a first MVD associated with the other reference picture list, wherein an MVD includes both a horizontal component and a vertical component; and at <NUM>, applying a range restriction on at least one of the first MVD and the second MVD; and at <NUM>, performing the conversion based on the determination.

In some examples, the first MVD is included in the bitstream representation of the block.

In some examples, the second MVD is derived from the first MVD in a symmetric motion vector difference (SMVD) mode.

In some examples, the horizontal component and/or the vertical component of the derived second MVD are restricted in a predefined range.

In some examples, the horizontal component and/or the vertical component of the derived second MVD are clipped or conditionally modified according to the predefined range.

In some examples, Mz is clipped as Mz=Clip3 ( -<NUM>N, <NUM>N-<NUM>, Mz), where Mz represents the horizontal component or the vertical component of the derived second MVD, N being an integer, wherein <MAT>.

In some examples, N= <NUM>, <NUM>, <NUM> or <NUM>.

In some examples, Mz is clipped as Mz=Clip3( -<NUM>N+<NUM>, <NUM>N-<NUM>, Mz), where Mz represents the horizontal component or the vertical component of the derived second MVD, N being an integer.

In some examples, Mz is modified as Mz = <NUM>N-<NUM> if Mz =<NUM>N, where Mz represents the horizontal component or the vertical component of the derived second MVD, N being an integer.

In some examples, the horizontal component and/or the vertical component of the derived second MVD are within a predefine range in a conformance bitstream.

In some examples, the horizontal component and/or the vertical component of the derived second MVD are within the range of [-<NUM>N, <NUM>N-<NUM>] inclusively in a conformance bitstream.

In some examples, the horizontal component and/or the vertical component of the derived second MVD are within the range of [-<NUM>N+<NUM>, <NUM>N-<NUM>] inclusively in a conformance bitstream.

In some examples, the horizontal component and/or the vertical component of the first MVD are clipped or conditionally modified according to a first predefined range so that the horizontal component and/or the vertical component of the derived second MVD are in a second predefined range.

In some examples, Mz is clipped as Mz=Clip3( -<NUM>N+<NUM>, <NUM>N-<NUM>, Mz), where Mz represents the horizontal component or the vertical component of the first MVD, N being an integer, wherein <MAT>.

In some examples, Mz is modified as Mz = -<NUM>N+<NUM> if Mz =-<NUM>N, where Mz represents the horizontal component or the vertical component of the first MVD, N being an integer.

In some examples, the horizontal component and/or the vertical component of the first MVD are within a first predefined range so that the horizontal component and/or the vertical component of the derived second MVD are in a second predefined range in a conformance bitstream.

In some examples, the first predefined range is different to the predefined range for a signaled MVD wherein the SMVD mode is not applied.

In some examples, the horizontal component and/or the vertical component of the first MVD is within the range of [-<NUM>N+<NUM>, <NUM>N-<NUM>] inclusively in a conformance bitstream, N being an integer.

In some examples, the first MVD is a signaled MVD, and the range of the signaled MVD is set differently for SMVD mode or other tools that derives MVD from another reference picture list and for non-SMVD coding tools.

In some examples, the signaled MVD is within a range of [-<NUM>N+SMVDFlag, <NUM>N-<NUM>], wherein SMVDFlag is equal to <NUM> when the SMVD mode is applied and SMVDFlag is equal to <NUM> when SMVD mode is not applied.

In some examples, clipping or constraint on the MVD is performed before the MVD is modified according to precision of MV associated with the MVD.

In some examples, clipping or constraint on the MVD is performed after the MVD is modified according to precision of MV associated with the MVD.

In some examples, clipping or constraint on the MVD depend on precision of MV associated with the MVD.

In some examples, the conversion generates the block of video from the bitstream representation.

In some examples, the conversion generates the bitstream representation from the block of video.

In the listing of examples in this present document, the term conversion may refer to the generation of the bitstream representation for the current video block or generating the current video block from the bitstream representation. The bitstream representation need not represent a contiguous group of bits and may be divided into bits that are included in header fields or in codewords representing coded pixel value information.

In the examples above, the applicability rule may be pre-defined and known to encoders and decoders.

It will be appreciated that the disclosed techniques may be embodied in video encoders or decoders to improve compression efficiency using techniques that include the use of various implementation rules of considerations regarding the use of a differential coding mode in intra coding, as described in the present document.

The disclosed and other solutions, examples, embodiments, modules and the functional operations described in this document can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this document and their structural equivalents, or in combinations of one or more of them.

While this patent document contains many specifics, these should not be construed as limitations on the scope of any subject matter or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular techniques. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment.

Claim 1:
A method of processing video data, comprising:
determining, for a conversion between a block of a current picture of a video and a bitstream of the video, that a first coding mode is applied to the block based on a derivation process of two target reference pictures of the current picture, wherein the conversion includes decoding the block of the video from the bitstream or encoding the block of the video into the bitstream, wherein the derivation process includes:
performing a first step for searching a forward target reference picture in a reference picture list <NUM> and searching a backward target reference picture in a reference picture list <NUM>,
in response to at least one of the two target reference pictures being not found in the first step, resetting both the target reference picture indices to be unavailable before a second step being invoked,
wherein resetting both the target reference picture indices to be unavailable comprises resetting both RefIdxSymL0 and RefldxSymL1 to -<NUM>, wherein RefIdxSymL0 indicates the target reference picture index in the reference picture list <NUM> and RefldxSymL1 indicates the target reference picture index in the reference picture list <NUM>, and
performing the second step for searching a backward target reference picture in the reference picture list <NUM> and searching a forward target reference picture in the reference picture list <NUM>; and
performing the conversion based on the determination,
wherein in the first coding mode, a syntax structure indicating a motion vector difference of a reference picture list X is not present in the bitstream, and the motion vector difference of the reference picture list X is derived based on a motion vector difference of a reference picture list (<NUM>-X), wherein X is equal to <NUM> or <NUM>.