WO2020003276A1 - Emm mode signaling - Google Patents

Abstract

Methods, devices and systems for using an extended merge mode (EMM) in video coding are described. An exemplary method of video processing includes constructing a list of EMM candidates; determining, based on a first set of bits in a bitstream representation of a current block, the motion information inherited by the current block from the list; determining, based on a second set of bits in the bitstream representation of a current block, the signaled motion information of the current block; and performing, based on the list of EMM candidates and the signaled motion information, a conversion between the current block and the bitstream representation, wherein the bitstream representation further comprises a field signaling a selective usage of an extended merge mode comprising the list of EMM candidates.

Description

EMM MODE SIGNALING

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] Under the applicable patent law and/or rules pursuant to the Paris Convention, this application is made to timely claim the priority to and benefit of International Patent Application No. PCT/CN2018/093646, filed on June 29, 2018. For all purposes under the U.S. law, the entire disclosure of the International Patent Application No. PCT/CN2018/093646 is incorporated by reference as part of the disclosure of this application.

TECHNICAL FIELD

[002] This document is related to video coding technologies.

BACKGROUND

[003] Digital video accounts for the largest bandwidth use on the internet and other digital communication networks. As the number of connected user devices capable of receiving and displaying video increases, it is expected that the bandwidth demand for digital video usage will continue to grow.

SUMMARY

[004] The disclosed techniques may be used by video decoder or encoder embodiments for using an extended merge mode in which some motion information may be inherited while some motion information may be signaled.

[005] In one example aspect, a method of video processing is disclosed. The method includes constmcting a list of extended merge mode (EMM) candidates; determining, based on a first set of bits in a bitstream representation of a current block, the motion information inherited by the current block from the list; determining, based on a second set of bits in the bitstream representation of a current block, the signaled motion information of the current block; and performing, based on the list of EMM candidates and the signaled motion information, a conversion between the current block and the bitstream representation, wherein the bitstream representation further comprises a field signaling a selective usage of an extended merge mode comprising the list of EMM candidates. [006] In another example aspect, the above-described method may be implemented by a video decoder apparatus that comprises a processor.

[007] In another example aspect, the above-described method may be implemented by a video encoder apparatus comprising a processor for decoding encoded video during video encoding process.

[008] In yet another example aspect, these methods may be embodied in the form of processor-executable instmctions and stored on a computer-readable program medium.

[009] These, and other, aspects are further described in the present document.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 shows an example of a derivation process for merge candidates list constmction.

[0011] FIG. 2 shows example positions of spatial merge candidates.

[0012] FIG. 3 shows examples of candidate pairs considered for redundancy check of spatial merge candidates.

[0013] FIGS. 4A and 4B show example positions for the second PU of Nx2N and 2NxN partitions.

[0014] FIG. 5 is an example illustration of motion vector scaling for temporal merge candidate.

[0015] FIG. 6 shows examples of candidate positions for temporal merge candidate CO and C 1.

[0016] FIG. 7 shows an example of combined bi-predictive merge candidate.

[0017] FIG. 8 shows an example derivation process for motion vector prediction candidates.

[0018] FIG. 9 shows an example illustration of motion vector scaling for spatial motion vector candidate.

[0019] FIG. 10 shows an example of neighboring samples used for deriving IC parameters.

[0020] FIG. 1 1 shows an example of simplified affine motion model.

[0021] FIG. 12 shows an example of affine MVF per sub-block.

[0022] FIG. 13 shows an example of MVP for AF_INTER.

[0023] FIGS. 14A and 14B show examples of candidates for AF_MERGE.

[0024] FIG. 15 shows an example of bilateral matching.

[0025] FIG. 16 shows an example of template matching.

[0026] FIG. 17 shows an example of unilateral ME in FRUC. [0027] FIG. 18 shows an example of DMVR based on bilateral template matching.

[0028] FIG. 19 shows examples of non-adjacent merge candidates.

[0029] FIG. 20 shows examples of non-adjacent merge candidates.

[0030] FIG. 21 shows examples of non-adjacent merge candidates.

[0031] FIG. 22 and FIG. 23 depict examples of ultimate motion vector expression technique of video coding.

[0032] FIG. 24 is a flowchart for an example of a video bitstream processing method.

[0033] FIG. 25 is a block diagram of an example of a video processing apparatus.

DETAILED DESCRIPTION

[0034] The present document provides various techniques that can be used by a decoder of video bitstreams to improve the quality of decompressed or decoded digital video. Furthermore, a video encoder may also implement these techniques during the process of encoding in order to reconstruct decoded frames used for further encoding.

[0035] Section headings are used in the present document for ease of understanding and do not limit the embodiments and techniques to the corresponding sections. As such, embodiments from one section can be combined with embodiments from other sections.

2. Technical framework

[0036] 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-l and MPEG-4 Visual, and the two organizations jointly produced the

H.262/MPEG-2 Video and H.264/MPEG-4 Advanced Video Coding (A VC) 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, 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). In April 2018, the Joint Video Expert Team (JVET) between VCEG (Q6/16) and ISO/IEC JTC1 SC29/WG11 (MPEG) was created to work on the VVC standard targeting at 50% bitrate reduction compared to HEVC. 2.1 Inter prediction in HEVC/H.265

[0037] 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 signaled using inter_pred_idc . Motion vectors may be explicitly coded as deltas relative to predictors.

[0038] When a CU is coded with skip mode, one PU is associated with the CU, and there are no significant residual coefficients, no coded motion vector delta or reference picture index. A merge mode is specified whereby the motion parameters for the current PU are obtained from neighboring PUs, including spatial and temporal candidates. The merge mode can be applied to any inter-predicted PU, not only for skip mode. The alternative to merge mode is the explicit transmission of motion parameters, where motion vector (to be more precise, motion vector difference compared to a motion vector predictor), corresponding reference picture index for each reference picture list and reference picture list usage are signaled explicitly per each PU. Such a mode is named Advanced motion vector prediction (AMVP) in this document.

[0039] When signaling indicates that one of the two reference picture lists is to be used, the PU is produced from one block of samples. This is referred to as‘uni-prediction’. Uni-prediction is available both for P-slices and B-slices.

[0040] When signaling indicates that both of the reference picture lists are to be used, the PU is produced from two blocks of samples. This is referred to as‘bi-prediction’. Bi-prediction is available for B-slices only.

[0041] The following text provides the details on the inter prediction modes specified in HEVC. The description will start with the merge mode.

2.1.1 Merge Mode

2.1.1.1 Derivation of candidates for merge mode

[0042] 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 constmction of this list is specified in the HEVC standard and can be summarized according to the following sequence of steps:

• Step 1 : Initial candidates derivation

o Step 1.1 : Spatial candidates derivation o Step 1.2: Redundancy check for spatial candidates

o Step 1.3: Temporal candidates derivation

• Step 2: Additional candidates insertion

o Step 2.1 : Creation of bi -predictive candidates

o Step 2.2: Insertion of zero motion candidates

[0043] These steps are also schematically depicted in FIG. 1. 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 1 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 (TUB). If the size of CU is equal to 8, all the PUs of the current CU share a single merge candidate list, which is identical to the merge candidate list of the 2Nx2N prediction unit.

[0044] In the following, the operations associated with the aforementioned steps are detailed.

2.1.1.2 Spatial candidate derivation

[0045] In the derivation of spatial merge candidates, a maximum of four merge candidates are selected among candidates located in the positions depicted in FIG. 2. The order of derivation is Ai_, Bi_, Bo_, Ao and B₂. Position B₂ is considered only when any PU of position Ai, Bi, Bo, Ao is not available (e.g. because it belongs to another slice or tile) or is intra coded. After candidate at position Ai is added, the addition of the remaining candidates is subject to a redundancy check which ensures that candidates with same motion information are excluded from the list so that coding efficiency is improved. To reduce computational complexity, not all possible candidate pairs are considered in the mentioned redundancy check. Instead only the pairs linked with an arrow in FIG. 3 are considered and a candidate is only added to the list if the corresponding candidate used for redundancy check has not the same motion information. Another source of duplicate motion information is the“ second PIT’ associated with partitions different from 2Nx2N. As an example, FIG. 4 depicts the second PU for the case of Nx2N and 2NxN, respectively. When the current PU is partitioned as Nx2N, candidate at position Ai is not considered for list construction. In 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 Bi is not considered when the current PU is partitioned as 2NxN.

2.1.1.3 Temporal candidate derivation

[0046] 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. 5, 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 0 and the other is for reference picture list 1, are obtained and combined to make the bi-predictive merge candidate.

[0047] In the co-located PU (Y) belonging to the reference frame, the position for the temporal candidate is selected between candidates Co and Ci, as depicted in FIG. 6. If PU at position Co is not available, is intra coded, or is outside of the current CTU row, position Ci is used. Otherwise, position Co is used in the derivation of the temporal merge candidate.

2.1.1.4 Additional candidate insertion

[0048] 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. 7 depicts the case when two candidates in the original list (on the left), which have mvLO and refldxLO or mvLl and refldxLl, are used to create a combined bi-predictive merge candidate added to the final list (on the right). There are numerous mles regarding the combinations which are considered to generate these additional merge candidates.

[0049] Zero motion candidates are inserted to fill the remaining entries in the merge candidates list and therefore hit the MaxNumMergeCand capacity. These candidates have zero spatial displacement and a reference picture index which starts from zero and increases every time a new zero motion candidate is added to the list. The number of reference frames used by these candidates is one and two for uni- and bi-directional prediction, respectively. Finally, no redundancy check is performed on these candidates.

2.1.1.5 Motion estimation regions for parallel processing

[0050] To speed up the encoding process, motion estimation can be performed in parallel whereby the motion vectors for all prediction units inside a given region are derived

simultaneously. The derivation of merge candidates from spatial neighbourhood may interfere with parallel processing as one prediction unit cannot derive the motion parameters from an adjacent PU until its associated motion estimation is completed. To mitigate the trade-off between coding efficiency and processing latency, HEVC defines the motion estimation region (MER) whose size is signalled in the picture parameter set using the

“log2_parallel_merge_level_minus2” syntax element. When a MER is defined, merge candidates falling in the same region are marked as unavailable and therefore not considered in the list constmction.

2.1.2 AMVP

[0051] 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 constmcted 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 signalling, the index of the best motion vector candidate is encoded using truncated unary binarization. The maximum value to be encoded in this case is 2 (see FIG. 8). In the following sections, details about derivation process of motion vector prediction candidate are provided.

2.1.2.1 Derivation of AMVP candidates

[0052] FIG. 8 summarizes derivation process for motion vector prediction candidate.

[0053] 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. 2.

[0054] For temporal motion vector candidate derivation, one motion vector candidate is selected from two candidates, which are derived based on two different co-located positions. After the first list of spatio-temporal candidates is made, duplicated motion vector candidates in the list are removed. If the number of potential candidates is larger than two, motion vector candidates whose reference picture index within the associated reference picture list is larger than 1 are removed from the list. If the number of spatio-temporal motion vector candidates is smaller than two, additional zero motion vector candidates is added to the list.

2.1.2.2 Spatial motion vector candidates

[0055] 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. 2, those positions being the same as those of motion merge. The order of derivation for the left side of the current PU is defined as A₀, Ai,and scaled Ao, scaled Ai. The order of derivation for the above side of the current PU is defined as Bo, Bi, B 2, scaled Bo, scaled Bi, scaled B₂. For each side there are therefore four cases that can be used as motion vector candidate, with two cases not required to use spatial scaling, and two cases where spatial scaling is used. The four different cases are summarized as follows.

• No spatial scaling

— (1) Same reference picture list, and same reference picture index (same POC)

— (2) Different reference picture list, but same reference picture index (same POC)

• Spatial scaling — (3) Same reference picture list, but different reference picture index (different POC)

— (4) Different reference picture list, and different reference picture index (different POC)

[0056] 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 neighboring 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.

[0057] 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. 9. 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.

2.1.2.3 Temporal motion vector candidates

[0058] 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. 6). The reference picture index is signalled to the decoder.

2.2 New inter prediction methods in JEM

2.2.1 Adaptive motion vector difference resolution

[0059] In HEVC, motion vector differences (MVDs) (between the motion vector and predicted motion vector of a PU) are signalled in units of quarter luma samples when use_integer_mv_flag is equal to 0 in the slice header. In the JEM, a locally adaptive motion vector resolution

(LAMVR) is introduced. In the JEM, MVD can be coded in units of quarter luma samples, integer luma samples or four luma samples. The MVD resolution is controlled at the coding unit (CU) level, and MVD resolution flags are conditionally signalled for each CU that has at least one non-zero MVD components.

[0060] For a CU that has at least one non-zero MVD components, a first flag is signalled to indicate whether quarter luma sample MV precision is used in the CU. When the first flag (equal to 1) indicates that quarter luma sample MV precision is not used, another flag is signalled to indicate whether integer luma sample MV precision or four luma sample MV precision is used.

[0061] When the first MVD resolution flag of a CU is zero, or not coded for a CU (meaning all MVDs in the CU are zero), the quarter luma sample MV resolution is used for the CU. When a CU uses integer-luma sample MV precision or four-luma-sample MV precision, the MVPs in the AMVP candidate list for the CU are rounded to the corresponding precision.

[0062] In the encoder, CU-level RD checks are used to determine which MVD resolution is to be used for a CU. That is, the CU-level RD check is performed three times for each MVD resolution. To accelerate encoder speed, the following encoding schemes are applied in the JEM.

• During RD check of a CU with normal quarter luma sample MVD resolution, the motion information of the current CU (integer luma sample accuracy) is stored. The stored motion information (after rounding) is used as the starting point for further small range motion vector refinement during the RD check for the same CU with integer luma sample and 4 luma sample MVD resolution so that the time-consuming motion estimation process is not duplicated three times.

• RD check of a CU with 4 luma sample MVD resolution is conditionally invoked. For a CU, when RD cost integer luma sample MVD resolution is much larger than that of quarter luma sample MVD resolution, the RD check of 4 luma sample MVD resolution for the CU is skipped.

2.2.2 Higher motion vector storage accuracy

[0063] In HEVC, motion vector accuracy is one-quarter pel (one -quarter luma sample and one- eighth chroma sample for 4:2:0 video). In the JEM, the accuracy for the internal motion vector storage and the merge candidate increases to 1/16 pel. The higher motion vector accuracy (1/16 pel) is used in motion compensation inter prediction for the CU coded with skip/merge mode.

For the CU coded with normal AMVP mode, either the integer-pel or quarter-pel motion is used, as described in section 2.2.1.

[0064] SHVC upsampling interpolation filters, which have same filter length and normalization factor as HEVC motion compensation interpolation filters, are used as motion compensation interpolation filters for the additional fractional pel positions. The chroma component motion vector accuracy is 1/32 sample in the JEM, the additional interpolation filters of 1/32 pel fractional positions are derived by using the average of the filters of the two neighbouring 1/16 pel fractional positions.

2.2.3 Local illumination compensation

[0065] Local Illumination Compensation (LIC) is based on a linear model for illumination changes, using a scaling factor a and an offset b. And it is enabled or disabled adaptively for each inter-mode coded coding unit (CU).

[0066] When LIC applies for a CU, a least square error method is employed to derive the parameters a and b by using the neighbouring samples of the current CU and their corresponding reference samples. More specifically, as illustrated in FIG. 10, the subsampled (2:1 subsampling) neighbouring samples of the CU and the corresponding samples (identified by motion information of the current CU or sub-CU) in the reference picture are used. The IC parameters are derived and applied for each prediction direction separately.

[0067] When a CU is coded with merge mode, the LIC flag is copied from neighbouring blocks, in a way similar to motion information copy in merge mode; otherwise, an LIC flag is signalled for the CU to indicate whether LIC applies or not.

[0068] When LIC is enabled for a picture, additional CU level RD check is needed to determine whether LIC is applied or not for a CU. When LIC is enabled for a CU, mean- removed sum of absolute difference (MR-SAD) and mean-removed sum of absolute Hadamard- transformed difference (MR-SATD) are used, instead of SAD and SATD, for integer pel motion search and fractional pel motion search, respectively.

[0069] To reduce the encoding complexity, the following encoding scheme is applied in the JEM.

[0070] LIC is disabled for the entire picture when there is no obvious illumination change between a current picture and its reference pictures. To identify this situation, histograms of a current picture and every reference picture of the current picture are calculated at the encoder. If the histogram difference between the current picture and every reference picture of the current picture is smaller than a given threshold, LIC is disabled for the current picture; otherwise, LIC is enabled for the current picture. 2.2.4 Affine motion compensation prediction

[0071] 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 the JEM, a simplified affine transform motion compensation prediction is applied. As shown FIG. 1 1, the affine motion field of the block is described by two control point motion vectors.

[0072] The motion vector field (MVF) of a block is described by the following equation:

where ( vo_x , vo_y) is motion vector of the top -left corner control point, and (vi_x, vi_y) is motion vector of the top-right corner control point.

[0073] In order to further simplify the motion compensation prediction, sub-block based affine transform prediction is applied. The sub-block size M x N is derived as in Equation 2, where MvPre is the motion vector fraction accuracy (1/16 in JEM), ( V2_x , V2_y) is motion vector of the bottom-left control point, calculated according to Equation 1.

[0074] After derived by Equation 2, M and N should be adjusted downward if necessary to make it a divisor of w and h, respectively.

[0075] To derive motion vector of each MxN sub-block, the motion vector of the center sample of each sub-block, as shown in FIG. 12, is calculated according to Equation 1 , and rounded to 1/16 fraction accuracy. Then the motion compensation interpolation filters are applied to generate the prediction of each sub-block with derived motion vector.

[0076] After MCP, the high accuracy motion vector of each sub-block is rounded and saved as the same accuracy as the normal motion vector. [0077] In the JEM, there are two affine motion modes: AF_INTER mode and AF_MERGE mode. For CUs with both width and height larger than 8, AF_INTER mode can be applied. An affine flag in CU level is signalled in the bitstream to indicate whether AF_INTER mode is used. In this mode, a candidate list with motion vector pair {(V_Q V_!) | v₀ = {v_A,v_B,v_c}, n₄ = {v_Dy_E}j is constmcted using the neighbour blocks. As shown in FIG. 13, v₀ is selected from the motion vectors of the block A, B or C. The motion vector from the neighbour block is scaled according to the reference list and the relationship among the POC of the reference for the neighbour block, the POC of the reference for the current CU and the POC of the current CU. And the approach to select v₄ from the neighbour block D and E is similar. If the number of candidate list is smaller than 2, the list is padded by the motion vector pair composed by duplicating each of the AMVP candidates. When the candidate list is larger than 2, the candidates are firstly sorted according to the consistency of the neighbouring motion vectors (similarity of the two motion vectors in a pair candidate) and only the first two candidates are kept. An RD cost check is used to determine which motion vector pair candidate is selected as the control point motion vector prediction (CPMVP) of the current CU. And an index indicating the position of the CPMVP in the candidate list is signalled in the bitstream. After the CPMVP of the current affine CU is determined, affine motion estimation is applied and the control point motion vector (CPMV) is found. Then the difference of the CPMV and the CPMVP is signalled in the bitstream.

[0078] When a CU is applied in AF_MERGE mode, it gets the first block coded with affine mode from the valid neighbour reconstmcted blocks. And the selection order for the candidate block is from left, above, above right, left bottom to above left as shown in FIG. 14A. If the neighbour left bottom block A is coded in affine mode as shown in FIG. 14B, the motion vectors v₂ , v₃ and v₄ of the top left corner, above right comer and left bottom comer of the CU which contains the block A are derived. And the motion vector v₀ of the top left comer on the current CU is calculated according to v₂ , v₃ and v₄. Secondly, the motion vector v₄ of the above right of the current CU is calculated.

[0079] After the CPMV of the current CU v₀ and V ₁ are derived, according to the simplified affine motion model Equation 1, the MVF of the current CU is generated. In order to identify whether the current CU is coded with AF_MERGE mode, an affine flag is signalled in the bitstream when there is at least one neighbour block is coded in affine mode. 2.2.5 Pattern matched motion vector derivation

[0080] Pattern matched motion vector derivation (PMMVD) mode is a special merge mode based on Frame -Rate Up Conversion (FRUC) techniques. With this mode, motion information of a block is not signalled but derived at decoder side.

[0081] A FRUC flag is signalled for a CU when its merge flag is tme. When the FRUC flag is false, a merge index is signalled and the regular merge mode is used. When the FRUC flag is true, an additional FRUC mode flag is signalled to indicate which method (bilateral matching or template matching) is to be used to derive motion information for the block.

[0082] At encoder side, the decision on whether using FRUC merge mode for a CU is based on RD cost selection as done for normal merge candidate. That is the two matching modes (bilateral matching and template matching) are both checked for a CU by using RD cost selection. The one leading to the minimal cost is further compared to other CU modes. If a FRUC matching mode is the most efficient one, FRUC flag is set to tme for the CU and the related matching mode is used.

[0083] Motion derivation process in FRUC merge mode has two steps. A CU-level motion search is first performed, then followed by a Sub-CU level motion refinement. At CU level, an initial motion vector is derived for the whole CU based on bilateral matching or template matching. First, a list of MV candidates is generated and the candidate which leads to the minimum matching cost is selected as the starting point for further CU level refinement. Then a local search based on bilateral matching or template matching around the starting point is performed and the MV results in the minimum matching cost is taken as the MV for the whole CU. Subsequently, the motion information is further refined at sub-CU level with the derived CU motion vectors as the starting points.

[0084] For example, the following derivation process is performed for a W c H CU motion information derivation. At the first stage, MV for the whole W x. H CU is derived. At the second stage, the CU is further split into M c M sub-CUs. The value of M is calculated as in (3), D is a predefined splitting depth which is set to 3 by default in the JEM. Then the MV for each sub-CU is derived.

[0085] As shown in the FIG. 15, the bilateral matching is used to derive motion information of the current CU by finding the closest match between two blocks along the motion trajectory of the current CU in two different reference pictures. Under the assumption of continuous motion trajectory, the motion vectors MV0 and MV1 pointing to the two reference blocks shall be proportional to the temporal distances, i.e., TD0 and TD1, between the current picture and the two reference pictures. As a special case, when the current picture is temporally between the two reference pictures and the temporal distance from the current picture to the two reference pictures is the same, the bilateral matching becomes mirror based bi-directional MV.

[0086] As shown in FIG. 16, template matching is used to derive motion information of the current CU by finding the closest match between a template (top and/or left neighbouring blocks of the current CU) in the current picture and a block (same size to the template) in a reference picture. Except the aforementioned FRUC merge mode, the template matching is also applied to AMVP mode. In the JEM, as done in HEVC, AMVP has two candidates. With template matching method, a new candidate is derived. If the newly derived candidate by template matching is different to the first existing AMVP candidate, it is inserted at the very beginning of the AMVP candidate list and then the list size is set to two (meaning remove the second existing AMVP candidate). When applied to AMVP mode, only CU level search is applied.

2.2.5.1 CU level MV candidate set

[0087] The MV candidate set at CU level consists of:

(i) Original AMVP candidates if the current CU is in AMVP mode

(ii) all merge candidates,

(iii) several MVs in the interpolated MV field.

(iv) top and left neighbouring motion vectors

[0088] When using bilateral matching, each valid MV of a merge candidate is used as an input to generate a MV pair with the assumption of bilateral matching. For example, one valid MV of a merge candidate is (MVa, refa) at reference list A. Then the reference picture refb of its paired bilateral MV is found in the other reference list B so that refa and refb are temporally at different sides of the current picture. If such a refb is not available in reference list B, refb is determined as a reference which is different from refa and its temporal distance to the current picture is the minimal one in list B. After refb is determined, MVb is derived by scaling MVa based on the temporal distance between the current picture and refa, refb.

[0089] Four MVs from the interpolated MV field are also added to the CU level candidate list. More specifically, the interpolated MVs at the position (0, 0), (W/2, 0), (0, H/2) and (W/2, H/2) of the current CU are added.

[0090] When FRUC is applied in AMVP mode, the original AMVP candidates are also added to CU level MV candidate set.

[0091] At the CU level, up to 15 MVs for AMVP CUs and up to 13 MVs for merge CUs are added to the candidate list.

2.2.5.2 Sub-CU level MV candidate set

[0092] The MV candidate set at sub-CU level consists of:

(i) an MV determined from a CU -level search,

(ii) top, left, top-left and top-right neighbouring MVs,

(iii) scaled versions of collocated MVs from reference pictures,

(iv) up to 4 ATMVP candidates,

(v) up to 4 STMVP candidates

[0093] The scaled MVs from reference pictures are derived as follows. All the reference pictures in both lists are traversed. The MVs at a collocated position of the sub-CU in a reference picture are scaled to the reference of the starting CU-level MV.

[0094] ATMVP and STMVP candidates are limited to the four first ones.

[0095] At the sub-CU level, up to 17 MVs are added to the candidate list.

2.2.5.3 Generation of interpolated MV field

[0096] Before coding a frame, interpolated motion field is generated for the whole picture based on unilateral ME. Then the motion field may be used later as CU level or sub-CU level MV candidates.

[0097] First, the motion field of each reference pictures in both reference lists is traversed at 4x4 block level. For each 4x4 block, if the motion associated to the block passing through a 4x4 block in the current picture (as shown in FIG. 17) and the block has not been assigned any interpolated motion, the motion of the reference block is scaled to the current picture according to the temporal distance TD0 and TD1 (the same way as that of MV scaling of TMVP in HE VC) and the scaled motion is assigned to the block in the current frame. If no scaled MV is assigned to a 4x4 block, the block’s motion is marked as unavailable in the interpolated motion field.

2.2.5.4 Interpolation and matching cost

[0098] When a motion vector points to a fractional sample position, motion compensated interpolation is needed. To reduce complexity, bi -linear interpolation instead of regular 8 -tap HEVC interpolation is used for both bilateral matching and template matching.

[0099] The calculation of matching cost is a bit different at different steps. When selecting the candidate from the candidate set at the CU level, the matching cost is the sum of absolute difference (SAD) of bilateral matching or template matching. After the starting MV is determined, the matching cost C of bilateral matching at sub-CU level search is calculated as follows:

where w is a weighting factor which is empirically set to 4, MV and MV^S indicate the current MV and the starting MV, respectively. SAD is still used as the matching cost of template matching at sub-CU level search.

[00100] In FRUC mode, MV is derived by using luma samples only. The derived motion will be used for both luma and chroma for MC inter prediction. After MV is decided, final MC is performed using 8-taps interpolation filter for luma and 4-taps interpolation filter for chroma.

2.2.5.5 MV refinement

[00101] MV refinement is a pattern based MV search with the criterion of bilateral matching cost or template matching cost. In the JEM, two search patterns are supported - an unrestricted center-biased diamond search (UCBDS) and an adaptive cross search for MV refinement at the CU level and sub-CU level, respectively. For both CU and sub-CU level MV refinement, the MV is directly searched at quarter luma sample MV accuracy, and this is followed by one-eighth luma sample MV refinement. The search range of MV refinement for the CU and sub-CU step are set equal to 8 luma samples. 2.2.5.6 Selection of prediction direction in template matching FRUC merge mode

[00102] In the bilateral matching merge mode, bi -prediction is always applied since the motion information of a CU is derived based on the closest match between two blocks along the motion trajectory of the current CU in two different reference pictures. There is no such limitation for the template matching merge mode. In the template matching merge mode, the encoder can choose among uni-prediction from listO, uni-prediction from listl or bi-prediction for a CU. The selection is based on a template matching cost as follows:

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

bi-prediction is used;

Otherwise, if costO <= costl

uni-prediction from listO is used;

Otherwise,

uni-prediction from listl is used;

where costO is the SAD of listO template matching, costl is the SAD of listl template matching and costBi is the SAD of bi-prediction template matching. The value of factor is equal to 1.25, which means that the selection process is biased toward bi-prediction.

[00103] The inter prediction direction selection is only applied to the CU-level template matching process.

2.2.6 Decoder-side motion vector refinement

[00104] In bi-prediction operation, for the prediction of one block region, two prediction blocks, formed using a motion vector (MV) of listO and a MV of listl, respectively, are combined to form a single prediction signal. In the decoder-side motion vector refinement (DMVR) method, the two motion vectors of the bi-prediction are further refined by a bilateral template matching process. The bilateral template matching applied in the decoder to perform a distortion-based search between a bilateral template and the reconstmction samples in the reference pictures in order to obtain a refined MV without transmission of additional motion information.

[00105] In DMVR, a bilateral template is generated as the weighted combination (i.e. average) of the two prediction blocks, from the initial MV0 of listO and MV1 of listl, respectively, as shown in FIG. 18. The template matching operation consists of calculating cost measures between the generated template and the sample region (around the initial prediction block) in the reference picture. For each of the two reference pictures, the MV that yields the minimum template cost is considered as the updated MV of that list to replace the original one. In the JEM, nine MV candidates are searched for each list. The nine MV candidates include the original MV and 8 surrounding MVs with one luma sample offset to the original MV in either the horizontal or vertical direction, or both. Finally, the two new MVs, i.e., MVO' and MV1' as shown in FIG. 18, 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.

[00106] DMVR is applied for the merge mode of bi-prediction with one MV from a reference picture in the past and another from a reference picture in the future, without the transmission of additional syntax elements. In the JEM, when LIC, affine motion, FRUC, or sub-CU merge candidate is enabled for a CU, DMVR is not applied.

2.3 Non-adjacent merge candidates

[00107] In J0021, Qualcomm proposes to derive additional spatial merge candidates from non- adjacent neighboring positions which are marked as 6 to 49 as in FIG. 19. The derived candidates are added after TMVP candidates in the merge candidate list.

[00108] In J0058, Tencent proposes to derive additional spatial merge candidates from positions in an outer reference area which has an offset of (-96, -96) to the current block.

[00109] As shown in FIG. 20, the positions are marked as A(i,j), B(i,j), C(i,j), D(i,j) and E(i,j). Each candidate B (i, j) or C (i, j) has an offset of 16 in the vertical direction compared to its previous B or C candidates. Each candidate A (i, j) or D (i, j) has an offset of 16 in the horizontal direction compared to its previous A or D candidates. Each E (i, j) has an offset of 16 in both horizontal direction and vertical direction compared to its previous E candidates. The candidates are checked from inside to the outside. And the order of the candidates is A (i, j), B (i, j), C (i, j), D (i, j), and E (i, j). To further study whether the number of merge candidates can be further reduced. The candidates are added after TMVP candidates in the merge candidate list. [00110] In J0059, the extended spatial positions from 6 to 27 as in FIG. 21 are checked according to their numerical order after the temporal candidate. To save the MV line buffer, all the spatial candidates are restricted within two CTU lines.

2.4 Related methods

[00111] Ultimate motion vector expression (UMVE) in J0024 can be either skip or direct (or merge) modes, which use proposed motion vector expression method with neighboring motion information. As the skip and the merge modes in HEVC, UMVE also makes a candidate list from neighboring motion information. Among those candidates in the list, a MV candidate is selected, and is further expanded by new motion vector expression method.

[00112] FIG. 22 shows an example of a UMVE search process, and FIG. 23 shows an example of UMVE search points.

[00113] UMVE provides a new motion vector expression with simplified signaling. The expression method includes starting point, motion magnitude, and motion direction.

[00114] Base candidate index defines the starting point. Base candidate index indicates the best candidate among candidates in the list as follows.

[00115] Distance index is motion magnitude information. Distance index indicates the pre defined distance from the starting point information. Pre-defined distance is as follows:

[00116] Direction index represents the direction of the MVD relative to the starting point. The direction index can represent of the four directions as shown below.

3. Discussion of drawbacks in existing implementations

[00117] In merge mode, motion information of a merge candidate is inherited by current block, including motion vector, reference pictures, prediction direction, LIC flag etc. Only a merge index is signaled, which is efficient in many cases. However, the inherited motion information, especially motion vector maybe not good enough.

[00118] On the other hand, in AM VP mode, all motion information is signaled, including motion vector (i.e., MVP index and MVD), reference pictures (i.e., reference index), prediction direction, LIC flag and MVD precision etc., which is bits consuming.

[00119] In the UMVE proposed by JVET-J0024, it proposes to code additional MVD. However, the MVD can only has non-zero component in either horizontal direction or vertical direction but not both direction. Meanwhile, it also signals the MVD information, i.e., the distance index or motion magnitude information.

4. Methods for extended merge mode (EMM) based on the disclosed technology

[00120] The techniques disclosed in the present document can be used by video encoder and decoder embodiments to implement an Extended Merge Mode (EMM), in which only few information is signaled and no special restriction is placed on MVD.

[00121] The detailed inventions below should be considered as examples to explain general concepts. These inventions should not be interpreted in a narrow way. Furthermore, these inventions can be combined in any manner.

[00122] It is proposed to split the motion information (such as prediction direction, reference indices/pictures, motion vectors, LIC flag, affine flag, Intra Block Copy (IBC) flag, MVD precision, MVD values) into two parts. The first part is directly inherited and the second part is signaled explicitly with/without predictive coding.

[00123] It is proposed that an EMM list is constmcted, and an index is signaled to indicate the first part of motion information of which candidate is inherited by the current block (e.g., PU/CU). Meanwhile, additional information (e.g., 2nd part of the motion information) like MVD is further signaled.

a. The first part of motion information includes one or more of the following information: prediction direction, reference pictures, motion vectors, LIC flag and MVD precision etc. b. The second part can be coded with predictive coding.

[00124] It is proposed that the motion information candidate list is constmcted by inserting motion information of spatial neighboring blocks, temporal neighboring blocks or non-adjacent blocks.

[00125] Alternatively, the prediction direction is not inherited and is explicitly signaled. In this case, it is proposed to constmct two or multiple motion information candidate lists.

[00126] It is proposed that MVD precision is inherited from neighboring blocks in merge modes and is stored.

[00127] The EMM mode is signaled as a special AMVP mode. When merge flag is false, an EMM flag is signaled to indicate whether EMM mode or AMVP mode is used.

a. Alternatively, EMM flag is signaled as a special merge mode. When the merge flag is true, an EMM flag is signaled to indicate whether EMM mode or merge mode is used.

b. Alternatively, there are only AMVP mode and EMM mode, without the normal merge mode.

c. Alternatively, there are only normal merge mode and EMM mode, without the AMVP mode.

d. Alternatively, a placeholder is inserted into the merge candidate list, and the EMM mode is used when the placeholder is selected.

e. Alternatively, additional merge candidates may be added to the conventional merge candidate list. When the selected merge index corresponds to these newly introduced merge candidates, the signaling of additional motion information is further allowed even it is coded as the merge mode.

f. Alternatively, EMM mode is coded in the syntax element inter prediction direction.

For example, inter_dir equal to 0 indicates that the EMM mode is used. [00128] Applying the EMM mode or not is signaled from the encoder to the decoder. For example, the selection can be signaled in Video Parameter Set (VPS), Sequence Parameter Set (SPS), Picture Parameter Set (PPS), Slice header, Coding Tree Unit (CTU), Coding Tree Block (CTB), Coding Unit (CU) or Prediction Unit (PU), region covering multiple CTU/CTB/CU/PUs.

[00129] The examples described above may be incorporated in the context of the methods described below, e.g., method 2400, which may be implemented at a video decoder or a video encoder.

[00130] FIG. 24 is a flowchart for an example method 2400 of processing a video bitstream. The method 2400 includes constructing (2402) a list of extended merge mode (EMM) candidates; determining (2404), based on a first set of bits in a bitstream representation of a current block, the motion information inherited by the current block from the list; determining (2406), based on a second set of bits in the bitstream representation of a current block, the signaled motion information of the current block; and performing (2408), based on the list of EMM candidates and the signaled motion information, a conversion between the current block and the bitstream representation, wherein the bitstream representation further comprises a field signaling a selective usage of an extended merge mode comprising the list of EMM candidates.

[00131] The following listing of examples provide embodiments that can addressed the technical problems described in the present document, among other problems.

[00132] 1. A method of video processing, comprising: constmcting a list of extended merge mode (EMM) candidates; determining, based on a first set of bits in a bitstream representation of a current block, the motion information inherited by the current block from the list; determining, based on a second set of bits in the bitstream representation of a current block, the signaled motion information of the current block; and performing, based on the list of EMM candidates and the signaled motion information, a conversion between the current block and the bitstream representation, wherein the bitstream representation further comprises a field signaling a selective usage of an extended merge mode comprising the list of EMM candidates.

[00133] 2. The method of example 1, wherein the field is signaled as a special advanced motion vector prediction (AM VP) mode.

[00134] 3. The method of example 2, wherein, in case a merge flag for the current block is set to false, the field indicates the usage of either the extended merge mode or the AMVP mode. [00135] 4. The method of example 1, wherein the field is signaled as a special merge mode.

[00136] 5. The method of example 2, wherein, in case a merge flag for the current block is set to true, the field indicates the usage of either the extended merge mode or a normal merge mode.

[00137] 6. The method of example 1, wherein a candidate list associated with the normal merge mode comprises a placeholder candidate, and wherein a selection of the placeholder candidate indicates the usage of the extended merge mode.

[00138] 7. The method of example 1, wherein the field is an inter prediction direction syntax element.

[00139] 8. The method of example 7, wherein the field is inter_dir, and wherein inter_dir equals zero indicates the usage of the extended merge mode.

[00140] 9. The method of example 1, further comprising: splitting the motion information into two parts, wherein the motion information comprises a prediction direction, one or more reference indices or pictures, one or more motion vectors, a local illumination compensation (LIC) flag, an affine flag, an intra block copy (IBC) flag, a motion vector difference (MVD) precision, or one or more MVD values.

[00141] 10. The method of example 9, wherein a first part of the two parts is directly inherited from the list of EMM candidates, and wherein a second part of the two parts is explicitly signaled.

[00142] 11. The method of any of examples 1 to 10, wherein the field is signaled in a video parameter set (VPS), sequence parameter set (SPS), picture parameter set (PPS), slice header, coding tree unit (CTU), coding tree block (CTB), coding unit (CU) or prediction unit (PU), or a region covering multiple CTUs, CTBs, CUs or PUs.

[00143] 12. An apparatus in a video system comprising a processor and a non-transitory memory with instmctions thereon, wherein the instructions upon execution by the processor, cause the processor to implement the method in any one of examples 1 to 11.

[00144] 13. A computer program product stored on a non-transitory computer readable media, the computer program product including program code for carrying out the method in any one of examples 1 to 11.

5. References [00145] [1] ITU-T and ISO/IEC,“High efficiency video coding”, Rec. ITU-T H.265 | ISO/IEC 23008-2 (in force edition).

[00146] [2] C. Rosewarne, B. Bross, M. Naccari, K. Sharman, G. Sullivan,“High Efficiency Video Coding (HEVC) Test Model 16 (HM 16) Improved Encoder Description Update 7,” JCTVC-Y 1002, Oct. 2016.

[00147] [3] J. Chen, E. Alshina, G. J. Sullivan, J.-R. Ohm, J. Boyce,“Algorithm description of Joint Exploration Test Model 7 (JEM7),” JVET-G1001, Aug. 2017.

[00148] [4] JEM-7.0: https://jvet.hhi.fraunhofer.de/svn/svn_HMJEMSoftware/tags/ HM-l6.6- JEM-7.0.

[00149] [5] A. Alshin, E. Alshina, etc.,“Description of SDR, HDR and 360° video coding technology proposal by Samsung, Huawei, GoPro, and HiSilicon - mobile application scenario,” JVET-J0024, Apr. 2018.

6. Embodiments of the disclosed technology

[00150] FIG. 25 is a block diagram of a video processing apparatus 2500. The apparatus 2500 may be used to implement one or more of the methods described herein. The apparatus 2500 may be embodied in a smartphone, tablet, computer, Internet of Things (IoT) receiver, and so on. The apparatus 2500 may include one or more processors 2502, one or more memories 2504 and video processing hardware 2506. The processor(s) 2502 may be configured to implement one or more methods (including, but not limited to, method 2400) described in the present document. The memory (memories) 2504 may be used for storing data and code used for implementing the methods and techniques described herein. The video processing hardware 2506 may be used to implement, in hardware circuitry, some techniques described in the present document.

[00151] In some embodiments, the video coding methods may be implemented using an apparatus that is implemented on a hardware platform as described with respect to FIG. 25.

[00152] 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. The disclosed and other embodiments can be implemented as one or more computer program products, i.e., one or more modules of computer program instmctions encoded on a computer readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable medium can be a machine -readable storage device, a machine-readable storage substrate, a memory device, a composition of matter effecting a machine-readable propagated signal, or a combination of one or more them. The term“data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. A propagated signal is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus.

[00153] A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

[00154] The processes and logic flows described in this document can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

[00155] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random-access memory or both. The essential elements of a computer are a processor for performing instmctions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Computer readable media suitable for storing computer program instmctions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

[00156] While this patent document contains many specifics, these should not be constmed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple

embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

[00157] Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.

[00158] Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.

Claims

1. A method of video processing, comprising:

constructing a list of extended merge mode (EMM) candidates;

determining, based on a first set of bits in a bitstream representation of a current block, the motion information inherited by the current block from the list;

determining, based on a second set of bits in the bitstream representation of a current block, the signaled motion information of the current block; and

performing, based on the list of EMM candidates and the signaled motion information, a conversion between the current block and the bitstream representation,

wherein the bitstream representation further comprises a field signaling a selective usage of an extended merge mode comprising the list of EMM candidates.

2. The method of claim 1 , wherein the field is signaled as a special advanced motion vector prediction (AMVP) mode.

3. The method of claim 2, wherein, in case a merge flag for the current block is set to false, the field indicates the usage of either the extended merge mode or the AMVP mode.

4. The method of claim 1 , wherein the field is signaled as a special merge mode.

5. The method of claim 2, wherein, in case a merge flag for the current block is set to true, the field indicates the usage of either the extended merge mode or a normal merge mode.

6. The method of claim 1 , wherein a candidate list associated with the normal merge mode comprises a placeholder candidate, and wherein a selection of the placeholder candidate indicates the usage of the extended merge mode.

7. The method of claim 1 , wherein the field is an inter prediction direction syntax element.

8. The method of claim 7, wherein the field is inter_dir, and wherein inter_dir equals zero indicates the usage of the extended merge mode.

9. The method of claim 1 , further comprising:

splitting the motion information into two parts, wherein the motion information comprises a prediction direction, one or more reference indices or pictures, one or more motion vectors, a local illumination compensation (LIC) flag, an affine flag, an intra block copy (IBC) flag, a motion vector difference (MVD) precision, or one or more MVD values.

10. The method of claim 9, wherein a first part of the two parts is directly inherited from the list of EMM candidates, and wherein a second part of the two parts is explicitly signaled.

11. The method of any of claims 1 to 10, wherein the field is signaled in a video parameter set (VPS), sequence parameter set (SPS), picture parameter set (PPS), slice header, coding tree unit (CTU), coding tree block (CTB), coding unit (CU) or prediction unit (PU), or a region covering multiple CTUs, CTBs, CUs or PUs.

12. An apparatus in a video system comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to implement the method recited in one or more of claims 1 to 11.

13. A computer program product stored on a non-transitory computer readable media, the computer program product including program code for carrying out the method recited in one or more of claims 1 to 11.