Patent Description:
Video coding systems may be used to compress digital video signals, e.g., to reduce the storage and/or transmission bandwidth needed for such signals. Video coding systems may include block-based, wavelet-based, and/or object-based systems. The systems employ video coding techniques, such as bidirectional motion compensated prediction (MCP), which may remove temporal redundancies by exploiting temporal correlations between pictures. Such techniques may increase the complexity of computations performed during encoding and/or decoding. The following documents disclose a simplification of the Bi-Directional Optical Flow computation in the VVC standard development: <NPL>); <NPL>); <NPL>); <NPL>".

The invention is set out in the set of appended claims. Bi-directional optical flow (BDOF) may be bypassed, for a current coding block, based or whether symmetric motion vector difference (SMVD) is used in motion vector coding for the current coding block.

A coding device (e.g., an encoder or a decoder) may determine that BDOF is enabled. The coding device may determine whether to bypass BDOF for the current coding block based at least in part on an SMVD indication for the current coding block. The coding device may obtain the SMVD indication that indicates whether SMVD is used in motion vector coding for the current coding block. If SMVD indication indicates that SMVD is used in the motion vector coding for the current coding block, the coding device may bypass BDOF for the current coding block. The coding device may reconstruct the current coding block without performing BDOF if it determines to bypass BDOF for the current coding block.

A motion vector difference (MVD) for the current coding block may indicate a difference between a motion vector predictor (MVP) for the current coding block and a motion vector (MV) for the current coding block. The MVP for the current coding block may be determined based on an MV of a spatial neighboring block of the current coding block and/or a temporal neighboring block of the current coding block.

If an SMVD indication indicates SMVD is used for the motion vector coding for the current coding block, the coding device may receive a first motion vector coding information associated with a first reference picture list. The coding device may determine a second motion vector coding information associated with a second reference picture list based on the first motion vector coding information associated with the first reference picture list and that the MVD associated with the first reference picture list and the MVD associated with the second reference picture list are symmetric.

In an example, if the SMVD indication indicates SMVD is used for the motion vector coding for the current coding block, the coding device may parse a first MVD associated with a first reference picture list in a bitstream. The coding device may determine a second MVD associated with a second reference picture list based on the first MVD and that the first MVD and the second MVD are symmetric to each other.

If the coding device determines not to bypass BDOF for the current coding block, the coding device may refine a motion vector of a (e.g., each) sub-block of the current coding block based at least in part on gradients associated with a location in the current coding block.

The coding device may receive a sequence-level SMVD indication that indicates whether SMVD is enabled for a sequence of pictures. If SMVD is enabled for the sequence of pictures, the coding device may obtain the SMVD indication associated with the current coding block based on the sequence-level SMVD indication.

A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings.

The coding device may be or may include a WTRU.

The SGW <NUM> may perform other functions, such as anchoring user planes during intereNode B handovers, triggering paging when DL data is available for the WTRUs 102a, 102b, 102c, managing and storing contexts of the WTRUs 102a, 102b, 102c, and the like.

11e DLS or an <NUM>. The IBSS mode of communication may sometimes be referred to herein as an "adhoc" mode of communication.

A PDU session type may be IP-based, non-IP based, Ethernetbased, and the like.

Video coding systems may be used to compress digital video signals, which may reduce the storage needs and/or the transmission bandwidth of video signals. Video coding systems may include block-based, wavelet-based, and/or object-based systems. Block-based video coding systems may include MPEG-<NUM>/<NUM>/<NUM> part <NUM>, H. <NUM>/MPEG-<NUM> part <NUM> AVC, VC-<NUM>, High Efficiency Video Coding (HEVC) and/or Versatile Video Coding (VVC).

Block-based video coding systems may include a block-based hybrid video coding framework. <FIG> is a diagram of an example block-based hybrid video encoding framework for an encoder. An encoder may include a WTRU. An input video signal <NUM> may be processed block-by-block. Block sizes (e.g., extended block sizes, such as a coding unit (CU)) may compress high resolution (e.g., 1080p and beyond) video signals. For example, a CU may include 64x64 pixels or more. A CU may be partitioned into prediction units (PUs), and/or separate predictions may be used. For an input video block (e.g., macroblock (MB) and/or a CU), spatial prediction <NUM> and/or temporal prediction <NUM> may be performed. Spatial prediction <NUM> (e.g., intra prediction) may use pixels from samples of coded neighboring blocks (e.g., reference samples) in the video picture/slice to predict the current video block. The spatial prediction <NUM> may reduce spatial redundancy, for example, that may be inherent in the video signal. Motion prediction <NUM> (e.g., inter prediction and/or temporal prediction) may use reconstructed pixels from the coded video pictures, for example, to predict the current video block. The motion prediction <NUM> may reduce temporal redundancy, for example, that may be inherent in the video signal. Motion prediction signals for a video block may be signaled by one or more motion vectors and/or may indicate the amount and/or the direction of motion between the current block and/or the current block's reference block. If multiple reference pictures are supported for a (e.g., each) video block, the video block's reference picture index may be sent. The reference picture index may be used to identify from which reference picture in a reference picture store <NUM> the motion prediction signal may derive.

After the spatial prediction <NUM> and/or motion prediction <NUM>, a mode decision block <NUM> in the encoder may determine a prediction mode (e.g., the best prediction mode), for example, based on a rate-distortion optimization. The prediction block may be subtracted from a current video block <NUM> and/or the prediction residual may be de-correlated using a transform <NUM> and/or a quantization <NUM> to achieve a bitrate, such as a target bit rate. The quantized residual coefficients may be inverse quantized at quantization <NUM> and/or inverse transformed at transform <NUM>, for example, to form the reconstructed residual, which may be added to the prediction block <NUM>, for example, to form a reconstructed video block. In-loop filtering (e.g., a de-blocking filter and/or adaptive loop filters) may be applied at loop filter <NUM> on the reconstructed video block before the reconstructed video block may be put in the reference picture store <NUM> and/or used to code video blocks (e.g., future video blocks). To form the output video bit-stream <NUM>, coding mode (e.g., inter or intra), prediction mode information, motion information, and/or quantized residual coefficients may be sent (e.g., may all be sent) to an entropy coding module <NUM>, for example, to be compressed and/or packed to form the bit-stream.

<FIG> is a diagram of an example block-based video decoding framework for a decoder. A decoder may include a WTRU. A video bit-stream <NUM> (e.g., the video bit-stream <NUM> in <FIG>) may be unpacked (e.g., first unpacked) and/or entropy decoded at an entropy decoding module <NUM>. The coding mode and prediction information may be sent to a spatial prediction module <NUM> (e.g., if intra coded) and/or to a motion compensation prediction module <NUM> (e.g., if inter coded and/or temporal coded) to form a prediction block. Residual transform coefficients may be sent to an inverse quantization module <NUM> and/or to an inverse transform module <NUM>, e.g., to reconstruct the residual block. The prediction block and/or the residual block may be added together at <NUM>. The reconstructed block may go through in-loop filtering at a loop filter <NUM>, for example, before the reconstructed block is stored in a reference picture store <NUM>. The reconstructed video <NUM> in the reference picture store <NUM> may be sent to a display device and/or used to predict video blocks (e.g., future video blocks).

The use of bi-directional motion compensated prediction (MCP) in video codecs may remove temporal redundancies by exploiting temporal correlations between pictures. A bi-prediction signal may be formed by combining two uni-prediction signals using a weight value (e.g., <NUM>). In certain videos, illuminance characteristics may change rapidly from one reference picture to another. Thus, prediction techniques may compensate for variations in illuminance over time (e.g., fading transitions) by applying global or local weights and offset values to one or more sample values in the reference pictures.

MCP in a bi-prediction mode may be performed using CU weights. As an example, MCP may be performed using bi-prediction with CU weights. An example of bi-prediction with CU weights (BCW) may include generalized bi-prediction (GBi). A bi-prediction signal may be calculated based on one or more of weight(s), motion-compensated prediction signal(s) corresponding to a motion vector associated with a reference picture list(s), and/or the like. In an example, the prediction signal at sample x (as given) in a bi-prediction mode may be calculated using Eq.<NUM>.

P[x] may denote the resulting prediction signal of a sample x located at a picture position x. Pi[x+vi] may denote the motion-compensated prediction signal of x using the motion vector (MV) vi for i-th list (e.g., list <NUM>, list <NUM>, etc.). w<NUM> and w<NUM> may denote the two weight values that are applied on a prediction signal(s) for a block and/or CU. As an example, w<NUM> and w<NUM> may denote the two weight values shared across the samples in a block and/or CU. A variety of prediction signals may be obtained by adjusting the weight value(s). As shown in Eq.<NUM>, a variety of prediction signals may be obtained by adjusting weight values w<NUM> and w<NUM>.

Some configurations of weight values w<NUM> and w<NUM> may indicate prediction such as uni-prediction and/or bi-prediction. For example, (w<NUM>, w<NUM>) = (<NUM>, <NUM>) may be used in associated with uni-prediction with reference list L0. (w<NUM>, w<NUM>) = (<NUM>, <NUM>) may be used in association with uni-prediction with reference list L1. (w<NUM>, w<NUM>) = (<NUM>, <NUM>) may be used in association with the bi-prediction with two reference lists (e.g., L1 and L2).

Weight(s) may be signaled at the CU level. In an example, the weight values w<NUM> and w<NUM> may be signaled per CU. Bi-prediction may be performed with the CU weights. A constraint for the weights may be applied to a pair of weights. The constraint may be preconfigured. For example, the constraint for the weights may include w<NUM>+ w<NUM>=<NUM>. A weight may be signaled. The signaled weight may be used to determine another weight. For example, with the constraint for CU weights, only one weight may be signaled. Signaling overhead may be reduced. Examples of pairs of weights may include {(<NUM>/<NUM>, <NUM>/<NUM>), (<NUM>/<NUM>, <NUM>/<NUM>), (<NUM>/<NUM>, <NUM>/<NUM>), (-<NUM>/<NUM>, <NUM>/<NUM>), (<NUM>/<NUM>, -<NUM>/<NUM>)}.

A weight may be derived based on a constraint for the weights, for example, when unequal weights are to be used. A coding device may receive a weight indication and determine a first weight based on the weight indication. The coding device may derive a second weight based on the determined first weight and the constraint for the weights.

Eq. <NUM> may be used. In an example, Eq. <NUM> may be produced based on Eq.<NUM> and the constraint of w<NUM>+ w<NUM>=<NUM>.

Weight values (e.g., w<NUM> and/or w<NUM>) may be discretized. Weight signaling overhead may be reduced. In an example, bi-prediction CU weight value w<NUM> may be discretized. The discretized weight value w<NUM> may include, for example, one or more of -<NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>, and/or the like. A weight indication may be used to indicate weights to be used for a CU, for example, for bi-prediction. An example of the weight indication may include a weight index. In an example, each weight value may be indicated by an index value.

<FIG> is a diagram of an example video encoder with support of BCW (e.g., GBi). An encoding device as described in the example shown in <FIG> may be or may include a WTRU. The encoder may include a mode decision module <NUM>, spatial prediction module <NUM>, a motion prediction module <NUM>, a transform module <NUM>, a quantization module <NUM>, an inverse quantization module <NUM>, an inverse transform module <NUM>, a loop filter <NUM>, a reference picture store <NUM> and an entropy coding module <NUM>. In examples. , some or all of the encoder's modules or components (e.g., the spatial prediction module <NUM>) may be the same as, or similar to, those described in connection with <FIG>. In addition, the spatial prediction module <NUM> and the motion prediction module <NUM> may be pixel-domain prediction modules. Thus, an input video bit-stream <NUM> may be processed in a similar manner as the input video bit-stream <NUM>, to output video bit-stream <NUM>. The motion prediction module <NUM> may further include support of bi-prediction with CU weights. As such, the motion prediction module <NUM> may combine two separate prediction signals in a weighted-averaging manner. Further, the selected weight index may be signaled in the input video bitstream <NUM>.

<FIG> is a diagram of an example module with support for bi-prediction with CU weights for an encoder. <FIG> illustrates a block diagram of an estimation module <NUM>. The estimation module <NUM> may be employed in a motion prediction module of an encoder, such as the motion prediction module <NUM>. The estimation module <NUM> may be used in connection with BCW (e.g., GBi). The estimation module <NUM> may include a weight value estimation module <NUM> and a motion estimation module <NUM>. The estimation module <NUM> may utilize a two-step process to generate inter prediction signal, such as a final inter prediction signal. The motion estimation module <NUM> may perform motion estimation using reference picture(s) received from a reference picture store <NUM> and by searching two optimal motion vectors (MVs) pointing to (e.g., two) reference blocks. The weight value estimation module <NUM> may search for the optimal weight index to minimize the weighted bi-prediction error between the current video block and bi-prediction prediction. The prediction signal of the generalized bi-prediction may be computed as a weighted average of the two prediction blocks.

<FIG> is a diagram of an example block-based video decoder with support for bi-prediction with CU weights. <FIG> illustrates a block diagram of an example video decoder that may decode a bit-stream from an encoder. The encoder may support BCW and/or share some similarities with the encoder described in connection with <FIG>. A decoder as described in the example shown in <FIG> may include a WTRU. As shown in <FIG>, the decoder may include an entropy decoder <NUM>, a spatial prediction module <NUM>, a motion prediction module <NUM>, a reference picture store <NUM>, an inverse quantization module <NUM>, an inverse transform module <NUM> and a loop filter module <NUM>. Some or all of the decoder's modules may be the same as, or similar to, those described in connection with <FIG>. For example, the prediction block and/or residual block may be added together at <NUM>. A video bit-stream <NUM> may be processed to generate the reconstructed video <NUM> that may be sent to a display device and/or used to predict video blocks (e.g., future video blocks). The motion prediction module <NUM> may further include support for BCW. The coding mode and/or prediction information may be used to derive a prediction signal using either spatial prediction or MCP with support for BCW. For BCW, the block motion information and/or weight value (e.g., in the form of an index indicating a weight value) may be received and decoded to generate the prediction block.

<FIG> is a diagram of an example module with support for bi-prediction with CU weights for a decoder. <FIG> illustrates a block diagram of a prediction module <NUM>. The prediction module <NUM> may be employed in a motion prediction module of a decoder, such as the motion prediction module <NUM>. The prediction module <NUM> may be used in connection with BCW. The prediction module <NUM> may include a weighted averaging module <NUM> and a motion compensation module <NUM>, which may receive one or more references pictures from a reference picture store <NUM>. The prediction module <NUM> may use the block motion information and weight value to compute a prediction signal of BCW as a weighted average of (e.g., two) motion compensated prediction blocks.

Bi-prediction in video coding may be based on a combination of multiple (e.g., two) temporal prediction blocks. In examples, CU and block may be used interchangeably. The temporal prediction blocks may be combined. In an example, two temporal prediction blocks that are obtained from the reference pictures that are reconstructed may be combined using averaging. Bi-prediction may be based on block-based motion compensation. A relatively small motion may be observed between the (e.g., two) prediction blocks in bi-prediction.

Bi-directional optical flow (BDOF) may be used, for example, to compensate the relatively small motion observed between prediction blocks. BDOF may be applied to compensate for such motion for a sample inside a block. In an example, BDOF may compensate for such motion for individual samples inside a block. This may increase the efficiency of motion compensated prediction.

BDOF may include refining motion vector(s) associated with a block. In examples, BDOF may include sample-wise motion refinement that is performed on top of block-based motion-compensated predictions when bi-prediction is used. BDOF may include deriving refined motion vector(s) for a sample. As an example of BDOF, the derivation of the refined motion vector for individual samples in a block may be based on the optical flow model.

BDOF may include refining a motion vector of a sub-block associated with a block based on one or more of the following: a location in a block; gradients (e.g., horizontal, vertical, and/or the like) associated with the location in the block; sample values associated with a corresponding reference picture list for the location; and/or the like. Eq. <NUM> may be used for deriving refined motion vector for a sample. As shown in Eq. <NUM>, I(k) (x, y) may denote the sample value at the coordinate (x, y) of the prediction block, derived from the reference picture list k (k = <NUM>, <NUM>). ∂I(k)(x,y)/∂x and ∂I(k)(x,y)/ ∂y may be the horizontal and vertical gradients of the sample. The motion refinement (vx,vy) at (x, y) may be derived using Eq. <NUM>. Eq. <NUM> may be based on an assumption that the optical flow model is valid.

<FIG> illustrates an example bidirectional optical flow. In <FIG>, (MVx0, MVy0) and (MVx1, MVy1) may indicate block-level motion vectors. The block-level motion vectors may be used to generate prediction blocks I(<NUM>) and I(<NUM>). The motion refinement parameters (vx,vy) at the sample location (x, y) may be calculated, for example, by minimizing the difference Δ between the motion vector values of the sample(s) after motion refinement (e.g., motion vector between the current picture and the backward reference picture A and motion vector between the current picture and the forward reference picture B in <FIG>). The difference Δ between the motion vector values of the samples after motion refinement may be calculated using, for example, Eq. <NUM>.

It may be assumed that the motion refinement is consistent for the samples, for example inside one unit (e.g., a 4x4 block). Such assumption may support the regularity of the derived motion refinement. The value of ( <MAT>) may be derived, for example, by minimizing Δ inside the 6x6 window Ω around each 4x4 block, as shown in Eq. <NUM>.

In examples, BDOF may include a progressive technique which may optimize the motion refinement in the horizontal direction (e.g., first) and in the vertical direction (e.g., second), e.g., to be used in association with Eq. <NUM>. This may result in Eq. <NUM>. <MAT> where <MAT> may be the floor function that outputs the greatest value that is less than or equal to the input. thBIO may be the motion refinement value (e.g., threshold value) to prevent the error propagation, e.g., due to coding noise and irregular local motion. As an example, the motion refinement value may be <NUM><NUM>-BD. The values of S<NUM>, S<NUM>, S<NUM>, S<NUM> and S<NUM> may be calculated, for example, as shown in Eq. <NUM> and Eq. <NUM> <MAT> where <MAT>.

The BDOF gradients in Eq. <NUM> in the horizontal and vertical directions may be obtained by calculating the difference between multiple neighboring samples at a sample position of the L0/L1 prediction block. In an example, the difference may be calculated between two neighboring samples horizontally or vertically depending on the direction of the gradient being derived at one sample position of each L0/L1 prediction block, for example, using Eq. <NUM>.

In Eq. <NUM>, L may be the bit-depth increase for the internal BDOF, e.g., to keep data precision. L may be set to <NUM>. The regulation parameters r and m in Eq. <NUM> may be defined as shown in Eq. <NUM> (e.g., to avoid division by a smaller value). <MAT> BD may be the bit depth of the input video. The bi-prediction signal (e.g., final bi-prediction signal) of the current CU may be calculated by interpolating the L0/L1 prediction samples along the motion trajectory, e.g., based on the optical flow Eq. <NUM> and the motion refinement derived by Eq. <NUM>. The bi-prediction signal of the current CU may be calculated using Eq. <NUM>. <MAT> shift and ooffset may be the offset and right shift that is applied to combine the L0 and L1 prediction signals for bi-prediction, which may be set equal to <NUM> - BD and <NUM> « (<NUM> - BD) + <NUM> · (<NUM> « <NUM>), respectively; rnd(. ) may be a rounding function that rounds the input value to the closest integer value.

There may be various types of motions within a particular video, such as zoom in/out, rotation, perspective motions and other irregular motions. A translational motion model and/or an affine motion model may be applied for MCP. The affine motion model may be four-parameter and/or six-parameter. A first flag for (e.g., each) inter coded CU may be signaled to indicate whether the translational motion model or the affine motion model is applied for inter prediction. If the affine motion model is applied, a second flag may be sent to indicate whether the model is four-parameter or six-parameter.

The four-parameter affine motion model may include two parameters for translation movement in the horizontal and vertical directions, one parameter for a zoom motion in the horizontal and vertical directions and/or one parameter for a rotation motion in the horizontal and vertical directions. A horizontal zoom parameter may be equal to a vertical zoom parameter. A horizontal rotation parameter may be equal to a vertical rotation parameter. The four-parameter affine motion model may be coded using two motion vectors at two control point positions defined at the top-left and top right corners of a (e.g., current) CU.

<FIG> illustrates an example four-parameter affine mode. <FIG> illustrates an example affine motion field of a block. As shown in <FIG>, the block may be described by two control point motion vectors (Vo, V<NUM>). Based on a control point motion, the motion field (vx, vy) of one affine coded block may be described in Eq. <NUM>.

In Eq. <NUM>, (v0x, v0y) may be a motion vector of the top-left corner control point. (v1x, v1y) may be a motion vector of the top-right corner control point. w may be the width of CU. The motion field of an affine coded CU may be derived at a 4x4 block level. For example, (vx, vy) may be derived for each of the 4x4 blocks within a current CU and applied to a corresponding 4x4 block.

The four parameters may be estimated iteratively. The motion vector pairs at step k may be denoted as <MAT>, the original luminance signal as I(i,j), and the prediction luminance signal as I'k(i,j). The spatial gradient <MAT> and <MAT> may be derived using a Sobel filter applied on the prediction signal I'k(i,j) in the horizontal and vertical directions, respectively. The derivative of Eq. <NUM> may be represented as Eq. <NUM>.

In Eq. <NUM>, (a, b) may be delta translational parameters and (c, d) may be delta zoom and rotation parameters at step k. The delta MV at control points may be derived with its coordinates as Eq. <NUM> and Eq. <NUM>. For example, (<NUM>, <NUM>), (w, <NUM>) may be coordinates for top-left and top-right control points, respectively. <MAT> <MAT>.

Based on an optical flow equation, the relationship between the change of luminance and the spatial gradient and temporal movement may be formulated as Eq. <NUM>.

Substituting <MAT> and <MAT> with Eq. <NUM> may produce Eq. <NUM> for parameters (a, b, c, d).

If the samples in the CU satisfy Eq. <NUM>, the parameter set (a, b, c, d) may be derived using, for example, the least square calculation. The motion vectors at two control points <MAT> at step (k+<NUM>) may be derived with Eqs. <NUM> and <NUM>, and they may be rounded to a specific precision (e.g., <NUM>/<NUM> pel). Using the iteration, the motion vectors at two control points may be refined until it converges when parameters (a, b, c, d) may be zeros or the iteration times meet a pre-defined limit.

The six-parameter affine motion model may include two parameters for translation movement in the horizontal and vertical directions, one parameter for a zoom motion, one parameter for a rotation motion in the horizontal direction, one parameter for a zoom motion and/or one parameter for a rotation motion in the vertical direction. The six-parameter affine motion model may be coded with three motion vectors at three control points. <FIG> illustrates an example six-parameter affine mode. As shown in <FIG>, three control points for the six-parameter affine coded CU may be defined at the top-left, top-right and/or bottom left corners of CU. The motion at the top-left control point may be related to a translational motion. The motion at the top-right control point may be related to rotation and zoom motions in the horizontal direction. The motion at the bottom-left control point may be related to rotation and zoom motions in the vertical direction. In the six-parameter affine motion model, the rotation and zoom motions in the horizontal direction may not be same as those motions in the vertical direction. In an example, the motion vector of each sub-block (vx, vy) may be derived from Eqs. <NUM> and <NUM> using three motion vectors as control points: <MAT> <MAT>.

<NUM> and <NUM>, (v2x, v2y) may be a motion vector of the bottom-left control point. (x, y) may be a center position of a sub-block. w and h may be a width and height of a CU.

The six parameters of the six-parameter affine model may be estimated, for example, in a similar way. For example, Eq. <NUM> may be produced based on Eq. <NUM>.

In Eq. <NUM>, for step k, (a, b) may be delta translation parameters. (c, d) may be delta zoom and rotation parameters for the horizontal direction. (e, f) may be delta zoom and rotation parameters for the vertical direction. For example, Eq. <NUM> may be produced based on Eq. <NUM>.

The parameter set (a, b, c, d, e, f) may be derived using the least square calculation by considering the samples within the CU. The motion vector of the top-left control point ( <MAT>) may be calculated using Eq. <NUM>. The motion vector of the top-right control point ( <MAT>) may be calculated using Eq. <NUM>. The motion vector of the top-right control point ( <MAT>) may be calculated using Eq. <NUM>. <MAT> <MAT>.

There may be a symmetric MV difference for bi-prediction. In some examples, the motion vector in forward reference picture and backward reference picture may be symmetric, e.g., due to the continuity of motion trajectory in bi-prediction.

SMVD may be an inter coding mode. With SMVD, the MVD of a first reference picture list (e.g., reference picture list <NUM>) may be symmetric to the MVD of a second reference picture list (e.g., reference picture list <NUM>). The motion vector coding information (e.g., MVD) of one reference picture list may be signaled, and the motion vector information of another reference picture list may not be signaled. The motion vector information of another reference picture list may be determined, for example, based on the signaled motion vector information and that the motion vector information of the reference pictures lists is symmetric. In an example, the MVD of reference picture list <NUM> may be signaled and the MVD of List <NUM> may not be signaled. The MV coded with this mode may be calculated using Eq. 24A. <MAT> where the subscripts indicate the reference picture list <NUM> or <NUM>, x indicates horizontal direction and y indicates vertical direction.

As shown in Eq. 24A, an MVD for the current coding block may indicate a difference between an MVP for the current coding block and an MV for the current coding block. A person of skilled in the art would appreciate that an MVP may be determined based on an MV of a spatial neighboring block(s) of the current coding block and/or a temporal neighboring block for the current coding block. Eq. 24A may be illustrated in <FIG> illustrates an example non-affine motion symmetric MVD (e.g., MVD1=-MVD0). As shown in Eq. 24A and illustrated in <FIG>, the MV of a current coding block may be equal to the sum of the MVP for the current coding block and the MVD of the current coding block (or negative MVD depending on the reference picture list). As shown in Eq. 24A and illustrated in <FIG>, the MVD of reference picture list <NUM> (MVD1) may be equal to negative of the MVD of reference picture list <NUM> (MVDQ) for SMVD. The MV predictor (MVP) of reference picture list <NUM> (MVP0) may or may not be symmetric to the MVP of reference picture list <NUM>(MVP1). The MVP0 may or may not be equal to negative of the MVP1. As shown in Eq. 24A, the MV of a current coding block may be equal to the sum of the MVP for the current coding block and the MVD of the current coding block. The MV of reference picture list <NUM> (MV0) may not be equal to negative of the MV of reference picture list <NUM> (MV1) based on Eq. 24A. The MV of reference picture list <NUM> MV0 may or may not be symmetric to the MV of reference picture list <NUM> MV1.

SMVD may be available for bi-prediction in the case that: reference picture list <NUM> includes a forward reference picture and reference picture list <NUM> includes a backward reference picture; or reference picture list <NUM> includes a backward reference picture and reference picture list <NUM> includes a forward reference picture.

With SMVD, the reference picture indices of reference picture list <NUM> and list <NUM> may not be signaled. They may be derived as follows. If reference picture list <NUM> includes a forward reference picture and reference picture list <NUM> includes a backward reference picture, the reference picture index in list <NUM> may be set to the nearest forward reference picture to the current picture, and, the reference picture index of list <NUM> may be set to the nearest backward reference picture to the current picture. If reference picture list <NUM> includes a backward reference picture and reference picture list <NUM> includes a forward reference picture, the reference picture index in list <NUM> may be set to the nearest backward reference picture to the current picture, and, the reference picture index of list <NUM> may be set to the nearest forward reference picture to the current picture.

For SMVD, a reference picture index may not need to be signaled for either list. One set of MVD for one reference picture list (e.g. list <NUM>) may be signaled. Signaling overhead may be reduced for bi-prediction coding.

In a merge mode, motion information may be derived and/or be used (e.g., directly used) to generate the prediction samples of the current CU. A merge mode with motion vector difference (MMVD) may be used. A merge flag may be signaled to specify whether MMVD is used for a CU. A MMVD flag may be signaled after sending a skip flag.

In MMVD, after a merge candidate is selected, the merge candidate may be refined by MVD information. The MVD information may be signaled. The MVD information may include one or more of a merge candidate flag, a distance index to specify motion magnitude, and/or an index for indication of motion direction. In MMVD, one of multiple candidates (e.g., the first two) candidates in the merge list may be selected to be used as the MV basis. The merge candidate flag may indicate which candidate is used.

The distance index may specify motion magnitude information and/or may indicate a pre-defined offset from the starting point (e.g., from the candidate selected to be the MV basis). <FIG> illustrates an example motion vector difference (MVD) search point(s). As shown in <FIG>, the center points may be the starting point MVs. As shown in <FIG>, the patterns in the dots may indicate different searching orders (e.g., from dots nearest the center MVs to the ones further away from the center MVs). As shown in <FIG>, an offset may be added to the horizontal component and/or vertical component of the starting point MV. Example relation of distance index and pre-defined offset is shown in Table <NUM>.

The direction index may represent the direction of the MVD relative to the starting point. The direction index may represent any of the four directions as shown in Table <NUM>. The meaning of MVD sign may vary according to the information of the starting point MV or MVs. When the starting point has a uni-prediction MV or a pair of bi-prediction MVs with both lists pointing to the same side of the current picture, the sign in Table <NUM> may specify the sign of the MV offset added to the starting MV or MVs. For example, when the picture order counts (POCs) of two references are both larger than the POC of the current picture, or are both smaller than the POC of the current picture, the sign may specify the sign of the MV offset added to the starting MV or MVs. When the starting point has a pair of bi-prediction MVs with both lists pointing to the different sides of the current picture (e.g., when the POC of one reference is larger than the POC of the current picture, and the POC of the other reference is smaller than the POC of the current picture), the sign in Table <NUM> may specify the sign of MV offset added to the list0 MV component of the starting point MV, and the sign of the MV offset added to the list1 MV may have the opposite value.

A symmetric mode for bi-prediction coding may be used. One or more features described herein may be used in association with the symmetric mode for bi-prediction coding, e.g., which, in examples, may increase coding efficiency and/or reduce complexity. The symmetric mode may include SMVD. One or more features described herein may be associated with synergizing SMVD with one or more other coding tools, e.g., bi-prediction with CU weights (BCW or BPWA), BDOF, and/or affine mode. One or more features described herein may be used in encoding (e.g., encoder optimization), which may include fast motion estimation for translational and/or affine motion.

SMVD coding features may include one or more of restrictions, signaling, SMVD search features, and/or the like.

The application of SMVD mode may be based on the CU size. For example, a restriction may disallow the SMVD for a relatively small CU (e.g., a CU having an area that is no greater than <NUM>). A restriction may disallow the SMVD for a relatively large CU (e.g., a CU bigger than 32x32). If a restriction disallows the SMVD for a CU, symmetric MVD signaling may be skipped or disabled for the CU, and/or a coding device (e.g., an encoder) may not search symmetric MVD.

The application of SMVD mode may be based on the POC distances between the current picture and reference pictures. The coding efficiency of SMVD may decrease for relatively large POC distance (e.g., POC distance greater than or equal to <NUM>). SMVD may be disabled if the POC distance between a reference picture (e.g., any reference picture) to the current picture is relatively large. If SMVD is disabled, the symmetric MVD signaling may be skipped or disabled, and/or a coding device (e.g., an encoder) may not search symmetric MVD.

The application of SMVD mode may be restricted to one or more temporal layers. In an example, a lower temporal layer may refer to reference pictures that have larger POC distance from the current picture, with a hierarchical GOP structure. The coding efficiency of the SMVD may decrease for a lower temporal layer. SMVD coding may be disallowed for relatively low temporal layers (e.g., temporal layer <NUM> and <NUM>). If SMVD is disallowed, the symmetric MVD signaling may be skipped or disabled, and/or a coding device (e.g., an encoder) may not search symmetric MVD.

In SMVD coding, the MVD of one of the reference picture lists may be signaled (e.g., explicitly signaled). In an example, a coding device (e.g., a decoder) may parse a first MVD associated with a first reference picture list in a bitstream. The coding device may determine a second MVD associated with a second reference picture list based on the first MVD and that the first MVD and the second MVD are symmetric to each other.

The coding device (e.g., the decoder) may identify the MVD of which reference picture list is signaled, for example, whether the MVD of reference picture list <NUM> or reference picture list <NUM> is signaled. In examples, the MVD of reference picture list <NUM> may be signaled (e.g., always signaled). The MVD of reference picture list <NUM> may be obtained (e.g., derived).

The reference picture list whose MVD is signaled (e.g., explicitly signaled) may be selected. One or more of the following may apply. An indication (e.g., a flag) may be signaled to indicate which reference picture list is selected. The reference picture list with a smaller POC distance to a current picture may be selected. If the POC distances for the reference picture lists are the same, a reference picture list may be predetermined to break the tie. For example, reference picture list <NUM> may be selected if the POC distances for the reference picture lists are the same.

The MVP index for a reference picture list (e.g., one reference picture list) may be signaled. In some examples, the indices of MVP candidates for both reference picture lists may be signaled (e.g., explicitly signaled). The MVP index for a reference picture list may be signaled (e.g., only the MVP index for one reference picture list), for example to reduce signaling overhead. The MVP index for the other reference picture list may be derived, for example as described herein. LX may be the reference picture list whose MVP index is signaled (e.g., explicitly signaled), and i may be the signaled MVP index. mvp' may be derived from the MVP of LX as shown in Eq. <NUM>. <MAT> where POCLX , POC<NUM>-LX and POCcurrmay be the POC of list LX reference picture, list (<NUM>-LX) reference picture, and current picture, respectively. And from the reference picture list (<NUM>-LX)'s MVP list, the MVP that is closest to mvp' may be selected, e.g., as shown in Eq. <NUM>, <MAT> where j may be the MVP index of reference picture list (<NUM>-LX). LX may be the reference picture list whose MVD is signaled (e.g., explicitly signaled).

Table <NUM> shows an example CU syntax that may support symmetric MVD signaling for a non-affine coding mode.

For example, an indication, such as the sym_mvd_flag flag may indicate whether SMVD is used in motion vector coding for the current coding block (e.g., bi-prediction coded CU).

An indication, such as refIdxSymL0, may indicate the reference picture index in reference picture list <NUM>. The refIdxSymL0 indication being set to -<NUM> may indicate that SMVD is not applicable, and the sym_mvd_flag may be absent.

An indication, such as refIdxSymL1, may indicate the reference picture index in reference picture list <NUM>. The refldxSymL1 indication having a value of -<NUM> may indicate that SMVD is not applicable, and the sym_mvd_flag may be absent.

In examples, SMVD may be applicable when reference picture list <NUM> includes a forward reference picture and reference picture list <NUM> includes a backward reference picture, or when reference picture list <NUM> includes a backward reference picture and reference picture list <NUM> includes a forward reference picture. Otherwise, SMVD may not be applicable. For example, when SMVD is not applicable, signaling of the SMVD indication (e.g., the sym_mvd_flag flag at the CU level), may be skipped. A coding device (e.g., a decoder) may perform one or more condition checks. As shown in Table <NUM>, two condition checks may be performed (e.g. refIdxSymL0 > -<NUM> and refIdxSymL1 >-<NUM>). The one or more condition checks may be performed to determine whether the SMVD indication is used. As shown in Table <NUM>, two condition checks may be performed (e.g. refIdxSymL0 > -<NUM> and refIdxSymL1 >-<NUM>) to determine whether the SMVD indication is received. In an example, in order for a decoder to check these conditions, the decoder may wait for the construction of the reference picture lists of the current picture (e.g., list0 and list1) before certain CU parsing. In some instances, even if both of the two checked conditions (e.g. refIdxSymL0 > -<NUM> and refIdxSymL1 >-<NUM>) are true, an encoder may not use the SMVD for the CU (e.g., to save the encoding complexity).

An SMVD indication may be at CU level and associated with a current coding block. The CU-level SMVD indication may be obtained based a higher level indication. In examples, the presence of the CU-level SMVD indication (e.g., the sym_mvd_flag flag), may be controlled (e.g., alternatively or additionally) by a higher level indication. For example, a SMVD enabled indication, such as the sym_mvd_enabled_flag flag may be signaled at the slice level, tile level, tile group level, or at the picture parameter set (PPS) level, at the sequence parameter set (SPS) level, and/or in any syntax level, where the reference picture lists are shared by CUs associated with the syntax level. For example, a slice level flag can be placed in the slice header. In an example, a coding device (e.g., a decoder) may receive a sequence-level SMVD indication indicating whether SMVD is enabled for a sequence of pictures. If SMVD is enabled for the sequence, the coding device may obtain an SMVD indication associated with the current coding block based on the sequence-level SMVD indication.

With the higher-level SMVD enablement indication, such as the sym_mvd_enabled_flag flag, the CU-level parsing may be performed without checking the one or more conditions (e.g., described herein). The SMVD can be enabled or disabled at the higher level than CU level (e.g. at the discretion of the encoder). Table <NUM> shows example syntax that may support SMVD mode.

There may be different ways for a coding device (e.g., an encoder) to determine whether to enable SMVD, and set the value of the sym_mvd_enabled_flag accordingly. One of more examples herein may be combined to determine whether to enable SMVD, and set the value of the sym_mvd_enabled_flag accordingly. For example, the encoder may determine whether to enable SMVD by checking the reference picture lists. If both forward and backward reference picture are present, the sym_mvd_enabled_flag flag may be set equal to true to enable SMVD. For example, the encoder may determine whether to enable SMVD based on the temporal distance(s) between the current picture and the forward and/or backward reference picture(s). When a reference picture is far away from current picture, SMVD mode may not be effective. If either the forward or backward reference picture is far away from the current picture, the encoder may disable SMVD. A coding device may set the value of the high-level SMVD enablement indication based on value (e.g., a threshold value) for temporal distances between a current picture and a reference picture. For example, to reduce the encoding complexity, the encoder may use (e.g., only use) the higher level control flag to enable SMVD for pictures having forward and backward reference pictures such that the maximum temporal distance of these two reference pictures to the current picture is smaller than a value (e.g., a threshold value). For example, the encoder may determine whether to enable SMVD based on a temporal layer (e.g., of the current picture). A relatively low temporal layer may indicate that the reference picture is far away from the current picture, and in that case SMVD may not be effective. The encoder may determine that the current picture belongs to a relatively low temporal layer (e.g., lower than a threshold, such as <NUM>, <NUM>), and may disable SMVD for such a current picture. For example, the sym_mvd_enabled_flag may be set to false to disable SMVD. For example, the encoder may determine whether to enable SMVD based on the statistics of previously coded pictures at the same temporal layer as the temporal layer of the current picture. The statistics may include the average POC distance for bi-prediction coded CUs (e.g., the average of the distance(s) between the current picture(s) and the temporal center of two reference pictures of the current picture(s)). R0, R1 may be the reference pictures for bi-prediction coded CU. poc(x) may be the POC of picture x. The POC distance distance (CUi) of two reference pictures and current picture (current_picture) may be calculated using Eq. <NUM>. The average POC distance for bi-prediction coded CUs AvgDist may be calculated using Eq. <NUM>. <MAT> <MAT> Variable N may indicate the total number of bi-prediction coded CU's that may have both forward and backward reference pictures. As an example, if AvgDist is smaller than a value (e.g., a predefined threshold), sym_mvd_enabled_flag may be set by the encoder to true to enable SMVD; otherwise, sym_mvd_enabled_flag may be set to false to disable SMVD.

In some examples, MVD value(s) may be signaled. In some examples, a combination of direction index and distance index may be signaled, and MVD value(s) may not be signaled. Example direction table and example distance table as shown in Table <NUM> and Table <NUM> may be used for signaling and deriving the MVD information. For example, a combination of distance index <NUM> and direction index <NUM> may indicate MVD (<NUM>/<NUM>, <NUM>).

A search for symmetric MVD may be performed, e.g., after uni-prediction search and bi-prediction search. A uni-prediction search may be used to search for an optimal MV pointing to a reference block for uni-prediction. A bi-prediction search may be used to search for two optimal MVs pointing to two reference blocks for bi-prediction. The search may be performed to find a candidate symmetric MVD, e.g., the best symmetric MVD. In an example, a set of search points may be iteratively evaluated, for the symmetric MVD search. An iteration may include evaluation of a set of search points. The set of search points may form a search pattern centered around, for example, a best MV of a previous iteration. For the first iteration, the search pattern may be centered around the initial MV. The selection of an initial MV may affect overall results. A set of initial MV candidates may be evaluated. The initial MV for the symmetric MVD search may be determined, for example, based on rate-distortion cost. In an example, the MV candidate with the lowest rate-distortion cost may be chosen to be the initial MV for the symmetric MVD search. The rate-distortion cost may be estimated, for example, by summing the bi-prediction error and the weighted rate of MVD coding for reference picture list <NUM>. The set of initial MV candidates may include one or more of the MV(s) obtained from uni-prediction search, the MV(s) obtained from the bi-prediction search, and the MVs from the advanced motion vector predictor (AMVP) list. At least one MV may be obtained for each reference picture from uni-prediction search.

Early termination may be applied, e.g., to reduce complexity. Early termination may be applied (e.g., by an encoder) if bi-prediction cost is larger than a value (e.g., a threshold value). In examples, e.g., before initial MV selection, the search for symmetric MVD may be terminated if the rate-distortion cost for the MV obtained from the bi-prediction search is larger than a value (e.g., a threshold value). For example, the value may be set to a multiple of (e.g., <NUM> times) the uni-prediction cost. In examples, e.g., after initial MV selection, the symmetric MVD search may be terminated if the rate-distortion cost associated with the initial MV is higher than a value (e.g., a threshold value). For example, the value may be set to a multiple of (e.g., <NUM> times of) the lowest among uni-prediction cost and bi-prediction cost.

There may be interaction between SMVD mode and other coding tools. One or more of the following may be performed: symmetric affine MVD coding; combine SMVD with bi-prediction with CU weights (BCW or BPWA); or combine SMVD with BDOF.

Symmetric affine MVD coding may be used. Affine motion model parameters may be represented by control point motion vectors. <NUM>-parameter affine model may be represented by two control point MVs and <NUM>-parameter affine model may be represented by three control point MVs. An example shown in Eq. <NUM> may be a <NUM>-parameter affine motion model represented by two control point MVs, e.g., the top-left control point MV (v0) and top-right control point MV (v1). The top-left control point MV may represent the translational motion. The top-left control point MV may have corresponding symmetric MVs, for example, associated with forward and backward reference pictures following the motion trajectory. SMVD mode may be applied to the top-left control point. The other control point MVs may represent a combination of zoom, rotate, and/or shear mapping. SMVD may not be applied to the other control point MVs.

SMVD may be applied to the top-left control point (e.g., only to the top-left control point), and the other control point MVs may be set to their respective MV predictor(s).

An MVD of a control point associated with a first reference picture list may be signaled. An MVD(s) of a control point(s) associated with a second reference picture list(s) may be obtained based on the MVD of the control point associated with the first reference picture list and that the MVD of the control point associated with the first reference picture list is symmetric to an MVD of a control point associated with the second reference picture list. In examples, when symmetric affine MVD mode is applied, the MVD of a control point associated with reference picture list <NUM> may be signaled (e.g., only the control point MVD of reference picture list <NUM> may be signaled). MVDs of control points associated with reference picture list <NUM> may be derived based on the symmetric property. The MVDs of control points associated with reference picture list <NUM> may not be signaled.

The control point MVs associated with reference picture lists may be derived. The control point MVs for control point <NUM> (top-left) of reference picture list <NUM> and reference picture list <NUM>, may be derived, for example, using Eq. <NUM>.

Eq. <NUM> may be illustrated in <FIG> illustrates an example affine motion symmetric MVD. As shown in Eq. <NUM> and illustrated in <FIG>, the top left control point MV of a current coding block may be equal to the sum of the MVP for the top left control point of the current coding block and the MVD for the top left control point of the current coding block (or negative MVD depending on the reference picture list). As shown in Eq. <NUM> and illustrated in <FIG>, the top left control point MVD associated with reference picture list <NUM> may be equal to negative of top left control point MVD associated with reference picture list <NUM> for symmetric affine MVD coding.

The MVs for other control points may be derived using at least the affine MVP prediction, for example, as shown in Eq. <NUM>. <MAT> In Eqs. <NUM>-<NUM>, the first dimension of the subscripts may indicate the reference picture list. The second dimension of the subscripts may indicate the control point index.

The translational MVD (e.g., mvdx<NUM>,<NUM>, mvdy<NUM>,<NUM>) of a first reference picture list may be applied to the top-left control point MV derivation of a second reference picture list (e.g., mvx<NUM>,<NUM>, mvy<NUM>,<NUM>). The translational MVD (e.g., mvdx<NUM>,<NUM>, mvdy<NUM>,<NUM>) of the first reference picture list may not be applied to other control point MV derivation of the second reference picture list (e.g., mvx<NUM>,j, mvy<NUM>,j). In some examples of symmetric affine MVD derivation, the translational MVD (mvdx<NUM>,<NUM>, mvdy<NUM>,<NUM>) of reference picture list <NUM> may be applied (e.g., only applied) to the top-left control point MV derivation of reference picture list <NUM>. The other control-point MVs of list <NUM> may be the same as the corresponding predictor(s), for example, as shown in Eq. <NUM>. <MAT> Table <NUM> shows example syntax that may be used for signaling the use of SMVD in combination with affine mode (e.g., symmetric affine MVD coding).

Whether affine SMVD is used may be determined, for example, based on an inter-affine indication and/or an SMVD indication. For example, when inter-affine indication inter_affine_flag is <NUM> and SMVD indication sym_mvd_flag[ x0 ][ y0 ] is <NUM>, affine SMVD may be applied. When inter-affine indication inter_affine_flag is <NUM> and SMVD indication sym_mvd_flag[ x0 ][ y0 ] is <NUM>, non-affine motion SMVD may be applied. If SMVD indication sym_mvd_flag[ x0 ][ y0 ] is <NUM>, SMVD may not be applied.

The MVDs of the control points in a reference picture list (e.g., reference picture list <NUM>) may be signaled. In an example, the MVD value(s) of a subset of the control points in a reference picture list may be signaled. For example, the MVD of the top-left control point in reference picture list <NUM> may be signaled. The signaling of the MVDs of the other control points in reference picture list <NUM> may be skipped, and, for example, the MVDs of these control points may be set to <NUM>. The MVs of the other control points may be derived based on the MVD of the top-left control point in reference picture list <NUM>. For example, the MVs of the control points, whose MVDs are not signaled, may be derived as shown in Eq. <NUM>.

Table <NUM> shows example syntax that may be used for signaling the information related to SMVD mode in combination with affine mode.

An indication, such as a top left MVD only flag, may indicate whether only the MVD of the top-left control point in a reference picture list (e.g., reference picture list <NUM>) is signaled, or whether the MVDs of the control points in the reference picture list are signaled. This indication may be signaled at the CU level. Table <NUM> shows example syntax that may be used for signaling the information related to SMVD mode in combination with affine mode.

In examples, a combination of direction index and distance index, as described herein with respect to MMVD, may be signaled. Example direction table and example distance table are shown in Table <NUM> and Table <NUM>. For example, a combination of distance index <NUM> and direction index <NUM> may indicate MVD (<NUM>/<NUM>, <NUM>).

In examples, an indication (e.g., a flag) may be signaled to indicate which reference picture list's MVDs are signaled. The other reference picture list's MVDs may not be signaled; they may be derived.

One or more restrictions described herein for translational motion symmetric MVD coding may be applied to affine motion symmetric MVD coding, for example, to reduce complexity and/or reduce signaling overhead.

With the symmetric affine MVD mode, signaling overhead may be reduced. Coding efficiency may be improved.

Bi-predictive motion estimation may be used to search for a symmetric MVD for an affine model. In an example, to find a symmetric MVD for affine model (e.g., the best symmetric MVD for affine model), the bi-predictive motion estimation may be applied after uni-prediction search. The reference pictures for reference picture list <NUM> and/or reference picture list <NUM> may be derived as described herein. The initial control point MVs may be selected from one or more of the uni-prediction searching results, bi-prediction searching results, and/or the MVs from the affine AMVP list. A control point MV (e.g., the control point MV with the lowest rate-distortion cost) may be selected to be the initial MV. A coding device (e.g., the encoder) may check one or more cases: in a first case, symmetric MVD may be signaled for reference picture list <NUM>, and control point MVs for reference picture list <NUM> may be derived based on symmetric mapping (using Eq. <NUM> and/or Eq.<NUM>); in a second case, symmetric MVD for reference picture list <NUM> may be signaled, and control point MVs for reference picture list <NUM> may be derived based on symmetric mapping. The first case may be used as an example herein. The symmetric MVD search technique may be applied based on uni-prediction searching results. Given the control point MV predictors in reference picture list <NUM>, an iterative search using a pre-defined search pattern (e.g., diamond pattern, cubic pattern, and/or the like) may be applied. At an iteration (e.g., each iteration), the MVD may be refined by the search pattern, and the control point MVs in reference picture list <NUM> and reference picture list <NUM> may be derived using Eq. <NUM> and/or Eq. <NUM>. The bi-prediction error corresponding to control point MVs of reference picture list <NUM> and reference picture list <NUM> may be evaluated. The rate-distortion cost may be estimated, for example, by summing the bi-prediction error and the weighted rate of MVD coding for reference picture list <NUM>. In an example, the MVD with a low (e.g., the lowest) rate-distortion cost during the searching candidates may be treated as the best MVD for the symmetric MVD search process. The other control point MVs such as top-right and bottom-left control point MVs may be refined, e.g., using the optical flow-based technique described herein.

Symmetric MVD search for symmetric affine MVD coding may be performed. In an example, a set of parameters such as translational parameters may be searched first, followed by non-translational parameters searching. In an example, optical flow search may be performed by (e.g., jointly) considering the MVD of reference picture list <NUM> and reference picture list <NUM>. For <NUM>-parameter affine model, an example optical flow equation for the list <NUM> MVDs may be shown in Eq. <NUM>. <MAT> where <MAT> may denote the list <NUM> prediction in the k-th iteration, and <MAT> and <MAT> may denote spatial gradients of the list <NUM> prediction.

Reference picture list <NUM> may have translation change (e.g., may have translation change only). The translation change may have the same magnitude as that of reference picture list <NUM> but in a reverse direction, which is the condition of symmetric affine MVD. An example optical flow equation for reference picture list <NUM> MVDs may be shown in Eq. <NUM>.

The BCW weights w<NUM> and w<NUM> to list <NUM> prediction and list <NUM> prediction may be applied respectively. An example optical equation for the symmetric affine model may be shown in Eq. <NUM>,<MAT>
where <MAT> <MAT> <MAT> <MAT> <MAT>.

Parameters a, b, c, d may be estimated (e.g., by the least mean square error calculation).

An example optical flow equation for symmetric <NUM>-parameter affine model may be shown in Eq. <NUM>.

Parameters a, b, c, d, e, f may be estimated by the least mean square error calculation. When joint optical-flow motion search is performed, the affine parameters may be optimized jointly. Performance may be improved.

Early termination may be applied, e.g., to reduce complexity. In examples, e.g., before initial MV selection, the search may be terminated if the bi-prediction cost is larger than a value (e.g., a threshold value). For example, the value can be set to a multiple of the cost of uni-prediction cost, e.g., <NUM> times the cost of uni-prediction cost. In examples, a coding device (e.g., the encoder) may, before the ME of symmetric affine MVD starts, compare the current best affine motion estimation (ME) cost (considering uni-prediction and bi-prediction affine search) with the non-affine ME cost. If the current best affine ME cost is larger than the non-affine ME cost multiplied by a value (e.g., a threshold value such as <NUM>), the coding device may skip the ME of symmetric affine MVD. In examples, e.g., after initial MV selection, the affine symmetric MVD search may be skipped if the cost of the initial MV is higher than a value (e.g., a threshold value). For example, the value may be a multiple (e.g., set to <NUM> times) of the lowest among uni-prediction cost and bi-prediction cost. In examples, the value may be set to a multiple (e.g., <NUM>) times of the non-affine ME cost.

SMVD mode may be combined with BCW. When the BCW is enabled for current CU, SMVD may be applied in one or more ways. In certain examples, SMVD may be enabled when (e.g., only when) the BCW weight is equal weight (e.g., <NUM>); and, SMVD may be disabled for other BCW weights. In such a case, the SMVD flag may be signaled before BCW weight index, and the signaling of BCW weight index may be conditionally controlled by the SMVD flag. When SMVD indication (e.g., SMVD flag) may have a value of <NUM>, the signaling of BCW weight index may be skipped. The decoder may infer the SMVD indication as having a value <NUM>, which may correspond to the equal weight for bi-predictive averaging. When the SMVD flag is <NUM>, the BCW weight index may be coded for bi-prediction mode. In the example where SMVD may be enabled when the BCW weight is equal weight and disabled for other BCW weights, the BCW index may sometimes be skipped. In some examples, the SMVD may be fully combined with BCW. The SMVD flag and BCW weight index may be signaled for explicit bi-prediction mode. The MVD search (e.g., of the encoder) for SMVD may consider the BCW weight index during bi-predictive averaging. The SMVD search may be based on an evaluation of one or more (e.g., all) possible BCW weights.

A coding tool (e.g., bi-directional optical flow (BDOF)) may be used in association with one or more other coding tools/modes. BDOF may be used in association with SMVD. Whether BDOF is applied for a coding block may depend on whether SMVD is used. SMVD may be based on the assumption of symmetric MVD at a coding block level. BDOF, if performed, may be used to refine sub-block MVs based on optical flow. Optical flow may be based on the assumption of symmetric MVD at a sub-block level.

BDOF may be used in association with SMVD. In an example, a coding device (e.g., a decoder or an encoder) may receive one or more indications that SMVD and/or BDOF are enabled. BDOF may be enabled for the current picture. The coding device may determine whether BDOF is to be bypassed or performed on a current coding block. The coding device may determine whether to bypass BDOF based on an SMVD indication (e.g., sym_mvd_flag[ x0 ][ y0 ]). BDOF may be used interchangeably with BIO in some examples.

The coding device may determine whether to bypass BDOF for a current coding block. BDOF may be bypassed on the current coding block if SMVD mode is used for motion vector coding of the current coding block, e.g., to reduce decoding complexity. If SMVD is not used for the motion vector coding of the current coding block, the coding device may determine whether to enable BDOF for the current coding block, for example, based on at least another condition.

The coding device may obtain an SMVD indication (e.g., sym_mvd_flag[ x0 ][ y0 ]). The SMVD indication may indicate whether SMVD is used in motion vector coding for the current coding block.

The current coding block may be reconstructed based on the determination of whether to bypass BDOF. An MVD may be signaled (e.g., explicitly signaled) at the CU level for the SMVD mode.

The coding device may be configured to perform motion vector coding using SMVD without BDOF based on the determination to bypass BDOF.

Claim 1:
A video decoding device comprising:
a processor configured to:
determine that bi-directional optical flow (BDOF) is enabled for a picture comprising a current coding block;
obtain a symmetric motion vector difference (SMVD) indication associated with the current coding block, the SMVD indication indicating whether SMVD is used in a determination of a motion vector (MV) for the current coding block;
determine whether to bypass BDOF for the current coding block based on the SMVD indication associated with the current coding block, wherein the processor is configured to determine, on a condition that SMVD is used in the determination of the MV for the current coding block, to bypass BDOF for the current coding block; and
reconstruct the current coding block based on the determination of whether to bypass BDOF.