KR20230159254A - Method for Inter-predicting Chroma Components Using Bi-prediction - Google Patents

Abstract

A method for inter prediction of chroma components using bidirectional prediction is disclosed. In this embodiment, the image decoding device generates bidirectional chroma prediction blocks for the current chroma block. The image decoding apparatus derives weights of the current chroma block using the first luma prediction block, the second luma prediction block, and the final luma reconstruction block of the luma block corresponding to the current chroma block. The image decoding device applies weights to the bidirectional chroma prediction blocks to generate the final chroma prediction block of the current chroma block.

Description

Inter prediction method for chroma components using bi-prediction {Method for Inter-predicting Chroma Components Using Bi-prediction}

The present disclosure relates to an inter prediction method of chroma components using bidirectional prediction.

The content described below simply provides background information related to this embodiment and does not constitute prior art.

Since video data has a larger amount of data than audio data or still image data, it requires a lot of hardware resources, including memory, to store or transmit it without processing for compression.

Therefore, typically, when storing or transmitting video data, an encoder is used to compress the video data and store or transmit it, and a decoder receives the compressed video data, decompresses it, and plays it. These video compression technologies include H.264/AVC, HEVC (High Efficiency Video Coding), and VVC (Versatile Video Coding), which improves coding efficiency by about 30% or more compared to HEVC.

However, the size, resolution, and frame rate of the image are gradually increasing, and the amount of data that needs to be encoded is also increasing accordingly, so a new compression technology with better coding efficiency and higher picture quality improvement effect than the existing compression technology is required.

In VVC technology, the Advanced Motion Vector Prediction (AMVP) mode of inter prediction assigns weights at the CU (Coding Unit) level to the two prediction signals derived by two motion vectors during bidirectional prediction. This method is called bi-prediction with CU-level weight (BCW). In the case of BCW, the decoder generates the final prediction signal by averaging the two prediction signals based on the weight. Therefore, in order to improve video coding efficiency and improve picture quality, efficient setting of these weights needs to be considered, especially for chroma components.

In the present disclosure, in inter prediction of chroma components, weights are calculated on the decoder side using up and down luma prediction blocks or template matching, and the calculated weights are applied to the bidirectional chroma prediction blocks to generate the final prediction block. The purpose is to provide a video coding method and device.

According to an embodiment of the present disclosure, in a method of predicting a current chroma block performed by an image decoding apparatus, generating bidirectional chroma prediction blocks for the current chroma block, wherein the bidirectional chroma prediction blocks are first Includes a chroma prediction block and a second chroma prediction block; Deriving weights of the current chroma block using a first luma prediction block, a second luma prediction block, and a final luma restoration block of the luma block corresponding to the current chroma block, wherein the weights are the first chroma weight. and a second chroma weight; and applying the weights to the bidirectional chroma prediction blocks to generate a final chroma prediction block of the current chroma block.

According to another embodiment of the present disclosure, in a method of predicting a current chroma block performed by an image encoding device, generating bidirectional chroma prediction blocks for the current chroma block, wherein the bidirectional chroma prediction blocks are Includes a first chroma prediction block and a second chroma prediction block; Deriving weights of the current chroma block using a first luma prediction block, a second luma prediction block, and a final luma restoration block of the luma block corresponding to the current chroma block, wherein the weights are the first chroma weight. and a second chroma weight; and applying the weights to the bidirectional chroma prediction blocks to generate a final chroma prediction block of the current chroma block.

According to another embodiment of the present disclosure, a computer-readable recording medium storing a bitstream generated by an image encoding method, the image encoding method comprising: generating bidirectional chroma prediction blocks for a current chroma block, where: The bidirectional chroma prediction blocks include a first chroma prediction block and a second chroma prediction block; Deriving weights of the current chroma block using a first luma prediction block, a second luma prediction block, and a final luma restoration block of the luma block corresponding to the current chroma block, wherein the weights are the first chroma weight. and a second chroma weight; and applying the weights to the bidirectional chroma prediction blocks to generate a final chroma prediction block of the current chroma block.

As described above, according to this embodiment, in inter prediction of chroma components, the decoder calculates the weight using the up-and-down luma prediction blocks or template matching, and applies the calculated weight to the bi-directional chroma prediction blocks. By providing a video coding method and device for generating a final prediction block, it is possible to improve video coding efficiency and video quality.

1 is an example block diagram of a video encoding device that can implement the techniques of the present disclosure.
Figure 2 is a diagram for explaining a method of dividing a block using the QTBTTT (QuadTree plus BinaryTree TernaryTree) structure.
3A and 3B are diagrams showing a plurality of intra prediction modes including wide-angle intra prediction modes.
Figure 4 is an example diagram of neighboring blocks of the current block.
Figure 5 is an example block diagram of a video decoding device that can implement the techniques of the present disclosure.
Figure 6 is an example diagram showing weighted chroma prediction.
Figure 7 is an example diagram showing the use of a luma block when predicting a chroma block, according to an embodiment of the present disclosure.
Figure 8 is an example diagram illustrating calculation of the difference between luma blocks and surrounding luma samples, according to an embodiment of the present disclosure.
Figure 9 is an example diagram illustrating template matching according to an embodiment of the present disclosure.
FIG. 10 is a flowchart illustrating a method for predicting a current chroma block performed by an image encoding device according to an embodiment of the present disclosure.
FIG. 11 is a flowchart illustrating a method for predicting a current chroma block performed by an image decoding apparatus according to an embodiment of the present disclosure.

Hereinafter, some embodiments of the present disclosure will be described in detail with reference to illustrative drawings. When adding reference numerals to components in each drawing, it should be noted that identical components are given the same reference numerals as much as possible even if they are shown in different drawings. Additionally, in describing the present embodiments, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present embodiments, the detailed description will be omitted.

1 is an example block diagram of a video encoding device that can implement the techniques of the present disclosure. Hereinafter, the video encoding device and its sub-configurations will be described with reference to the illustration in FIG. 1.

The image encoding device includes a picture division unit 110, a prediction unit 120, a subtractor 130, a transform unit 140, a quantization unit 145, a rearrangement unit 150, an entropy encoding unit 155, and an inverse quantization unit. It may be configured to include (160), an inverse transform unit (165), an adder (170), a loop filter unit (180), and a memory (190).

Each component of the video encoding device may be implemented as hardware or software, or may be implemented as a combination of hardware and software. Additionally, the function of each component may be implemented as software and a microprocessor may be implemented to execute the function of the software corresponding to each component.

One image (video) consists of one or more sequences including a plurality of pictures. Each picture is divided into a plurality of regions and encoding is performed for each region. For example, one picture is divided into one or more tiles and/or slices. Here, one or more tiles can be defined as a tile group. Each tile or/slice is divided into one or more Coding Tree Units (CTUs). And each CTU is divided into one or more CUs (Coding Units) by a tree structure. Information applied to each CU is encoded as the syntax of the CU, and information commonly applied to CUs included in one CTU is encoded as the syntax of the CTU. Additionally, information commonly applied to all blocks within one slice is encoded as the syntax of the slice header, and information applied to all blocks constituting one or more pictures is a picture parameter set (PPS) or picture parameter set. Encoded in the header. Furthermore, information commonly referenced by multiple pictures is encoded in a sequence parameter set (SPS). And, information commonly referenced by one or more SPSs is encoded in a video parameter set (VPS). Additionally, information commonly applied to one tile or tile group may be encoded as the syntax of a tile or tile group header. Syntax included in the SPS, PPS, slice header, tile, or tile group header may be referred to as high level syntax.

The picture division unit 110 determines the size of the CTU (Coding Tree Unit). Information about the size of the CTU (CTU size) is encoded as SPS or PPS syntax and transmitted to the video decoding device.

The picture division unit 110 divides each picture constituting the image into a plurality of CTUs (Coding Tree Units) with a predetermined size, and then repeatedly divides the CTUs using a tree structure. (recursively) Divide. A leaf node in the tree structure becomes a coding unit (CU), the basic unit of encoding.

The tree structure is QuadTree (QT), in which the parent node is divided into four child nodes (or child nodes) of the same size, or BinaryTree, in which the parent node is divided into two child nodes. , BT), or a TernaryTree (TT) in which the parent node is divided into three child nodes in a 1:2:1 ratio, or a structure that mixes two or more of these QT structures, BT structures, and TT structures. there is. For example, a QuadTree plus BinaryTree (QTBT) structure may be used, or a QuadTree plus BinaryTree TernaryTree (QTBTTT) structure may be used. Here, BTTT may be combined and referred to as MTT (Multiple-Type Tree).

Figure 2 is a diagram to explain a method of dividing a block using the QTBTTT structure.

As shown in Figure 2, the CTU can first be divided into a QT structure. Quadtree splitting can be repeated until the size of the splitting block reaches the minimum block size (MinQTSize) of the leaf node allowed in QT. The first flag (QT_split_flag) indicating whether each node of the QT structure is split into four nodes of the lower layer is encoded by the entropy encoder 155 and signaled to the video decoding device. If the leaf node of QT is not larger than the maximum block size (MaxBTSize) of the root node allowed in BT, it may be further divided into either the BT structure or the TT structure. In the BT structure and/or TT structure, there may be multiple division directions. For example, there may be two directions in which the block of the node is divided: horizontally and vertically. As shown in Figure 2, when MTT splitting begins, a second flag (mtt_split_flag) indicates whether the nodes have been split, and if split, an additional flag indicating the splitting direction (vertical or horizontal) and/or the splitting type (Binary). Or, a flag indicating Ternary) is encoded by the entropy encoding unit 155 and signaled to the video decoding device.

Alternatively, prior to encoding the first flag (QT_split_flag) indicating whether each node is split into four nodes of the lower layer, a CU split flag (split_cu_flag) indicating whether the node is split is encoded. It could be. If the CU split flag (split_cu_flag) value indicates that it is not split, the block of the corresponding node becomes a leaf node in the split tree structure and becomes a CU (coding unit), which is the basic unit of coding. When the CU split flag (split_cu_flag) value indicates splitting, the video encoding device starts encoding from the first flag in the above-described manner.

When QTBT is used as another example of a tree structure, there are two types: a type that horizontally splits the block of the node into two blocks of the same size (i.e., symmetric horizontal splitting) and a type that splits it vertically (i.e., symmetric vertical splitting). Branches may exist. A split flag (split_flag) indicating whether each node of the BT structure is divided into blocks of a lower layer and split type information indicating the type of division are encoded by the entropy encoder 155 and transmitted to the video decoding device. Meanwhile, there may be an additional type that divides the block of the corresponding node into two asymmetric blocks. The asymmetric form may include dividing the block of the corresponding node into two rectangular blocks with a size ratio of 1:3, or may include dividing the block of the corresponding node diagonally.

A CU can have various sizes depending on the QTBT or QTBTTT division from the CTU. Hereinafter, the block corresponding to the CU (i.e., leaf node of QTBTTT) to be encoded or decoded is referred to as the 'current block'. Depending on the adoption of QTBTTT partitioning, the shape of the current block may be rectangular as well as square.

The prediction unit 120 predicts the current block and generates a prediction block. The prediction unit 120 includes an intra prediction unit 122 and an inter prediction unit 124.

In general, each current block in a picture can be coded predictively. Typically, prediction of the current block is done using intra prediction techniques (using data from the picture containing the current block) or inter prediction techniques (using data from pictures coded before the picture containing the current block). It can be done. Inter prediction includes both one-way prediction and two-way prediction.

The intra prediction unit 122 predicts pixels within the current block using pixels (reference pixels) located around the current block within the current picture including the current block. There are multiple intra prediction modes depending on the prediction direction. For example, as shown in FIG. 3A, the plurality of intra prediction modes may include two non-directional modes including a planar mode and a DC mode and 65 directional modes. The surrounding pixels and calculation formulas to be used are defined differently for each prediction mode.

For efficient directional prediction of the rectangular-shaped current block, the directional modes (67 to 80, -1 to -14 intra prediction modes) shown by dotted arrows in FIG. 3B can be additionally used. These may be referred to as “wide angle intra-prediction modes”. In Figure 3b, the arrows point to corresponding reference samples used for prediction and do not indicate the direction of prediction. The predicted direction is opposite to the direction indicated by the arrow. Wide-angle intra prediction modes are modes that perform prediction in the opposite direction of a specific directional mode without transmitting additional bits when the current block is rectangular. At this time, among the wide-angle intra prediction modes, some wide-angle intra prediction modes available for the current block may be determined according to the ratio of the width and height of the rectangular current block. For example, wide-angle intra prediction modes with angles smaller than 45 degrees (intra prediction modes 67 to 80) are available when the current block is in the form of a rectangle whose height is smaller than its width, and wide-angle intra prediction modes with angles larger than -135 degrees are available. Intra prediction modes (-1 to -14 intra prediction modes) are available when the current block has a rectangular shape with a width greater than the height.

The intra prediction unit 122 can determine the intra prediction mode to be used to encode the current block. In some examples, intra prediction unit 122 may encode the current block using multiple intra prediction modes and select an appropriate intra prediction mode to use from the tested modes. For example, the intra prediction unit 122 calculates rate-distortion values using rate-distortion analysis for several tested intra-prediction modes and has the best rate-distortion characteristics among the tested modes. You can also select intra prediction mode.

The intra prediction unit 122 selects one intra prediction mode from a plurality of intra prediction modes and predicts the current block using surrounding pixels (reference pixels) and an operation formula determined according to the selected intra prediction mode. Information about the selected intra prediction mode is encoded by the entropy encoding unit 155 and transmitted to the video decoding device.

The inter prediction unit 124 generates a prediction block for the current block using a motion compensation process. The inter prediction unit 124 searches for a block most similar to the current block in a reference picture that has been encoded and decoded before the current picture, and generates a prediction block for the current block using the searched block. Then, a motion vector (MV) corresponding to the displacement between the current block in the current picture and the prediction block in the reference picture is generated. Typically, motion estimation is performed on the luma component, and a motion vector calculated based on the luma component is used for both the luma component and the chroma component. Motion information including information about the reference picture and information about the motion vector used to predict the current block is encoded by the entropy encoding unit 155 and transmitted to the video decoding device.

The inter prediction unit 124 may perform interpolation on a reference picture or reference block to increase prediction accuracy. That is, subsamples between two consecutive integer samples are interpolated by applying filter coefficients to a plurality of consecutive integer samples including the two integer samples. If the process of searching for the block most similar to the current block is performed for the interpolated reference picture, the motion vector can be expressed with precision in decimal units rather than precision in integer samples. The precision or resolution of the motion vector may be set differently for each target area to be encoded, for example, slice, tile, CTU, CU, etc. When such adaptive motion vector resolution (AMVR) is applied, information about the motion vector resolution to be applied to each target area must be signaled for each target area. For example, if the target area is a CU, information about the motion vector resolution applied to each CU is signaled. Information about motion vector resolution may be information indicating the precision of a differential motion vector, which will be described later.

Meanwhile, the inter prediction unit 124 may perform inter prediction using bi-prediction. In the case of bidirectional prediction, two reference pictures and two motion vectors indicating the positions of blocks most similar to the current block within each reference picture are used. The inter prediction unit 124 selects the first reference picture and the second reference picture from reference picture list 0 (RefPicList0) and reference picture list 1 (RefPicList1), respectively, and searches for a block similar to the current block within each reference picture. Create a first reference block and a second reference block. Then, the first reference block and the second reference block are averaged or weighted to generate a prediction block for the current block. Then, motion information including information about the two reference pictures used to predict the current block and information about the two motion vectors is transmitted to the entropy encoding unit 155. Here, reference picture list 0 may be composed of pictures before the current picture in display order among the restored pictures, and reference picture list 1 may be composed of pictures after the current picture in display order among the restored pictures. there is. However, it is not necessarily limited to this, and in terms of display order, relief pictures after the current picture may be additionally included in reference picture list 0, and conversely, relief pictures before the current picture may be additionally included in reference picture list 1. may be included.

Various methods can be used to minimize the amount of bits required to encode motion information.

For example, if the reference picture and motion vector of the current block are the same as the reference picture and motion vector of the neighboring block, the motion information of the current block can be transmitted to the video decoding device by encoding information that can identify the neighboring block. This method is called ‘merge mode’.

In the merge mode, the inter prediction unit 124 selects a predetermined number of merge candidate blocks (hereinafter referred to as 'merge candidates') from neighboring blocks of the current block.

As shown in FIG. 4, the surrounding blocks for deriving merge candidates include the left block (A0), bottom left block (A1), top block (B0), and top right block (B1) adjacent to the current block in the current picture. ), and all or part of the upper left block (B2) can be used. Additionally, a block located within a reference picture (which may be the same or different from the reference picture used to predict the current block) rather than the current picture where the current block is located may be used as a merge candidate. For example, a block co-located with the current block within the reference picture or blocks adjacent to the co-located block may be additionally used as merge candidates. If the number of merge candidates selected by the method described above is less than the preset number, the 0 vector is added to the merge candidates.

The inter prediction unit 124 uses these neighboring blocks to construct a merge list including a predetermined number of merge candidates. A merge candidate to be used as motion information of the current block is selected from among the merge candidates included in the merge list, and merge index information is generated to identify the selected candidate. The generated merge index information is encoded by the entropy encoding unit 155 and transmitted to the video decoding device.

Merge skip mode is a special case of merge mode. After performing quantization, when all transformation coefficients for entropy encoding are close to zero, only peripheral block selection information is transmitted without transmitting residual signals. By using merge skip mode, relatively high coding efficiency can be achieved in low-motion images, still images, screen content images, etc.

Hereinafter, merge mode and merge skip mode are collectively referred to as merge/skip mode.

Another method for encoding motion information is AMVP (Advanced Motion Vector Prediction) mode.

In AMVP mode, the inter prediction unit 124 uses neighboring blocks of the current block to derive predicted motion vector candidates for the motion vector of the current block. The surrounding blocks used to derive predicted motion vector candidates include the left block (A0), bottom left block (A1), top block (B0), and top right block adjacent to the current block in the current picture shown in FIG. All or part of B1), and the upper left block (B2) can be used. Additionally, a block located within a reference picture (which may be the same or different from the reference picture used to predict the current block) rather than the current picture where the current block is located will be used as a surrounding block used to derive prediction motion vector candidates. It may be possible. For example, a collocated block located at the same location as the current block within the reference picture or blocks adjacent to the block at the same location may be used. If the number of motion vector candidates is less than the preset number by the method described above, the 0 vector is added to the motion vector candidates.

The inter prediction unit 124 derives predicted motion vector candidates using the motion vectors of the neighboring blocks, and determines a predicted motion vector for the motion vector of the current block using the predicted motion vector candidates. Then, the predicted motion vector is subtracted from the motion vector of the current block to calculate the differential motion vector.

The predicted motion vector can be obtained by applying a predefined function (eg, median, average value calculation, etc.) to the predicted motion vector candidates. In this case, the video decoding device also knows the predefined function. In addition, since the neighboring blocks used to derive predicted motion vector candidates are blocks for which encoding and decoding have already been completed, the video decoding device also already knows the motion vectors of the neighboring blocks. Therefore, the video encoding device does not need to encode information to identify the predicted motion vector candidate. Therefore, in this case, information about the differential motion vector and information about the reference picture used to predict the current block are encoded.

Meanwhile, the predicted motion vector may be determined by selecting one of the predicted motion vector candidates. In this case, information for identifying the selected prediction motion vector candidate is additionally encoded, along with information about the differential motion vector and information about the reference picture used to predict the current block.

The subtractor 130 generates a residual block by subtracting the prediction block generated by the intra prediction unit 122 or the inter prediction unit 124 from the current block.

The transform unit 140 converts the residual signal in the residual block having pixel values in the spatial domain into transform coefficients in the frequency domain. The conversion unit 140 may convert the residual signals in the residual block by using the entire size of the residual block as a conversion unit, or divide the residual block into a plurality of subblocks and perform conversion by using the subblocks as a conversion unit. You may. Alternatively, the residual signals can be converted by dividing them into two subblocks, a transform area and a non-transformation region, and using only the transform region subblock as a transform unit. Here, the transformation area subblock may be one of two rectangular blocks with a size ratio of 1:1 based on the horizontal axis (or vertical axis). In this case, a flag indicating that only the subblock has been converted (cu_sbt_flag), directional (vertical/horizontal) information (cu_sbt_horizontal_flag), and/or position information (cu_sbt_pos_flag) are encoded by the entropy encoding unit 155 and signaled to the video decoding device. do. In addition, the size of the transform area subblock may have a size ratio of 1:3 based on the horizontal axis (or vertical axis), and in this case, a flag (cu_sbt_quad_flag) that distinguishes the corresponding division is additionally encoded by the entropy encoding unit 155 to encode the image. Signaled to the decryption device.

Meanwhile, the transformation unit 140 can separately perform transformation on the residual block in the horizontal and vertical directions. For transformation, various types of transformation functions or transformation matrices can be used. For example, a pair of transformation functions for horizontal transformation and vertical transformation can be defined as MTS (Multiple Transform Set). The conversion unit 140 may select a conversion function pair with the best conversion efficiency among MTSs and convert the residual blocks in the horizontal and vertical directions, respectively. Information (mts_idx) about the transformation function pair selected from the MTS is encoded by the entropy encoder 155 and signaled to the video decoding device.

The quantization unit 145 quantizes the transform coefficients output from the transform unit 140 using a quantization parameter, and outputs the quantized transform coefficients to the entropy encoding unit 155. The quantization unit 145 may directly quantize a residual block related to a certain block or frame without conversion. The quantization unit 145 may apply different quantization coefficients (scaling values) depending on the positions of the transform coefficients within the transform block. The quantization matrix applied to the quantized transform coefficients arranged in two dimensions may be encoded and signaled to the video decoding device.

The rearrangement unit 150 may rearrange coefficient values for the quantized residual values.

The rearrangement unit 150 can change a two-dimensional coefficient array into a one-dimensional coefficient sequence using coefficient scanning. For example, the realignment unit 150 can scan from DC coefficients to coefficients in the high frequency region using zig-zag scan or diagonal scan to output a one-dimensional coefficient sequence. . Depending on the size of the transformation unit and the intra prediction mode, a vertical scan that scans a two-dimensional coefficient array in the column direction or a horizontal scan that scans the two-dimensional block-type coefficients in the row direction may be used instead of the zig-zag scan. That is, the scan method to be used among zig-zag scan, diagonal scan, vertical scan, and horizontal scan may be determined depending on the size of the transformation unit and the intra prediction mode.

The entropy encoding unit 155 uses various encoding methods such as CABAC (Context-based Adaptive Binary Arithmetic Code) and Exponential Golomb to encode the one-dimensional quantized transform coefficients output from the reordering unit 150. A bitstream is created by encoding the sequence.

In addition, the entropy encoder 155 encodes information such as CTU size, CU split flag, QT split flag, MTT split type, and MTT split direction related to block splitting, so that the video decoding device can encode blocks in the same way as the video coding device. Allow it to be divided. In addition, the entropy encoding unit 155 encodes information about the prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and generates intra prediction information (i.e., intra prediction) according to the prediction type. Information about the mode) or inter prediction information (coding mode of motion information (merge mode or AMVP mode), merge index in case of merge mode, information on reference picture index and differential motion vector in case of AMVP mode) is encoded. Additionally, the entropy encoding unit 155 encodes information related to quantization, that is, information about quantization parameters and information about the quantization matrix.

The inverse quantization unit 160 inversely quantizes the quantized transform coefficients output from the quantization unit 145 to generate transform coefficients. The inverse transform unit 165 restores the residual block by converting the transform coefficients output from the inverse quantization unit 160 from the frequency domain to the spatial domain.

The adder 170 restores the current block by adding the restored residual block and the prediction block generated by the prediction unit 120. Pixels in the restored current block are used as reference pixels when intra-predicting the next block.

The loop filter unit 180 restores pixels to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. that occur due to block-based prediction and transformation/quantization. Perform filtering on them. The filter unit 180 is an in-loop filter and may include all or part of a deblocking filter 182, a Sample Adaptive Offset (SAO) filter 184, and an Adaptive Loop Filter (ALF) 186. .

The deblocking filter 182 filters the boundaries between restored blocks to remove blocking artifacts caused by block-level encoding/decoding, and the SAO filter 184 and alf(186) perform deblocking filtering. Additional filtering is performed on the image. The SAO filter 184 and alf 186 are filters used to compensate for the difference between the restored pixel and the original pixel caused by lossy coding. The SAO filter 184 improves not only subjective image quality but also coding efficiency by applying an offset in units of CTU. In comparison, the ALF 186 performs filtering on a block basis, distinguishing the edge and degree of change of the block and applying different filters to compensate for distortion. Information about filter coefficients to be used in ALF may be encoded and signaled to a video decoding device.

The restored block filtered through the deblocking filter 182, SAO filter 184, and ALF 186 is stored in the memory 190. When all blocks in one picture are reconstructed, the reconstructed picture can be used as a reference picture for inter prediction of blocks in the picture to be encoded later.

Figure 5 is an example block diagram of a video decoding device that can implement the techniques of the present disclosure. Hereinafter, the video decoding device and its sub-configurations will be described with reference to FIG. 5.

The image decoding device includes an entropy decoding unit 510, a rearrangement unit 515, an inverse quantization unit 520, an inverse transform unit 530, a prediction unit 540, an adder 550, a loop filter unit 560, and a memory ( 570).

Like the video encoding device of FIG. 1, each component of the video decoding device may be implemented as hardware or software, or may be implemented as a combination of hardware and software. Additionally, the function of each component may be implemented as software and a microprocessor may be implemented to execute the function of the software corresponding to each component.

The entropy decoder 510 decodes the bitstream generated by the video encoding device, extracts information related to block division, determines the current block to be decoded, and provides prediction information and residual signals needed to restore the current block. Extract information, etc.

The entropy decoder 510 extracts information about the CTU size from a Sequence Parameter Set (SPS) or Picture Parameter Set (PPS), determines the size of the CTU, and divides the picture into CTUs of the determined size. Then, the CTU is determined as the highest layer of the tree structure, that is, the root node, and the CTU is divided using the tree structure by extracting the division information for the CTU.

For example, when dividing a CTU using the QTBTTT structure, first extract the first flag (QT_split_flag) related to the division of the QT and split each node into four nodes of the lower layer. And, for the node corresponding to the leaf node of QT, the second flag (MTT_split_flag) and split direction (vertical / horizontal) and/or split type (binary / ternary) information related to the split of MTT are extracted and the corresponding leaf node is divided into MTT. Divide by structure. Accordingly, each node below the leaf node of QT is recursively divided into a BT or TT structure.

As another example, when splitting a CTU using the QTBTTT structure, first extract the CU split flag (split_cu_flag) indicating whether to split the CU, and if the corresponding block is split, extract the first flag (QT_split_flag). It may be possible. During the division process, each node may undergo 0 or more repetitive MTT divisions after 0 or more repetitive QT divisions. For example, MTT division may occur immediately in the CTU, or conversely, only multiple QT divisions may occur.

As another example, when dividing a CTU using the QTBT structure, the first flag (QT_split_flag) related to the division of the QT is extracted and each node is divided into four nodes of the lower layer. And, for the node corresponding to the leaf node of QT, a split flag (split_flag) indicating whether to further split into BT and split direction information are extracted.

Meanwhile, when the entropy decoding unit 510 determines the current block to be decoded using division of the tree structure, it extracts information about the prediction type indicating whether the current block is intra-predicted or inter-predicted. When prediction type information indicates intra prediction, the entropy decoder 510 extracts syntax elements for intra prediction information (intra prediction mode) of the current block. When prediction type information indicates inter prediction, the entropy decoder 510 extracts syntax elements for inter prediction information, that is, information indicating a motion vector and a reference picture to which the motion vector refers.

Additionally, the entropy decoding unit 510 extracts information about quantized transform coefficients of the current block as quantization-related information and information about the residual signal.

The reordering unit 515 re-organizes the sequence of one-dimensional quantized transform coefficients entropy decoded in the entropy decoding unit 510 into a two-dimensional coefficient array (i.e., in reverse order of the coefficient scanning order performed by the image encoding device). block).

The inverse quantization unit 520 inversely quantizes the quantized transform coefficients and inversely quantizes the quantized transform coefficients using a quantization parameter. The inverse quantization unit 520 may apply different quantization coefficients (scaling values) to quantized transform coefficients arranged in two dimensions. The inverse quantization unit 520 may perform inverse quantization by applying a matrix of quantization coefficients (scaling values) from an image encoding device to a two-dimensional array of quantized transform coefficients.

The inverse transform unit 530 inversely transforms the inverse quantized transform coefficients from the frequency domain to the spatial domain to restore the residual signals, thereby generating a residual block for the current block.

In addition, when the inverse transformation unit 530 inversely transforms only a partial area (subblock) of the transformation block, a flag (cu_sbt_flag) indicating that only the subblock of the transformation block has been transformed, and directionality (vertical/horizontal) information of the subblock (cu_sbt_horizontal_flag) ) and/or extracting the position information (cu_sbt_pos_flag) of the subblock, and inversely transforming the transformation coefficients of the corresponding subblock from the frequency domain to the spatial domain to restore the residual signals, and for the area that has not been inversely transformed, a “0” value is used as the residual signal. By filling , the final residual block for the current block is created.

In addition, when MTS is applied, the inverse transform unit 530 determines a transformation function or transformation matrix to be applied in the horizontal and vertical directions, respectively, using the MTS information (mts_idx) signaled from the video encoding device, and uses the determined transformation function. Inverse transformation is performed on the transformation coefficients in the transformation block in the horizontal and vertical directions.

The prediction unit 540 may include an intra prediction unit 542 and an inter prediction unit 544. The intra prediction unit 542 is activated when the prediction type of the current block is intra prediction, and the inter prediction unit 544 is activated when the prediction type of the current block is inter prediction.

The intra prediction unit 542 determines the intra prediction mode of the current block among a plurality of intra prediction modes from the syntax elements for the intra prediction mode extracted from the entropy decoder 510, and provides a reference around the current block according to the intra prediction mode. Predict the current block using pixels.

The inter prediction unit 544 uses the syntax elements for the inter prediction mode extracted from the entropy decoder 510 to determine the motion vector of the current block and the reference picture to which the motion vector refers, and uses the motion vector and the reference picture to determine the motion vector of the current block. Use it to predict the current block.

The adder 550 restores the current block by adding the residual block output from the inverse transform unit and the prediction block output from the inter prediction unit or intra prediction unit. Pixels in the restored current block are used as reference pixels when intra-predicting a block to be decoded later.

The loop filter unit 560 may include a deblocking filter 562, a SAO filter 564, and an ALF 566 as an in-loop filter. The deblocking filter 562 performs deblocking filtering on the boundaries between restored blocks to remove blocking artifacts that occur due to block-level decoding. The SAO filter 564 and the ALF 566 perform additional filtering on the reconstructed block after deblocking filtering to compensate for the difference between the reconstructed pixel and the original pixel caused by lossy coding. The filter coefficient of ALF is determined using information about the filter coefficient decoded from the non-stream.

The restoration block filtered through the deblocking filter 562, SAO filter 564, and ALF 566 is stored in the memory 570. When all blocks in one picture are reconstructed, the reconstructed picture is later used as a reference picture for inter prediction of blocks in the picture to be encoded.

This embodiment relates to encoding and decoding of images (videos) as described above. More specifically, in inter prediction of chroma components, weights are calculated on the decoder side using up and down luma prediction blocks or template matching, and the calculated weights are applied to the bidirectional chroma prediction blocks to generate the final prediction block. Provides a video coding method and device.

The following embodiments may be performed by the inter prediction unit 124 in a video encoding device. Additionally, it may be performed by the inter prediction unit 544 in a video decoding device.

The video encoding device can generate signaling information related to this embodiment in terms of bit rate distortion optimization in inter prediction of chroma components. The video encoding device can encode signaling information using the entropy encoding unit 155 and then transmit it to the video decoding device. The image decoding device can decode signaling information related to inter prediction of chroma components from the bitstream using the entropy decoding unit 510.

In the following description, the term 'target block' may be used with the same meaning as a current block or a coding unit (CU), or may mean a partial area of a coding unit.

Additionally, the fact that the value of one flag is true indicates that the flag is set to 1. Additionally, the value of one flag being false indicates a case where the flag is set to 0.

Hereinafter, embodiments according to the present invention will be described focusing on a video decoding device, but can be similarly applied to a video encoding device.

I. Intra prediction techniques

Hereinafter, intra prediction techniques for reference in this embodiment will be described. Several techniques are introduced to improve coding efficiency using intra prediction.

In the VVC technology, the intra prediction mode of the luma block has 65 subdivided directional modes (i.e., 2 to 66) in addition to the non-directional mode (i.e., Planar and DC), as illustrated in FIG. 3A. The 65 directional modes, Planar and DC, are collectively referred to as 67 IPMs.

In VVC technology, CCLM (Cross-component Linear Model prediction) and MMLM (Multi-model Linear Model prediction) are technologies that predict chroma samples by identifying the correlation between luma components and chroma components.

When the CCLM mode is applied for intra prediction of the current chroma block, the image decoding device determines an area (hereinafter, 'corresponding luma area') in the luma image corresponding to the current chroma block. For prediction of a linear model, left reference pixels and top reference pixels of the corresponding luma area, and left reference pixels and top reference pixels of the target chroma block may be used. Hereinafter, the left reference pixels and the top reference pixels are integrated into reference pixels and surrounding pixels. Or expressed as adjacent pixels. Additionally, reference pixels of the chroma channel are indicated as chroma reference pixels, and reference pixels of the luma channel are indicated as luma reference pixels.

In CCLM prediction, a linear model is derived between the reference pixels of the corresponding luma area and the reference pixels of the chroma block, and then the linear model is applied to the restored pixels of the corresponding luma area to create a predictor of the target chroma block. A prediction block is created. For example, four pairs of pixels, which are a combination of pixels in the surrounding pixel lines of the current chroma block and pixels in the corresponding luma area, can be used to derive a linear model. The image decoding device may derive α and β representing a linear model for four pairs of pixels, as shown in Equation 1.

Here, for the corresponding luma pixels among the four pairs of pixels, X _a and X _b each represent the average value of the two minimum values and the average value of the two maximum values. Additionally, for chroma pixels, Y _a and Y _b each represent the average value of two minimum values and the average value of two maximum values. Afterwards, the image decoding device uses a linear model to determine the predictor of the current chroma block Pred _C [x][y] from the pixel value rec' _L [x][y] of the corresponding luma area, as shown in Equation 2. can be created.

Meanwhile, as described above, the CCLM mode is divided into three modes: CCLM_LT, CCLM_L, and CCLM_T, depending on the positions of surrounding pixels used in the derivation process of the linear model. CCLM_LT mode uses two pixels in each direction among the surrounding pixels adjacent to the left and top of the current chroma block. CCLM_L mode uses 4 pixels from surrounding pixels adjacent to the left of the current chroma block. Lastly, CCLM_T mode uses four pixels from among the surrounding pixels adjacent to the top of the current chroma block.

In VVC technology, Weighted Chroma Prediction (WCP) calculates weights for weighting the surrounding chroma samples using a difference vector between a luma sample and surrounding luma samples. Thereafter, a predictor of the chroma sample is generated by weighting the surrounding chroma samples using the calculated weights, as illustrated in FIG. 6 .

First, W is defined as the width of the chroma block, and H is defined as the height of the chroma block. If necessary depending on the chroma format, the restored luma block and surrounding luma samples may be downsampled. The video decoding device uses x=0, … , W-1, y=0, … , for H-1, DeltaL[k], which is the absolute value of the difference between the luma sample rec' _L [x][y] in the restored luma block and the surrounding luma vector Ref _L [k] of length K, is expressed in equation Calculate as in 3.

Here, k is an index indicating an element of the surrounding luma vector.

Afterwards, the video decoding device calculates the weight vector Weight[k] using a normalization process as shown in Equation 4.

Using the surrounding sample vector Ref _C [k] of the chroma block and the weight vector Weight[k], the image decoding device sets x=0,... , W-1, y=0, … , the predictor Pred _C [x][y] of the chroma sample for H-1 can be calculated as in Equation 5.

II. BCW(Bi-prediction with CU-level Weight)

In VVC technology, the Advanced Motion Vector Prediction (AMVP) mode of inter prediction assigns weights at the CU (Coding Unit) level to the two prediction signals derived by two motion vectors during bidirectional prediction. This method is called bi-prediction with CU-level weight (BCW). In the case of BCW, the video decoding device generates the final prediction signal by averaging the two prediction signals based on the weight as shown in Equation 6.

Here, P ₀ and P ₁ represent two bidirectional prediction signals.

In the case of low delay encoding, the weight w may be one of five weights {-2, 3, 4, 5, 10}. On the other hand, if low-latency encoding is not used, the weight w may be one of three weights {3, 4, 5}. The weight of this CU level is determined by the weight index transmitted for each CU in AMVP mode. Additionally, in the case of merge mode, the CU level weight may be determined based on the merge candidate index.

As described above, in existing two-way inter prediction, fixed weights are used for the luma component and chroma component. Hereinafter, when bidirectionally predicting a chroma component, a method of calculating the weight of the chroma component at the decoder using the restored luma component in addition to the chroma component will be described.

III. Realization examples according to the present disclosure

Similar to the luma signal, weighting may be assigned at the CU level in bidirectional prediction of the chroma signal. As shown in Equation 7, BCW can be implemented in which the final prediction signals are generated by weighting the two prediction signals derived by two motion vectors.

Hereinafter, w0 and w1 are referred to as the first chroma weight and the second chroma weight.

In this embodiment, the image decoding device adaptively calculates the weight of the chroma component instead of using the fixed weight as described above for the chroma component. At this time, similar to the existing BCW using fixed weights, the adaptive weight derivation method will be controlled based on the CU level on/off flag (hereinafter referred to as 'chroma weight derivation flag'). You can. For example, when the chroma weight derivation flag is true, the image decoding device derives the weights of chroma prediction blocks according to this implementation example. On the other hand, if the chroma weight derivation flag is false, the image decoding device derives the weights of the chroma prediction blocks according to the existing method. Here, the existing method can use fixed weights as described above.

<Implementation Example 1> Chroma BCW using weighted chroma prediction

In this implementation, the video decoding device performs bi-prediction with CU-level weight (BCW) on chroma components using weighted chroma prediction (WCP). Here, the WCP calculates the weight using the difference vector between the luma sample and surrounding luma samples. Afterwards, the calculated weight is used for weighted prediction of the corresponding chroma sample.

In this implementation, WCP can be applied as in Realization Example 1-1 or Realization Example 1-2 to perform BCW of the chroma component.

<Realization Example 1-1> Using the difference between bidirectional luma prediction blocks and the final luma restoration block

When BCW is performed on the luma component, bidirectional prediction blocks pred0 _L and pred1 _L and the final luma restoration block pred _L ^rec are generated. Here, pred0 _L and pred1 _L represent the forward and backward prediction blocks of the luma block, respectively. Additionally, pred _L ^rec corresponds to the output of the adder 550. The image decoding device can determine the weights of the two prediction signals in bidirectional prediction of the chroma component using pred0 _L , pred1 _L, and pred _L ^rec .

Figure 7 is an example diagram showing the use of a luma block when predicting a chroma block, according to an embodiment of the present disclosure.

The video decoding device calculates Delta _p0 and Delta _p1 , which are absolute values of the difference between the bidirectional luma prediction blocks and the final luma restoration block, as shown in Equation 8.

As an example, if the size of the chroma block is W×H and the size of the corresponding luma block is larger than the chroma block, the video decoding device may downsample Delta _p0 and Delta _p1 to the size of W×H. Alternatively, the luma block may first be downsampled to a size of W×H, and then the process of Equation 8 may be applied. Here, W represents the width of the chroma block, and H represents the height of the chroma block. Hereinafter, Delta _p0 and Delta _p1 are referred to as the first differential block and the second differential block, respectively.

The final chroma prediction block Pred _C can be generated using pred0 _C and pred1 _C , which are bidirectional prediction blocks of chroma components. Here, pred0 _C and pred1 _C represent the forward prediction block and backward prediction block of the chroma block, respectively. At this time, the weights w0 and w1 of Equation 7 can be calculated for the chroma component using Delta _p0 and Delta _p1 . In this case, w0 and w1 may be determined differently for each chroma sample Pred _C [x][y].

First, the video decoding device calculates sum[x][y] as shown in Equation 9.

Afterwards, Delta _p0 [x][y] and Delta p1 [x][y] are divided by sum[x][y] so that the sum of w0[x][y] and _w1 [x][y] is 1. As a result, normalized w0[x][y] and w1[x][y] are generated. Hereinafter, w0[x][y] and w1[x][y] are referred to as the first weight block and the second weight block. Each video decoding device has x=0,... , W-1, y=0, … , bidirectional prediction is performed on the chroma component as shown in Equation 10 using the normalized weight for H-1.

Here, Pred _C [x][y] represents the final predicted sample of the chroma component.

Meanwhile, in Equation 10, weights for each chroma sample based on luma blocks are used. As another example, final prediction samples of chroma components may be generated using one weight for each prediction block. For example, the video decoding device calculates the average μ _p0 of Delta _p0 and the average μ _p1 of Delta _p1 , then calculates w0=μ _p0 /(μ _p0 +μ _p1 ), w1=μ _p1 /(μ _p0 +μ _p1 ). Calculate. Hereinafter, μ _p0 and μ _p1 are referred to as the first average and the second average, respectively. The video decoding device is, x=0,... , W-1, y=0, … , bidirectional prediction of the chroma component is performed as shown in Equation 11 using normalized weights for H-1.

<Realization Example 1-2> Using the difference between luma blocks and surrounding luma samples

As described above, when BCW is performed on the luma component, bidirectional prediction blocks pred0 _L and pred1 _L , and pred _L ^rec , the final luma restoration block, are generated. At this time, pixel vectors around each block are defined as Ref ₀ , Ref ₁ , and Ref _L. The image decoding device calculates Delta _p0 , Delta _p1 , and Delta _L , which are difference vectors between each luma block and surrounding pixels, as shown in Equation 12. Hereinafter, Delta _p0 , Delta _p1 , and Delta _L are respectively referred to as the first difference vector, second difference vector, and luma difference vector.

Afterwards, the video decoding device calculates the weight of the chroma component using Delta _p0 , Delta _p1 , and Delta _L. First, define Diff ₀ = Delta _L - Delta _p0 and Diff ₁ = Delta _L - Delta _p1 . Hereinafter, Diff ₀ and Diff ₁ are referred to as the first bi-difference vector and the second bi-difference vector. When the number of pixels around the prediction block is K, Diff ₀ and Diff ₁ are each vectors of size 1×K. The example of FIG. 8 schematically shows the process of calculating Diff ₀ and Diff ₁ from differences between luma blocks and surrounding luma samples.

For example, the image decoding device can use Diff ₀ and Diff ₁ to calculate the weight of the chroma component as follows. The video decoding device concatenates Diff ₀ and Diff ₁ of size 1×K to generate Diff _p of size 1×2K as shown in Equation 13. Hereinafter, Diff _p is called the combined difference-difference vector.

Afterwards, the image decoding device normalizes each element of Diff _p to calculate the weight vector Weight[k] as shown in Equation 14.

Here, concat represents an operation that combines two vectors.

For the normalized weight vector Weight, Weight0 will be defined as k=0 to K-1 elements in Weight[k], and Weight1 will be defined as k=K to 2K-1 elements in Weight[k]. You can. Afterwards, the image decoding device can calculate the weights w0 and w1 of Equation 7 for the chroma component by calculating the sum of the vector elements of Weight0 and Weight1.

The image decoding device performs bidirectional prediction of the chroma component using the calculated weights. x=0, … , W-1, y=0, … , the final predicted sample Pred _C [x][y] of the chroma component can be calculated for H-1 as shown in Equation 15.

Here, pred0 _C and pred1 _C represent the forward prediction block and backward prediction block of the chroma block, respectively.

<Realization Example 2> Chroma BCW using template matching

In this implementation, the video decoding device calculates the weights w0 and w1 of Equation 7 using template matching. That is, as shown in the example of FIG. 9, the video decoding device can calculate the template matching cost using blocks surrounding the prediction block in each prediction direction and then normalize the calculated template matching cost to calculate the weight.

For example, in order to predict an N×N chroma block, an image decoding device may define (N+2)×2 samples at the top of the block and 2×N samples on the left side of the block as templates. If the template matching cost is defined as SSD (Sum of Squared Differences), the video decoding device can calculate the template matching cost as shown in Equation 16.

Here, T _curr and T _ref represent the template areas of the current block and reference block. In addition to SSD, SAD (Sum of Absolute Differences) based on |T _curr (i,j) - T _ref (i,j)| can be defined as the template matching cost.

(N+2) For example, 2N samples at the top of the block and 2N samples on the left may be defined as a template.

Hereinafter, the template matching costs of pred0 and pred1 prediction blocks are defined as TC _pred0 and TC _pred1 , respectively. Hereinafter, TC _pred0 and TC _pred1 are referred to as the first template matching cost and the second template matching cost, respectively. After normalizing the two matching costs, the video decoding device can calculate the weights w0 and w1 as shown in Equation 17 to set the weight of the block with a small matching cost to be larger.

Afterwards, the video decoding device can use the normalized w0 as the weight of pred0 and the normalized w1 as the weight of pred1.

In the above-described example, neighboring samples of the chroma block are used when calculating the template matching cost, but the method is not necessarily limited to this. For example, instead of using neighboring samples of a chroma block, neighboring samples of a luma block may be used. In this case, instead of using the cost function as shown in Equation 10, the template matching cost can be calculated using a cost function including a first-order linear function. Here, a linear linear function can be used to convert luma sample values into chroma sample values, as in the CCLM example described above.

Hereinafter, using the illustrations of FIGS. 10 and 11, a method of deriving weights of the current chroma block and generating a prediction block of the current chroma block using the derived weights will be described in bidirectional inter prediction.

FIG. 10 is a flowchart illustrating a method for predicting a current chroma block performed by an image encoding device according to an embodiment of the present disclosure.

The image encoding device generates bidirectional luma prediction blocks and a final luma reconstruction block for the luma block (S1000).

Here, the bidirectional luma prediction blocks include a first luma prediction block and a second luma prediction block. For example, the first luma prediction block may be a forward luma prediction block, and the second luma prediction block may be a backward luma prediction block.

The image encoding device generates bidirectional chroma prediction blocks for the current chroma block corresponding to the luma block (S1002).

Here, the bidirectional chroma prediction blocks include a first chroma prediction block and a second chroma prediction block. For example, the first chroma prediction block may be a forward chroma prediction block, and the second chroma prediction block may be a backward chroma prediction block.

The video encoding device derives the weights of the current chroma block using the final luma restoration block and the bidirectional luma prediction blocks (S1004). Here, the weights of the current chroma block include the first chroma weight w0 and the second chroma weight w1.

As an example, to derive weights, an image encoding device may use the difference between bidirectional luma prediction blocks and the final luma reconstruction block.

The image encoding device generates a first differential block Delta _p0 by calculating the absolute value of the difference between the final luma restoration block pred _L ^rec and the first luma prediction block pred0 _L. Additionally, the image encoding device calculates the absolute value of the difference between the final luma restoration block and the second luma prediction block pred1 _L and generates the second difference block Delta _p1 .

At this time, if the luma block is larger than the current chroma block, the image encoding device may downsample the bidirectional luma prediction blocks and the final luma restoration block and then generate a first differential block and a second differential block. Alternatively, the image encoding device may first generate the first differential block and the second differential block, and then downsample the first differential block and the second differential block.

The image encoding device normalizes the first differential block and the second differential block based on the sum of the first differential block and the second differential block, so that the first weight block w0[x][y] and the second weight block w1[x ][y] is created.

As described above, weights for each pixel may be used, but one weight may be used for each prediction block. The image encoding device calculates a first average, which is the average of the elements of the first differential block, and a second average, which is the average of the elements of the second differential block. Thereafter, the image encoding device may normalize the first average and the second average to generate a first chroma weight w0 and a second chroma weight w1.

As another example, an image encoding device may use the difference between luma blocks and surrounding luma samples.

The image encoding device generates a first difference vector Delta _p0 by calculating the absolute value of the difference between the pixels in the first luma prediction block and the surrounding pixels Ref ₀ of the first luma prediction block. The image encoding device generates a luma difference vector Delta _L by calculating the absolute value of the difference between the pixels in the final luma restoration block and the surrounding pixels Ref _L of the final luma restoration block. Additionally, the image encoding device generates a second difference vector Delta _p1 by calculating the absolute value of the difference between a pixel in the second luma prediction block and surrounding pixels Ref ₁ of the second luma prediction block.

The video encoding device generates a first bi-difference vector Diff ₀ by calculating the absolute value of the difference between the first difference vector and the luma difference vector. Additionally, the video encoding device generates a second difference vector Diff ₁ by calculating the absolute value of the difference between the second difference vector and the luma difference vector.

The image encoding device combines the first difference vector and the second difference vector to generate a combined difference vector Diff _p . The image encoding device may normalize each element of the combined difference-in-difference vector and generate a first chroma weight w0 and a second chroma weight w1 using the normalized elements.

As another example, to derive weights, the image encoding apparatus calculates the first template matching cost TC _pred0 and the second template matching cost TC _pred0 using blocks surrounding the prediction block in each prediction direction. After normalizing the calculated template matching costs, the video encoding device may calculate the first chroma weight w0 and the second chroma weight w1 according to Equation 17 to set the weight of the block with a small matching cost to be larger.

The image encoding device applies weights to the bidirectional chroma prediction block to generate the first final chroma prediction block of the current chroma block (S1006).

When using pixel-specific weights, as shown in Equation 10, the image encoding device applies the first weight block and the second weight block to the first chroma prediction block pred0 _C and the second chroma prediction block pred0 _C for each pixel to produce the first final You can create a chroma prediction block.

When using weights for each prediction block, as shown in Equation 11, the image encoding device applies the first chroma weight and the second chroma weight to the first chroma prediction block and the second chroma prediction block to generate the first final chroma prediction block. can do.

The image encoding device generates a second final chroma prediction block of the current chroma block by applying fixed weights to the bidirectional chroma prediction blocks (S1008). For example, the video encoding device may generate the second final chroma prediction block of the current chroma block using pre-fixed weights according to BCW.

The image encoding device selects the optimal chroma prediction block among the first final chroma prediction block and the second final chroma prediction block (S1010). For example, an image encoding device can select an optimal chroma prediction block in terms of optimizing distortion with respect to the original chroma block.

As described above, the flag indicating whether to derive chroma weights is called the chroma weight derivation flag.

The image encoding device determines a chroma weight derivation flag depending on whether the optimal chroma prediction block is the first final chroma prediction block or the second final chroma prediction block (S1012).

The video encoding device encodes the chroma weight derivation flag (S1014).

FIG. 11 is a flowchart illustrating a method for predicting a current chroma block performed by an image decoding apparatus according to an embodiment of the present disclosure.

The image decoding apparatus generates bidirectional luma prediction blocks of the luma block and a final luma reconstruction block (S1100).

Here, the bidirectional luma prediction blocks include a first luma prediction block and a second luma prediction block. For example, the first luma prediction block may be a forward luma prediction block, and the second luma prediction block may be a backward luma prediction block.

The image decoding device generates bidirectional chroma prediction blocks for the current chroma block corresponding to the luma block (S1102).

Here, the bidirectional chroma prediction blocks include a first chroma prediction block and a second chroma prediction block. For example, the first chroma prediction block may be a forward chroma prediction block, and the second chroma prediction block may be a backward chroma prediction block.

The video decoding device decodes the chroma weight derivation flag (S1104).

The video decoding device checks the chroma weight derivation flag (S1106).

If the chroma weight derivation flag is true, the video decoding device performs the following steps (S1108 and S1110).

The image decoding device uses the final luma restoration block and the bidirectional luma prediction blocks to derive the weights of the current chroma block (S1108). Here, the weights of the current chroma block include a first chroma weight and a second chroma weight.

Since the method by which the video decoding device derives the weights can use the same method as that of the video encoding device, further description is omitted.

The image decoding device applies weights to the bidirectional chroma prediction block to generate the final chroma prediction block of the current chroma block (S1110).

On the other hand, if the chroma weight induction flag is false, the image decoding device generates a final chroma prediction block by applying pre-fixed weights to the bidirectional chroma prediction blocks (S1120). For example, an image decoding device may generate the final chroma prediction block of the current chroma block using pre-fixed weights according to BCW.

In the flowchart/timing diagram of this specification, each process is described as being executed sequentially, but this is merely an illustrative explanation of the technical idea of an embodiment of the present disclosure. In other words, a person skilled in the art to which an embodiment of the present disclosure pertains may change the order described in the flowchart/timing diagram and execute one of the processes without departing from the essential characteristics of the embodiment of the present disclosure. Since the above processes can be applied in various modifications and variations by executing them in parallel, the flowchart/timing diagram is not limited to a time series order.

It should be understood from the above description that the example embodiments may be implemented in many different ways. The functions or methods described in one or more examples may be implemented in hardware, software, firmware, or any combination thereof. It should be understood that the functional components described herein are labeled as "...units" to particularly emphasize their implementation independence.

Meanwhile, various functions or methods described in this embodiment may be implemented with instructions stored in a non-transitory recording medium that can be read and executed by one or more processors. Non-transitory recording media include, for example, all types of recording devices that store data in a form readable by a computer system. For example, non-transitory recording media include storage media such as erasable programmable read only memory (EPROM), flash drives, optical drives, magnetic hard drives, and solid state drives (SSD).

The above description is merely an illustrative explanation of the technical idea of the present embodiment, and those skilled in the art will be able to make various modifications and variations without departing from the essential characteristics of the present embodiment. Accordingly, the present embodiments are not intended to limit the technical idea of the present embodiment, but rather to explain it, and the scope of the technical idea of the present embodiment is not limited by these examples. The scope of protection of this embodiment should be interpreted in accordance with the claims below, and all technical ideas within the equivalent scope should be interpreted as being included in the scope of rights of this embodiment.

124: Inter prediction unit
155: Entropy encoding unit
510: Entropy decoding unit
544: Inter prediction unit

Claims (16)

In the method of predicting the current chroma block performed by the video decoding device,
generating bidirectional chroma prediction blocks for the current chroma block, wherein the bidirectional chroma prediction blocks include a first chroma prediction block and a second chroma prediction block;
Deriving weights of the current chroma block using a first luma prediction block, a second luma prediction block, and a final luma restoration block of the luma block corresponding to the current chroma block, wherein the weights are the first chroma weight. and a second chroma weight; and
Applying the weights to the bidirectional chroma prediction blocks to generate a final chroma prediction block of the current chroma block.
A method comprising: According to paragraph 1,
Decoding a flag indicating whether to derive weights of the current chroma block; and
Steps to check the flag
It further includes,
If the value of the flag is true, deriving the weights is performed. According to paragraph 1,
The method further comprising generating the first luma prediction block, the second luma prediction block, and the final luma restoration block for the luma block. According to paragraph 1,
The step of deriving the weights is,
generating a first differential block by calculating an absolute value of a difference between the final luma restoration block and the first luma prediction block; and
Generating a second difference block by calculating an absolute value of the difference between the final luma restoration block and the second luma prediction block.
A method comprising: According to paragraph 4,
The step of deriving the weights is,
A method characterized in that normalizing the first differential block and the second differential block based on the sum of the first differential block and the second differential block to generate a first weight block and a second weight block. According to clause 5,
The step of generating the final chroma prediction block is,
A method, characterized in that applying the first weight block and the second weight block to the first chroma prediction block and the second chroma prediction block for each pixel. According to paragraph 4,
If the luma block is larger than the current chroma block, downsampling the first luma prediction block, the second luma prediction block, and the final luma restoration block, then generating the first differential block, and the second A method, characterized in that performing the step of generating a differential block. According to clause 4,
The step of deriving the weights is,
A first average, which is the average of the elements of the first differential block, and a second average, which is the average of the elements of the second differential block, are calculated, the first average and the second average are normalized, and the first chroma weight and Method, characterized in that generating the second chroma weight. According to paragraph 1,
The step of deriving the weights is,
generating a first difference vector by calculating an absolute value of a difference between a pixel in the first luma prediction block and neighboring pixels of the first luma prediction block;
generating a luma difference vector by calculating an absolute value of a difference between a pixel in the final luma restoration block and surrounding pixels of the final luma restoration block; and
Generating a second difference vector by calculating the absolute value of the difference between pixels in the second luma prediction block and surrounding pixels of the second luma prediction block.
A method comprising: According to clause 9,
The step of deriving the weights is,
generating a first bi-difference vector by calculating an absolute value of a difference between the first difference vector and the luma difference vector; and
Generating a second difference vector by calculating an absolute value of the difference between the second difference vector and the luma difference vector.
A method further comprising: According to clause 10,
The step of deriving the weights is,
combining the first difference vector and the second difference vector to generate a combined difference vector; and
Normalizing each element of the combined difference-in-difference vector and generating the first chroma weight and the second chroma weight using the normalized elements.
A method further comprising: In the method of predicting the current chroma block performed by an image encoding device,
generating bidirectional chroma prediction blocks for the current chroma block, wherein the bidirectional chroma prediction blocks include a first chroma prediction block and a second chroma prediction block;
Deriving weights of the current chroma block using a first luma prediction block, a second luma prediction block, and a final luma restoration block of the luma block corresponding to the current chroma block, wherein the weights are the first chroma weight. and a second chroma weight; and
Applying the weights to the bidirectional chroma prediction blocks to generate a first final chroma prediction block of the current chroma block.
A method comprising: According to clause 12,
generating a second final chroma prediction block of the current chroma block by applying fixed weights to the bidirectional chroma prediction blocks; and
Selecting an optimal chroma prediction block among the first final chroma prediction block and the second final chroma prediction block.
A method further comprising: According to clause 13,
determining a flag indicating whether to derive the weights depending on whether the optimal chroma prediction block is the first final chroma prediction block or the second final chroma prediction block; and
Encoding the flag
A method further comprising: According to clause 12,
The method further comprising generating the first luma prediction block, the second luma prediction block, and the final luma restoration block for the luma block. A computer-readable recording medium storing a bitstream generated by an image encoding method, the image encoding method comprising:
generating bidirectional chroma prediction blocks for a current chroma block, wherein the bidirectional chroma prediction blocks include a first chroma prediction block and a second chroma prediction block;
Deriving weights of the current chroma block using a first luma prediction block, a second luma prediction block, and a final luma restoration block of the luma block corresponding to the current chroma block, wherein the weights are the first chroma weight. and a second chroma weight; and
Applying the weights to the bidirectional chroma prediction blocks to generate a final chroma prediction block of the current chroma block.
A recording medium comprising:

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