CN114009031A

CN114009031A - Method for restoring chrominance block and apparatus for decoding image

Info

Publication number: CN114009031A
Application number: CN202080042779.7A
Authority: CN
Inventors: 沈东圭; 朴时奈; 朴俊泽; 林和平
Original assignee: Hyundai Motor Co; Industry Academic Collaboration Foundation of Kwangwoon University; Kia Corp
Current assignee: Hyundai Motor Co; Industry Academic Collaboration Foundation of Kwangwoon University; Kia Corp
Priority date: 2019-05-15
Filing date: 2020-05-15
Publication date: 2022-02-01
Also published as: KR20200132762A

Abstract

A method for restoring a chrominance block and an apparatus for decoding a picture are disclosed. According to one embodiment of the present invention, a method for restoring a chroma block of a target block to be restored includes the steps of: decoding, from a bitstream, information regarding a correlation between first residual samples, which are residual samples of a first chrominance component, and second residual samples, which are residual samples of a second chrominance component, prediction information of the first residual samples and a chrominance block; generating predicted samples of the first chrominance component and predicted samples of the second chrominance component based on the prediction information; deriving second residual samples by applying information on the correlation to the first residual samples; and restoring a chroma block of the first chroma component by adding the predicted sample of the first chroma component and the first residual sample, and restoring a chroma block of the second chroma component by adding the predicted sample of the second chroma component and the second residual sample.

Description

Method for restoring chrominance block and apparatus for decoding image

Technical Field

The present invention relates to encoding and decoding of video, and more particularly, to a method and video decoding apparatus for reconstructing a chrominance block, which improve encoding and decoding efficiency by efficiently predicting residual samples of chrominance components.

Background

Since video data has a large data amount compared to audio data or still image data, a large amount of hardware resources (including memory) are required to store or transmit the data in its original form before compression processing.

Accordingly, storing or transmitting video data is typically accompanied by compressing it by using an encoder before a decoder can receive, decompress, and reproduce the compressed video data. Existing Video compression techniques include H.264/AVC and High Efficiency Video Coding (HEVC), which increases the Coding Efficiency of H.264/AVC by approximately 40%.

However, the increasing size, resolution and frame rate of video pictures and the resulting increase in the amount of data to be encoded requires a new and superior compression technique with better coding efficiency improvement and higher picture quality improvement than existing compression techniques.

Disclosure of Invention

Technical problem

In view of this need, the present invention seeks to provide an improved video encoding and decoding technique. In particular, an aspect of the present invention relates to a technique for improving encoding and decoding efficiency by deriving one of a Cb chrominance component and a Cr chrominance component from the other.

Technical scheme

According to an aspect of the present invention, there is provided a method for reconstructing a chrominance block of a target block to be reconstructed. The method comprises the following steps: decoding, from a bitstream, correlation information between first and second residual samples, the first residual samples, and prediction information of a chroma block, wherein the first residual samples are residual samples of a first chroma component, and the second residual samples are residual samples of a second chroma component. The method further comprises: predicted samples of the first chrominance component and predicted samples of the second chrominance information are generated based on the prediction information, and second residual samples are derived by applying correlation information to the first residual samples. The method further comprises: the chrominance block of the first chrominance component is reconstructed by adding first residual samples of the first chrominance component and the predicted samples, and the chrominance block of the second chrominance component is reconstructed by adding second residual samples of the second chrominance component and the predicted samples.

According to another aspect of the present invention, there is provided a video decoding apparatus for reconstructing a chrominance block of a target block to be reconstructed. The video decoding apparatus includes a decoding unit configured to decode, from a bitstream, correlation information between first and second residual samples, the first residual samples being residual samples of a first chroma component, and prediction information of a chroma block, and the second residual samples being residual samples of a second chroma component. The video decoding apparatus further includes a prediction unit configured to generate a predicted sample of the first chroma component and a predicted sample of the second chroma information based on the prediction information, and a chroma component reconstruction unit configured to derive the second residual sample by applying the correlation information to the first residual sample. The video decoding apparatus further includes an adder configured to reconstruct a chroma block of the first chroma component by adding first residual samples of the first chroma component and the predicted samples, and to reconstruct a chroma block of the second chroma component by adding second residual samples of the second chroma component and the predicted samples.

Advantageous effects

As described above, according to some embodiments of the present invention, since any one of a Cb chrominance component and a Cr chrominance component is derived without signaling, compression performance of encoding and decoding is improved.

Drawings

Fig. 1 is a block diagram illustrating a video encoding device capable of implementing the techniques of this disclosure.

Fig. 2 is a schematic diagram for explaining a method of dividing blocks by using a QTBTTT structure.

Fig. 3a is a diagram illustrating a plurality of intra prediction modes.

Fig. 3b is a diagram illustrating a plurality of intra prediction modes including a wide-angle intra prediction mode.

Fig. 4 is a block diagram illustrating a video decoding apparatus that can implement the techniques of this disclosure.

Fig. 5 is an exemplary block diagram of a video encoding apparatus that may implement an example of a residual block reconstruction method for a chrominance component.

Fig. 6 is a flowchart illustrating an example of a residual block reconstruction method for a chrominance component implemented in the video encoding apparatus of fig. 5.

Fig. 7 is a block diagram illustrating an example of a video decoding apparatus that can implement a residual block reconstruction method for a chrominance component.

Fig. 8 is a flowchart illustrating an example of a residual block reconstruction method for a chrominance component implemented in the video decoding apparatus of fig. 7.

Fig. 9 is an exemplary block diagram of a video encoding apparatus capable of implementing another example of a residual block reconstruction method for a chrominance component.

Fig. 10 is a flowchart illustrating an example of a residual block reconstruction method for a chrominance component implemented in the video encoding apparatus of fig. 9.

Fig. 11 is a block diagram illustrating a video decoding apparatus that can implement another example of a residual block reconstruction method for a chrominance component.

Fig. 12 is a flowchart illustrating an example of a residual block reconstruction method implemented in the video decoding apparatus of fig. 11.

Fig. 13 and 14 are flowcharts illustrating other examples of the residual block reconstruction method for the chrominance component.

Fig. 15 is a flowchart illustrating an example of performance conditions of a residual reconstruction method for a chrominance component.

Detailed Description

Hereinafter, some embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same reference numerals preferably denote the same elements, although the elements are shown in different drawings. Furthermore, in the following description of some embodiments, specific descriptions of related known components and functions may be omitted for the sake of clarity and conciseness when it is considered that the subject matter of the present invention is obscured.

Fig. 1 is a block diagram illustrating a video encoding device capable of implementing the techniques of this disclosure. Hereinafter, a video encoding apparatus and elements of the apparatus will be described with reference to fig. 1.

The video encoding device includes: the image divider 110, the predictor 120, the subtractor 130, the transformer 140, the quantizer 145, the rearrangement unit 150, the entropy encoder 155, the inverse quantizer 160, the inverse transformer 165, the adder 170, the filtering unit 180, and the memory 190.

Each element of the video encoding apparatus may be implemented in hardware or software or a combination of hardware and software. The functions of the respective elements may be implemented as software, and the microprocessor may be implemented to perform the software functions corresponding to the respective elements.

One video includes a plurality of images. Each image is divided into a plurality of regions, and encoding is performed on each region. For example, an image is segmented into one or more tiles (tiles) or/and slices (slices). Here, one or more tiles may be defined as a tile group. Each tile or slice is partitioned into one or more Coding Tree Units (CTUs). Each CTU is divided into one or more Coding Units (CUs) by a tree structure. Information applied to each CU is encoded as syntax of the CU, and information commonly applied to information included in one CTU is encoded as syntax of the CTU. In addition, information commonly applied to all blocks in one slice is encoded as syntax of a slice header, and information applied to all blocks constituting one Picture is encoded in a Picture Parameter Set (PPS) or a Picture header. Further, information commonly referred to by a plurality of pictures is encoded in a Sequence Parameter Set (SPS). In addition, information commonly referenced by one or more SPS's is encoded in a Video Parameter Set (VPS). Information commonly applied to one tile or tile group may be encoded as syntax of a tile header or tile group header.

The picture partitioner 110 determines the size of the Coding Tree Unit (CTU). Information on the size of the CTU (CTU size) is encoded into the syntax of the SPS or PPS and transmitted to the video decoding apparatus.

The image divider 110 divides each image constituting the video into a plurality of CTUs having a predetermined size, and then recursively divides the CTUs using a tree structure. In the tree structure, leaf nodes serve as Coding Units (CUs), which are basic units of coding.

The tree structure may be a QuadTree (QT), a Binary Tree (BT), i.e., a node (or parent node) divided into four slave nodes (or child nodes) of the same size, a Ternary Tree (TT), i.e., a node divided into two slave nodes, or a structure formed of two or more QT structures, BT structures, and TT structures, and the Ternary Tree (TT), i.e., a node divided into three slave nodes at a ratio of 1:2: 1. For example, a QuadTree plus binary tree (QTBT) structure may be used, or a QuadTree plus binary tree TernaryTree (QTBTTT) structure may be used. Here, BTTTs may be collectively referred to as a multiple-type tree (MTT).

Fig. 2 exemplarily shows a QTBTTT split tree structure. As shown in fig. 2, the CTU may be first partitioned into QT structures. The QT split may be repeated until the size of the split block reaches the minimum block size MinQTSize of the leaf nodes allowed in QT. A first flag (QT _ split _ flag) indicating whether each node of the QT structure is divided into four nodes of a lower layer is encoded by the entropy encoder 155 and signaled to the video decoding apparatus. When the leaf node of QT is not larger than the maximum block size of the root node allowed in BT (MaxBTSize), it may be further partitioned into one or more BT structures or TT structures. The BT structure and/or the TT structure may have a plurality of splitting directions. For example, there may be two directions, i.e., a direction of dividing a block of a node horizontally and a direction of dividing a block vertically. As shown in fig. 2, when MTT segmentation starts, a second flag (MTT _ split _ flag) indicating whether a node is segmented, a flag indicating a segmentation direction (vertical or horizontal) in the case of segmentation, and/or a flag indicating a segmentation type (binary or trifurcate) are encoded by the entropy encoder 155 and signaled to the video decoding apparatus.

Alternatively, a CU partition flag (split _ CU _ flag) indicating whether a node is divided may be encoded before encoding a first flag (QT _ split _ flag) indicating whether each node is divided into 4 nodes of a lower layer. When the value of the CU partition flag (split _ CU _ flag) indicates that no partition is performed, the block of the node becomes a leaf node in the partition tree structure and is used as a Coding Unit (CU), which is a basic unit of coding. When the value of the CU partition flag (split _ CU _ flag) indicates that the partition is performed, the video encoding apparatus starts encoding from the first flag in the above-described manner.

When using QTBT as another example of the tree structure, there may be two types of partitioning, i.e., a type of partitioning a block horizontally into two blocks of the same size (i.e., symmetric horizontal partitioning) and a type of partitioning a block vertically into two blocks of the same size (i.e., symmetric vertical partitioning). A partition flag (split _ flag) indicating whether each node of the BT structure is partitioned into blocks of a lower layer and partition type information indicating that the partition type is encoded are encoded by the entropy encoder 155 and transmitted to the video decoding apparatus. There may be additional types of partitioning a block of a node into two asymmetric blocks. The asymmetric division type may include a type in which a block is divided into two rectangular blocks at a size ratio of 1:3, or a type in which a block of a node is divided diagonally.

CUs may have various sizes according to QTBT or QTBTTT partitioning of CTUs. Hereinafter, a block corresponding to a CU to be encoded or decoded (i.e., a leaf node of the QTBTTT) is referred to as a "current block". When QTBTTT partitioning is employed, the shape of the current block may be square or rectangular.

The predictor 120 predicts the current block to generate a prediction block. The predictor 120 includes an intra predictor 122 and an inter predictor 124.

In general, each current block in an image may be predictively encoded. In general, prediction of a current block is performed using an intra prediction technique, which uses data from an image including the current block, or an inter prediction technique, which uses data of an image encoded before the image including the current block. Inter prediction includes unidirectional prediction and bidirectional prediction.

The intra prediction unit 122 predicts pixels in the current block using pixels (reference pixels) located around the current block in the current picture including the current block. Depending on the prediction direction, there are multiple intra prediction modes. For example, as shown in fig. 3a, the plurality of intra prediction modes may include 2 non-directional modes and 65 directional modes, and the 2 non-directional modes include a plane (planar) mode and a DC mode. The adjacent pixels and equations to be used are defined differently for each prediction mode. The following table lists the intra prediction mode numbers and their names.

For efficient directional prediction of the rectangular-shaped current block, directional modes (intra prediction modes 67 to 80 and-1 to-14) indicated by dotted arrows in fig. 3b may be additionally used. These modes may be referred to as "wide-angle intra prediction modes". In fig. 3b, the arrows indicate the corresponding reference samples used for prediction, rather than the prediction direction. The prediction direction is opposite to the direction indicated by the arrow. The wide-angle intra prediction mode is a mode in which prediction is performed in a direction opposite to a specific direction mode without additional bit transmission when the current block is rectangular in shape. In this case, in the wide-angle intra prediction mode, some wide-angle intra prediction modes available for the current block may be determined based on a ratio of a width to a height of the rectangular current block. For example, when the current block has a rectangular shape whose height is smaller than its width, wide-angle intra prediction modes (intra prediction modes 67 to 80) having angles smaller than 45 degrees may be used. When the current block has a rectangular shape having a width greater than its height, wide-angle intra prediction modes (intra prediction modes-1 to-14) having angles greater than-135 degrees may be used.

The intra predictor 122 may determine an intra prediction mode to be used when encoding the current block. In some examples, the intra predictor 122 may encode the current block using several intra prediction modes and select an appropriate intra prediction mode to use from the tested modes. For example, the intra predictor 122 may calculate a rate-distortion value using a rate-distortion (rate-distortion) analysis of several tested intra prediction modes, and may select an intra prediction mode having the best rate-distortion characteristic among the tested modes.

The intra predictor 122 selects one intra prediction mode from among a plurality of intra prediction modes, and predicts the current block using the determined neighboring pixels (reference pixels) and equations according to the selected intra prediction mode. The information on the selected intra prediction mode is encoded by the entropy encoder 155 and transmitted to the video decoding apparatus.

The inter predictor 124 generates a prediction block of the current block through motion compensation. The inter predictor 124 searches for a block most similar to the current block in a reference picture, which has been encoded and decoded earlier than the current picture, and generates a prediction block of the current block using the searched block. Then, the inter predictor generates a motion vector (motion vector) corresponding to a displacement (displacement) between the current block in the current picture and the prediction block in the reference picture. In general, motion estimation is performed on a luminance component, and a motion vector calculated based on the luminance component is used for both the luminance component and the chrominance component. The motion information including information on the reference image and information on the motion vector for predicting the current block is encoded by the entropy encoder 155 and transmitted to the video decoding apparatus.

The subtractor 130 subtracts the prediction block generated by the intra predictor 122 or the inter predictor 124 from the current block to generate a residual block.

The transformer 140 may partition the residual block into one or more transform blocks, perform transformation on the transform blocks, and transform the residual values of the transform blocks from the pixel domain to the frequency domain. In the frequency domain, a transform block is referred to as a coefficient block containing one or more transform coefficient values. A two-dimensional (2D) transform kernel may be used for the transform, and a one-dimensional (1D) transform kernel may be used for each of the horizontal transform and the vertical transform. The transform kernel may be based on a Discrete Cosine Transform (DCT), a Discrete Sine Transform (DST), or the like.

The transformer 140 may transform the residual signal in the residual block by using the entire size of the residual block as a transform unit. Also, the transformer 140 may partition the residual block into two sub-blocks in a horizontal direction or a vertical direction, and may perform transformation on only one of the two sub-blocks. Accordingly, the size of the transform block may be different from the size of the residual block (and thus the size of the prediction block). Non-zero residual sample values may not be present or very sparse in the untransformed sub-blocks. The residual samples of the untransformed sub-blocks may not be signaled and may all be considered "0" by the video decoding apparatus. There may be several partition types depending on the partition direction and the partition ratio. The transformer 140 may provide information on an encoding mode (or a transform mode) of the residual block to the entropy encoder 155 (e.g., the information on the encoding mode includes information indicating whether to transform the residual block or a subblock of the residual block, information indicating a partition type selected to partition the residual block into subblocks, information for identifying a subblock to be transformed, and the like). The entropy encoder 155 may encode information on an encoding mode (or a transform mode) of the residual block.

The quantizer 145 quantizes the transform coefficient output from the transformer 140, and outputs the quantized transform coefficient to the entropy encoder 155. The quantizer 145 may quantize the associated residual block for a particular block or frame directly without transformation.

The rearrangement unit 150 may perform rearrangement of the coefficient values using the quantized transform coefficients. The rearranging unit 150 may change the two-dimensional coefficient array into a one-dimensional coefficient sequence using coefficient scanning (coeffient scanning). For example, the rearranging unit 150 may scan coefficients from DC coefficients to coefficients in a high frequency region by zigzag scanning (zig-zag scan) or diagonal scanning (diagonalscan) to output a one-dimensional coefficient sequence. Depending on the size of the transform unit and the intra prediction mode, the utilized zigzag scanning may be replaced by a vertical scanning for scanning the two-dimensional coefficient array in the column direction and a horizontal scanning for scanning the two-dimensional block-shaped coefficients in the row direction. In other words, the scanning method to be used may be determined in zigzag scanning, diagonal scanning, vertical scanning, and horizontal scanning according to the size of the transform unit and the intra prediction mode.

The entropy encoder 155 encodes the sequence of one-dimensionally quantized transform coefficients output from the rearranging unit 150 by using various encoding methods such as Context-based Adaptive Binary Arithmetic Code (CABAC), Exponential Golomb (explicit Golomb), and the like, thereby generating a bitstream.

The entropy encoder 155 encodes information related to block division (e.g., CTU size, CU division flag, QT division flag, MTT division type, and MTT division direction) so that the video decoding apparatus can divide blocks in the same manner as the video encoding apparatus. In addition, the entropy encoder 155 encodes information on a prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and encodes intra prediction information (i.e., information on an intra prediction mode) or inter prediction information (information on a reference picture index and a motion vector) according to the prediction type.

The inverse quantizer 160 inversely quantizes the quantized transform coefficient output from the quantizer 145 to generate a transform coefficient. The inverse transformer 165 transforms the transform coefficients output from the inverse quantizer 160 from the frequency domain to the spatial domain and reconstructs a residual block.

The adder 170 adds the reconstructed residual block and the prediction block generated by the predictor 120 to reconstruct the current block. The pixels in the reconstructed current block are used as reference pixels when performing intra prediction of a subsequent block.

The filtering unit 180 filters the reconstructed pixels to reduce block artifacts (blocking artifacts), ringing artifacts (ringing artifacts), and blurring artifacts (blurring artifacts) generated due to block-based prediction and transform/quantization. The filtering unit 180 may include a deblocking filter 182 and a Sample Adaptive Offset (SAO) filter 184.

The deblocking filter 180 filters the boundaries between the reconstructed blocks to remove block artifacts caused by block-wise encoding/decoding, and the SAO filter 184 performs additional filtering on the deblocking filtered video. The SAO filter 184 is a filter for compensating a difference between a reconstructed pixel and an original pixel caused by a lossy coding (lossy coding).

The reconstructed block filtered through the deblocking filter 182 and the SAO filter 184 is stored in the memory 190. Once all blocks in a picture are reconstructed, the reconstructed picture can be used as a reference picture for inter-predicting blocks in subsequent pictures to be encoded.

Fig. 4 is an exemplary functional block diagram of a video decoding device capable of implementing the techniques of this disclosure. Hereinafter, a video decoding apparatus and its components will be described with reference to fig. 4.

The video decoding apparatus may include: an entropy decoder 410, a reordering unit 415, an inverse quantizer 420, an inverse transformer 430, a predictor 440, an adder 450, a filtering unit 460, and a memory 470.

Similar to the video encoding apparatus of fig. 1, each element of the video decoding apparatus may be implemented in hardware, software, or a combination of hardware and software. Further, the function of each element may be implemented as software, and the microprocessor may be implemented to perform the software function corresponding to each element.

The entropy decoder 410 determines a current block to be decoded by decoding a bitstream generated by a video encoding apparatus and extracting information related to block division, and extracts prediction information required to reconstruct the current block, information regarding a residual signal, and the like.

The entropy decoder 410 extracts information on the CTU size from a Sequence Parameter Set (SPS) or a Picture Parameter Set (PPS), determines the size of the CTU, and partitions the picture into CTUs of the determined size. Then, the decoder determines the CTU as the highest layer (i.e., root node) of the tree structure and extracts partitioning information about the CTU to partition the CTU using the tree structure.

For example, when a CTU is divided using a QTBTTT structure, a first flag (QT _ split _ flag) related to the division of QT is extracted to divide each node into four nodes of a sub-layer. For nodes corresponding to leaf nodes of the QT, a second flag (MTT _ split _ flag) related to the splitting of the MTT and information on the splitting direction (vertical/horizontal) and/or the splitting type (binary/trifurcate) are extracted, thereby splitting the corresponding leaf nodes in the MTT structure. Thus, each node below the leaf node of the QT is recursively split in BT or TT structure.

As another example, when a CTU is divided using a QTBTTT structure, a CU division flag (split _ CU _ flag) indicating whether or not to divide a CU may be extracted. When the corresponding block is divided, a first flag (QT _ split _ flag) may be extracted. In a split operation, after zero or more recursive QT splits, zero or more recursive MTT splits may occur per node. For example, a CTU may undergo MTT segmentation directly without QT segmentation, or only multiple times.

As another example, when a CTU is divided using a QTBT structure, a first flag (QT _ split _ flag) related to QT division is extracted, and each node is divided into four nodes of a lower layer. Then, a partition flag (split _ flag) indicating whether or not a node corresponding to a leaf node of the QT is further partitioned with BT and partition direction information are extracted.

Once the current block to be decoded is determined through tree structure division, the entropy decoder 410 extracts information on a prediction type indicating whether the current block is intra-predicted or inter-predicted. When the prediction type information indicates intra prediction, the entropy decoder 410 extracts a syntax element of intra prediction information (intra prediction mode) of the current block. When the prediction type information indicates inter prediction, the entropy decoder 410 extracts syntax elements for the inter prediction information, that is, information indicating a motion vector and a reference picture referred to by the motion vector.

On the other hand, the entropy decoder 410 extracts information on an encoding mode of the residual block (e.g., information on whether to encode the residual block or encode only a sub-block of the residual block, information indicating a partition type selected to partition the residual block into sub-blocks, information for identifying the encoded residual sub-block, a quantization parameter, etc.) from the bitstream. Also, the entropy decoder 410 extracts information regarding transform coefficients of the quantized current block as information regarding a residual signal.

The rearrangement unit 415 may change the sequence of quantized 1D transform coefficients entropy-decoded by the entropy decoder 410 into a 2D coefficient array (i.e., block) in the reverse order of coefficient scanning performed by the video encoding apparatus.

The inverse quantizer 420 inversely quantizes the quantized transform coefficient, and the inverse transformer 430 generates a reconstructed residual block of the current block via a reconstructed residual signal by inversely transforming the inversely quantized transform coefficient from a frequency domain to a spatial domain based on information on a coding mode of the residual block.

When the information on the encoding mode of the residual block indicates that the residual block of the current block is encoded in the video encoding apparatus, the inverse transformer 430 generates a reconstructed residual block of the current block by performing inverse transformation on the inversely quantized transform coefficients using the size of the current block (and thus the size of the residual block to be restored) as a transform unit.

Further, when the information on the encoding mode of the residual block indicates that only one sub-block of the residual block is encoded in the video encoding apparatus, the inverse transformer 430 generates a reconstructed residual block of the current block by reconstructing a residual signal of the transformed sub-block and by setting a residual signal of an untransformed sub-block to "0" through inverse transformation of the inversely quantized transform coefficient using the size of the transformed sub-block as a transform unit.

The predictor 440 may include an intra predictor 442 and an inter predictor 444. The intra predictor 442 is activated when the prediction type of the current block is intra prediction, and the inter predictor 444 is activated when the prediction type of the current block is inter prediction.

The intra predictor 442 determines an intra prediction mode of the current block among a plurality of intra prediction modes based on syntax elements of the intra prediction modes extracted from the entropy decoder 410, and predicts the current block using reference pixels around the current block according to the intra prediction mode.

The inter predictor 444 determines a motion vector of the current block and a reference picture referred to by the motion vector using syntax elements of the inter prediction mode extracted by the entropy decoder 410 and predicts the current block based on the motion vector and the reference picture.

The adder 450 reconstructs the current block by adding the residual block output from the inverse transformer 430 to the prediction block output from the inter predictor 444 or the intra predictor 442. When intra-predicting a block to be subsequently decoded, pixels in the reconstructed current block are used as reference pixels.

The filtering unit 460 may include a deblocking filter 462 and an SAO filter 464. The deblocking filter 462 deblocks the boundaries between the reconstructed blocks to remove block artifacts caused by block-by-block decoding. The SAO filter 464 performs additional filtering on the reconstructed block after deblocking filtering the corresponding offset in order to compensate for a difference between the reconstructed pixel and the original pixel caused by the lossy coding. The reconstructed block filtered through the deblocking filter 462 and the SAO filter 464 is stored in the memory 470. When all blocks in a picture are reconstructed, the reconstructed picture is used as a reference picture for inter prediction of blocks in pictures to be subsequently encoded.

In the conventional video encoding/decoding method, in order to reduce the complexity of prediction of the chrominance components, each chrominance component is predicted in the same manner as the prediction process of the luminance component, or is predicted in a simplified manner of the prediction process of the luminance component. However, this conventional method has a problem that color distortion occurs.

The present invention proposes an encoding and decoding method for efficiently predicting chrominance components in a chrominance block of a target block to be reconstructed, i.e., a current block.

The method proposed herein is a method in which information on a residual sample (or residual signal) of one of a Cb chrominance component and a Cr chrominance component is encoded and signaled, and information on a residual sample of the other chrominance component is derived without being encoded and signaled.

Herein, the residual samples of the chrominance component to be derived may be referred to as "second residual samples of the second chrominance component", and the residual samples of the chrominance component to be encoded and signaled to derive the second residual samples may be referred to as "first residual samples of the first chrominance component".

The first chrominance component may be one of a Cb chrominance component and a Cr chrominance component, and the second chrominance component may be the other of the Cb chrominance component and the Cr chrominance component. For example, when the residual samples of the Cb chroma component are encoded and signaled, and the residual samples of the Cr chroma component are derived, the residual samples of the Cb chroma component may be referred to as first residual samples, and the residual samples of the Cr chroma component may be referred to as second residual samples. As another example, when residual samples of the Cr chroma component are encoded and signaled, and residual samples of the Cb chroma component are derived, the residual samples of the Cr chroma component may be referred to as first residual samples, and the residual samples of the Cb chroma component may be referred to as second residual samples.

The method of deriving the second residual samples can be divided into: 1) an implementation that uses information about correlation between a first residual sample and a second residual sample; 2) determining whether to activate or apply an implementation of the second residual sample derivation scheme, etc. In addition, the embodiments using the related information may be classified into different embodiments according to whether the difference between the chromaticities is used. Hereinafter, terms used herein will be first defined, and then each embodiment will be described in detail.

Related information

The correlation information refers to information for deriving a second residual sample from a first residual sample, and may be adaptively determined according to a range of luminance component values of the current block to be encoded. The correlation information may include multiplication information, or may include multiplication information and offset information.

The related information may be defined and signaled at various positions in the bitstream to the video decoding apparatus and may be decoded from the positions in the bitstream. For example, the relevant information may be defined and signaled at one or more locations in a high-level syntax (HLS) such as SPS, PPS, and picture level. As another example, the related information may be signaled at a lower level such as tile group level, tile level, CTU level, unit block level (CU, TU, PU), and the like. As another example, a difference value (difference correlation information) with correlation information signaled via HLS may be signaled at a lower level.

Depending on the embodiment, the relevant information may not be signaled directly, but some information from which the relevant information can be derived in the video decoding apparatus may be signaled. For example, table information including a fixed value of the correlation information may be signaled, and an index value indicating correlation information used to derive the second residual sample among the fixed values in the table information may be signaled. As another example, the table information may not be signaled, but may be predefined between the video encoding device and the video decoding device. The index value may be defined and signaled at one or more of a tile group level, a tile level, and a cell block level.

The correlation information is information for deriving the second residual samples and may therefore be signaled when the derivation of the second residual samples is applied. Accordingly, the related information may be decoded from the bitstream when a first syntax element to be described below indicates that derivation of the second residual samples is allowed, or may be decoded from the bitstream when a second syntax element to be described below indicates that derivation of the second residual samples is applied.

Multiplication information

The multiplication information refers to information indicating a multiplication factor between the first residual sample and the second residual sample. When applying the multiplication factor to (a value of) a first residual sample, a value equal to or within a range corresponding to (a value of) a second residual sample may be derived. The multiplication factor may represent a proportional relationship, a weight relationship, a sign relationship, etc. between the first residual sample and the second residual sample. Accordingly, the multiplication factor may be an integer such as-1 or a fraction such as 1/2 or-1/2.

When multiplication information is signaled in the form of a flag 0 or 1 and the multiplication factor represents a sign relationship between the first residual sample and the second residual sample, the multiplication information may represent the multiplication factor by the method shown in equation 1.

[ equation 1]

Multiplication factor 1-2 × (multiplication information)

Multiplication information (i.e., a flag) equal to 0 indicates that the first residual sample and the second residual sample have the same sign relationship, and a multiplication factor of "1" may be applied to the first residual sample. Multiplication information (i.e., a flag) equal to 1 indicates that the first residual sample and the second residual sample have different sign relationships, and a multiplication factor of "-1" may be applied to the first residual sample.

Offset information

The offset information refers to information indicating an offset factor between a first residual sample (to which a multiplication factor is applied) and a second residual sample. When an offset factor is applied to a first residual sample (to which a multiplication factor is applied), a value equal to or within a range corresponding to a second residual sample may be derived. The offset factor may be an integer such as-1, 0 or 1 or a fraction such as 1/2 or-1/2.

For the case where the offset factor is equal to 0, if only multiplication information is included in the correlation information, the offset information may not be signaled, and if the multiplication information and the offset information are included in the correlation information, the offset information may indicate that the offset factor is equal to 0.

Difference between chromaticities

The inter-chroma difference refers to a difference between a first residual sample and a second residual sample (i.e., refers to a value obtained by subtraction between the first residual sample and the second residual sample). More specifically, the inter-chroma difference value corresponds to a value derived by subtraction between a first residual sample (to which the correlation information is applied) and a second residual sample. For example, if the correlation information includes only multiplication information, the inter-chroma difference value may be derived by performing a subtraction between the first residual sample (to which the multiplication factor is applied) and the second residual sample. As another example, if the correlation information includes multiplication information and offset information, the inter-chroma difference value may be derived by performing subtraction between the first residual sample (to which the multiplication factor and the offset factor are applied) and the second residual sample.

Embodiment 1

Embodiment 1 is a method of utilizing both the correlation information and the difference between chromaticities. Embodiment 1 may be divided into the following sub-embodiments according to the steps of the encoding step performed according to the process of deriving the inter-chroma difference values and the related information and the decoding step performed according to the process of deriving the second residual samples.

Embodiments 1 to 1

In embodiment 1-1, the process of deriving the inter-chroma difference value and the related information is performed before the step of transforming the residual samples, and the process of deriving the second residual sample is performed after the step of inverse quantizing the residual samples.

Exemplary block diagrams and flowcharts for a video encoding apparatus for carrying out embodiment 1-1 are shown in fig. 5 and fig. 6, respectively, and exemplary block diagrams and flowcharts for a video decoding apparatus for carrying out embodiment 1-1 are shown in fig. 7 and fig. 8, respectively.

The subtractor 130 may obtain a first residual sample and a second residual sample (S610). In particular, a first residual sample may be obtained by subtraction between a prediction block (or predicted sample) of a first chroma component and a chroma block of the first chroma component, and a second residual sample may be obtained by subtraction between a prediction block of a second chroma component and a chroma block of the second chroma component. A prediction block of the chrominance component may be derived through prediction by the predictor 120, and information for prediction, i.e., prediction information, may be derived in this process. The process of generating predicted samples and the process of deriving prediction information may be equally applied to other embodiments of the present description.

The chrominance component predictor 510 may determine whether to derive a second residual sample from the first residual sample (S620).

The chrominance component predictor 510 may determine one of a method of encoding both the first and second residual samples (i.e., a general method) and a method of deriving the second residual sample (i.e., a second residual sample derivation method) for the chrominance block. For example, the chrominance component predictor 510 calculates a rate-distortion value through a rate-distortion analysis of a general method and a derivation method, and may select or determine one having the best rate-distortion characteristic for the chrominance block. The process of determining whether to derive the second residual sample may be equally applied to other embodiments of the present description.

When a second residual sample derivation method (i.e., a method of deriving second residual samples) is selected for the chroma block, the chroma component predictor 510 may modify the first residual samples (S630). The modification of the first residual samples may be achieved by applying the correlation information to the first residual samples.

The chrominance component predictor 510 may derive an inter-chrominance difference value using the modified first and second residual samples (S640). The inter-chroma difference value may be derived by subtracting the modified first and second residual samples.

Operations S630 and S640 may be performed by the following equation 2.

[ equation 2]

Cro_r＝Cro_resi2-(W*Cro_resi1+Offset)

In equation 2, Cro _ resil denotes a first residual sample, Cro _ resi2 denotes a second residual sample, Cro _ r denotes an inter-chroma difference, W denotes a multiplication factor, and Offset denotes an Offset factor. Referring again to equation 2 with emphasis on the second residual samples, Cro _ resi2 may be a primary signal of the second residual samples (i.e., a primary residual signal of the second chroma component), and Cro _ r may be a secondary signal of the second residual samples (i.e., a secondary residual signal of the second chroma component).

The transformer 140 may transform the inter-chroma difference value and the first residual sample, and the quantizer 145 may quantize the transformed inter-chroma difference value and the transformed first residual sample (S650). Here, the inter-chroma difference value may be quantized by the "quantization parameter changed with QP _ C _ offset" according to the quantization parameter of the first residual sample or luminance component. QP _ C _ offset may be determined by various methods. For example, QP _ C _ offset may be adaptively determined according to one or more of a range of luminance component values (a range of luminance values), a size of a chroma block, and a range of quantization parameters of a luminance component. As another example, QP _ C _ offset may be determined as a value preset in the video encoding apparatus and the video decoding apparatus. As another example, the video encoding apparatus may determine QP _ C _ offset as an arbitrary value, perform quantization processing, and signal the value of QP _ C _ offset used in the quantization processing to the video decoding apparatus. The quantization method using QP _ C _ offset can also be applied to other embodiments of the present specification.

The transformed and quantized inter-chroma difference values, the first residual samples, the correlation information, and the prediction information may be encoded and signaled to a video decoding apparatus (S660). Here, the second residual samples are not signaled.

The entropy decoder 410 may decode the inter-chroma difference value, the first residual sample, the correlation information, and the prediction information from the bitstream (S810). The inverse quantizer 420 may inverse quantize the inter-chrominance difference value and the first residual sample, and the inverse transformer 430 may inverse transform the inverse-quantized inter-chrominance difference value and the inverse-quantized first residual sample (S820).

The predictor 440 may generate (or reconstruct) predicted samples (or predicted blocks) of the first chrominance component and predicted samples of the second chrominance component based on the prediction information (S820).

The chrominance component reconstructing unit 710 may determine whether to derive the second residual sample from the first residual sample (whether to activate (allow) and/or apply a second residual sample derivation method) (S830). A detailed description of operation S830 will be described below by a separate embodiment.

When it is determined that the second residual sample is to be derived, the chrominance component reconstructing unit 710 may modify the first residual sample using the (inverse transformed) correlation information (S840). In addition, the chrominance component reconstructing unit 710 may derive a second residual sample using the modified first residual sample and the inverse-transformed inter-chrominance difference value (S850). The second residual sample may be derived by adding the modified first residual sample and the inverse transformed inter-chroma difference value.

Operations S630 and S640 may be performed by the following equation 3.

[ equation 3]

Cro_resi2＝(W*Cro_resi1+Offset)+Cro_r

The adder 450 may reconstruct a chroma block of the first chroma component by adding the first residual sample of the first chroma component and the prediction block, and may reconstruct a chroma block of the second chroma component by adding the derived second residual sample of the second chroma component and the prediction block (S860).

Embodiments 1 to 2

In embodiment 1-2, the process of deriving the inter-chroma difference value and the related information is performed after the step of quantizing the residual samples, and the process of deriving the second residual sample is performed before the step of inverse quantizing the residual samples.

Exemplary block diagrams and flowcharts for a video encoding apparatus for performing embodiments 1-2 are shown in fig. 9 and 10, respectively, and exemplary block diagrams and flowcharts for a video decoding apparatus for performing embodiments 1-2 are shown in fig. 11 and 12, respectively.

The subtractor 130 may obtain a first residual sample and a second residual sample (S1010). The residual samples of each chrominance component may be obtained by subtracting the prediction block and the chrominance block of each chrominance component, and the prediction block and the prediction information of each chrominance component are derived through the prediction process of the predictor 120.

The transformer 140 may transform the first and second residual samples, and the quantizer 145 may quantize the transformed first and second residual samples (S1020). Here, the second residual sample may be quantized to a value obtained by adding a quantization offset used to quantize the second residual sample to the quantization parameter of the first residual sample. The quantization offset may be determined by various methods. For example, the quantization offset may be adaptively determined according to one or more of a range of luma component values (a range of luma values), a size of the first residual sample values, and a bit depth of the second residual samples. As another example, the quantization offset may be determined as a value preset in the video encoding apparatus and the video decoding apparatus. The video decoding apparatus may determine the quantization parameter using a delta-QP signaled from the video encoding apparatus, add the quantization offset to the quantization parameter to derive the quantization parameter for the second residual sample, and then inverse-quantize the second residual sample using the derived quantization parameter. The quantization/inverse quantization method using the quantization offset may be applied to other embodiments of the present specification.

According to an embodiment, a quantized coefficient "0" may be derived by a quantization process for the second residual sample (i.e., no residual signal may be present in the quantization process). In this case, information or syntax elements indicating that the quantized coefficient "0" is derived may be signaled from the video encoding apparatus to the video decoding apparatus.

On the other hand, one or more of a quantization parameter value (first value) of a first residual sample to which a quantization offset is not added, a value (second value) obtained by adding the quantization offset and the quantization parameter of the first residual sample, and an average value of the first value and the second value may be used for in-loop filtering processing of a second residual sample. For example, one or more of the first value, the second value, and the mean value may be used as a parameter to determine the in-loop filtering strength of the second residual sample, or may be used as a parameter to determine an index in a table for determining the boundary strength. The method of using one or more of the first value, the second value, and the average value in the loop filtering process may be applied to other embodiments of the present specification.

The chrominance component predictor 510 may determine whether to derive a second residual sample from the first residual sample (S1030). When it is determined that the second residual sample is to be derived, the chrominance component predictor 510 may modify the quantized first residual sample (S1040). The modification of the first residual samples may be performed by applying the correlation information to the quantized first residual samples.

The chrominance component predictor 510 may derive an inter-chrominance difference value using the modified first residual samples and the quantized second residual samples (S1050). The inter-chroma difference may be derived by performing a subtraction between the modified first residual samples and the quantized second residual samples.

Operations S1040 and S1050 may be performed by equation 4 below.

[ equation 4]

Q(T(Cro_r))＝Q(T(Cro_resi2))-(W*Q(T(Cro-resi1))+Offset)

In equation 4, Q (T (Cro _ resi1)) denotes the transformed and quantized first residual sample, Q (T (Cro _ resi2)) denotes the transformed and quantized second residual sample, and Q (T (Cro _ r)) denotes an inter-chroma difference value derived from the transformed and quantized first residual sample and the transformed and quantized second residual sample.

The inter-chroma difference value, the first residual sample, the correlation information, and the prediction information may be encoded and signaled to the video decoding apparatus (S1060). Here, the second residual samples are not signaled.

The entropy decoder 410 may decode the inter-chroma difference value, the first residual sample, the correlation information, and the prediction information from the bitstream (S1210). The predictor 440 may generate (or reconstruct) predicted samples of the first chrominance component (a prediction block) and predicted samples of the second chrominance component based on the prediction information (S1220).

The chrominance component reconstructing unit 710 may determine whether to derive the second residual sample from the first residual sample (i.e., whether to activate and/or apply the second residual sample deriving method) (S1230). A detailed description of operation S1230 will be described below by way of a separate embodiment.

When it is determined that the second residual sample is to be derived, the chrominance component reconstructing unit 710 may modify the first residual sample using the correlation information (S1240). In addition, the chrominance component reconstructing unit 710 may derive a second residual sample using the modified first residual sample and the inter-chrominance difference value (S1250). The second residual sample may be derived by adding the modified first residual sample and the inter-chroma difference value.

Operations S1240 and S1250 may be performed by equation 5 below.

[ equation 5]

Q(T(Cro_resi2))＝(W*Q(T(Cro_resi1))+Offset)+Q(T(Cro_r))

The inverse quantizer 420 may inverse quantize the first residual sample and the derived second residual sample, and may inverse transform the inverse quantized first residual sample and the inverse quantized second residual sample (S1260). The adder 450 may reconstruct a chrominance block of the first chrominance component by adding the inverse-transformed first residual sample of the first chrominance component and the prediction block, and may reconstruct a chrominance block of the second chrominance component by adding the inverse-transformed second residual sample of the second chrominance component and the prediction block (S1270).

Embodiment 2

Embodiment 2 is a method of predicting and deriving a second residual sample using correlation information without using an inter-chroma difference value.

Embodiment 2 is different from embodiment 1 in that an inter-chroma difference value is not used, and a process of deriving an inter-chroma difference value (S640 or S1050) is not performed.

In addition to this difference, the remaining processing of embodiment 1 can also be performed in embodiment 2. Accordingly, as in embodiment 1-1, the process of deriving the relevant information in the video encoding apparatus may be performed before the step of transforming the residual samples, and the process of deriving the second residual samples in the video decoding apparatus may be performed after the step of inverse transforming the residual samples. Further, in embodiment 1-2, the process of deriving the relevant information in the video encoding apparatus may be performed after the step of quantizing the residual samples, and the process of deriving the second residual samples in the video decoding apparatus may be performed before the step of inverse quantizing the residual samples. However, the remaining steps other than the step of transforming/quantizing the residual samples and the step of inverse quantizing/inverse transforming the residual samples will be described below.

Fig. 13 and 14 show flowcharts illustrating an example of embodiment 2.

The subtractor 130 may subtract the prediction block of the first chrominance component and the chrominance block of the first chrominance component to obtain a first residual sample, and may subtract the prediction block of the second chrominance component and the chrominance block of the second chrominance component to obtain a second residual sample (S1310).

The chrominance component predictor 510 may determine whether to derive a second residual sample from the first residual sample (S1320). When it is determined that the second residual sample is to be derived, the chrominance component predictor 510 may derive correlation information using the first residual sample and the second residual sample (S1330).

On the other hand, according to an embodiment, when only multiplication information is included in the related information, the second residual sample derivation method may include the following three modes.

1) Mode 1: the values of the Cb residual samples are signaled and derived by applying a multiplication factor of-1/2 or +1/2 to the values of the Cb residual samples.

2) Mode 2: the value of the Cb residual sample is signaled and derived by applying a multiplication factor of-1 or +1 to the value of the Cb residual sample.

3) Mode 3: the values of the Cr residual samples are signaled and the values of the Cb residual samples are derived by applying a multiplication factor of-1/2 or +1/2 to the values of the Cr residual samples.

In addition, the second residual sample derivation method may further include a mode in which an offset factor is applied to each of the first to third modes when offset information is also included in the correlation information.

In this embodiment, the chrominance component predictor 510 may determine a mode having the best rate-distortion characteristic among the above modes as a mode for a chrominance block. The chrominance component predictor 510 may integrally perform the process of determining one of the above-described general method and the second residual sample derivation method and the process of determining one of the modes of the second residual sample derivation method. For example, the chrominance component predictor 510 may determine a mode or method having the best rate-distortion characteristic among the modes of the general method and the second residual sample derivation method for the chrominance block.

The first residual sample, the correlation information, and the prediction information may be encoded and signaled to a video decoding apparatus (S1340). Here, the second residual sample and the inter-chroma difference are not signaled.

The entropy decoder 410 may decode the first residual sample, the related information, and the prediction information from the bitstream (S1410).

The predictor 440 may generate (or reconstruct) predicted samples of the first chrominance component (a prediction block) and predicted samples of the second chrominance component based on the prediction information (S1420).

The chrominance component reconstructing unit 710 may determine whether to derive the second residual sample from the first residual sample (i.e., whether to activate and/or apply the second residual sample deriving method) (S1430). A detailed description of operation S1430 will be described below by way of a separate embodiment.

When it is determined that the second residual sample is to be derived, the chrominance component reconstructing unit 710 may derive the second residual sample by applying the correlation information to the first residual sample (S1440). For example, when the correlation information includes multiplication information, the second residual sample may be derived by applying a multiplication factor indicated by the multiplication information to the first residual sample. As another example, when the related information includes multiplication information and offset information, the second residual sample may be derived by applying an offset factor indicated by the offset information to the first residual sample to which the multiplication factor is applied.

Operation S1440 may be performed by equation 6 below.

[ equation 6]

Cro_resi2＝W＊Cro_resi1+Offset

Comparing equation 6 with equations 2 to 5, it can be seen that in embodiment 2, the inter-chromaticity difference (i.e., Cro _ r ═ 0) is not used. Accordingly, the transform/inverse transform process, the quantization/inverse quantization process, and the encoding/decoding process are not performed on the inter-chroma difference values.

The adder 450 may reconstruct a chroma block of the first chroma component by adding the first residual sample of the first chroma component and the prediction block, and may reconstruct a chroma block of the second chroma component by adding the derived second residual sample of the second chroma component and the prediction block (S1450).

Embodiment 3

Embodiment 3 is a method of determining whether to derive a second residual sample from a first residual sample (i.e., whether to allow (activate) and/or apply the second residual sample derivation method).

Whether to perform the second residual sample derivation method may be determined by various criteria. The various criteria may include: 1) a value of a syntax element (e.g., flag) indicating whether derivation of the second residual sample is allowed and/or applied (i.e., on or off); 2) a prediction mode of the target block; 3) the range of luminance component values, etc.

Standard 1: syntax element indicating on/off

The first syntax element and/or the second syntax element may be employed to indicate whether to derive the second residual samples.

A first syntax element, which is a syntax element indicating whether the second residual sample derivation method is allowed (or activated) (i.e., on or off), may be defined and signaled at various positions of the bitstream to the video decoding apparatus. For example, the first syntax element may be defined and signaled at a CTU level or higher, or may be defined and signaled at one or more levels of a unit block (PU, TU, CU) level, a tile group level, and a picture level.

A second syntax element, which is a syntax element indicating whether to apply the second residual sample derivation method to the target block (chroma block), i.e., whether to switch on or off the target block, may be defined and signaled at various positions of the bitstream to the video decoding apparatus. For example, the second syntax element may be defined and signaled at a CTU level or higher, or may be defined and signaled at one or more levels of a unit block (PU, TU, CU) level, a tile group level, and a picture level.

According to an embodiment, the first syntax element may be defined and signaled at a relatively higher level in the bitstream and the second syntax element may be defined and signaled at a relatively lower level in the bitstream. In this case, when the second residual sample derivation method is turned off at a higher level, the second syntax element is not signaled at a lower level, and even when the second residual sample derivation method is turned on at a higher level, whether to be turned on at the lower level or turned off can be selectively determined. Accordingly, the bit efficiency of the second residual sample derivation method can be improved.

Fig. 13 shows an example of determining whether to turn on or off the second residual sample derivation method.

The video encoding apparatus may determine whether the second residual sample derivation method is allowed, and may set a value of the first syntax element based on a result of the determination and signal the first syntax element to the video decoding apparatus. Further, the video encoding apparatus may determine whether to apply the second residual sample derivation method, and may set a value of the second syntax element as a result of the determination and signal the second syntax element to the video decoding apparatus.

The video decoding apparatus may decode a first syntax element from the bitstream (S1510), and may determine whether the second residual sample derivation method is allowed according to a value of the first syntax element (S1520).

When the second syntax element indicates that the second residual sample derivation method is allowed (i.e., the first syntax element is 1; S1520), the video decoding apparatus may decode the second syntax element from the bitstream (S1530). Also, the video decoding apparatus may determine whether to apply the second residual sample derivation method according to the value of the second syntax element (S1540).

When the second syntax element indicates that the second residual sample derivation method is applied to the target block (i.e., the second syntax element is 0; S1540), the video decoding apparatus may derive the second residual sample based on the correlation information and the first residual sample for the target block (or the correlation information, the first residual sample, and the inter-chroma difference value) (S1550).

When the first syntax element indicates that the second residual sample derivation method is not allowed in operation S1520 (i.e., the first syntax element is 0), or when the second syntax element indicates that the second residual sample derivation method is not applied in operation S1540, derivation of the second residual sample is not performed on the target block.

Standard 2: prediction mode of target block

Whether to turn on or off the second residual sample derivation method may be considered or adaptively determined according to a prediction mode of a target block (chroma block).

For example, when the chroma block is predicted in one of an intra mode, an inter mode, an IBC mode, and a palette mode, the derivation of the second residual samples may be turned on or off. As another example, when a chroma block is predicted in two or more of an intra mode, an inter mode, an IBC mode, and a palette mode (when a chroma block is predicted in one of the two or more modes), derivation of the second residual samples may be turned on or off.

As yet another example, when a chroma block is predicted by a cross-component linear model (CCLM) or a Direct Mode (DM) in an intra prediction mode, the derivation of the second residual samples may be turned on or off. In this case, only when a chroma block is predicted by CCLM or DM, information indicating the turn-on or turn-off of the derivation of the second residual samples may be signaled to the video decoding apparatus.

As yet another example, when a chroma block is predicted by a bi-prediction mode or a merge mode in an inter mode, and when the chroma block is predicted with reference to a zeroth reference picture, derivation of the second residual samples may be turned on or off. The information indicating the switching on or off of the derivation of the second residual samples may be signaled to the video decoding apparatus only when the chroma block is predicted by the bi-prediction mode or the merge mode, or only when the chroma block is predicted with reference to the zeroth reference picture.

The example considering the prediction mode of the chroma block may be combined with the above example using the first syntax element and the second syntax element. For example, in operation S1520, when the first syntax element is equal to 1 and the prediction mode of the chroma block corresponds to a derived prediction mode that turns on the second residual samples, the second syntax element may be decoded from the bitstream. (S1530). That is, whether to decode the second syntax element may be determined in consideration of a prediction mode of the chroma block.

Standard 3: range of values of luminance component

The range of values of the luminance component (range of luminance values) may be divided into two or more parts, and it may be determined whether to apply the second residual sample derivation method according to which part of the divided parts the value of the luminance component of the target block belongs to.

For example, in the case of dividing the range of values of the luma component into two parts (a first part and a second part), when the values of the luma component of the target block belong to the first part, the second residual sample derivation method is not applied, and when the values of the luma component of the target block belong to the second part, the second residual sample derivation method may be applied, and vice versa.

Of the two or more portions, a portion to which the second residual sample derivation method is not applied may correspond to a "visually perceived portion" to which the user's vision may acutely react, and a portion to which the second residual sample derivation method is applied may not correspond to the "visually perceived portion". Accordingly, the second residual sample derivation method may be selectively applied only to portions other than the visually perceived portion, and not to the visually perceived portion, so that deterioration of subjective image quality may be prevented.

The partial value indicating the range of the first portion and the one or more partial values indicating the partial values of the range of the second portion may be signaled from the video encoding device to the video decoding device. According to an embodiment, a partial value may be preset between a video encoding device and a video decoding device without signaling.

Standard 4: quantification of results

The second residual sample derivation method may be selectively applied when the quantized coefficients of the second residual samples have very small values (i.e., when a small number of quantized coefficients appear or exist) due to the accuracy of prediction of the second chrominance component. Also in this case, quantization of only a portion, but not all, of the second residual samples may not be omitted (i.e., only some of the second residual samples are signaled).

Other criteria

The second residual sample derivation method may be applied to the target block or not when a delta-QP (DQP) of the luma component is greater than or equal to a preset value, when a transform skip mode is not applied to the chroma block, or when a block differential coded modulation (BDPCM) mode is not applied to the target block.

The second residual sample derivation method may not be applied to the target block when the picture including the target block is a progressive random access (GRA) picture or an Instantaneous Decoding Recording (IDR) picture for random access.

Although exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications and changes are possible without departing from the spirit and scope of the invention. For the sake of brevity and clarity, exemplary embodiments have been described. Accordingly, it will be understood by those of ordinary skill that the scope of the embodiments is not limited by the embodiments explicitly described above, but is included in the claims and their equivalents.

Cross Reference to Related Applications

The present application claims priority from korean patent application No.10-2019-0056974 filed on 5/15/2019 and korean patent application No.10-2020-0058335 filed on 5/15/2020, the entire contents of which are incorporated herein by reference.

Claims

1. A method for reconstructing a chroma block of a target block to be reconstructed, the method comprising:

decoding, from a bitstream, correlation information between first and second residual samples, the first residual samples being residual samples of a first chroma component, and prediction information of a chroma block, the second residual samples being residual samples of a second chroma component;

generating predicted samples of the first chrominance component and predicted samples of the second chrominance component based on the prediction information;

deriving second residual samples by applying correlation information to the first residual samples; and

the chrominance block of the first chrominance component is reconstructed by adding first residual samples of the first chrominance component and the predicted samples, and the chrominance block of the second chrominance component is reconstructed by adding second residual samples of the second chrominance component and the predicted samples.

2. The method of claim 1, wherein the first chroma component is one of a Cb chroma component and a Cr chroma component, and the second chroma component is the other of the Cb chroma component and the Cr chroma component.

3. The method of claim 1, wherein decoding comprises: the related information is decoded from the picture level of the bitstream.

4. The method of claim 1, wherein the correlation information comprises multiplication information indicating a multiplication factor between a first residual sample and a second residual sample, and

wherein deriving the second residual samples comprises: the second residual samples are derived by applying a multiplication factor indicated by the multiplication information to the first residual samples.

5. The method of claim 4, wherein the correlation information further comprises offset information indicating an offset factor between a first residual sample and a second residual sample,

wherein deriving the second residual samples comprises: the second residual samples are derived by applying the offset factor indicated by the offset information to the first residual samples to which the multiplication factor is applied.

6. The method of claim 1, wherein:

the decoding includes:

decoding a first syntax element from a Sequence Parameter Set (SPS) level of a bitstream when the first syntax element indicates that derivation of the second residual sample is allowed, the first syntax element indicating whether derivation of the second residual sample is allowed; and

decoding a second syntax element from a level in the bitstream below the SPS level when the first syntax element indicates that derivation of the second residual sample is allowed, the second syntax element indicating whether to apply derivation of the second residual sample to the chroma block,

wherein the deriving of the second residual samples comprises: deriving the second residual sample when the second syntax element indicates that the derivation of the second residual sample is applied to the chroma block.

7. The method of claim 6, wherein decoding a second syntax element comprises: the second syntax element is decoded in consideration of a prediction mode of the target block.

8. A video decoding device for reconstructing a chroma block of a target block to be reconstructed, the video decoding device comprising:

a decoder configured to decode, from a bitstream, correlation information between first residual samples and second residual samples, the first residual samples being residual samples of a first chroma component, and prediction information of a chroma block, the second residual samples being residual samples of a second chroma component;

a predictor configured to generate predicted samples of the first chroma component and predicted samples of the second chroma component based on the prediction information;

a chrominance component reconstructing unit configured to derive second residual samples by applying correlation information to the first residual samples; and

an adder configured to reconstruct a chroma block of the first chroma component by adding first residual samples of the first chroma component and the predicted samples, and configured to reconstruct a chroma block of the second chroma component by adding second residual samples of the second chroma component and the predicted samples.

9. The video decoding apparatus of claim 8, wherein the first chroma component is one of a Cb chroma component and a Cr chroma component, and the second chroma component is the other of the Cb chroma component and the Cr chroma component.

10. The video decoding apparatus of claim 8, wherein the decoder is further configured to decode the related information from a picture level of the bitstream.

11. The video decoding device of claim 8, wherein:

the correlation information comprises multiplication information indicating a multiplication factor between the first residual sample and the second residual sample, an

The chrominance component reconstructing unit is further configured to derive the second residual sample by applying a multiplication factor indicated by the multiplication information to the first residual sample.

12. The video decoding device of claim 11, wherein:

the correlation information further comprises offset information indicating an offset factor between the first residual sample and the second residual sample, an

The chrominance component reconstructing unit is configured to derive the second residual sample by applying an offset factor indicated by the offset information to the first residual sample to which the multiplication factor is applied.

13. The video decoding device of claim 8, wherein:

the decoder is further configured to:

decoding a first syntax element for a Sequence Parameter Set (SPS) level of a bitstream, the first syntax element indicating whether derivation of second residual samples is allowed; and

decoding a second syntax element from a level in the bitstream below the SPS level when the first syntax element indicates that derivation of the second residual sample is allowed, the second syntax element indicating whether to apply derivation of the second residual sample to the chroma block, and

the chroma component reconstruction unit is further configured to derive the second residual samples when the second syntax element indicates that derivation of the second residual samples is applied to the chroma block.

14. The video decoding apparatus of claim 13, wherein the decoder is further configured to decode the second syntax element in consideration of a prediction mode of the target block.