KR20230105646A - Method for Template-based Intra Mode Derivation for Chroma Component - Google Patents

Abstract

As a disclosure of a template-based intra mode derivation method for chroma components, this embodiment extends a template-based intra mode derivation (TIMD) method of luma components to intra prediction of chroma components, but complexity Provided is a video coding method and apparatus using a TIMD method for a chroma component with reduced .

Description

Template-based intra mode derivation method for chroma component {Method for Template-based Intra Mode Derivation for Chroma Component}

The present disclosure relates to a template-based intra mode derivation method for chroma components.

The contents described below merely provide background information related to the present embodiment and do not constitute prior art.

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

Therefore, when video data is stored or transmitted, an encoder is used to compress and store or transmit the video data, and a decoder receives, decompresses, and reproduces the compressed video data. Examples of such video compression technologies include H.264/AVC, High Efficiency Video Coding (HEVC), and Versatile Video Coding (VVC), which has improved coding efficiency by about 30% or more compared to HEVC.

However, since the size, resolution, and frame rate of video are gradually increasing, and the amount of data to be encoded accordingly increases, a new compression technology with higher encoding efficiency and higher picture quality improvement effect than existing compression technologies is required.

A template-based intra mode derivation (TIMD) method is a technique for deriving an optimal intra prediction mode for a current block based on a template. By applying the TIMD method to all candidate modes in the MPM (Most Probable Mode) list and comparing them, an optimal intra mode for the current block (as a coding unit block, also called CU) can be derived. The mode derived using the TIMD technique can be used as one additional mode for the current block. The neighboring neighboring templates (template, L1×N and M×L2) of the current block used in the TIMD method and the reference of the template used to generate the prediction template are as illustrated in FIG. 6 .

The TIMD method generates a prediction template by applying the directionality of the corresponding mode to reference pixels of the template for each candidate mode in the MPM list, and then pixels of the generated prediction template and pixels of the restored template Calculate the Sum of Absolute Transformed Differences (SATD) between Among all candidate modes in the MPM list, the mode with the smallest sum of absolute transform differences is the intra mode of the current block selected according to the TIMD method.

As described above, the TIMD method used for encoding the luma component may be extended and used for encoding the chroma component. Similar to the TIMD method for luma components, the TIMD method for chroma components derives an intra mode using a template composed of neighboring samples of a current chroma component block. The TIMD method for the chroma component may use a coding unit level flag (CU level flag) indicating whether the current chroma component block uses the TIMD method.

On the other hand, the TIMD method for the chroma component has a problem that the coding efficiency is not high and the coding efficiency is high in complexity (in particular, the complexity of the decoder). Therefore, it is necessary to consider a TIMD method for chroma components, which can improve video coding efficiency and reduce complexity.

The present disclosure extends a template-based intra mode derivation (TIMD) method of luma components to intra prediction of chroma components in order to improve video quality and improve video encoding efficiency, but with reduced complexity. , and to provide a video coding method and apparatus using the TIMD method for chroma components.

According to an embodiment of the present disclosure, a method of deriving an intra mode of a current chroma component block, performed by an image decoding apparatus, includes generating a candidate mode list of a chroma component for the current chroma component block; checking whether a template-based intra mode derivation method is used for the current chroma component block; checking whether a cross-component linear model (CCLM) is used when the template-based intra mode derivation method is used; determining an intra mode of the current chroma component block by applying the template-based intra mode derivation method to CCLM modes in the candidate mode list when the CCLM is used; and determining an intra mode of the current chroma component block by applying the template-based intra mode derivation method to directional modes in the candidate mode list when the CCLM is not used. provides

According to another embodiment of the present disclosure, a method for deriving an intra mode of a current chroma component block, performed by an image encoding apparatus, includes generating a candidate mode list of a chroma component for the current chroma component block; determining an optimal first mode by performing intra prediction on the current chroma component block; determining an optimal second mode by performing a template-based intra mode derivation method on modes in the candidate mode list; and determining whether the first mode is optimal for the second mode, wherein if the second mode is optimal, determining the second mode as an intra mode for the current chroma component block, setting a flag indicating use of a template-based intra mode derivation method to be true; and setting a flag indicating use of CCLM according to whether the second mode is included in cross-component linear model (CCLM) modes.

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 a candidate mode list of chroma components for a current chroma component block; ; determining an optimal first mode by performing intra prediction on the current chroma component block; determining an optimal second mode by performing a template-based intra mode derivation method on modes in the candidate mode list; and determining whether the first mode is optimal for the second mode, wherein if the second mode is optimal, determining the second mode as an intra mode for the current chroma component block, setting a flag indicating use of a template-based intra mode derivation method to be true; and setting a flag indicating use of CCLM according to whether the second mode is included in cross-component linear model (CCLM) modes.

As described above, according to the present embodiment, a template-based intra mode derivation (TIMD) method of luma components is extended to intra prediction of chroma components, but complexity is reduced. TIMD for chroma components By providing a video coding method and apparatus using the method, it is possible to improve video coding efficiency and improve video quality.

1 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of this disclosure.
2 is a diagram for explaining a method of dividing a block using a QTBTTT structure.
3A and 3B are diagrams illustrating a plurality of intra prediction modes including wide-angle intra prediction modes.
4 is an exemplary diagram of neighboring blocks of a current block.
5 is an exemplary block diagram of a video decoding apparatus capable of implementing the techniques of this disclosure.
6 is an exemplary view showing references to templates used to generate neighboring templates of a current block used in the TIMD method and prediction templates.
7 is a flowchart illustrating a TIMD method for a chroma component performed by an image encoding apparatus according to an embodiment of the present disclosure.
8 is a flowchart illustrating a TIMD method for a chroma component performed by an image decoding apparatus according to an embodiment of the present disclosure.
9 is an exemplary diagram illustrating a current chroma block and its neighboring blocks.

DETAILED DESCRIPTION Some embodiments of the present disclosure are described in detail below with reference to exemplary drawings. In adding reference numerals to components of each drawing, it should be noted that the same components have the same numerals as much as possible even if they are displayed on different drawings. In addition, 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 exemplary block diagram of an image encoding apparatus capable of implementing the techniques of this disclosure. Hereinafter, with reference to the illustration of FIG. 1, an image encoding device and sub-components of the device will be described.

The image encoding apparatus 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. 160, an inverse transform unit 165, an adder 170, a loop filter unit 180, and a memory 190.

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

One image (video) is composed of one or more sequences including a plurality of pictures. Each picture is divided into a plurality of areas and encoding is performed for each area. For example, one picture is divided into one or more tiles and/or slices. Here, one or more tiles may 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 coded as a CU syntax, and information commonly applied to CUs included in one CTU is coded as a CTU syntax. In addition, information commonly applied to all blocks in one slice is coded as syntax of a slice header, and information applied to all blocks constituting one or more pictures is a picture parameter set (PPS) or picture coded in the header. Furthermore, information commonly referred to by a plurality of pictures is coded into a Sequence Parameter Set (SPS). Also, information commonly referred to by one or more SPSs is coded into a video parameter set (VPS). Also, information commonly applied to one tile or tile group may be encoded as 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 divider 110 determines the size of a coding tree unit (CTU). Information on the size of the CTU (CTU size) is encoded as SPS or PPS syntax and transmitted to the video decoding apparatus.

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

As a tree structure, a quad tree (QT) in which a parent node (or parent node) is divided into four subnodes (or child nodes) of the same size, or a binary tree (BinaryTree) in which a parent node is divided into two subnodes , BT), or a TernaryTree (TT) in which a parent node is split into three subnodes at a ratio of 1:2:1, or a structure in which two or more of these QT structures, BT structures, and TT structures are mixed. 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 to be referred to as MTT (Multiple-Type Tree).

2 is a diagram for explaining a method of dividing a block using a QTBTTT structure.

As shown in FIG. 2, the CTU may first be divided into QT structures. Quadtree splitting can be repeated until the size of the splitting block reaches the minimum block size (MinQTSize) of leaf nodes allowed by QT. A first flag (QT_split_flag) indicating whether each node of the QT structure is split into four nodes of a 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 a BT structure or a TT structure. A plurality of division directions may exist in the BT structure and/or the TT structure. For example, there may be two directions in which blocks of the corresponding node are divided horizontally and vertically. As shown in FIG. 2, when MTT splitting starts, a second flag (mtt_split_flag) indicating whether nodes are split, and if split, a flag indicating additional split direction (vertical or horizontal) and/or split type (Binary or Ternary) is encoded by the entropy encoding unit 155 and signaled to the video decoding apparatus.

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

As another example of a tree structure, when QTBT is used, the block of the corresponding node is divided into two blocks of the same size horizontally (i.e., symmetric horizontal splitting) and the type that splits vertically (i.e., symmetric vertical splitting). Branches may exist. A split flag (split_flag) indicating whether each node of the BT structure is split into blocks of a lower layer and split type information indicating a split type are encoded by the entropy encoder 155 and transmitted to the video decoding device. Meanwhile, a type in which a block of a corresponding node is divided into two blocks having an asymmetric shape may additionally exist. The asymmetric form may include a form in which the block of the corresponding node is divided into two rectangular blocks having a size ratio of 1:3, or a form in which the block of the corresponding node is divided in a diagonal direction may be included.

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

The prediction unit 120 predicts a 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. In general, prediction of a current block uses an intra-prediction technique (using data from a picture containing the current block) or an inter-prediction technique (using data from a picture coded before the picture containing the current block). can be performed Inter prediction includes both uni-prediction and bi-prediction.

The intra predictor 122 predicts pixels in the current block using pixels (reference pixels) located around the current block in the current picture including the current block. A plurality of intra prediction modes exist according to 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. Depending on each prediction mode, the neighboring pixels to be used and the arithmetic expression are defined differently.

For efficient directional prediction of the rectangular current block, directional modes (numbers 67 to 80 and -1 to -14 intra prediction modes) indicated by dotted arrows in FIG. 3B may be additionally used. These may be referred to as “wide angle intra-prediction modes”. In FIG. 3B , arrows indicate corresponding reference samples used for prediction and do not indicate prediction directions. The prediction direction is opposite to the direction the arrow is pointing. Wide-angle intra prediction modes are modes that perform prediction in the opposite direction of a specific directional mode without additional bit transmission when the current block is rectangular. At this time, among the wide-angle intra prediction modes, some wide-angle intra prediction modes usable for the current block may be determined by the ratio of the width and height of the rectangular current block. For example, wide-angle intra prediction modes (67 to 80 intra prediction modes) having an angle smaller than 45 degrees are usable when the current block has a rectangular shape with a height smaller than a width, and a wide angle having an angle greater than -135 degrees. Intra prediction modes (-1 to -14 intra prediction modes) are available when the current block has a rectangular shape where the width is greater than the height.

The intra prediction unit 122 may determine an intra prediction mode to be used for encoding the current block. In some examples, the intra prediction unit 122 may encode the current block using several intra prediction modes and select an appropriate intra prediction mode to be used from the tested modes. For example, the intra predictor 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. Intra prediction mode can also be selected.

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

The inter prediction unit 124 generates a prediction block for a current block using a motion compensation process. The inter-prediction unit 124 searches for a block most similar to the current block in the encoded and decoded reference picture prior to the current picture, and generates a prediction block for the current block using the searched block. Then, a motion vector (MV) corresponding to displacement between the current block in the current picture and the prediction block in the reference picture is generated. In general, motion estimation is performed on a 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 reference picture information and motion vector information used to predict the current block is encoded by the entropy encoding unit 155 and transmitted to the video decoding apparatus.

The inter-prediction unit 124 may perform interpolation on a reference picture or reference block in order 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. When a process of searching for a block most similar to the current block is performed for the interpolated reference picture, the motion vector can be expressed with precision of decimal units instead of integer sample units. The precision or resolution of the motion vector may be set differently for each unit of a target region to be encoded, for example, a slice, tile, CTU, or CU. When such adaptive motion vector resolution (AMVR) is applied, information on motion vector resolution to be applied to each target region must be signaled for each target region. For example, when the target region is a CU, information on motion vector resolution applied to each CU is signaled. Information on the motion vector resolution may be information indicating the precision of differential motion vectors, which will be described later.

Meanwhile, the inter prediction unit 124 may perform inter prediction using bi-prediction. In the case of bi-directional prediction, two reference pictures and two motion vectors representing positions of blocks most similar to the current block within each reference picture are used. The inter prediction unit 124 selects a first reference picture and a 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. A first reference block and a second reference block are generated. Then, a prediction block for the current block is generated by averaging or weighted averaging the first reference block and the second reference block. Further, motion information including information on two reference pictures used to predict the current block and information on two motion vectors is delivered to the encoder 150. Here, reference picture list 0 may include pictures prior to the current picture in display order among restored pictures, and reference picture list 1 may include pictures after the current picture in display order among restored pictures. there is. However, it is not necessarily limited to this, and in order of display, ups and downs pictures subsequent to the current picture may be additionally included in reference picture list 0, and conversely, ups and downs pictures prior to the current picture may be additionally included in reference picture list 1. may also be included.

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

For example, when the reference picture and motion vector of the current block are the same as those of the neighboring block, the motion information of the current block can be delivered to the video decoding apparatus by encoding information capable of identifying 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.

Neighboring blocks for deriving merge candidates include a left block (A0), a lower left block (A1), an upper block (B0), and an upper right block (B1) adjacent to the current block in the current picture, as shown in FIG. ), and all or part of the upper left block A2 may be used. Also, a block located in a reference picture (which may be the same as or different from a reference picture used to predict the current block) other than the current picture in which the current block is located may be used as a merge candidate. For example, a block co-located with the current block in the reference picture or blocks adjacent to the co-located block may be additionally used as a merge candidate. If the number of merge candidates selected by the method described above is less than the preset number, a 0 vector is added to the merge candidates.

The inter prediction unit 124 constructs a merge list including a predetermined number of merge candidates using these neighboring blocks. Among the merge candidates included in the merge list, a merge candidate to be used as motion information of the current block is selected, and merge index information for identifying the selected candidate is generated. The generated merge index information is encoded by the encoder 150 and transmitted to the video decoding apparatus.

Merge skip mode is a special case of merge mode. After performing quantization, when all transform coefficients for entropy encoding are close to zero, only neighboring block selection information is transmitted without transmitting residual signals. By using the merge skip mode, it is possible to achieve a relatively high encoding efficiency in low-motion images, still images, screen content images, and the like.

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

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

In the AMVP mode, the inter prediction unit 124 derives predictive motion vector candidates for the motion vector of the current block using neighboring blocks of the current block. Neighboring blocks used to derive predictive motion vector candidates include a left block A0, a lower left block A1, an upper block B0, and an upper right block adjacent to the current block in the current picture shown in FIG. B1), and all or part of the upper left block (A2) may be used. In addition, a block located in a reference picture (which may be the same as or different from the reference picture used to predict the current block) other than the current picture where the current block is located will be used as a neighboring block used to derive motion vector candidates. may be For example, a collocated block co-located with the current block within the reference picture or blocks adjacent to the collocated block may be used. If the number of motion vector candidates is smaller than the preset number according to the method described above, a 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, a differential motion vector is calculated by subtracting the predicted motion vector from the motion vector of the current block.

The predicted motion vector may be obtained by applying a predefined function (eg, median value, average value operation, etc.) to predicted motion vector candidates. In this case, the video decoding apparatus also knows the predefined function. In addition, since a neighboring block used to derive a predicted motion vector candidate is a block that has already been encoded and decoded, the video decoding apparatus also knows the motion vector of the neighboring block. Therefore, the video encoding apparatus does not need to encode information for identifying a predictive motion vector candidate. Therefore, in this case, information on differential motion vectors and information on reference pictures 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, along with information on differential motion vectors and information on reference pictures used to predict the current block, information for identifying the selected predictive motion vector candidate is additionally encoded.

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

The transform unit 140 transforms the residual signal in the residual block having pixel values in the spatial domain into transform coefficients in the frequency domain. The transform unit 140 may transform residual signals in the residual block by using the entire size of the residual block as a transform unit, or divide the residual block into a plurality of subblocks and use the subblocks as a transform unit to perform transformation. You may. Alternatively, the residual signals may be divided into two subblocks, a transform region and a non-transform region, and transform the residual signals using only the transform region subblock as a transform unit. Here, the transformation region subblock may be one of two rectangular blocks having a size ratio of 1:1 based on a horizontal axis (or a vertical axis). In this case, a flag (cu_sbt_flag) indicating that only subblocks have been transformed, directional (vertical/horizontal) information (cu_sbt_horizontal_flag), and/or location 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 region 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) for distinguishing the corresponding division is additionally encoded by the entropy encoder 155 to obtain an image It is signaled to the decryption device.

Meanwhile, the transform unit 140 may individually transform the residual block in the horizontal direction and the vertical direction. For the transformation, various types of transformation functions or transformation matrices may be used. For example, a pair of transformation functions for horizontal transformation and vertical transformation may be defined as a multiple transform set (MTS). The transform unit 140 may select one transform function pair having the highest transform efficiency among the MTS and transform the residual blocks in the horizontal and vertical directions, respectively. Information (mts_idx) on a pair of transform functions selected from the MTS is encoded by the entropy encoding unit 155 and signaled to the video decoding device.

The quantization unit 145 quantizes 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 related residual block without transformation for a certain block or frame. The quantization unit 145 may apply different quantization coefficients (scaling values) according to positions of transform coefficients in the transform block. A quantization matrix applied to the two-dimensionally arranged quantized transform coefficients may be coded and signaled to the video decoding apparatus.

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

The reordering unit 150 may change a 2D coefficient array into a 1D coefficient sequence using coefficient scanning. For example, the reordering unit 150 may output a one-dimensional coefficient sequence by scanning DC coefficients to coefficients in a high frequency region using a zig-zag scan or a diagonal scan. . Depending on the size of the transformation unit and intra prediction mode, instead of zig-zag scan, vertical scan that scans a 2D coefficient array in a column direction and horizontal scan that scans 2D block-shaped coefficients in a row direction may be used. That is, a scan method to be used among zig-zag scan, diagonal scan, vertical scan, and horizontal scan may be determined according to the size of the transform unit and the intra prediction mode.

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

In addition, the entropy encoding unit 155 encodes information such as CTU size, CU splitting flag, QT splitting flag, MTT splitting type, and MTT splitting direction related to block splitting so that the video decoding apparatus can divide the block in the same way as the video encoding apparatus. make it possible to divide In addition, the entropy encoding unit 155 encodes information about a prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and encodes intra prediction information (ie, intra prediction) according to the prediction type. mode) or inter prediction information (motion information encoding mode (merge mode or AMVP mode), merge index in case of merge mode, reference picture index and differential motion vector information in case of AMVP mode) are encoded. Also, the entropy encoding unit 155 encodes information related to quantization, that is, information about quantization parameters and information about quantization matrices.

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 transforms transform coefficients output from the inverse quantization unit 160 from a frequency domain to a spatial domain to restore a residual block.

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

The loop filter unit 180 reconstructs pixels in order to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. caused by block-based prediction and transformation/quantization. perform filtering on 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 boundary between reconstructed blocks to remove blocking artifacts caused by block-by-block 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 the alf 186 are filters used to compensate for a difference between a reconstructed pixel and an original pixel caused by lossy coding. The SAO filter 184 improves not only subjective picture quality but also coding efficiency by applying an offset in units of CTUs. In contrast, the ALF 186 performs block-by-block filtering. Distortion is compensated for by applying different filters by distinguishing the edge of the corresponding block and the degree of change. Information on filter coefficients to be used for ALF may be coded and signaled to the video decoding apparatus.

The reconstruction block filtered through the deblocking filter 182, the SAO filter 184, and the 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.

5 is an exemplary block diagram of a video decoding apparatus capable of implementing the techniques of this disclosure. Hereinafter, referring to FIG. 5, a video decoding device and sub-elements of the device will be described.

The image decoding apparatus 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) may be configured.

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

The entropy decoding unit 510 determines a current block to be decoded by extracting information related to block division by decoding the bitstream generated by the video encoding apparatus, and provides prediction information and residual signals necessary for restoring the current block. extract information, etc.

The entropy decoding unit 510 determines the size of the CTU by extracting information about the CTU size from a sequence parameter set (SPS) or a picture parameter set (PPS), and divides the picture into CTUs of the determined size. Then, the CTU is divided using the tree structure by determining the CTU as the top layer of the tree structure, that is, the root node, and extracting division information for the CTU.

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

As another example, when a CTU is split using the QTBTTT structure, a CU split flag (split_cu_flag) indicating whether the CU is split is first extracted, and when the corresponding block is split, a first flag (QT_split_flag) is extracted. may be During the splitting process, each node may have zero or more iterative MTT splits after zero or more repetitive QT splits. For example, the CTU may immediately undergo MTT splitting, or conversely, only QT splitting may occur multiple times.

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

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

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

The reordering unit 515 converts the sequence of 1-dimensional quantized transform coefficients entropy-decoded in the entropy decoding unit 510 into a 2-dimensional coefficient array (ie, in the reverse order of the coefficient scanning performed by the image encoding apparatus). block) can be changed.

The inverse quantization unit 520 inverse quantizes the quantized transform coefficients and inverse quantizes the quantized transform coefficients using a quantization parameter. The inverse quantization unit 520 may apply different quantization coefficients (scaling values) to the two-dimensionally arranged quantized transform coefficients. The inverse quantization unit 520 may perform inverse quantization by applying a matrix of quantization coefficients (scaling values) from the image encoding device to a 2D array of quantized transformation coefficients.

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

In addition, when the inverse transform unit 530 inverse transforms only a partial region (subblock) of a transform block, a flag (cu_sbt_flag) indicating that only a subblock of the transform block has been transformed, and direction information (vertical/horizontal) information (cu_sbt_horizontal_flag) of the transform block ) and/or the location information (cu_sbt_pos_flag) of the subblock, and inversely transforms the transform coefficients of the corresponding subblock from the frequency domain to the spatial domain to restore the residual signals. By filling , the final residual block for the current block is created.

In addition, when the MTS is applied, the inverse transform unit 530 determines transform functions or transform matrices to be applied in the horizontal and vertical directions, respectively, using MTS information (mts_idx) signaled from the video encoding device, and uses the determined transform functions. Inverse transform is performed on the transform coefficients in the transform 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 element for the intra prediction mode extracted from the entropy decoding unit 510, and references the current block according to the intra prediction mode. The current block is predicted using pixels.

The inter prediction unit 544 determines the motion vector of the current block and the reference picture referred to by the motion vector by using the syntax element for the inter prediction mode extracted from the entropy decoding unit 510, and converts the motion vector and the reference picture. 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 reconstructed 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, an SAO filter 564, and an ALF 566 as in-loop filters. The deblocking filter 562 performs deblocking filtering on boundaries between reconstructed blocks in order to remove blocking artifacts generated by block-by-block 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. ALF filter coefficients are determined using information on filter coefficients decoded from the non-stream.

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

This embodiment relates to encoding and decoding of images (video) as described above. More specifically, a template-based intra mode derivation (TIMD) method of luma components is extended to intra prediction of chroma components, but a video coding method using a TIMD method for chroma components with reduced complexity, and provide the device.

The following embodiments may be performed by the intra prediction unit 122 in a video encoding device. Also, it may be performed by the intra prediction unit 542 in the video decoding device.

The video encoding apparatus may generate signaling information related to the present embodiment in terms of bit rate distortion optimization in encoding of the current block. The image encoding device may encode the image using the entropy encoding unit 155 and transmit it to the image decoding device. The video decoding apparatus may decode signaling information related to decoding of the current block from the bitstream using the entropy decoding unit 510 .

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

Also, a value of one flag being true indicates a case in which the flag is set to 1. In addition, a false value of one flag indicates a case in which the flag is set to 0.

I. Intra prediction of chroma components

Several techniques are introduced to improve coding efficiency using intra prediction.

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

Meanwhile, according to the prediction direction used by the luma block, the chroma block may also use the intra prediction of the subdivided directional mode in a limited manner. However, in intra prediction of a chroma block, various directional modes other than horizontal and vertical directions that can be used by a luma block cannot always be used. In order to be able to use these various directional modes, the prediction mode of the current chroma block must be set to Direct Mode (DM). By setting the DM mode in this way, the current chroma block can use a directionality mode other than the horizontal and vertical luma blocks.

When encoding a chroma block, intra prediction modes that are used most frequently or that are most basically used to maintain image quality include planar, DC, vertical, horizontal, and DM. At this time, in the DM, the intra prediction mode of the luma block spatially corresponding to the current chroma block is used as the intra prediction mode of the chroma block.

The video encoding apparatus may signal whether the intra prediction mode of the chroma block is DM to the video decoding apparatus. At this time, there may be several methods for transferring the DM to the video decoding device. For example, the video encoding apparatus may indicate whether it is a DM by setting intra_chroma_pred_mode, which is information for indicating the intra prediction mode of a chroma block, to a specific value and then transmitting the data to the video decoding apparatus.

When the chroma block is encoded in the intra prediction mode, the video encoding apparatus determines the intra prediction mode of the chroma block according to Table 1. IntraPredModeC can be set.

Hereinafter, in order to distinguish intra_chroma_pred_mode and IntraPredModeC, which are information related to the intra prediction mode of a chroma block, they are expressed as a chroma intra prediction mode indicator and a chroma intra prediction mode, respectively.

Here, lumaIntraPredMode is an intra prediction mode (hereinafter referred to as 'luma intra prediction mode') of a luma block corresponding to the current chroma block. lumaIntraPredMode represents one of the prediction modes illustrated in FIG. 3A. For example, in Table 1, lumaIntraPredMode = 0 indicates Planar prediction mode, and lumaIntraPredMode = 1 indicates DC prediction mode. Cases in which lumaIntraPredMode is 18, 50, and 66 represent directional modes referred to as horizontal, vertical, and VDIA, respectively. Meanwhile, intra_chroma_pred_mode = 0, 1, 2, and 3 indicate planar, vertical, horizontal, and DC prediction modes, respectively. DM is when intra_chroma_pred_mode = 4, and the IntraPredModeC value, which is a chroma intra prediction mode, is set equal to the value of lumaIntraPredMode.

Meanwhile, a process of parsing the intra prediction mode of the chroma block, performed by the video decoding apparatus, is shown in Table 2.

The video decoding apparatus parses cclm_mode_flag indicating whether to use a cross-component linear model (CCLM) mode. If cclm_mode_flag is set to 1 and the CCLM mode is used, the video decoding apparatus parses cclm_mode_idx indicating the CCLM mode. At this time, one of three modes may be indicated as the CCLM mode according to the value of cclm_mode_idx. On the other hand, when cclm_mode_flag is 0 and the CCLM mode is not used, the video decoding apparatus parses intra_chroma_pred_mode indicating the intra prediction mode as described above.

When the CCLM mode is applied for intra prediction of the current chroma block, the image decoding apparatus determines an area (hereinafter referred to as 'corresponding luma area') in a luma image corresponding to the current chroma block. For prediction of the linear model, left reference pixels and upper reference pixels of the corresponding luma area and left reference pixels and upper reference pixels of the target chroma block may be used. Hereinafter, left reference pixels and top reference pixels are combined to obtain reference pixels and neighboring pixels. Or expressed in adjacent pixels. In addition, reference pixels of a chroma channel are represented as chroma reference pixels, and reference pixels of a luma channel are represented as luma reference pixels.

In CCLM prediction, a predictor of a target chroma block is obtained by deriving a linear model between reference pixels of a corresponding luma region and reference pixels of a chroma block, and then applying the corresponding linear model to reconstructed pixels of the corresponding luma region. A prediction block is generated. For example, four pairs of pixels obtained by combining pixels in neighboring pixel lines of the current chroma block and pixels in a corresponding luma region may be used to derive a linear model. The image decoding apparatus may derive α and β representing a linear model as shown in Equation 1 for four pairs of pixels.

Here, for the corresponding luma pixels among the four pairs of pixels, X _a and X _b respectively represent an average value of two minimum values and an average value of two maximum values. Also, for chroma pixels, Y _a and Y _b indicate an average value of two minimum values and an average value of two maximum values, respectively. Thereafter, as shown in Equation 2, the video decoding apparatus generates a predictor pred _C (i, j) of the current chroma block from the pixel value rec' _L (i, j) of the corresponding luma region using a linear model. can do.

Meanwhile, as described above, the CCLM mode is classified into three modes, CCLM_LT, CCLM_L, and CCLM_T, according to the positions of neighboring pixels used in the linear model derivation process. The CCLM_LT mode uses two pixels in each direction among neighboring pixels adjacent to the left and top of the current chroma block. The CCLM_L mode uses 4 pixels among neighboring pixels adjacent to the left side of the current chroma block. Finally, the CCLM_T mode uses 4 pixels among neighboring pixels adjacent to the top of the current chroma block.

The most probable mode (MPM) technique uses intra prediction modes of neighboring blocks when intra prediction of a current block is performed. The video encoding apparatus generates an MPM list to include intra prediction modes derived from predefined positions spatially adjacent to a current block. When the MPM mode is applied, the video encoding apparatus may transmit intra_luma_mpm_flag, which is a flag indicating whether to use the MPM list, to the video decoding apparatus. If intra_luma_mpm_flag does not exist, it is inferred as 1. In addition, the video encoding apparatus may improve encoding efficiency of the intra prediction mode by transmitting an MPM index, intra_luma_mpm_idx, instead of the index of the prediction mode.

II. Embodiments According to the Invention

This embodiment proposes a method of reducing complexity while extending a template-based intra mode derivation (TIMD) method of luma components to intra prediction of chroma components. Hereinafter, according to convenience, a video encoding device or an image decoding device will be mainly described. However, the following information may be similarly applied to an image encoding device and an image decoding device.

When the existing TIMD method for chroma components is applied, the video encoding apparatus determines the candidate mode list of the chroma component by using the method of generating the candidate mode list of the existing chroma component, and for each candidate mode in the candidate mode list of the determined chroma component. TIMD method is applied to determine the optimal mode. That is, the video encoding apparatus generates a prediction template by applying the directionality of the corresponding mode or the intra prediction method of the corresponding mode to reference samples of the template for each candidate mode in the candidate mode list of the chroma component. Then, the image encoding apparatus calculates the sum of absolute transformed differences (SATD) between the samples of the generated prediction template and the samples of the already reconstructed template, and the sum of the calculated absolute transform differences is the smallest. The mode is determined as an intra mode of the current chroma component block selected according to the TIMD method for chroma components. Here, SATD is a method of calculating SAD (Sum of Absolute Differences) after converting samples to be compared into samples in the frequency domain.

Here, the chroma component represents a Cb component or a Cr component. Hereinafter, 'candidate mode lists of chroma components', 'chroma candidate mode lists' and 'candidate mode lists' may be used interchangeably.

When the video encoding apparatus determines the intra mode of the current chroma component block using the TIMD method, the video decoding apparatus performs the same process as the video encoding apparatus. That is, the video decoding apparatus constructs a candidate mode list of chroma components and determines an optimal mode by applying the TIMD method to all candidate modes in the candidate mode list of the generated chroma component. Accordingly, the complexity of the image decoding apparatus may increase.

<Example 1> TIMD method for chroma component considering complexity

Embodiment 1 proposes a method of reducing the complexity of an image decoding apparatus using a TIMD method for a chroma component by proposing a syntax structure as shown in Table 3.

As shown in Table 3, in this embodiment, sps_chroma_timd_flag, which is a syntax indicating whether the TIMD method for chroma components is used, is specified to be transmitted/parsed at the Sequence Parameter Set (SPS) level, but is not necessarily limited thereto. Syntax indicating whether or not the TIMD method is used for chroma components can be transmitted/parsed at an arbitrary level such as a slice header, a tile, a picture parameter set (PPS), a picture header, and the like.

As shown in Table 3, when sps_chroma_timd_flag is true, that is, when the TIMD method for the chroma component is used when encoding/decoding the current chroma component, chroma_timd_flag, a syntax indicating whether the current chroma component block uses the TIMD method, sent/parsed. When chroma_timd_flag is false, that is, when the current chroma component block does not use the TIMD method, the mode of the current chroma component block is determined using the intra mode transmission/parsing method of the existing chroma component. On the other hand, if chroma_timd_flag is true, that is, if the current chroma component block uses the TIMD method, cclm_flag, which is a syntax indicating whether to use a cross-component linear model (CCLM), is transmitted/parsed.

If the current chroma component block uses CCLM, three CCLM modes, i.e. The mode for the current chroma block is derived by applying the TIMD method to the LT_CCLM mode using the top and left neighboring samples, the T_CCLM mode using the top neighboring samples, and the L_CCLM mode using the left neighboring samples. When the current chroma block does not use CCLM, the mode for the current chroma component block is derived by applying the TIMD method to the candidate modes in the candidate mode list generated based on the directional mode.

As shown in Table 3, when sps_chroma_timd_flag is false, the mode of the current chroma component block is determined using the intra mode transmission/parsing method of the existing chroma component.

In the method according to Table 3, if the current chroma component block uses the TIMD method, the candidate modes to which the TIMD method is applied are divided into two candidate mode groups (normal direction mode group and CCLM mode group) using cclm_flag indicating whether CCLM is used or not. Separated. Then, the TIMD method is applied by selecting one of the two candidate mode groups. That is, the TIMD method for the existing chroma component determines an optimal mode by applying the TIMD method to all chroma candidate modes in the generated candidate mode list when the current chroma component block uses the TIMD method. However, the method according to the present embodiment divides the candidate modes in the generated candidate mode list into two candidate mode groups, selects one candidate mode group based on whether CCLM is used, and then assigns a candidate mode in the selected candidate mode group. The TIMD method is applied to Therefore, since the method according to the present embodiment applies the TIMD method to a smaller number of candidate modes than the conventional method, the complexity of the video decoding apparatus can be reduced.

Hereinafter, the TIMD method for chroma components will be described using the illustrations of FIGS. 7 and 8 .

7 is a flowchart illustrating a TIMD method for a chroma component performed by an image encoding apparatus according to an embodiment of the present disclosure.

The image encoding apparatus generates a candidate mode list of chroma components for the current chroma component block (S700). The image encoding apparatus may generate a candidate mode list of chroma components according to Embodiment 2 to be described later.

The image encoding apparatus determines an optimal first mode by performing intra prediction on the current chroma component block (S702). The video encoding apparatus may determine the first mode in terms of bit rate distortion optimization.

The video encoding apparatus determines an optimal second mode by applying the TIMD method to the modes in the candidate mode list (S704). The video encoding apparatus may determine the second mode in terms of bit rate distortion optimization.

The video encoding apparatus checks whether the first mode is optimal for the second mode (S706).

When the first mode is optimal, the video encoding apparatus determines the first mode as a mode for the current chroma component block (S708) and sets the aforementioned chroma_timd_flag to false (S710) to obtain the TIMD method for the current chroma component block. Indicates whether or not to use Thereafter, the video encoding apparatus encodes intra mode information and chroma_timd_flag related to the first mode.

When the second mode is optimal, the video encoding apparatus determines the second mode as a mode for the current chroma component block (S720), sets the aforementioned chroma_timd_flag to true (S722), and calculates the TIMD method for the current chroma component block. Indicates whether or not to use

The video encoding device checks whether the second mode is included in the CCLM modes (S724). Here, the CCLM modes include LT_CCLM mode using upper and left peripheral samples, T_CCLM mode using upper peripheral samples, and L_CCLM mode using left peripheral samples.

When the second mode is included in the CCLM modes, the video encoding apparatus sets the aforementioned cclm_flag to true (S726) to indicate the use of CCLM for the current chroma component block.

When the second mode is included in directional modes other than CCLM modes, the video encoding apparatus sets the aforementioned cclm_flag to false (S728) to indicate use of the directional mode for the current chroma component block.

After that, the video encoding device encodes chroma_timd_flag and cclm_flag.

8 is a flowchart illustrating a TIMD method for a chroma component performed by an image decoding apparatus according to an embodiment of the present disclosure.

The image decoding apparatus generates a candidate mode list of chroma components for the current chroma component block (S800). The video decoding apparatus may generate a candidate mode list of chroma components according to Embodiment 2 to be described later.

The video decoding apparatus checks whether the TIMD method is used for the current chroma component block (S802). For example, the video decoding apparatus may check whether the TIMD method is used for the current chroma component block by parsing the aforementioned chroma_timd_flag from the bitstream.

When the current chroma component block does not use the TIMD method, the video decoding apparatus parses the intra mode information of the chroma component from the bitstream and uses the parsed intra mode information to obtain the current chroma component block as in the conventional method. Determines the mode for (S804). For example, the intra mode information may be an index indicating an intra mode of the current chroma component block.

If the current chroma component block uses the TIMD method, the video decoding apparatus checks whether CCLM is used (S806). For example, the video decoding apparatus may check whether CCLM is used for the current chroma component block by parsing the aforementioned cclm_flag from the bitstream.

When CCLM is used, the video decoding apparatus determines an optimal mode for the current chroma component block by applying the TIMD method to the CCLM modes in the candidate mode list (S808). Here, the CCLM modes include LT_CCLM mode using upper and left peripheral samples, T_CCLM mode using upper peripheral samples, and L_CCLM mode using left peripheral samples. On the other hand, when CCLM is not used, the optimal mode for the current chroma component block is determined by applying the TIMD method to the directional modes in the candidate mode list (S810).

According to steps S804, S808, and S810, the video decoding apparatus may determine an intra mode of the current chroma component block.

In conclusion, Embodiment 1 proposes a method of reducing the complexity of the video decoding apparatus by limiting the number of candidate modes to which the TIMD method for chroma components is applied in the video decoding apparatus using cclm_flag.

<Example 2> Construction of candidate list of TIMD method for chroma component

A candidate mode list for an intra mode of an existing chroma component block is constructed using direct mode (DM), default mode (planar, DC, vertical, horizontal), and CCLM mode. In this case, the direct mode is an intra mode of a luma block at the same location as the current chroma component block.

When using the method of constructing a candidate mode list for an intra mode of an existing chroma component block, candidate modes in the candidate mode list cannot be efficiently constructed because a default mode is simply used without considering correlation with neighboring modes. . In addition, since the TIMD method for a chroma component derives an optimal mode using the candidate mode list configured in this way, encoding efficiency may be limited. Therefore, Embodiment 2 proposes a method of efficiently constructing a candidate mode list of chroma components for conventional chroma component coding in addition to TIMD for chroma components.

<Example 2-1> Constructing a chroma candidate mode list using a luma component MPM list construction method

This embodiment proposes a method of constructing a chroma candidate mode list using the same method as the method of constructing an MPM list of a luma component. The video decoding apparatus first adds the direct mode and the CCLM mode to the chroma candidate mode list in consideration of the characteristics of the chroma component block, and then configures the chroma candidate mode list by applying the MPM list construction method of the luma component to the directional mode. .

9 is an exemplary diagram illustrating a current chroma block and its neighboring blocks.

In the example of FIG. 9 , upper blocks (AL, A1, A2, A3, A4, and A5) and left blocks (L1, L2, L3, L4, and L5) of the current block are the minimum unit including intra mode information. represents blocks. In the same way as the method of constructing the MPM list of the luma component, the video decoding apparatus constructs a chroma candidate mode list using intra mode information of the upper right A4 block and intra mode information of the lower left L4 block. When constructing the chroma candidate mode list, a redundancy check is performed in the same way as the method of constructing the MPM list of the luma component, and then modes that do not overlap may be added to the chroma candidate mode list. In this case, the size of the chroma candidate mode list, that is, the number of candidates included in the chroma candidate mode list may be arbitrarily determined. When the size of the chroma candidate mode list is large, coding efficiency increases but complexity also increases. Conversely, when the size of the chroma candidate mode list is small, complexity decreases but coding efficiency also decreases. Accordingly, the size of an appropriate chroma candidate mode list may be arbitrarily determined in consideration of encoding efficiency and complexity.

<Example 2-2> Constructing a chroma candidate mode list based on neighboring blocks

This embodiment proposes a method of constructing a chroma candidate mode list using intra mode information of neighboring blocks around a current chroma block. The video decoding apparatus first adds the direct mode and the CCLM mode to the chroma candidate mode list in consideration of the characteristics of the chroma component block, and then configures the chroma candidate mode list using intra mode information of neighboring blocks of the current chroma component block. do.

As in the example of FIG. 9 , the video decoding apparatus searches neighboring blocks of a current block according to a preset order and constructs a chroma candidate mode list using an intra mode of the searched block. In this case, the search order may be determined in an arbitrary order, and the size of the chroma candidate mode list may be arbitrarily determined. When adding an intra mode of a block to the chroma candidate mode list according to the search order, a redundancy check is performed, and then a non-redundant mode is added to the chroma candidate mode list. Also in this embodiment, the size of a suitable chroma candidate mode list may be arbitrarily determined in consideration of encoding efficiency and complexity.

When constructing the chroma candidate mode list using the method according to Example 2-1 and Example 2-2, if the chroma candidate mode list is not fully filled, the chroma candidate mode list is constructed using a preset default mode. can At this time, the default mode may be arbitrarily determined.

The two methods proposed in Embodiment 2, that is, the method of constructing a chroma candidate mode list using the method of constructing an MPM list of luma components and the method of constructing a chroma candidate mode list based on neighboring blocks, both construct a chroma component When constructing a candidate mode list for chroma, direct mode and CCLM mode are added to the beginning of the chroma candidate mode list. Thereafter, the remaining part of the chroma candidate mode list is configured by selecting an arbitrary method among the method of Example 2-1 and the method of Example 2-1.

The method for constructing the candidate mode list of chroma components according to the second embodiment may be selected and applied using any one of the following three methods. Here, the three methods include i) template-based intra mode derivation method for chroma components, ii) intra coding of existing chroma components, and iii) template-based intra mode derivation method for chroma components and intra coding of existing chroma components. . Here, the third method may be applied, for example, when chroma_timd_flag is used.

chroma_timd_flag

<Example 3> Combination of Example 1 and Example 2

Embodiment 1 proposes a method for reducing the complexity of an image decoding apparatus for a TIMD method for a chroma component. The complexity of the video decoding apparatus is reduced, but coding efficiency can be reduced at the same time. Embodiment 2 proposes a method for constructing a candidate mode list of chroma components to increase the coding efficiency of conventional chroma component coding in addition to the TIMD method for chroma components.

Since Embodiment 1 and Embodiment 2 may be complementary to each other, Embodiment 3 proposes a method of combining the method of Embodiment 1 and the method of Embodiment 2. Embodiment 3 uses the method of Embodiment 2 to configure a chroma candidate mode list for conventional chroma component encoding in addition to the TIMD method for chroma components. Then, Embodiment 3 uses a method for reducing the complexity of the video decoding apparatus according to the TIMD method for chroma components proposed in Example 1 based on the configured chroma candidate mode list, thereby increasing encoding efficiency and at the same time video decoding apparatus complexity can be reduced.

In the flow chart/timing diagram of the present specification, it is described that each process is sequentially executed, but this is merely an example of the technical idea of one embodiment of the present disclosure. In other words, those skilled in the art to which an embodiment of the present disclosure belongs may change and execute the order described in the flowchart/timing diagram within the range that does not deviate from the essential characteristics of the embodiment of the present disclosure, or one of each process Since the above process can be applied by performing various modifications and variations in parallel, the flow chart/timing chart is not limited to a time-series sequence.

In the above description, it should be understood that the exemplary embodiments may be implemented in many different ways. 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 in this specification have been labeled "...unit" to particularly emphasize their implementation independence.

Meanwhile, various functions or methods described in this embodiment may be implemented as 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 in which data is stored in a form readable by a computer system. For example, the non-transitory recording medium includes storage media such as an erasable programmable read only memory (EPROM), a flash drive, an optical drive, a magnetic hard drive, and a solid state drive (SSD).

The above description is merely an example of the technical idea of the present embodiment, and various modifications and variations can be made to those skilled in the art without departing from the essential characteristics of the present embodiment. Therefore, the present embodiments are not intended to limit the technical idea of the present embodiment, but to explain, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of this embodiment should be construed according to the claims below, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of rights of this embodiment.

122: intra prediction unit
155: entropy encoding unit
510: entropy decoding unit
542: intra prediction unit

Claims (14)

A method for deriving an intra mode of a current chroma component block, performed by an image decoding apparatus, comprising:
generating a candidate mode list of chroma components for the current chroma component block;
checking whether a template-based intra mode derivation method is used for the current chroma component block;
checking whether a cross-component linear model (CCLM) is used when the template-based intra mode derivation method is used;
determining an intra mode of the current chroma component block by applying the template-based intra mode derivation method to CCLM modes in the candidate mode list when the CCLM is used; and
When the CCLM is not used, determining an intra mode of the current chroma component block by applying the template-based intra mode derivation method to directional modes in the candidate mode list.
Characterized in that, the method comprising a. According to claim 1,
If the template-based intra mode derivation method is not used,
The method further comprising decoding intra mode information of the chroma component from a bitstream and determining an intra mode of the current chroma component block using the intra mode information. According to claim 1,
The template-based intra mode derivation method,
After generating a prediction template by applying the prediction method of each candidate mode among the candidate modes to the reference samples of the template, the sum of absolute transformation differences between the samples of the prediction template and the samples of the restored template (sum of absolute transformed differences) to determine a mode with the smallest sum of absolute transform differences as an intra mode of the current chroma component block, wherein the candidate modes are the CCLM modes or the directional modes. According to claim 1,
The step of checking whether the template-based intra mode derivation method is used,
Decoding a flag indicating whether to use the template-based intra mode derivation method from a bitstream, and checking the flag. According to claim 1,
The step of checking whether the CCLM is used,
A method characterized by decoding a flag indicating whether to use the CCLM from a bitstream and checking the flag. According to claim 1,
Generating the candidate mode list includes:
adding the direct mode and the CCLM modes to a candidate mode list of the chroma component; and
constructing a candidate mode list of the chroma component using intra mode information of an upper right block and intra mode information of a lower left block with respect to the current chroma component block;
Characterized in that, the method comprising a. According to claim 6,
Generating the candidate mode list includes:
Characterized in that by performing redundancy check, a non-redundant mode is added to a candidate mode list of the chroma component. According to claim 6,
Generating the candidate mode list includes:
characterized in that, when the candidate mode list of the chroma component is not filled, the candidate mode list of the chroma component is constructed using a preset default mode. According to claim 1,
Generating the candidate mode list includes:
adding the direct mode and the CCLM modes to a candidate mode list of the chroma component; and
Searching neighboring blocks of the current chroma component block according to a predetermined order and constructing a candidate mode list of the chroma component using intra mode information of the searched block.
Characterized in that, the method comprising a. A method for deriving an intra mode of a current chroma component block, performed by an image encoding apparatus, comprising:
generating a candidate mode list of chroma components for the current chroma component block;
determining an optimal first mode by performing intra prediction on the current chroma component block;
determining an optimal second mode by performing a template-based intra mode derivation method on modes in the candidate mode list; and
Checking whether the first mode is optimal for the second mode,
When the second mode is optimal,
determining the second mode as an intra mode for the current chroma component block and setting a flag indicating use of the template-based intra mode derivation method to be true; and
Setting a flag indicating use of CCLM according to whether the second mode is included in cross-component linear model (CCLM) modes.
Characterized in that, the method comprising a. According to claim 10,
The method further comprising encoding a flag indicating use of the template-based intra mode derivation method and a flag indicating use of the CCLM. According to claim 10,
If the first mode is optimal, determining the first mode as an intra mode for the current chroma component block and setting a flag indicating use of the template-based intra mode derivation method to false Characterized in that, the method. According to claim 12,
The method of claim 1, further comprising encoding intra mode information related to the first mode and a flag indicating use of the template-based intra mode derivation method. A computer-readable recording medium storing a bitstream generated by an image encoding method, the image encoding method comprising:
generating a candidate mode list of chroma components for the current chroma component block;
determining an optimal first mode by performing intra prediction on the current chroma component block;
determining an optimal second mode by performing a template-based intra mode derivation method on modes in the candidate mode list; and
Checking whether the first mode is optimal for the second mode,
When the second mode is optimal,
determining the second mode as an intra mode for the current chroma component block and setting a flag indicating use of the template-based intra mode derivation method to be true; and
Setting a flag indicating use of CCLM according to whether the second mode is included in cross-component linear model (CCLM) modes.
Characterized in that it comprises, a recording medium.

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