KR20230010804A - Video Encoding and Decoding Using Intra Prediction - Google Patents

Abstract

The present invention relates to a method for encoding information on an intra prediction mode of a chroma block. The method comprises the steps of: configuring a candidate set including a plurality of candidates for the intra prediction mode of the chroma block, wherein the plurality of candidates include at least one intra prediction mode induced from a luma block corresponding to the chroma block; and encoding chroma intra mode information for indicating the intra prediction mode of the chroma block among the plurality of candidates belonging to the candidate set. The present invention relates to a technique of selecting one or more candidates from intra prediction modes of at least some sub-blocks among a plurality of sub-blocks when the luma block is divided into the plurality of sub-blocks. The present invention is to provide an improved technique for determining and encoding the intra prediction mode of a current block in intra prediction coding.

Description

Video Encoding and Decoding Using Intra Prediction {Video Encoding and Decoding Using Intra Prediction}

The present invention relates to encoding or decoding of an image using intra prediction.

The contents described in this part merely provide background information on the present embodiment and do not constitute prior art.

A plurality of intra prediction modes exist for intra prediction and coding for predicting pixel values included in the current picture using pixel information in the current picture. The video encoding apparatus selects a final mode for the current block to be encoded among a plurality of intra prediction modes and transmits information about the corresponding mode to the video decoding apparatus. At this time, the most probable mode (MPM) is used to efficiently express the selected intra mode.

1 is a diagram illustrating intra modes usable for intra prediction in HEVC. In the case of HEVC, as shown in FIG. 11, there are a total of 35 intra modes including 33 angular modes with directionality and 2 modes without directionality.

In the case of a luma block composed of luminance components in a block to be coded, in order to encode the final intra mode, three MPMs for the block are used using the neighboring blocks of the block and statistically most frequently used modes. select

A 1-bit MPM flag indicating whether the final mode of the corresponding block is the same as MPM is transmitted, and if the final mode is MPM, an MPM index value is additionally transmitted. If the final mode is not MPM, which of the remaining modes is transmitted explicitly.

Meanwhile, in the case of a chroma block composed of chrominance components, intra prediction mode candidates composed of some of a total of 35 intra prediction modes are configured, and an intra prediction mode of the chrominance block is selected from among the candidates. Then, information for indicating a selected candidate among the candidates is signaled. These intra prediction mode candidates are composed of a planar mode, a vertical mode, a horizontal mode, a DC mode, and an intra prediction mode (DM, direct mode) of a luminance block corresponding to a chrominance block.

As the resolution of an image gradually increases from 64x64 to 256x256, etc., the unit of an encoding target block is gradually increasing, and accordingly, the possibility of adding many intra modes increases. Against this background, the present invention intends to provide an improved method for determining an intra prediction mode.

An object of the present invention is to provide an improved technique for determining and encoding an intra prediction mode of a current block in intra prediction coding.

One aspect of the present invention provides a method performed by a video decoding apparatus to intra-predict a chroma block divided from a chroma block having a block division structure different from that of a corresponding luminance coding tree block. do. The method includes decoding a flag indicating whether the chrominance block is predicted from the reconstructed pixel values in a corresponding luminance block from a bitstream; and determining an intra prediction mode of the chrominance block and predicting the chrominance block according to the flag, wherein when the flag indicates that the chrominance block is not predicted from the reconstructed pixel values in the luminance block, the The predicting of the chrominance block may include decoding mode information for selecting an intra prediction mode of the chrominance block from a candidate set of predefined intra prediction modes, the candidate set being derived from the luminance block. Including DM mode, which is an intra prediction mode; and determining an intra prediction mode of the chrominance block using the mode information.

Another aspect of the present invention provides a method performed by an image encoding apparatus to intra-predict a chroma block divided from a chroma block having a block division structure different from a corresponding luminance coding tree block. do. The method may further include determining an intra prediction mode of the chrominance block; predicting the chrominance block using an intra prediction mode of the chrominance block; and encoding information on an intra prediction mode of the chrominance block, wherein the encoding of the information on the intra prediction mode of the chrominance block comprises: predicting from reconstructed pixel values in a luminance block to which the chrominance block corresponds. Encoding a flag indicating whether or not it is; and a mode for selecting an intra prediction mode of the chrominance block from a candidate set of predefined intra prediction modes when the flag indicates that the chrominance block is not predicted from reconstructed pixel values in the luminance block. and encoding information, wherein the candidate set includes a DM mode, which is an intra prediction mode derived from the luminance block.

Another aspect of the present invention is to store a bitstream generated by an encoding method for intra-predicting a chroma block divided from a chroma block having a block division structure different from a corresponding luminance coding tree block, A recording medium readable by a decoder is provided. The encoding method may further include determining an intra prediction mode of the chrominance block; predicting the chrominance block using an intra prediction mode of the chrominance block; and encoding information on an intra prediction mode of the chrominance block, wherein the encoding of the information on the intra prediction mode of the chrominance block comprises: predicting from reconstructed pixel values in a luminance block to which the chrominance block corresponds. Encoding a flag indicating whether or not it is; and a mode for selecting an intra prediction mode of the chrominance block from a candidate set of predefined intra prediction modes when the flag indicates that the chrominance block is not predicted from reconstructed pixel values in the luminance block. and encoding information, wherein the candidate set includes a DM mode, which is an intra prediction mode derived from the luminance block.

1 is a diagram showing intra modes usable for intra prediction in HEVC;
2 is a block diagram of an image encoding apparatus according to an embodiment of the present invention;
3 is an example of block division using a QTBT structure;
4 is an exemplary view of a plurality of intra prediction modes according to an embodiment of the present invention;
5 is an exemplary view showing neighboring blocks of a current block;
6 is an exemplary view showing a block division structure for a luminance component and a chrominance component of a CTU;
7 is an exemplary view showing an intra prediction mode of each luminance block in a block division structure for a luminance component of a CTU;
8 is an exemplary diagram showing a z-scan sequence;
9 is an exemplary diagram for inducing a direct mode (DM) of the present invention;
10 is another exemplary diagram for inducing a DM of the present invention;
11 is another exemplary view for inducing a DM of the present invention;
12 is another exemplary view for inducing a DM of the present invention;
13 is another exemplary view for inducing a DM of the present invention;
14 is a block diagram of a video decoding apparatus according to an embodiment of the present invention.

Hereinafter, some embodiments of the present invention will be described in detail through exemplary drawings. It should be noted that in adding identification codes to components of each drawing, the same components have the same symbols as much as possible even if they are displayed on different drawings. In addition, in describing the present invention, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present invention, the detailed description will be omitted.

2 is a block diagram of an image encoding apparatus according to an embodiment of the present invention.

The image encoding apparatus includes a block division unit 210, a prediction unit 220, a subtractor 230, a transform unit 240, a quantization unit 245, an encoding unit 250, an inverse quantization unit 260, an inverse transform unit ( 265), an adder 270, a filter unit 280 and a memory 290. Each component of the image encoding apparatus may be implemented as a hardware chip or may be implemented as software and a microprocessor may execute software functions corresponding to each component.

The block divider 210 divides each picture constituting an image into a plurality of Coding Tree Units (CTUs), and then recursively divides the CTUs using a tree structure. 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 (QuadTree, QT) in which a parent node splits into four subnodes, or a QTBT (QuadTree plus BinaryTree) structure can be used.

In the QTBT (QuadTree plus BinaryTree) structure, the CTU is first partitioned into the QT structure. Then, the leaf nodes of QT can be further split by BT. Split information generated by the block division unit 210 by dividing the CTU by the QTBT structure is encoded by the encoder 250 and transmitted to the video decoding apparatus.

In QT, a first flag (QT split flag, QT_split_flag) indicating whether a block of a corresponding node is split is encoded. If the first flag is 1, the block of the corresponding node is divided into four blocks of the same size, and if 0, the corresponding node is not further divided by QT.

In BT, a second flag (BT split flag, BT_split_flag) indicating whether a block of a corresponding node is split is encoded. In BT, a plurality of partition types can exist. For example, there may be two types of horizontally dividing a block of a corresponding node into two equal-sized blocks and a vertical dividing type. Alternatively, 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. In this way, when a BT has a plurality of split types, when the second flag indicating that the block is split is encoded, split type information indicating the split type of the corresponding block is additionally encoded.

3 is an exemplary diagram of block division using a QTBT structure. (a) of FIG. 3 is an example in which a block is divided by a QTBT structure, and (b) is a tree structure representation. In FIG. 3, the solid line represents the division by the QT structure, and the dotted line represents the division by the BT structure. In addition, in relation to the layer description in FIG. 3 (b), those without parentheses represent QT layers, and those with parentheses represent BT layers. In the BT structure represented by a dotted line, numbers represent partition type information. 1 represents horizontal division and 0 represents vertical division.

Hereinafter, a block corresponding to a CU to be encoded or decoded is referred to as a 'current block'.

The prediction unit 220 predicts a current block and generates a prediction block. The prediction unit 220 includes an intra prediction unit 222 and an inter prediction unit 224 .

The intra predictor 222 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 prediction directions, and neighboring pixels to be used and operation expressions are defined differently according to each prediction mode. In particular, the intra prediction unit 222 may determine an intra prediction mode to be used for encoding the current block. In some examples, the intra prediction unit 222 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 222 calculates rate-distortion values using rate-distortion analysis for several tested intra-prediction modes, and selects a rate-distortion having the best rate-distortion characteristics among the tested modes. Intra prediction mode can also be selected.

4 shows an example of intra prediction modes according to an embodiment of the present invention.

As shown in FIG. 4, intra prediction modes may include two non-directional modes (planar mode and DC mode) and 65 directional modes.

The intra prediction unit 222 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 encoder 250 and transmitted to the video decoding apparatus.

The inter-prediction unit 224 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 corresponding to displacement between the current block in the current picture and the prediction block in the reference picture is generated. Motion information including information on reference pictures and motion vectors used to predict the current block is encoded by the encoder 250 and transmitted to the video decoding apparatus.

The subtractor 230 subtracts the prediction block generated by the intra prediction unit 222 or the inter prediction unit 224 from the current block to generate a residual block.

The transform unit 240 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 240 may transform the residual signals in the residual block by using the size of the current block as a transform unit, or divide the residual block into a plurality of smaller subblocks and transform the residual signals into subblock size transform units. can also be converted. There may be various methods of dividing a residual block into smaller subblocks. For example, it may be divided into subblocks having the same predefined size, or a QT (quadtree) method partitioning using a residual block as a root node may be used.

The quantization unit 245 quantizes transform coefficients output from the transform unit 240 and outputs the quantized transform coefficients to the encoder 250 .

The encoder 250 encodes the quantized transform coefficients using a coding scheme such as CABAC to generate a bitstream. Also, the encoder 250 encodes information such as CTU size, QT splitting flag, BT splitting flag, and splitting type related to block splitting so that the video decoding apparatus can divide the block in the same way as the video encoding apparatus.

The encoder 250 encodes prediction type information indicating whether the current block is coded by intra prediction or inter prediction, and syntax elements for intra prediction mode or inter prediction information according to the prediction type. encode

The inverse quantization unit 260 inversely quantizes the quantized transform coefficients output from the quantization unit 245 to generate transform coefficients. The inverse transform unit 265 transforms transform coefficients output from the inverse quantization unit 260 from a frequency domain to a spatial domain to restore a residual block.

The adder 270 restores the current block by adding the restored residual block and the predicted block generated by the predictor 220. Pixels in the reconstructed current block are used as reference pixels when intra-predicting the next block.

The filter unit 280 performs deblocking filtering on boundaries between reconstructed blocks in order to remove blocking artifacts generated by block-by-block encoding/decoding, and stores them in the memory 290 . 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.

Meanwhile, pixels of the current block may be composed of a luminance component and two chrominance components (Cb and Cr). In this case, the current block includes one luminance block composed of luminance components and two chrominance blocks composed of respective chrominance components. The luminance block and the chrominance block may be independently predicted and encoded according to the above-described encoding method. That is, the luminance block and the chrominance block are independently predicted, and the residual signals of the luminance block and the chrominance block are independently encoded.

The present disclosure relates to determining and encoding an intra prediction mode of each of a luminance block and a chrominance block in encoding a luminance block and a chrominance block. A method of determining the intra prediction mode of the luminance block may be different from a method of determining the intra prediction mode of the chrominance block.

In the case of a luminance block, the video encoding apparatus 200 determines an intra prediction mode of the luminance block and classifies all intra prediction modes into a plurality of candidate sets. In addition, luma intra mode information generates information about which group the intra prediction mode of the luma block belongs to among a plurality of candidate sets and index information for indicating the intra prediction mode of the luma block within the corresponding group as luma intra mode information. Encode. A plurality of candidate sets may be generated by classifying all intra prediction modes of the current block according to the frequency of occurrence. For example, the plurality of candidate sets include the following groups.

(1) First group (MPM group):

It consists of most probable modes (MPMs) with the highest probability of being selected as the intra prediction mode of the luminance block. MPMs are derived from intra prediction modes of neighboring blocks adjacent to the luminance block. The neighboring blocks are a luminance block (L) adjacent to the left side of the luminance block, a luminance block (A) adjacent to the top side, a luminance block (BL) adjacent to the lower left corner, a block (AR) adjacent to the upper right corner of the luminance block, and a luminance block adjacent to the upper left corner ( AL) included.

(2) Second group (Selected Mode group):

It is composed of frequently appearing modes among modes other than MPMs. The second group includes modes having a mode number that is a multiple of 4 when rearranging mode numbers in ascending order, excluding MPM, among all intra prediction modes, or directional modes belonging to the first group with an offset (e.g., -1 , +1) is composed of modes generated by adding

(3) Third group (non-Selected Mode group):

Among all intra prediction modes, it is composed of modes other than modes belonging to the first group and the second group.

Meanwhile, in the case of a color difference block, the image encoding apparatus 200 configures a candidate set including a plurality of candidates for an intra prediction mode of the color difference block. Then, an intra prediction mode of a color difference block among a plurality of candidates belonging to the candidate set is determined, and chroma intra mode information for indicating a selected candidate among the plurality of candidates is encoded.

A candidate set may be composed of the following four types of intra prediction modes.

(1) LM (linear mode):

This is an intra mode for deriving a prediction block for a chrominance block using reconstructed pixel values in the luminance block. After obtaining the scaling factor α value and the offset β value by using the correlation between the luminance block and the chrominance block, a prediction block for the chrominance block is derived using these two values and the reconstructed pixel value in the luminance block. do. A specific formula is shown in Equation 1.

Here, Pred _c (i,j) is the predicted chrominance block corresponding to the current block, rec _L (i,j) is the downsampled reconstructed luminance block corresponding to the current block, α is a scaling factor, and β is an offset indicates α and β are either a block unit in which intra prediction is performed, a block of the top layer corresponding to the root node in a tree structure for block partition, or a sequence unit that is a slice, a picture, or a group of multiple pictures. It may be included in the bitstream in units of and delivered to the decoding device. Alternatively, α and β may be identically calculated by the encoding device and the decoding device using pixel values in already reconstructed luminance blocks and chrominance blocks adjacent to the current block. In this case, signaling for α and β is not required.

(2) DM (direct mode):

This means an intra prediction mode of a luminance block corresponding to a chrominance block. In the tree structure division from CTU to CU, the luminance component and chrominance component can be divided into the same structure. In this case, since there is only one luminance block corresponding to the color difference block, there is one DM. However, the luminance component and the chrominance component may be divided into different structures. Referring to FIG. 6, FIG. 6(a) shows a block division structure of a CTU composed of luminance components, and FIG. 6(b) shows a block division structure of a CTU composed of chrominance components. It can be seen that the luminance block located at the upper right of FIG. 6(a) corresponding to the color difference block located at the upper right of FIG. 6(b) is divided into a total of 6 subblocks. Since each subblock becomes one CU, each subblock can be intra predicted using different intra prediction modes. Accordingly, intra prediction modes of the luminance block corresponding to the chrominance block may be plural.

Therefore, when a luminance block corresponding to a chrominance block is divided into a plurality of subblocks, a method for deriving one or more intra prediction modes from the intra prediction modes of the plurality of subblocks is required, which will be described in detail later.

(3) neighboring mode:

Intra prediction modes of neighboring chrominance blocks spatially adjacent to the chrominance block. For example, positions of neighboring color difference blocks may be determined as shown in FIG. 5 . When deriving candidates from intra prediction modes of neighboring chrominance blocks, the priorities for selecting neighboring chrominance blocks are left (L), top (A), bottom left (BL), top right (AR), and top left ( AL) may be in order.

(4) default mode:

Modes that are frequently used stochastically may be included in the candidate set in the order of planar, DC, vertical, horizontal, and diagonal modes. Alternatively, first, the planar and DC modes are included in the candidate set in order, and then modes obtained by adding -1 or +1 to angular modes (direction modes) already included in the candidate set are included in the candidate set. If the number of candidates in the candidate set is still insufficient, the candidate set is filled in the order of vertical, horizontal, and diagonal modes.

A candidate set is constructed using the intra prediction modes derived from the four types described above. The number m of candidates in the candidate set may have a value in the range of 5 to 8. Among the four types, the order of filling the intra prediction modes of the chrominance block in the candidate set may be variously determined.

As an example, intra prediction mode candidates of a chrominance block may be filled in a candidate set in the order of DM, LM, neighboring mode, and default mode. For example, if the number of candidate sets is defined as 6, 2 DMs and 1 LM are derived, and 3 modes are derived from available neighboring blocks L, AR, and AL based on the priority of the neighboring mode , Since a total of 6 candidates have already been derived, the default mode is not used. Therefore, the candidate set is filled in the order of intra prediction modes of two DMs, one LM, and neighboring blocks L, AR, and AL. If two modes are obtained from one DM and one LM and available neighboring blocks A and AL, two candidates are derived in the order of planar and DC based on the priority in the default mode. The candidate set is filled in the order of 1 DM, 1 LM, neighboring block A, intra prediction mode of AL, planar, DC. If 5 DMs and 1 LM are derived, since all 6 candidates have already been derived, the neighboring mode and default mode are not included in the candidate set.

As another example, intra prediction mode candidates of the chrominance block may be filled in the candidate set in the order of LM, DM, neighboring mode, and default mode.

m intra prediction mode candidates belonging to the candidate set must not overlap. When adding a new intra prediction mode as a candidate to the candidate set, it is checked whether the new intra prediction mode already exists in the candidate set, and if not, the new intra prediction mode is added to the candidate set.

Meanwhile, the LM may always be included in the candidate set. In this case, the number of normal intra prediction modes excluding the LM mode among m candidates is m-1. For example, if the number of candidate sets is predefined as 6, up to 5 general intra prediction modes are derived from DM, neighboring modes, and default modes.

Hereinafter, a method of deriving DM when the luminance component and the chrominance component of the CTU are divided into different structures will be described.

In order to derive one or more DMs, the video encoding apparatus 200 must store intra prediction modes of the luminance block. Intra prediction modes of each luminance block may be stored in memory in an NxN block size. N may be appropriately set in consideration of memory capacity and the like. For example, the value of N may have a value of 1 (that is, the intra prediction mode of the luminance block is stored in units of pixels) and may have a minimum block size among block sizes available for intra prediction.

FIG. 7 is an exemplary diagram illustrating an intra prediction mode of each luminance block in the block division structure of FIG. 6 (a).

In (a) of FIG. 7, intra prediction modes (A to H) of each luminance block are displayed in different patterns. 7(b) shows an example of storing intra prediction modes of luma blocks in a memory in units of 4x4 block size. When the size of the CTU is 32x32, intra prediction modes for a total of 64 regions are stored.

In this embodiment, when a luminance block corresponding to a chrominance block is divided into a plurality of subblocks, the image encoding apparatus 200 determines intra prediction modes of at least some of the subblocks present in the luminance block. One or more candidates for the intra prediction mode of the chrominance block, ie, DM, are selected. As a rule for selecting a DM, one or more of the priorities of intra prediction modes of at least some subblocks or a predefined order of scanning at least some subblocks may be used.

Here, at least some of the subblocks may be all of a plurality of subblocks existing in the luminance block or may be subblocks at predefined positions. The subblocks at predefined positions include a pixel (center, CR) located at the center point of the luminance block corresponding to the chrominance block, a pixel located at the upper left (top left, TL) within the luminance block, and a pixel located at the upper right within the luminance block ( top right (TR), a pixel positioned at the lower left of the luminance block (bottom left, BL), and a pixel positioned at the lower right (bottom right, BR) within the luminance block. CR refers to a top left center (TLC) pixel located at the top left of the center point of the luminance block. However, it is not limited thereto, and in addition to the TLC, at least one of the top right center (TRC), bottom left center (BLC), and bottom right center (BRC) pixels. may be selected as this CR.

Meanwhile, the priority may be an area of a block covered by an intra prediction mode of each of at least some subblocks within a luminance block. For example, assume that at least some subblocks are blocks covering pixels located in CR, TL, TR, BL, and BR. Referring to (a) of FIG. 11, intra prediction modes of CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located in the upper right of FIG. 6 (b) are CR(G) and TL (C), TR(C), BL(A), and BR(G) (expressed as “position (mode)”). There are a total of three intra prediction modes (G, C, A) corresponding to CR, TL, TR, BL, and BR, and the areas occupied by these modes in the luminance block are in the order of C, G, and A. Therefore, modes C, G, and A are included in the candidate set in the order.

Alternatively, the priority may be the frequency at which an intra prediction mode identical to each of the intra prediction modes of at least some subblocks within a luminance block occurs. For example, assume that at least some subblocks are blocks covering pixels located in CR, TL, TR, BL, and BR. Referring to (a) of FIG. 11, intra prediction modes of CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located in the upper right of FIG. 6 (b) are CR(G) and TL (C), TR (C), BL (A), and BR (G). Therefore, intra prediction modes corresponding to CR, TL, TR, BL, and BR are a total of three (G, C, and A). When counting the frequency at which the same mode as each intra prediction mode occurs within the luminance block is counted in units of block size stored in memory (eg, 4x4 block units as shown in FIG. 7(b)), these modes The frequency of occurrence is 8 times for mode C, 4 times for mode G, and 1 time for mode A. Modes C, G, and A are included in the candidate set in order of increasing frequency. Here, it has been described that the frequency count is counted in units of the block size in which the intra prediction mode is stored in the memory, but the present invention is not necessarily limited thereto. For example, counting may be performed in units of subblocks divided into a tree structure within a luminance block.

Alternatively, the priority may be defined as the number of repetitions between intra prediction modes of at least some subblocks. For example, referring to FIG. 11, intra prediction modes corresponding to CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located in the upper right of FIG. 6 (b) are CR(G) and TL (C), TR (C), BL (A), and BR (G). Accordingly, the number of repetitions between intra prediction modes of at least some subblocks is 2 times for mode C, 2 times for mode G, and 1 time for mode A. Therefore, mode C and mode G are included in the candidate set before mode A.

A z-scan order may be used as a predefined scan order. The z-scan order means the order of upper left, upper right, lower left, and lower right. When a block is divided into sub-blocks as shown in FIG. 8, the processing order between sub-blocks is also a z-scan order, and the processing order within a sub-block is also a z-scan order. However, the predefined scan order is not limited to the z-scan order. In the case of using subblocks at predefined positions, the predefined positions may be scanned using a scan order different from the z-scan order. For example, in the case of subblocks covering pixels located in CR, TL, TR, BL, and BR, the scan order of CR, TL, TR, BL, and BR, or the scan order of TL, CR, TR, BL, and BR may be used.

Hereinafter, a method of sequentially selecting candidates for an intra prediction mode of a chrominance block from a plurality of subblocks in a luminance block according to a priority order or a predefined scan order will be described through various embodiments. For convenience of description, it is assumed that the block division structures of the CTU composed of luminance components and the CTU composed of chrominance components are as shown in FIGS. 6(a) and 6(b), respectively. As exemplified above, assuming that up to five normal intra prediction modes can be selected from DM, neighboring mode, and default mode, except for LM, among them, since DM has the highest priority, up to five DMs can be set as candidates.

Example #1

In this embodiment, one or more DMs are selected based on priority according to z-scan order from all intra prediction modes of all subblocks existing in a luminance block.

9 is an exemplary diagram for inducing a DM according to the first embodiment. Capital letters A to H shown in (a) of FIG. 9 indicate intra prediction modes, and numbers written in (b) of FIG. 9 indicate the number of DMs in each color difference block.

Referring to (a) of FIG. 9, the luminance block corresponding to the chrominance block located in the upper left of (b) of FIG. 9 has two subblocks having different intra prediction modes. A total of two DMs are selected in the order of intra prediction mode A and intra prediction mode B according to the z-scan order. The remaining three candidates are selected from neighboring mode and default mode.

Meanwhile, a total of 6 subblocks exist in the luminance block corresponding to the chrominance block located at the upper right of FIG. 9(b), and the 6 subblocks have different intra prediction modes. In the case of the color difference block located at the upper right, a total of five DMs are selected in the order of intra prediction modes C, D, E, A, and F. Since the maximum number of DMs is 5, the intra prediction mode G is not selected as a DM.

Example #2

In this embodiment, one or more DMs are sequentially selected according to priority and z-scan order from all intra prediction modes of all subblocks present in a luminance block. DMs are selected in the order of highest priority, and if the priority is the same, they are selected according to the z-scan order. In this embodiment, any one of the above-described block area, frequency, and overlapping number may be used as a priority, but here, the description is based on the block area.

10 is an exemplary diagram for inducing a DM according to the second embodiment. Capital letters A to H shown in (a) of FIG. 10 represent intra prediction modes, and numbers in (b) of FIG. 10 represent the number of DMs of each color difference block.

Referring to FIG. 10(a), the luminance block corresponding to the chrominance block located in the upper left of FIG. 10(b) has two subblocks having different intra prediction modes. Areas of blocks covered by two intra prediction modes within a luminance block are the same. Accordingly, a total of two DMs are selected in the order of intra prediction mode A and intra prediction mode B according to the z-scan order. The remaining three candidates are selected from neighboring mode and default mode.

Meanwhile, a total of 6 subblocks exist in the luminance block corresponding to the chrominance block located at the upper right of FIG. 10(b), and the 6 subblocks have different intra prediction modes. The block area covered by each intra prediction mode within a luminance block is larger in the order of intra prediction modes C and G. Block areas covered by intra prediction modes D, E, A, and F are equal to each other. In this embodiment, first, two DMs are selected in the order of intra prediction modes C and G according to the block area, and intra prediction mode D among intra prediction modes D, E, A, and F covering the same area is selected according to the z-scan order. Select three DMs in the order of , E, and A. Since the maximum number of DMs is 5, intra prediction mode F is not selected as a DM. Therefore, in this embodiment, a total of five DMs are selected in the order of intra prediction modes C, G, D, E, and A.

Example #3

In this embodiment, one or more DMs are selected from subblocks at a predefined position among all subblocks existing in a luminance block based on a priority order and a predefined scan order. Subblocks at predefined locations include subblocks covering pixels located at CR, TL, TR, BL, and BR within the luminance block. In this embodiment, DMs are selected in the order of highest priority, and when the priority is the same, they are selected according to a predefined scan order. This embodiment may use any one of block area, frequency, or overlap count as a priority, and as a predefined scan order, a Z-scan order, or a scan order in the order of CR, TL, TR, BL, BR, or A scan order of TL, CR, TR, BL, and BR may be used. Hereinafter, an example of using a Z-scan order as a predefined scan order and using a block area or the number of overlaps as a priority will be described.

11 is an exemplary view for inducing a DM according to the third embodiment. Uppercase letters A to H shown in (a) of FIG. 11 represent intra prediction modes, and numbers in (b) of FIG. 11 represent the number of DMs in each color difference block.

In the case of using the block area as the priority order, referring to FIG. 11(a), CR, TL, TR, BL, BR within the luminance block corresponding to the color difference block located in the upper left of FIG. 11(b) There are two intra prediction modes corresponding to each position. Since the block area covered by the two intra prediction modes within the luminance block is the same, the present embodiment selects two DMs in the order of intra prediction modes A and B according to the Z-scan order. The remaining three candidates are selected from neighboring mode and default mode. Meanwhile, in the luminance block corresponding to the chrominance block located at the upper right of FIG. 11(b), there are a total of three intra prediction modes corresponding to positions of CR, TL, TR, BL, and BR. Three DMs are selected in the order of intra prediction modes C, G, and A according to the block area covered by each intra prediction mode within the luminance block. The remaining two candidates are selected from neighboring mode and default mode.

On the other hand, when using the number of overlaps rather than the block area as the priority, each position of CR, TL, TR, BL, BR within the luminance block corresponding to the color difference block located in the upper left of FIG. 11 (b) corresponds to The intra prediction modes that are performed are CR (B), TL (A), TR (B), BL (A), and BR (B). The number of repetitions between intra prediction modes in CR, TL, TR, BL, and BR is 2 times for mode A and 3 times for mode B. Therefore, in this embodiment, two DMs are selected in the order of intra prediction modes B and A. The remaining three candidates are selected from neighboring mode and default mode. On the other hand, intra prediction modes corresponding to positions of CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located in the upper right of FIG. Same as TR (C), BL (A), and BR (G). The number of repetitions between intra prediction modes in CR, TL, TR, BL, and BR is 1 time for mode A, 2 times for mode C, and 2 times for mode G. Therefore, modes C and G are selected by the DM first rather than mode A. Between modes C and G having the same number of repetitions, the intra prediction mode C is first selected as the DM according to the Z-scan order. Accordingly, three DMs are selected in the order of C, G, and A. The remaining two candidates are selected from neighboring mode and default mode.

Although the present embodiment described above considers both the priority order and the predefined scan order, the present embodiment may use only the predefined scan order without considering the block area. For example, it is also possible to determine the DM in the scan order of CR, TL, TR, BL, and BR, or the scan order of TL, CR, TR, BL, and BR. In the case of using the scan order of CR, TL, TR, BL, and BR, the DM of the color difference block located in the upper left of FIG. 11(b) is selected in the order of intra prediction modes G, C, and A.

Example #4

This embodiment is the same as embodiment #3 in that one or more DMs are selected from subblocks at a predefined position among all subblocks within the luminance block based on a priority order and a predefined scan order. However, in this embodiment, as subblocks at predefined locations, subblocks covering pixels located in TLC, TRC, BLC, and BRC as well as TL, TR, BL, and BR are used in the luminance block. Hereinafter, using a z-scan order as a predefined scan order and using a block area or the number of overlaps as a priority will be described as an example.

12 is an exemplary diagram for inducing a DM according to the fourth embodiment. Capital letters A to H in (a) of FIG. 9 represent intra prediction modes, and numbers in (b) of FIG. 12 represent the number of DMs in each color difference block.

First, the case of using the block area as the priority order will be described. Referring to (a) of FIG. 12, the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC in the luminance block corresponding to the color difference block located in the upper left of (b) of FIG. There are two intra prediction modes. Since the block area covered by the two intra prediction modes in the luminance block corresponding to the chrominance block located at the upper left is the same, the DM is selected in the order of intra prediction modes A and B in the order of Z-scan. The remaining three candidates are selected from neighboring mode and/or default mode.

Meanwhile, in the luminance block corresponding to the chrominance block located in the upper right of FIG. 12 (b), there are a total of four intra prediction modes corresponding to the locations of TL, TR, BL, BR, TLC, TRC, BLC, and BRC. According to the block area covered by each intra prediction mode, first two DMs are selected in the order of intra prediction modes C and G. Since the intra prediction modes corresponding to the BL and BLC locations cover the same block area, the intra prediction modes E and A are sequentially selected by the DM in the order of BLC and BL according to the Z-scan order. That is, in this embodiment, a total of four DMs are selected in the order of intra prediction modes C, G, E, and A. The remaining one candidate is selected from neighboring mode and/or default mode.

In the case of using the number of repetitions as the priority order, the locations of TL, TR, BL, BR, TLC, TRC, BLC, and BRC within the luminance block corresponding to the color difference block located in the upper left of FIG. 12 (b) Intra prediction modes corresponding to are the same as TL (A), TR (B), BL (A), BR (B), TLC (A), TRC (B), BLC (A), and BRC (B). Since the number of repetitions between the two intra prediction modes corresponding to the locations of TL, TR, BL, BR, TLC, TRC, BLC, and BRC is the same as 4 times, DM in the order of intra prediction modes A and B in the order of Z-scan choose The remaining three candidates are selected from neighboring mode and/or default mode.

Meanwhile, intra prediction modes corresponding to positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC in the luminance block corresponding to the chrominance block located in the upper right of FIG. , TR (C), BL (A), BR (G), TLC (C), TRC (C), BLC (E), BRC (G). The number of repetitions between intra prediction modes corresponding to each position is 4 times for C, 1 time for A, 2 times for G, and 1 time for E. According to the number of repetitions, first two DMs are selected in the order of intra prediction modes C and G. Since intra prediction modes A and E corresponding to BL and BLC locations have the same number of repetitions, intra prediction modes E and A are sequentially selected as DM in the order of BLC and BL according to the Z-scan order. That is, in this embodiment, a total of four DMs are selected in the order of intra prediction modes C, G, E, and A. The remaining one candidate is selected from neighboring mode and/or default mode.

Example #5

This embodiment is the same as embodiment #3 in that one or more DMs are selected from subblocks at a predefined position among all subblocks within the luminance block based on a priority order and a predefined scan order. However, in this embodiment, subblocks covering pixels located at TLC, TRC, BLC, and BRC within the luminance block are used as subblocks at predefined positions. Hereinafter, using a z-scan order as a predefined scan order and using a block area or the number of overlaps as a priority will be described as an example.

13 is an exemplary view for inducing a DM according to the fifth embodiment. Capital letters A to H in (a) of FIG. 9 represent intra prediction modes, and numbers in (b) of FIG. 13 represent the number of DMs in each color difference block.

First, the case of using the block area as the priority order will be described. Referring to (a) of FIG. 13, there are two intra prediction modes corresponding to the positions of TLC, TRC, BLC, and BRC in the luminance block corresponding to the color difference block located in the upper left of (b) of FIG. 13. . Since the block area covered by the two intra prediction modes in the luminance block corresponding to the chrominance block located at the upper left is the same, the DM is selected in the order of intra prediction modes A and B in the order of Z-scan. The remaining three modes are selected from neighboring mode and/or default mode.

Meanwhile, in the luminance block corresponding to the chrominance block located at the upper right of FIG. 13(b), there are a total of three intra prediction modes corresponding to the positions of TLC, TRC, BLC, and BRC. Block areas covered by each intra prediction mode are different. Accordingly, three DMs are selected in the order of intra prediction modes C, G, and E according to the block area. The remaining two candidates are selected from neighboring mode and/or default mode.

In the case of using the number of duplicates as the priority order, the intra prediction modes corresponding to the positions of TLC, TRC, BLC, and BRC in the luminance block corresponding to the chrominance block located in the upper left of FIG. 13 (b) are 2 dogs exist. Looking at intra prediction modes of subblocks at each position, they are the same as TLC (A), TRC (B), BLC (A), and BRC (B). Since the number of repetitions between intra prediction modes corresponding to each position is the same, the DM is selected in the order of intra prediction modes A and B according to the Z-scan order. The remaining three modes are selected from neighboring mode and/or default mode.

Meanwhile, in the luminance block corresponding to the chrominance block located in the upper right of FIG. E), same as BRC (G). The number of repetitions between intra prediction modes at each location is 2 times for C and 1 time for E and G respectively. Therefore, DM is selected first in the intra prediction mode C according to the number of repetitions. Since the number of repetitions of intra prediction modes corresponding to BLC and BRC locations is the same, intra prediction modes E and G are sequentially selected by DM in the order of BLC and BRL according to the Z-scan order. That is, in this embodiment, a total of three DMs are selected in the order of intra prediction modes C, E, and G. The remaining two candidates are selected from neighboring mode and/or default mode.

The image encoding apparatus 200 assigns a mode index to a predefined number of candidates belonging to a candidate set and encodes an index corresponding to a candidate selected as an intra prediction mode of a color difference block as index intra mode information. Here, the mode index is given in the order of LM, DM, neighboring mode, default mode, or DM, LM, neighboring mode, default mode in the same manner as described above. The mode index may be binarized so that an index of a candidate selected first in a candidate set has a smaller number of bits than an index of a candidate selected later. In this case, the indices of the last two candidates selected from the candidate set may have the same number of bits and may be binarized so that the remaining bits excluding the least significant bit (LSB) are identical to each other.

Table 3 below shows a method of binarizing the mode index when the number of candidates belonging to the candidate set is 6.

Codeword #1 is a binarization result using a truncated unary (TU) method. For example, when it is assumed that indexes are assigned in the order of LM, DM, neighboring mode, and default mode, the first bin of codeword #1 indicates whether the intra prediction mode of the color difference block is LM. If the first bin has a first value (0 in Table 1), it indicates LM, and if it has a second value (1 in Table 1), it indicates not LM. Therefore, in the case of LM, the mode index is represented as 0.

If the intra prediction mode of the chrominance block is not LM, the first bin is represented by 1. Normal modes derived in the order of DM, neighboring mode, and default mode are identified by second and subsequent bins. The mode index of the intra prediction mode included in the first candidate set among the normal modes is binarized with a total of 2 bits, and the second bin is represented by 0, and the second bin of the mode indices of candidates subordinate to the first candidate is all 1. is expressed as Among the normal modes, the mode index for the intra prediction mode included in the second candidate set is binarized to a total of 3 bits by increasing the number of bits by 1 compared to the first candidate, and the third bin is represented by 0. The third bin of the mode indices of candidates having a lower priority than the second candidate are all represented by 1. Among the general modes, the mode index for the intra prediction mode included in the third candidate set is expressed as a total of 4 bits by increasing the number of bits by 1 compared to the second candidate, and the fourth bin is expressed as 0. Through this process, mode indices for normal modes included in the candidate set are binarized. However, among the normal modes, the last two candidates included in the candidate set, that is, the fourth candidate and the fifth candidate, are binarized with the same number of bits, and are binarized so that the remaining bits except for the lowest bin are the same. Referring to Table 1, the first four bins of the fourth and fifth candidates among the normal modes are equal to 1, and only the fifth bin, which is the lowest bin, is represented by 0 and 1, respectively.

In the above, four types (LM, DM, neighboring mode, default mode) are sequentially applied to select a predetermined number (eg, 6) of candidates, and which of the candidates is used as the intra prediction mode of the chrominance block. It has been described as encoding a mode index indicating . However, the present invention is not necessarily limited thereto.

For example, in Table 1 described above, the first bin indicates whether LM is used as an intra prediction mode of a color difference block. Therefore, it is also possible to indicate whether LM is used as an intra prediction mode of a chrominance block through a separate flag instead of the first bin, which is obvious to those skilled in the art. In this case, if the flag indicates that LM is not used as an intra prediction mode of a chrominance block, a predefined number of normal modes are selected in the order of DM, neighboring mode, and default mode, and among the normal modes A mode index indicating which of the candidates is used as the intra prediction mode of the chrominance block may be binarized using the above-described binarization method.

As another example of encoding the color difference intra prediction mode, the image encoding apparatus 200 encodes information (eg, a flag) indicating whether DM is used as the intra prediction mode of the color difference block, and then, when the DM is used, A predetermined number of DMs may be selected, and a first index indicating which candidate among the predetermined number of DMs is used as the intra prediction mode of the chrominance block may be binarized using the above-described binarization method. When DM is not used, the second index indicating which candidate among the predetermined number of modes derived from LM, neighboring mode, and default mode is used as the intra prediction mode of the color difference block is used by the binarization method described above. can be binarized.

8 illustrates an image decoding apparatus according to an embodiment of the present invention.

The image decoding apparatus includes a decoder 1410, an inverse quantization unit 1420, an inverse transform unit 1430, a predictor 1440, an adder 1450, a filter unit 1460, and a memory 1470. Like the video encoding apparatus of FIG. 2 , each component of the video decoding apparatus may be implemented as a hardware chip or implemented as software and a microprocessor may execute software functions corresponding to each component.

The decoder 1410 decodes the bitstream received from the video encoding apparatus, extracts block division-related information, determines a current block to be decoded, predictive information required for restoring the current block, and information on a residual signal, etc. extract

The decoder 1410 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 a picture into CTUs of the determined size. In addition, 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 splitting a CTU using a QTBT structure, first, a first flag (QT_split_flag) related to splitting of QT is extracted and each node is split into four nodes of a lower layer. Then, for a node corresponding to a leaf node of QT, a second flag (BT_split_flag) related to BT splitting and split type information are extracted to split the corresponding leaf node into a BT structure.

Meanwhile, when the decoder 1410 determines a current block to be decoded through partitioning of a tree structure, 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 decoder 1410 decodes a syntax element for intra prediction information (intra prediction mode) of the current block and transmits it to the intra prediction unit 1444.

Also, the decoder 1410 extracts information on quantized transform coefficients of the current block as information on the residual signal.

The inverse quantization unit 1420 inversely quantizes the quantized transform coefficients, and the inverse transform unit 1430 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.

The prediction unit 1440 includes an intra prediction unit 642 and an inter prediction unit 644 . The intra predictor 1442 is activated when intra prediction is the prediction type of the current block, and the inter predictor 1444 is activated when intra prediction is the prediction type of the current block.

The intra prediction unit 1442 determines the intra prediction mode of the current block using the syntax element for the intra prediction mode extracted from the decoder 1410, and uses reference pixels around the current block according to the intra prediction mode to determine the current block. predict a block

The inter predictor 1444 determines motion information of the current block using the syntax elements of the inter prediction information extracted from the decoder 1410, and predicts the current block using the determined motion information.

The adder 1450 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 filter unit 1460 performs deblocking filtering on boundaries between reconstructed blocks in order to remove blocking artifacts generated by block-by-block decoding, and stores them in the memory 1470. 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 decoded later.

Meanwhile, when the current block is composed of a luminance component and a chrominance component, the luminance component and chrominance component of the current block are independently predicted and restored. In this case, a syntax element for intra prediction information (intra prediction mode) decoded by the decoder 1410 predicts a chrominance block composed of luminance intra mode information and chrominance components for predicting a luminance block composed of luminance components of the current block. Color difference intra mode information for

The intra predictor 1442 predicts a luminance block using luminance intra mode information. Luminance intra mode information includes information indicating a group to which an intra prediction mode of a luma block belongs and an index indicating an intra prediction mode of the luma block among intra prediction modes in the group.

The intra prediction unit 1442 derives intra prediction mode candidates constituting the corresponding group in the same manner as the video encoding apparatus according to information indicating the group to which the intra prediction mode of the luminance block belongs. If the information indicating the group indicates an MPM group, an MPM group is created. If a Selected Mode group is indicated, a Selected Mode group is created. If a non-Selected Mode group is indicated, a non-Selected Mode group is created. Then, a candidate identified by the index within the corresponding group is set as an intra prediction mode of the luminance block, and the luminance block is predicted using the set intra prediction mode. The predicted luminance block is added with the residual block for the luminance component to restore the luminance component of the current block.

In addition, the intra prediction unit 1442 predicts a color difference block using the color difference intra mode information. The intra prediction unit 1442 configures a candidate set for the intra prediction mode of the color difference block in the same manner as the image encoding apparatus 200 . That is, a candidate set is configured using at least one type among DM, LM, neighboring mode, and default mode. Since the method of deriving each type of mode has already been described in the video encoding apparatus, a detailed description thereof will be omitted to avoid duplication of description. The intra predictor 1442 selects one candidate as an intra prediction mode of a chrominance block from among candidates in the candidate set using the chrominance intra mode information, and predicts the chrominance block using the selected intra prediction mode.

As an example, when m candidates are selected in the order of DM, LM, neighboring mode, default mode or LM, DM, neighboring mode, default mode, the color difference intra mode information decoded from the decoder 1410 is m It includes information indicating an intra prediction mode of a chrominance block among candidates. The intra prediction unit 1442 constructs a candidate set by sequentially selecting m candidates in the order of DM, LM, neighboring mode, default mode or LM, DM, neighboring mode, default mode. Among m candidates in the candidate set, a candidate indicated by the chrominance intra mode information is determined as an intra prediction mode of the chrominance block.

As another example, the color difference intra mode information includes information indicating whether the intra prediction mode of the color difference block is LM, and if it is not LM, a predetermined number of normal modes derived from DM, neighboring mode, and default mode ), information indicating the intra prediction mode of the color difference block may be included. In this case, the intra prediction unit 1442 selects LM as the intra prediction mode of the chrominance block when the information indicating whether the LM is used indicates that LM is used. When the information indicating whether the LM is used does not indicate use of the LM, the intra prediction unit 1442 configures a candidate set by sequentially selecting a predetermined number of normal modes from DM, neighboring mode, and default mode. The intra prediction mode of the color difference block is selected from among the candidates in the candidate set according to the information indicating the intra prediction mode of the color difference block.

As another example, information indicating whether the intra prediction mode of the chrominance intra chrominance block is DM is included. When the intra prediction mode of the color difference block is DM, the color difference intra mode information includes information (first index) indicating the intra prediction mode of the color difference block among the predetermined number of DMs, and when it is not DM, LM, neighboring mode, It may include information (second index) indicating an intra prediction mode of a chrominance block among a predetermined number of modes derived from the default mode. In this case, the intra prediction unit 1442 derives a predetermined number of DMs as a candidate set when the information indicating whether the DM is a DM indicates the use of the DM, and the intra prediction mode of the chrominance block according to the first index among the DMs. choose with On the other hand, if the information indicating whether the DM is the DM does not indicate the use of the DM, the intra prediction unit 1442 configures a candidate set by sequentially selecting a predetermined number of normal modes from LM, neighboring mode, and default mode. Then, the intra prediction mode of the chrominance block is selected from the candidates in the candidate set by the second index.

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.

Claims (10)

In a method performed by an image decoding apparatus to intra-predict a chroma block divided from a chroma block having a block division structure different from a corresponding luminance coding tree block,
decoding a flag indicating whether the chrominance block is predicted from reconstructed pixel values in a corresponding luminance block from a bitstream; and
determining an intra prediction mode of the chrominance block according to the flag and predicting the chrominance block;
When the flag indicates that the chrominance block is not predicted from reconstructed pixel values in the luminance block, predicting the chrominance block comprises:
Decoding mode information for selecting an intra prediction mode of the chrominance block from a candidate set of predefined intra prediction modes, the candidate set including a DM mode as an intra prediction mode derived from the luminance block box; and
Determining an intra prediction mode of the chrominance block using the mode information
A method comprising a. According to claim 1,
wherein the candidate set includes at least two of a planar mode, a DC mode, a horizontal mode, and a vertical mode. According to claim 2,
The mode information is binarized so that the smallest number of bits is allocated to the DM mode. According to claim 1,
When the flag indicates that the chrominance block is predicted from reconstructed pixel values in the luminance block, intra-predicting the chrominance block comprises:
deriving prediction parameters using the chrominance samples around the chrominance block and the luminance samples around the luminance block; and
predicting the chrominance block using reconstructed pixel values in the luminance block and the prediction parameters;
A method comprising a. According to claim 1,
The method of claim 1, wherein the DM mode is set using an intra prediction mode of a block covering at least one sample at a predefined position in the luminance block. According to claim 5,
wherein the sample at the at least one predefined location comprises a center sample within the luminance block. According to claim 5,
The method of claim 1, wherein the DM mode is an intra prediction mode of a first effective block in a predefined scan order among blocks each covering the sample at the at least one predefined position. According to claim 5,
The DM mode is determined based on priorities of intra prediction mode candidates of blocks each covering the at least one predefined position sample,
The priority is
The area of a block covered by each of the intra prediction mode candidates, or
The frequency at which the same intra prediction mode as each of the intra prediction mode candidates occurs within the luma block, or
The number of repetitions between the intra prediction mode candidates
A method characterized in that it is determined according to. A method performed by an image encoding apparatus to intra-predict a chroma block divided from a chrominance coding tree block having a block division structure different from a corresponding luminance coding tree block,
determining an intra prediction mode of the color difference block;
predicting the chrominance block using an intra prediction mode of the chrominance block; and
Encoding information about an intra prediction mode of the color difference block,
The step of encoding information on the intra prediction mode of the color difference block,
encoding a flag indicating whether the chrominance block is predicted from reconstructed pixel values in a corresponding luminance block; and
Mode information for selecting an intra prediction mode of the chrominance block from a candidate set of predefined intra prediction modes when the flag indicates that the chrominance block is not predicted from reconstructed pixel values in the luminance block. Including the step of encoding,
The method of claim 1, wherein the candidate set includes a DM mode, which is an intra prediction mode derived from the luminance block. A record, readable by a decoder, for storing a bitstream generated by an encoding method of intra-predicting a chroma block divided from a chroma block having a block division structure different from that of the corresponding luminance coding tree block. in the media,
The encoding method,
determining an intra prediction mode of the color difference block;
predicting the chrominance block using an intra prediction mode of the chrominance block; and
Encoding information about an intra prediction mode of the color difference block,
The step of encoding information on the intra prediction mode of the color difference block,
encoding a flag indicating whether the chrominance block is predicted from reconstructed pixel values in a corresponding luminance block; and
Mode information for selecting an intra prediction mode of the chrominance block from a candidate set of predefined intra prediction modes when the flag indicates that the chrominance block is not predicted from reconstructed pixel values in the luminance block. Including the step of encoding,
The recording medium of claim 1 , wherein the candidate set includes a DM mode, which is an intra prediction mode derived from the luminance block.

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