KR20240074735A - Video Encoding and Decoding Using Intra Prediction - Google Patents

Abstract

The present invention relates to a method of encoding information about an intra prediction mode of a chroma block, comprising the steps of configuring a candidate set including a plurality of candidates for an intra prediction mode of the chroma block, the plurality of candidates includes one or more intra prediction modes derived from a luma block corresponding to the chrominance block; And encoding chrominance intra mode information for indicating an intra prediction mode of the chrominance block among the plurality of candidates belonging to the candidate set, wherein when the luminance block is divided into a plurality of sub-blocks, the plurality of sub-blocks It relates to a technique for selecting the one or more candidates from intra prediction modes of at least some subblocks of a block.

Description

Encoding and decoding video using intra prediction {Video Encoding and Decoding Using Intra Prediction}

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

The content described in this section simply provides background information for this embodiment and does not constitute prior art.

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

Figure 1 is a diagram showing intra modes available for intra prediction in HEVC. In the case of HEVC, there are a total of 35 intra modes, including 33 directional angular modes and 2 non-directional modes, as shown in FIG. 11.

In the case of a luma block consisting of a luminance component in the encoding target block, three MPMs for the block are used using the neighboring blocks of the block and the statistically most frequently used modes to encode the final intra mode. Select .

A 1-bit MPM flag indicating whether the final mode of the 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 clearly transmitted.

Meanwhile, in the case of a chroma block consisting of a chrominance component, intra prediction mode candidates consisting of some of a total of 35 intra prediction modes are formed, and the intra prediction mode of the chrominance block is selected from among the candidates. And, information to indicate the candidate selected among the candidates is signaled. These intra prediction mode candidates consist of planar mode, vertical mode, horizontal mode, DC mode, and intra prediction mode (DM, direct mode) of the luminance block corresponding to the chrominance block.

As the resolution of the video increases from 64x64 to 256x256, the unit of encoding target block also increases, and the possibility of adding more intra modes increases. Against this background, the present invention seeks to provide an improved method for determining an intra prediction mode.

The purpose of the present invention is to provide an improved technology for determining and encoding the intra prediction mode of the current block in intra prediction coding.

One aspect of the present invention provides a method performed by an image decoding apparatus to intra-predict a chroma block divided from a chrominance coding treeblock having a block division structure different from the corresponding luminance coding treeblock. do. The method includes 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 a chrominance block includes decoding mode information for selecting an intra prediction mode of the chrominance block from a candidate set of predefined intra prediction modes, wherein the candidate set is derived from the luminance block. Includes 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 device to intra-predict a chroma block divided from a chrominance coding treeblock having a block division structure different from the corresponding luminance coding treeblock. do. The method includes 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 about the intra prediction mode of the chrominance block, wherein the step of encoding the information about the intra prediction mode of the chrominance block includes predicting the chrominance block from the reconstructed pixel values in the corresponding luminance block. Encoding a flag indicating whether or not 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 of intra-predicting a chroma block divided from a chrominance coding treeblock having a block division structure different from the corresponding luminance coding treeblock, A recording medium readable by a decoder is provided. The encoding method includes 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 about the intra prediction mode of the chrominance block, wherein the step of encoding the information about the intra prediction mode of the chrominance block includes predicting the chrominance block from the reconstructed pixel values in the corresponding luminance block. Encoding a flag indicating whether or not 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 available for intra prediction in HEVC;
2 is a block diagram of an image encoding device according to an embodiment of the present invention;
Figure 3 is an example of block division using the QTBT structure,
Figure 4 is an illustration of a plurality of intra prediction modes according to an embodiment of the present invention;
Figure 5 is an example diagram showing neighboring blocks of the current block,
Figure 6 is an example diagram showing a block division structure for the luminance component and chrominance component of the CTU;
Figure 7 is an example diagram showing the intra prediction mode of each luminance block in the block division structure for the luminance component of the CTU;
Figure 8 is an example diagram showing the z-scan order;
Figure 9 is an example diagram for inducing DM (direct mode) of the present invention;
Figure 10 is another example for deriving DM of the present invention,
Figure 11 is another example for deriving DM of the present invention,
Figure 12 is another example for deriving DM of the present invention,
Figure 13 is another example for deriving DM of the present invention,
Figure 14 is a block diagram of an image decoding device according to an embodiment of the present invention.

Hereinafter, some embodiments of the present invention will be described in detail through illustrative drawings. In adding identification codes to components in each drawing, it should be noted that the same components are given the same codes as much as possible even if they are shown in different drawings. Additionally, 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.

Figure 2 is a block diagram of a video encoding device according to an embodiment of the present invention.

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

The block division unit 210 divides each picture constituting the image into a plurality of CTUs (Coding Tree Units) and then recursively divides the CTUs using a tree structure. A leaf node in the tree structure becomes a CU (coding unit), the basic unit of encoding. The tree structure is either QuadTree (QT), where the upper node is divided into four lower nodes, or QTBT (QuadTree), which is a combination of the QT structure and BinaryTree (BT) structure, where the upper node is divided into two lower nodes. plus BinaryTree) structure can be used.

In the QTBT (QuadTree plus BinaryTree) structure, the CTU is first divided into the QT structure. Afterwards, the leaf nodes of QT can be further divided by BT. Segmentation information generated by the block division unit 210 by dividing the CTU using the QTBT structure is encoded by the encoder 250 and transmitted to the video decoding device.

In QT, a first flag (QT split flag, QT_split_flag) indicating whether the block of the corresponding node is split is encoded. If the first flag is 1, the block of the node is divided into four blocks of the same size, and if the first flag is 0, the node is no longer divided by QT.

In BT, a second flag (BT split flag, BT_split_flag) indicating whether the block of the corresponding node is split is encoded. In BT, multiple split types may exist. For example, there may be two types: a type that divides the block of the node horizontally into two blocks of the same size and a type that divides it vertically. Alternatively, there may be an additional type that divides the block of the corresponding node into two asymmetric blocks. The asymmetric form may include dividing the block of the corresponding node into two rectangular blocks with a size ratio of 1:3, or may include dividing the block of the corresponding node diagonally. In this case, when the BT has multiple partition types, when the second flag indicating that the block is partitioned is encoded, partition type information indicating the partition type of the corresponding block is additionally encoded.

Figure 3 is an example of block division using the QTBT structure. Figure 3 (a) is an example of a block being divided by the QTBT structure, and (b) is this expressed in a tree structure. In Figure 3, the solid line represents division by the QT structure, and the dotted line represents division by the BT structure. In addition, with respect to the layer notation in Figure 3 (b), those without parentheses indicate a QT layer, and those with parentheses indicate a BT layer. In the BT structure represented by a dotted line, numbers represent split type information. 1 represents horizontal division and 0 represents vertical division.

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

The prediction unit 220 predicts the 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 prediction unit 222 predicts pixels within the current block using pixels (reference pixels) located around the current block within the current picture including the current block. There are multiple intra prediction modes depending on the prediction direction, and the surrounding pixels and calculation formulas to be used are defined differently for each prediction mode. In particular, the intra prediction unit 222 can determine the intra prediction mode to be used to encode the current block. In some examples, intra prediction unit 222 may encode the current block using multiple intra prediction modes and select an appropriate intra prediction mode to use from the tested modes. For example, the intra prediction unit 222 calculates rate-distortion values using rate-distortion analysis for several tested intra prediction modes, and selects the rate-distortion value with the best rate-distortion characteristics among the tested modes. You can also select intra prediction mode.

Figure 4 shows examples 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 a plurality of intra prediction modes and predicts the current block using surrounding pixels (reference pixels) and an operation formula determined according to the selected intra prediction mode. Information about the selected intra prediction mode is encoded by the encoder 250 and transmitted to the video decoding device.

The inter prediction unit 224 searches for a block most similar to the current block in a reference picture that has been encoded and decoded before the current picture, and generates a prediction block for the current block using the searched block. Then, a motion vector corresponding to the displacement between the current block in the current picture and the prediction block in the reference picture is generated. Motion information including information about reference pictures and motion vectors used to predict the current block is encoded by the encoder 250 and transmitted to the video decoding device.

The subtractor 230 generates a residual block by subtracting the prediction block generated by the intra prediction unit 222 or the inter prediction unit 224 from the current block.

The converter 240 converts the residual signal in the residual block with pixel values in the spatial domain into a transform coefficient in the frequency domain. The conversion unit 240 may convert the residual signals in the residual block using the size of the current block as a conversion unit, or divide the residual block into a plurality of smaller subblocks and convert the residual signals into a conversion unit of the subblock size. You can also convert it. There may be various ways to divide the residual block into smaller subblocks. For example, it may be divided into predefined subblocks of the same size, or QT (quadtree) type division may be used with the residual block as the root node.

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

The encoder 250 generates a bitstream by encoding the quantized transform coefficients using an encoding method such as CABAC. In addition, the encoder 250 encodes information such as CTU size, QT split flag, BT split flag, and split type related to block splitting, so that the video decoding device can split the block in the same way as the video coding device.

The encoder 250 encodes information about the prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and syntax elements for the intra prediction mode or inter prediction information depending on 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 restores the residual block by converting the transform coefficients output from the inverse quantization unit 260 from the frequency domain to the spatial domain.

The addition unit 270 restores the current block by adding the restored residual block and the prediction block generated by the prediction unit 220. Pixels in the restored current block are used as reference pixels when intra-predicting the next block.

The filter unit 280 performs deblocking filtering on the boundaries between restored blocks to remove blocking artifacts caused by block-level encoding/decoding and stores them in the memory 290. When all blocks in one picture are reconstructed, the reconstructed picture is later used as a reference picture for inter prediction of blocks in the picture to be encoded.

Meanwhile, the pixels of the current block may be composed of a luminance component and two chrominance components (Cb, 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 can be predicted and encoded independently of each other according to the encoding method described above. That is, the luminance block and the chrominance block are predicted independently from each other, and the residual signals of the luminance block and the chrominance block are also encoded independently.

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

In the case of a luminance block, the image encoding device 200 determines the intra prediction mode of the luminance block and classifies all intra prediction modes into a plurality of candidate sets. In addition, information about which group the intra prediction mode of the luminance block belongs to among a plurality of candidate sets and index information to indicate the intra prediction mode of the luminance block within the group are generated as luma intra mode information. Encode. A plurality of candidate sets can be created by classifying all intra prediction modes of the current block according to their frequency of occurrence. For example, the plurality of candidate sets includes the following groups.

(1) Group 1 (MPM Group):

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

(2) Second group (Selected Mode group):

It consists of modes that frequently appear among modes other than MPMs. The second group includes modes with mode numbers that are multiples of 4 when rearranging the mode numbers in ascending order, excluding MPM among all intra prediction modes, or an offset (e.g., -1) to the directional modes belonging to the first group. It consists of modes created by adding , +1).

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

It consists of the remaining modes excluding the modes belonging to the first and second groups among all intra prediction modes.

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

The candidate set may consist of the following four types of intra prediction modes.

(1) LM(linear mode):

This is an intra mode that derives a prediction block for the chrominance block using reconstructed pixel values in the luminance block. After calculating the scaling factor α value and the offset β value 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. The specific formula is as 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 restored luminance block corresponding to the current block, α is the scaling factor, and β is the offset. represents. α and β are either a block unit on which intra prediction is performed, a block of the highest layer corresponding to the root node in a tree structure for block partition, or a sequence unit that is a slice, picture, or group of multiple pictures. It can be included in the bitstream in units of and transmitted to the decoding device. Alternatively, α and β may be equally calculated by the encoding device and the decoding device using pixel values in the already restored luminance block and chrominance block around the current block. In this case signaling for α and β is not required.

(2) DM (direct mode):

This refers to the intra prediction mode of the luminance block corresponding to the 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, there is one luminance block corresponding to the chrominance block, so 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 in the upper right corner of FIG. 6(a), which corresponds to the chrominance block located in the upper right corner of FIG. 6(b), is divided into a total of six subblocks. Since each subblock becomes one CU, each subblock can be intra predicted using different intra prediction modes. Accordingly, there may be a plurality of intra prediction modes of the luminance block corresponding to the chrominance block.

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

(3) neighboring mode:

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

(4) default mode:

Probabilistically frequently used modes can be included in the candidate set in the order of planar, DC, vertical, horizontal, and diagonal modes. Alternatively, planar and DC modes are first included in the candidate set in that order, and then modes obtained by adding -1 or +1 to the angular modes (directional 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 candidate set with the intra prediction mode of the color difference block can be determined in various ways.

As an example, the intra prediction mode candidates of the color difference block can be filled in the 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 the 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 following order: two DMs, one LM, and the intra prediction modes of neighboring blocks L, AR, and AL. If two modes are obtained from one DM, one LM, and available surrounding 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 following order: 1 DM, 1 LM, surrounding block A, AL's intra prediction mode, planar, and DC. If 5 DMs and 1 LM are derived, all 6 candidates have already been derived, so the neighboring mode and default mode are not included in the candidate set.

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

The 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, check whether the new intra prediction mode already exists in the candidate set, and if not, add the new intra prediction mode to the candidate set.

Meanwhile, LM may always be included in the candidate set. In this case, among the total m candidates, the number of normal intra prediction modes excluding LM mode 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.

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

In order to derive one or more DMs, the image encoding device 200 must store intra prediction modes of the luminance block. The intra prediction modes of each luminance block can be stored in memory in an NxN block size. N can be set appropriately considering memory capacity, etc. For example, the value of N may have a value of 1 (that is, the intra prediction mode of the luminance block is stored on a pixel basis) and may have the minimum block size among block sizes available for intra prediction.

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

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

In this embodiment, when the luminance block corresponding to the chrominance block is divided into a plurality of subblocks, the image encoding device 200 uses intra prediction modes of at least some of the subblocks present in the luminance block. Select one or more candidates, that is, DM, for the intra prediction mode of the color difference block. 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 for 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. Subblocks at predefined positions include a pixel located at the center point of the luminance block corresponding to the chrominance block (center, CR), a pixel located at the top left within the luminance block (top left, TL), and a pixel located at the top right within the luminance block ( It can be selected from subblocks covering the top right (TR), the pixel located at the bottom left of the luminance block (bottom left, BL), and the pixel located at the bottom right of the luminance block (bottom right, BR). CR refers to the pixel (top left center, TLC) located at the top left based on the center point of the luminance block. However, it is not limited to this, and in addition to TLC, at least one of the pixels located in the upper right center (TRC), the pixel located in the lower left center (BLC), and the pixel located in the lower right center (bottom right center, BRC) It may also be selected as this CR.

Meanwhile, the priority may be the area of the block covered by the intra prediction mode of each of at least some subblocks within the luminance block. For example, assume that at least some subblocks are blocks that cover pixels located in CR, TL, TR, BL, and BR. Referring to Figure 11 (a), the intra prediction modes of CR, TL, TR, BL, and BR within the luminance block corresponding to the chrominance block located in the upper right corner of Figure 6 (b) are CR(G), 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 these modes occupy within 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 that order.

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

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

As a predefined scan order, z-scan order can be used. The z-scan order means top left, top right, bottom left, and bottom right. When a block is divided into subblocks as shown in FIG. 8, the processing order between sub-blocks is also z-scan order, and the processing order within the sub-block is also z-scan order. However, the predefined scan order is not limited to the z-scan order. When 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 is CR, TL, TR, BL, and BR, or the scan order is TL, CR, TR, BL, and BR. may also be used.

Hereinafter, a method of sequentially selecting candidates for the intra prediction mode of a chrominance block from a plurality of subblocks in a luminance block according to priority or a predefined scan order will be described through various embodiments. For convenience of explanation, it is assumed that the block division structures of the CTU composed of the luminance component and the CTU composed of the chrominance component are the same as Figure 6(a) and Figure 6(b), respectively. As previously exemplified, assuming that up to 5 normal intra prediction modes can be selected from DM, neighboring mode, and default mode excluding LM, among them, DM has the highest priority, so up to 5 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 the luminance block.

Figure 9 is an exemplary diagram for inducing DM according to Example 1. The uppercase letters (A to H) shown in (a) of FIG. 9 indicate the intra prediction mode, and the numbers shown 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 corner 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, there are a total of 6 sub-blocks in the luminance block corresponding to the chrominance block located in the upper right corner of Figure 9(b), and the 6 sub-blocks have different intra prediction modes. For the color difference block located in the upper right, a total of 5 DMs are selected in the order of intra prediction modes C, D, E, A, and F. Since the maximum number of DMs is 5, 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 existing in the luminance block. DMs are selected in order of highest priority, and if the priorities are the same, they are selected in z-scan order. In this embodiment, any one of the above-described block area, frequency, and overlap count can be used as a priority, but here, the description is made based on the block area.

Figure 10 is an exemplary diagram for inducing DM according to Example 2. The uppercase letters (A to H) shown in (a) of FIG. 10 indicate the intra prediction mode, and the numbers shown in (b) of FIG. 10 indicate the number of DMs in each color difference block.

Referring to (a) of FIG. 10, the luminance block corresponding to the chrominance block located in the upper left corner of (b) of FIG. 10 has two subblocks having different intra prediction modes. The area of the block covered by the two intra prediction modes within the luminance block is the same. Therefore, 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, there are a total of 6 sub-blocks in the luminance block corresponding to the chrominance block located in the upper right corner of Figure 10 (b), and the 6 sub-blocks have different intra prediction modes. The block area covered by each intra prediction mode within the luminance block is larger in the order of intra prediction modes C and G. And, the block areas covered by intra prediction modes D, E, A, and F are the same. In this embodiment, two DMs are first selected in the order of intra prediction modes C and G according to the block area, and among intra prediction modes D, E, A, and F that cover the same area, intra prediction mode D is selected according to z-scan order. Select three DMs in the following order: , 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 predefined positions among all subblocks existing in a luminance block based on priority and a predefined scan order. Subblocks at predefined positions include subblocks covering pixels located at CR, TL, TR, BL, and BR within the luminance block. In this embodiment, DMs are selected in order of highest priority, and if the priorities are the same, DMs are selected according to a predefined scan order. This embodiment can use any of the block area, frequency, or number of duplicates as the priority, and as a predefined scan order, the Z-scan order, or the scan order in the order of CR, TL, TR, BL, and BR, or The scan order of TL, CR, TR, BL, BR, etc. can be used. Hereinafter, an example will be given of using the Z-scan order as a predefined scan order and the block area or number of overlaps as the priority.

Figure 11 is an exemplary diagram for inducing DM according to Example 3. The capital letters (A to H) shown in (a) of FIG. 11 indicate the intra prediction mode, and the numbers shown in (b) of FIG. 11 indicate the number of DMs in each color difference block.

When using block area as a priority, referring to (a) of FIG. 11, the values of CR, TL, TR, BL, and BR within the luminance block corresponding to the chrominance block located at the upper left of FIG. 11 (b) There are two intra prediction modes corresponding to each location. Since the block area covered by the two intra prediction modes within the luminance block is the same, this 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 in the upper right corner of Figure 11 (b), there are a total of three intra prediction modes corresponding to the positions of CR, TL, TR, BL, and BR. Within the luminance block, 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. 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, and BR within the luminance block corresponding to the chrominance block located in the upper left corner of Figure 11 (b) The intra prediction modes used are CR (B), TL (A), TR (B), BL (A), and BR (B). The number of overlaps 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. Meanwhile, in the luminance block corresponding to the chrominance block located in the upper right corner of Figure 11 (b), the intra prediction modes corresponding to the positions of CR, TL, TR, BL, and BR are CR (G), TL (C), Same as TR (C), BL (A), BR (G). The number of overlaps 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 as DM before mode A. Between modes C and G with the same number of overlaps, intra prediction mode C is selected as DM first according to the Z-scan order. Therefore, three DMs are selected in the order C, G, and A. The remaining two candidates are selected from neighboring mode and default mode.

In the above-described embodiment, both the priority and the predefined scan order are considered, but this embodiment may use only the predefined scan order without considering the block area. For example, it is possible to determine the DM by the scan order of CR, TL, TR, BL, and BR, or by the scan order of TL, CR, TR, BL, and BR. When using the scan order of CR, TL, TR, BL, and BR, the DM of the color difference block located in the upper left of Figure 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 predefined positions among all subblocks existing in the luminance block based on priority and a predefined scan order. However, in this embodiment, subblocks at predefined positions that cover pixels located at TLC, TRC, BLC, and BRC as well as TL, TR, BL, and BR within the luminance block are used. Below, an example will be given of using the z-scan order as a predefined scan order and the block area or number of overlaps as the priority.

Figure 12 is an exemplary diagram for inducing DM according to Example 4. The capital letters (A to H) shown in (a) of FIG. 9 indicate the intra prediction mode, and the numbers shown in (b) of FIG. 12 indicate the number of DMs in each color difference block.

First, the case of using block area as priority will be explained. Referring to (a) of FIG. 12, the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC within the luminance block corresponding to the chrominance block located in the upper left corner of (b) of FIG. 12. There are two intra prediction modes. Since the block area covered by the two intra prediction modes within the luminance block corresponding to the chrominance block located in the upper left corner is the same, DM is selected in the order of intra prediction modes A and B in the Z-scan order. 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 corner of Figure 12 (b), there are a total of four intra prediction modes corresponding to the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC. According to the block area covered by each intra prediction mode, two DMs are selected first in the order of intra prediction modes C and G. Since the intra prediction modes corresponding to the BL and BLC positions cover the same block area, 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 candidate is selected from neighboring mode and/or default mode.

To explain the case of using the number of duplicates as priority, the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC within the luminance block corresponding to the chrominance block located in the upper left corner of Figure 12 (b). The corresponding intra prediction modes are TL (A), TR (B), BL (A), BR (B), TLC (A), TRC (B), BLC (A), and BRC (B). Since the number of overlaps between the two intra prediction modes corresponding to the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC is the same at 4, DM is performed in the order of intra prediction modes A and B in Z-scan order. Choose. 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 corner of Figure 12 (b), the intra prediction modes corresponding to the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC are TL (C) , TR (C), BL (A), BR (G), TLC (C), TRC (C), BLC (E), BRC (G). The number of overlaps 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. Depending on the number of overlaps, two DMs are selected first in the order of intra prediction mode C and G. Since the intra prediction modes A and E corresponding to the BL and BLC positions have the same number of overlaps, 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 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 predefined positions among all subblocks existing in the luminance block based on priority 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. Below, an example will be given of using the z-scan order as a predefined scan order and the block area or number of overlaps as the priority.

Figure 13 is an exemplary diagram for inducing DM according to Example 5. The capital letters (A to H) shown in (a) of FIG. 9 indicate the intra prediction mode, and the numbers shown in (b) of FIG. 13 indicate the number of DMs in each color difference block.

First, the case of using block area as priority will be explained. Referring to Figure 13 (a), there are two intra prediction modes corresponding to the positions of TLC, TRC, BLC, and BRC within the luminance block corresponding to the chrominance block located in the upper left corner of Figure 13 (b). . Since the block area covered by the two intra prediction modes within the luminance block corresponding to the chrominance block located in the upper left corner is the same, DM is selected in the order of intra prediction modes A and B in 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 corner of Figure 13 (b), there are a total of three intra prediction modes corresponding to the positions of TLC, TRC, BLC, and BRC. The block area covered by each intra prediction mode is different. Therefore, 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.

To explain the case of using the number of duplicates as a priority, 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 corner of Figure 13 (b) are 2. Dogs exist. Looking at the intra prediction modes of subblocks at each location, they are TLC (A), TRC (B), BLC (A), and BRC (B). Since the number of overlaps between intra prediction modes corresponding to each position is the same, 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 corner of Figure 13 (b), the intra prediction modes corresponding to the positions of TLC, TRC, BLC, and BRC are TLC (C), TRC (C), and BLC ( E), same as BRC (G). The number of overlaps between intra prediction modes at each location is 2 times for C and 1 time each for E and G. Therefore, DM selects intra prediction mode C first according to the number of overlaps. Since the number of overlapping intra prediction modes corresponding to the BLC and BRC positions is the same, intra prediction modes E and G are sequentially selected as 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 the candidate set and encodes the index corresponding to the candidate selected as the intra prediction mode of the chrominance 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 the index of a candidate selected first in the candidate set has a smaller number of bits than the index of a candidate selected later. At this time, the indices for the two candidates last selected from the candidate set may be binarized so that they have the same number of bits and the remaining bits except the least significant bit (LSB) are the same.

Table 3 below shows how to binarize the mode index when the number of candidates in the candidate set is 6.

Codeword #1 is the result of binarization using the TU (truncated unary) method. For example, when indexing is given 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 the first value (0 in Table 1), it indicates that it is LM, and if it has the second value (1 in Table 1), it indicates that it is not LM. Therefore, in the case of LM, the mode index is expressed as 0.

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

In the above, four types (LM, DM, neighboring mode, default mode) are sequentially applied to select a predetermined number (e.g., 6) of candidates, and which of the candidates is used as the intra prediction mode of the color difference block. It was explained that the mode index indicating is encoded. However, the present invention is not necessarily limited thereto.

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

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

Figure 8 shows an image decoding device according to an embodiment of the present invention.

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

The decoder 1410 decodes the bitstream received from the video encoding device, extracts information related to block division, determines the current block to be decoded, and provides prediction information and residual signal information necessary to restore the current block. Extract .

The decoder 1410 extracts information about the CTU size from the SPS (Sequence Parameter Set) or PPS (Picture Parameter Set), determines the size of the CTU, and divides the picture into CTUs of the determined size. Then, the CTU is determined as the highest layer of the tree structure, that is, the root node, and the CTU is divided using the tree structure by extracting the division information about the CTU. For example, when dividing a CTU using the QTBT structure, first extract the first flag (QT_split_flag) related to the division of the QT and split each node into four nodes of the lower layer. And, for the node corresponding to the leaf node of the QT, the second flag (BT_split_flag) and split type information related to the split of the BT are extracted and the leaf node is split into a BT structure.

Meanwhile, when the decoder 1410 determines the current block to be decoded by dividing the tree structure, it extracts information about the 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 the syntax element for the intra prediction information (intra prediction mode) of the current block and transmits it to the intra prediction unit 1444.

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

The inverse quantization unit 1420 inversely quantizes the quantized transform coefficients, and the inverse transformation unit 1430 inversely transforms the inverse quantized transform coefficients from the frequency domain to the spatial domain to restore the residual signals, thereby generating a residual block for the current block.

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

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 predict the current block. Predict blocks.

The inter prediction unit 1444 determines motion information of the current block using syntax elements for 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 restored current block are used as reference pixels when intra-predicting a block to be decoded later.

The filter unit 1460 deblocks and filters the boundaries between restored blocks to remove blocking artifacts that occur due to block-level decoding and stores them in the memory 1470. When all blocks in one picture are reconstructed, the reconstructed picture is later used as a reference picture for inter prediction of blocks in the picture to be decoded.

Meanwhile, when the current block consists of a luminance component and a chrominance component, the luminance component and the chrominance component of the current block are predicted and restored independently. In this case, the syntax element for the 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 the luminance component of the current block. Color difference intra mode information is included for this purpose.

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

The intra prediction unit 1442 derives intra prediction mode candidates constituting the group in the same manner as the image encoding device 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 it indicates a Selected Mode group, a Selected Mode group is created, and if it indicates a non-Selected Mode group, a non-Selected Mode group is created. Then, the candidate identified by the index within the corresponding group is set to the 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 to the residual block for the luminance component, and the luminance component of the current block is restored.

Additionally, the intra prediction unit 1442 predicts a chrominance block using chrominance intra mode information. The intra prediction unit 1442 configures a candidate set for the intra prediction mode of the chrominance block in the same manner as the image encoding device 200. That is, a candidate set is constructed using at least one type among DM, LM, neighboring mode, and default mode. Since the method for deriving each type of mode has already been described in the video encoding device, detailed description will be omitted to avoid duplication. The intra prediction unit 1442 selects one candidate from among the candidates in the candidate set as the intra prediction mode of the chrominance block 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 the total m. Among the candidates, it includes information indicating the intra prediction mode of the color difference block. The intra prediction unit 1442 constructs a candidate set by sequentially selecting m candidates in the following order: DM, LM, neighboring mode, default mode, or LM, DM, neighboring mode, default mode. Among the m candidates in the candidate set, the candidate indicated by the color difference intra mode information is determined as the intra prediction mode of the color difference block.

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

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

Claims (7)

In a method performed by an image decoding apparatus to intra-predict a chroma block divided from a chrominance coding treeblock having a block division structure different from the corresponding luminance coding treeblock,
Decoding partition information indicating a partition structure of the chrominance coding tree block from a bitstream, and determining a chrominance block to be decoded based on the partition information;
decoding a flag indicating whether the target chrominance block is predicted from reconstructed pixel values in a corresponding luminance block from the bitstream; and
Determining an intra prediction mode of the target chrominance block according to the flag and predicting the target chrominance block,
When the flag indicates that the target chrominance block is not predicted from reconstructed pixel values in the luminance block, predicting the target chrominance block includes:
Decoding mode information for selecting an intra prediction mode of the target chrominance block from a candidate set of predefined intra prediction modes, the candidate set being a DM (direct) intra prediction mode derived from the luminance block. includes mode); and
A step of determining an intra prediction mode of the target chrominance block using the mode information,
The method is characterized in that the DM is set using an intra prediction mode of a block covering samples at at least one predefined position in the luminance block. According to paragraph 1,
The method of claim 1, wherein the candidate set further includes at least two of a planar mode, a DC mode, a horizontal mode, and a vertical mode. According to paragraph 1,
When the flag indicates that the target chrominance block is predicted from reconstructed pixel values in the luminance block, intra-predicting the target chrominance block includes:
Deriving prediction parameters using the reconstructed chrominance samples around the target chrominance block and the reconstructed luminance samples around the luminance block; and
Predicting the target chrominance block using the restored pixel values in the luminance block and the prediction parameters.
A method comprising: According to paragraph 1,
and wherein the at least one predefined location sample includes a center sample within the luminance block. According to paragraph 1,
The DM is determined based on the priorities of intra prediction mode candidates of blocks each covering a sample at the at least one predefined location,
The above priorities are:
The area of the block covered by each of the intra prediction mode candidates, or
The frequency with which the same intra prediction mode as each of the intra prediction mode candidates occurs within the luminance block, or
Number of overlaps between the intra prediction mode candidates
A method characterized in that it is determined according to. A method performed by an image encoding device to intra-predict a chroma block divided from a chrominance coding treeblock having a block division structure different from the corresponding luminance coding treeblock, comprising:
determining a division structure of the chrominance coding tree block and a chrominance block to be encoded;
Encoding segmentation information indicating a segmentation structure of the chrominance coding tree block into a bitstream;
determining an intra prediction mode of the target chrominance block;
predicting the target chrominance block using an intra prediction mode of the target chrominance block; and
Including encoding information about the intra prediction mode of the target chrominance block,
The step of encoding information about the intra prediction mode of the target chrominance block is:
encoding a flag indicating whether the target chrominance block is predicted from reconstructed pixel values in a corresponding luminance block; and
When the flag indicates that the target chrominance block is not predicted from reconstructed pixel values in the luminance block, for selecting an intra prediction mode of the target chrominance block from a candidate set of predefined intra prediction modes. Including the step of encoding mode information,
The candidate set includes a direct mode (DM), which is an intra prediction mode derived from the luminance block,
The method is characterized in that the DM is set using an intra prediction mode of a block covering samples at at least one predefined position in the luminance block. In a method of transmitting information about a chroma block divided from a chrominance coding tree block having a block division structure different from the corresponding luminance coding tree block to an image decoding device,
Encoding information about the chrominance block into a bitstream using intra prediction; and
Including transmitting the bitstream to the video decoding device,
The step of encoding information about the chrominance block into a bitstream is:
determining a division structure of the chrominance coding tree block and a chrominance block to be encoded;
encoding segmentation information indicating a segmentation structure of the chrominance coding tree block into the bitstream;
determining an intra prediction mode of the target chrominance block;
predicting the target chrominance block using an intra prediction mode of the target chrominance block; and
Including encoding information about the intra prediction mode of the target chrominance block,
The step of encoding information about the intra prediction mode of the target chrominance block is:
encoding a flag indicating whether the target chrominance block is predicted from reconstructed pixel values in a corresponding luminance block; and
When the flag indicates that the target chrominance block is not predicted from reconstructed pixel values in the luminance block, for selecting an intra prediction mode of the target chrominance block from a candidate set of predefined intra prediction modes. Including encoding mode information,
The candidate set includes a direct mode (DM), which is an intra prediction mode derived from the luminance block,
The method is characterized in that the DM is set using an intra prediction mode of a block covering samples at at least one predefined position in the luminance block.