WO2019135629A1 - Method for reconstructing chroma block, and image decoding apparatus using same - Google Patents

Abstract

A method for reconstructing a chroma block and an image decoding apparatus using same are disclosed. Provided according to one embodiment of the present invention is a method for partitioning a chroma block, corresponding to a coding tree unit (CTU), by an image decoding apparatus, the method for reconstructing a chroma block comprising the steps of: decoding partition information of a luma block, corresponding to the CTU, and partition information of the chroma block from a bit stream; and reconstructing the chroma block on the basis of the partition information of the luma block and the partition information of the chroma block.

Description

Method for restoring chroma block and image decoding apparatus using the same

More particularly, the present invention relates to a chroma-block reconstruction method that improves the efficiency of encoding and decoding by sharing division information of a chroma block and division information of a luma block, and a video decoding apparatus using the same .

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

Accordingly, when moving picture data is stored or transmitted, the moving picture data is compressed and stored or transmitted using an encoder. The decoder receives compressed moving picture data, decompresses and reproduces the moving picture data. There are H.264 / AVC and HEVC (High Efficiency Video Coding) which improves the coding efficiency by about 40% compared to H.264 / AVC.

However, since the image size, resolution, and frame rate are gradually increasing and the amount of data to be encoded is also increased, a new compression technique is required that has higher coding efficiency and higher image quality than existing compression techniques.

An object of the present invention is to provide an improved image encoding and decoding technique in order to meet such a demand. In particular, one aspect of the present invention is to provide an improved image encoding and decoding technology for encoding and decoding by reducing the number of bits required for division information of a chroma block. And relates to techniques for improving the efficiency of decoding.

According to an aspect of the present invention, there is provided a method of restoring a chroma block corresponding to a coding tree unit (CTU) in an image decoding apparatus, the method comprising: dividing a luma block corresponding to the CTU from a bitstream; Information and the division information of the chroma block; And reconstructing the chroma block based on division information of the luma block and division information of the chroma block.

According to another aspect of the present invention, there is provided an image decoding apparatus, comprising: a bitstream extracting unit that extracts, from a bitstream, division information of a luma block corresponding to a coding tree unit (CTU), and division information of a chroma block corresponding to the CTU And determines a division structure of the luma block by using the division information of the luma block and determines a division structure of the chroma block based on the division structure of the luma block and the division information of the chroma block, ; And an image reconstructor for reconstructing an image based on the division structure of the luma block and the division structure of the chroma block.

As described above, according to the embodiment of the present invention, since the division structure of the chroma block can be expressed by a smaller number of bits by reflecting the similarity or the shared relation between the luma block and the chroma block in block division, And the efficiency for decoding can be improved.

1 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of the present disclosure;

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

3 is a diagram for explaining a plurality of intra prediction modes,

4 is an exemplary block diagram of an image decoding apparatus capable of implementing the techniques of the present disclosure;

5 is a diagram for explaining a case where a chroma block and a luma block are independently divided for a single CTU,

Figures 6-14 illustrate various embodiments of the present invention that share partition information of chroma blocks and luma blocks for certain portions of a single CTU.

Hereinafter, some embodiments of the present invention will be described in detail with reference to exemplary drawings. It should be noted that, in adding the identification codes to the constituent elements of the respective drawings, the same constituent elements are denoted by the same reference symbols as possible even if they are displayed on different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

1 is an exemplary block diagram of an image encoding apparatus in which the techniques of the present disclosure may be implemented.

1, the image encoding apparatus includes a block division unit 110, a prediction unit 120, a subtractor 130, a transform unit 140, a quantization unit 145, an encoding unit 150, An inverse transform unit 165, an adder 170, a filter unit 180,

Each component of the image encoding apparatus may be implemented by hardware or software, or may be implemented by a combination of hardware and software. In addition, the functions of the respective components may be implemented in software, and the microprocessor may be implemented in such a manner that each function of each software (component) is executed.

One video (video) is composed of a plurality of pictures. Each picture is divided into a plurality of areas and coding is performed for each area. For example, one picture is divided into one or more slices or / and tiles, and each slice or tile is divided into one or more CTU (Coding Tree Unit). Each CTU is divided into one or more CUs (Coding Units) by a tree structure.

The information applied to each CU is encoded in the Syntax of the CU, and the information commonly applied to the CUs contained in one CTU is encoded into the syntax of the CTU. In addition, information that is commonly applied to all blocks in one tile is encoded into a syntax of a tile or is encoded into a syntax of a group of tiles to which the tile belongs, and information applied to all blocks constituting one picture includes a picture parameter Picture parameter set (PPS) or a picture header.

In addition, information that is commonly referred to by a plurality of pictures is encoded into a sequence parameter set (SPS), and information that is commonly referred to by one or more SPSs is encoded into a video parameter set (VPS).

The block dividing unit 110 determines the size of the Coding Tree Unit (CTU). The information on the size of the CTU (CTU size) is encoded by the syntax of SPS or PPS and transmitted to the image decoding apparatus.

The block dividing unit 110 divides each picture constituting an image into a plurality of CTUs (Coding Tree Units) having a predetermined size, and thereafter repeatedly divides the divided CTUs using a tree structure And recursively. A leaf node in a tree structure becomes a coding unit (CU) which is a basic unit of coding.

In the tree structure, a quad tree (QT) in which an upper node (or a parent node) is divided into four lower nodes (or child nodes) having the same size, a binary tree in which an upper node is divided into two lower nodes Tree, BT), and a tertiary tree (TT) in which an upper node is divided into three lower nodes at a ratio of 1: 2: 1.

Also, a structure in which two or more of the QT structure, the BT structure, and the TT structure are mixed may be included. For example, a Quad Tree plus Binary Tree (QTBT) structure may be used, and a Quad Tree plus Binary Tree Ternary Tree (QTBTTT) structure may be used.

2 is a diagram for explaining a method of dividing a block using a QTBTTT structure. As shown in FIG. 2, the CTU can be first divided into a QT structure. The quadtree partitioning can be repeated until the size of the splitting block reaches the minimum block size (MinQTSize) of the leaf node allowed in QT.

If the leaf node of the QT is not larger than the maximum block size (MaxBTSize) of the root node allowed in BT, it may be further divided into one or more of a BT structure or a TT structure. In the BT structure and / or the TT structure, a plurality of dividing directions may exist.

For example, in some examples, there may be two types of horizontally dividing a block of the node (i.e., horizontal splitting) and a vertically dividing type (i.e., vertical splitting).

2, when the BTTT segmentation is performed, a flag indicating whether the nodes are divided, a flag indicating a dividing direction (vertical or horizontal), and / or a flag indicating a division type (Binary or Ternary) Lt; / RTI > device.

On the other hand, there may be additional types of dividing the blocks of the node into two asymmetric blocks. The asymmetric shape may include a shape in which a block of the node is divided into two rectangular blocks having a size ratio of 1: 3, a shape in which a block of the corresponding node is divided in a diagonal direction, and the like.

When QTBT is used as another example of a tree structure, after the CTU is first divided into the QT structure, the leaf nodes of the QT can be further divided into the BT structure. The CU may have various sizes depending on the QTBT or QTBTTT segment from the CTU. Hereinafter, a block corresponding to a CU to be encoded or decoded (i.e., a leaf node of QTBTTT) is referred to as a 'current block'.

The prediction unit 120 generates a prediction block by predicting the current block. The prediction unit 120 may include an intra prediction unit 122 and an inter prediction unit 124. In general, the current blocks in a picture may each be predictively coded. The prediction of the current block can be performed using an intra prediction technique using data of a picture containing the current block or an inter prediction technique using data of a picture coded prior to the picture including the current block.

The intra prediction unit 122 predicts pixels in the current block using pixels (reference pixels) located in the vicinity of the current block in the current picture including the current block. There are a plurality of intra prediction modes according to the prediction direction. For example, as shown in FIG. 3, a plurality of intraprediction modes may include a non-directional mode including a planar mode and a DC mode, and 65 directional modes. The neighboring pixels to be used and the calculation expression are defined differently according to each prediction mode.

The intra prediction unit 122 may determine an intra prediction mode to be used for coding the current block. In some examples, the intra-prediction unit 122 may encode the current block using a plurality of intra-prediction modes and may select an appropriate intra-prediction mode to use from the tested modes.

For example, the intra-prediction unit 122 may calculate rate-distortion values using a rate-distortion analysis for various tested intra-prediction modes, and may employ rate- The intra prediction mode may be selected.

The intra prediction unit 122 selects one intra prediction mode from a plurality of intra prediction modes, and predicts the current block using neighboring pixels (reference pixels) determined by the selected intra prediction mode and an equation. The information on the selected intra prediction mode is encoded by the encoding unit 150 and transmitted to the video decoding apparatus.

The inter-prediction unit 124 searches for a block most similar to the current block in the reference picture encoded and decoded prior to the current picture through a motion estimation process, And generates a prediction block for the block. Inter prediction can be generally classified into uni-directional prediction and bi-directional prediction according to the prediction direction.

The inter prediction unit 124 generates a motion vector corresponding to a displacement between the current block in the current picture and the prediction block in the reference picture. In general, motion estimation is performed on a luma component, and motion vectors calculated based on luma components are used for both luma components and chroma components.

The motion information including the information on the reference picture used for predicting the current block and the information on the motion vector is encoded by the encoding unit 150 and transmitted to the video decoding apparatus.

Various methods can be used to reduce or minimize the amount of bits required to encode motion information. Examples of these various methods include a Skip mode, a Merge mode, and an Amvp (Adaptive (advanced) motion vector predictor) mode.

The subtracter 130 generates a residual block by subtracting the current block from the prediction block generated by the intra prediction unit 122 or the inter prediction unit 124. The transform unit 140 transforms the residual And converts the residual signal in the block into a transform coefficient in the frequency domain.

The transform unit 140 may transform the residual signals in the residual block using the size of the current block as a transform unit, divide the residual block into a plurality of smaller subblocks, transform the residual signals into subblock- You may.

There are various ways of dividing the residual block into smaller sub-blocks. For example, it may be divided into predefined subblocks of the same size, or a quad tree (QT) scheme using the residual block as the root node.

The quantization unit 145 quantizes the transform coefficients output from the transform unit 140, and outputs the quantized transform coefficients to the encoding unit 150.

The encoding unit 150 generates a bitstream by encoding the quantized transform coefficients using a coding scheme such as CABAC. Also, the encoding unit 150 encodes and signals information such as a CTU size, a QT division flag, a BTTT division flag, a dividing direction, and a division type associated with the block division so that the video decoding apparatus divides the blocks .

Further, the encoding unit 150 encodes information on a prediction type indicating whether the current block is coded by intra-prediction or inter-prediction, and outputs the intra-prediction information (that is, Or information on inter prediction information (information on reference pictures and motion vectors).

The inverse quantization unit 160 dequantizes the quantized transform coefficients output from the quantization unit 145 to generate transform coefficients. The inverse transform unit 165 transforms the transform coefficients output from the inverse quantization unit 160 from the frequency domain to the spatial domain and restores the residual block.

The adder 170 adds the reconstructed residual block and the prediction block generated by the predictor 120 to reconstruct the current block. The pixels in the reconstructed current block are used as reference pixels for intra prediction of the next-order block.

The filter unit 180 may be configured to reduce the blocking artifacts, ringing artifacts, blurring artifacts, and the like caused by block-based prediction and transform / . The filter unit 180 may include a deblocking filter 182 and an SAO filter 184.

The deblocking filter 180 filters the boundary between the restored blocks to remove blocking artifacts caused by coding / decoding on a block basis, and the SAO filter 184 performs additional filtering on the deblocking filtered image .

The SAO filter 184 corresponds to a filter used to compensate for the difference between the reconstructed pixel and the original pixel due to lossy coding. The restored block filtered through the deblocking filter 182 and the SAO filter 184 is stored in the memory 190. [ When all the blocks in one picture are reconstructed, the reconstructed picture is used as a reference picture for inter prediction of a block in a picture to be coded later.

4 is an exemplary block diagram of an image decoding apparatus capable of implementing the techniques of this disclosure;

4, the image decoding apparatus includes an inverse quantization unit 420, an inverse transform unit 430, a prediction unit 440, an adder 450, a filter unit 460, and a memory 470 A reconstructor 4000, and a decoding unit 410. [

1, each component may be implemented as a hardware chip, and the functions of the respective components may be implemented by software, and the microprocessor may be configured to execute each function of each software .

The decoding unit 410 decodes the bitstream received from the image encoding apparatus and extracts information related to the block division (division information of the luma block and / or division information of the chroma block) and uses the extracted current block to decode And extracts prediction information necessary for restoring the current block and information about the residual signal.

The decoding unit 410 extracts information on a CTU size from an SPS (Sequence Parameter Set) or a PPS (Picture Parameter Set) to determine the size of the CTU, and divides the picture into CTUs of a predetermined size. In addition, the decoding unit 410 determines the CTU as the top layer of the tree structure, that is, the root node, extracts the segmentation information from the bitstream, and then divides or restores the block using the segmentation information.

For the node corresponding to the leaf node of the QT division, the decoding unit 410 extracts the information about whether the BT is divided into blocks and the division type (division direction) of the block, and divides the corresponding leaf node into the BT structure.

As another example, when dividing or restoring a block using the QTBTTT structure, the decoding unit 410 extracts information (flag) about whether or not the QT is divided, divides each node into four nodes of a lower layer, The node corresponding to the leaf node (the node where the QT division no longer occurs) is further divided into BT or TT, information on the dividing direction, and split type information for distinguishing whether the BT structure or the TT structure is extracted It divides recursively into BT or TT structures.

The decoding unit 410 of the present invention determines the division structure of the chroma block based on the division structure of the luma block, which will be described later in detail.

When the current block to be decoded is determined using the division information, the decoding unit 410 extracts information on the prediction type indicating whether the current block is intra-predicted or inter-predicted.

When the prediction type information indicates intra prediction, the decoding unit 410 extracts the syntax element for the intra prediction information (intra prediction mode) of the current block. When the prediction type information indicates inter prediction, the decoding unit 410 extracts syntax elements for inter prediction information, that is, information indicating a motion vector and a reference picture referred to by the motion vector.

Meanwhile, the decoding unit 410 extracts information on the quantized transform coefficients of the current block as information on the residual signal.

The inverse quantization unit 420 dequantizes the quantized transform coefficients, and the inverse transform unit 430 inversely transforms the dequantized transform coefficients from the frequency domain to the spatial domain to generate residual blocks for the current block by restoring the residual signals .

The prediction unit 440 may include an intra prediction unit 442 and an inter prediction unit 444. The intra prediction unit 342 is activated when the prediction type of the current block is intra prediction, The unit 344 is activated when the prediction type of the current block is intra prediction.

The intra prediction unit 442 determines an intra prediction mode of the current block among the plurality of intra prediction modes using the syntax element for the intra prediction mode extracted from the decoding unit 410, The current block is predicted using reference pixels.

The image restorer 4000 sequentially restores the current blocks corresponding to the CUs, thereby restoring the picture composed of the CTUs and CTUs constituted by the CUs.

The inter prediction unit 444 determines a motion vector of the current block and a reference picture referenced by the motion vector using a syntax element for the intra prediction mode extracted from the decoding unit 410, Predicts the current block.

The adder 450 adds the residual block output from the inverse transform unit 430 and the prediction block output from the inter prediction unit 444 or the intra prediction unit 442 to reconstruct the current block. The pixels in the reconstructed current block are used as reference pixels for intraprediction of a block to be decoded later.

The filter unit 460 includes a deblocking filter 462 and an SAO filter 464. The deblocking filter 462 performs deblocking filtering on the boundary between the restored blocks to remove blocking artifacts caused by decoding on a block-by-block basis. The SAO filter 464 performs additional filtering on the reconstructed block after deblocking filtering to compensate for the difference between the reconstructed pixel and the original pixel resulting from lossy coding.

The reconstructed block filtered through the deblocking filter 462 and the SAO filter 464 is stored in the memory 470. When all the blocks in one picture are reconstructed, And is used as a reference picture for prediction.

FIG. 5 is a view for explaining a case in which a luma block and a chroma block are independently divided for a single CTU. FIGS. 6 to 14 illustrate a case where a chroma block and a luma block share division information And dividing the remaining parts independently of one another, according to various embodiments of the present invention.

Hereinafter, with reference to FIGS. 5 to 14, it is assumed that the chroma block and the luma block share division information and the division information is not shared (when the luma block and the chroma block are independently divided) To indicate the division structure of the number of bits required).

The comparison of the number of required bits is explained mainly for the case of QTBT segmentation for convenience of explanation and understanding, but the embodiment of the present invention sharing the segmentation information can also be applied to the case of QTBTTT segmentation.

5 (a) shows an example of a chroma block that is independently divided into QTBTs, FIG. 5 (b) shows a tree structure of the divided structure of FIG. 5 (a) ).

Although not shown in the figure, the luma blocks are independently divided to have a separate tree structure for the divided structure. That is, the partition tree for the chroma block and the partition tree for the luma block exist separately.

As shown in FIG. 5 (b), the chroma block is represented by "1" when the chroma block is QT divided and is represented by "0" when the chroma block is not QT divided. , "10" in the case of BT division in the horizontal direction, and "11" in the case of BT division in the vertical direction.

With reference to the example shown in FIG. 5, when dividing a chroma block independently, one bit at depth 0, 8 bits at depth 1, 12 bits at depth 2, A total of 28 bits are needed to represent the partition structure of the chroma block, such as 2 bits at depth 5 and depth 4 at depth 3.

Hereinafter, various embodiments of the present invention in which the chroma block and the luma block share division information for a certain portion in a single CTU, and each of the other portions are separately described will be described in detail.

Before explaining the various embodiments of the present invention, a process of determining the division structure of the luma block and the chroma block by the decoding unit 410 will be described. The decoding unit 410 of the present invention decodes the division information of the luma block corresponding to the CTU and the division information of the chroma block corresponding to the CTU in the bit stream received from the image encoding apparatus, And determines the division structure of the block.

In addition, the decoding unit 410 of the present invention determines (divides the chroma block) the division structure of the chroma block based on the division structure of the luma block (division information of the luma block) for a certain portion sharing the division information, For another part that does not share information (independently dividing), the division structure of the chroma block is determined (chroma block restoration) based on the division information of the chroma block.

That is, the decoding unit 410 of the present invention decodes the division information of the luma block corresponding to the CTU and the division information of the chroma block from the bitstream, and restores the chroma block based on the decoded information.

According to the embodiment, the decoding unit 410 of the present invention decodes the division information of the luma block corresponding to the CTU in the bit stream, determines the division structure of the luma block using this information, And decode the division information of the chroma block to be decoded.

Each embodiment of the present invention described below can be performed by the encoding unit 110 of the image encoding apparatus illustrated in FIG. 1 and / or the decoding unit 410 of the image decoding apparatus illustrated in FIG. 4, And / or using one or more other units of the image decoding apparatus.

Example 1

FIGS. 6 and 7 are diagrams for explaining Embodiment 1 of the present invention which does not allow division of a chroma block when a luma block is not divided and permits division of a chroma block only when the luma block is divided.

In the case of the first embodiment, the division structure of the chroma block may correspond to a subset of the luma block division structure. That is, the division structure of the chroma block shares a part of the division structure of the luma block, and independent division of only the chroma block is not allowed.

6 (a) and 7 (a) show one embodiment of a divided structure of luma blocks, and Figs. 6 (b) and 7 (b) show an embodiment of a divided structure of chroma blocks, Figs. 6 (c) and 7 (c) show the tree structure of the division structure of the chroma block through the first embodiment.

First, in FIG. 6, since the chroma block is divided because the luma block is divided on the basis of the depth 0 (QT, BT, and TT irrespective of the QT, BT, and TT), the chroma block is divided. (&Quot; 1 ").

In the case of the left top block, the luma block is divided in the horizontal direction and then divided in the vertical direction with respect to the upper block. Since the chroma block is divided only in the horizontal direction, the division information of the chroma block is divided (&Quot; 1 "), and not to the upper block (" 0 ").

Since the luma block is not divided in the upper right block and the lower left block based on the depth 1, the division of the chroma block is not allowed, so that the division information of the chroma block does not include any information or index about that division of the chroma block is not allowed Do not. That is, in this case, the image encoding apparatus does not signal the division information of the chroma block (" - "). In the case of the right lower block, since the luma block is divided and the chroma block is also divided, the division information of the chroma block is divided ("1").

Since the chroma block is not divided, the division information of the chroma block is not divided (" 0 "), whereas the lower right block of the luma block is divided in the horizontal and vertical directions based on the depth 2.

When calculating the number of bits of the chroma block division information on the basis of the first embodiment shown in Fig. 6, only 5 bits are required in total to represent the division structure of the chroma block.

Next, in Fig. 7, the luma block and the chroma block have the same division structure. Therefore, the partition information of the chroma block indicates that the chroma block is also divided ("1") at the node where the luma block is partitioned, and the luma block does not show any information about the partition of the chroma block at the unpartitioned leaf node ").

In this case, if the number of bits for representing the division structure of the chroma block is calculated based on the tree structure shown in Fig. 7 (c), a total of 6 bits are required.

When the division information of the chroma block to which Embodiment 1 is applied is signaled from the image encoding apparatus, the decoding unit 410 determines the division structure of the chroma block using the division information and divides the chroma block accordingly.

Specifically, if the luma block is not divided (at a specific node), the decoding unit 410 can not divide the chroma block, and the chrominance block division information for the corresponding node is not decoded from the bitstream, .

Alternatively, when the luma block is divided (at a specific node), the image decoding apparatus determines whether to divide the chroma block based on the division information of the chroma block. If the division information of the chroma block indicates that the chroma block is divided, It is determined that the block is divided (the division type shares the division direction and division type information of the luma block), and when the division information of the chroma block indicates that the chroma block is not divided, it is determined that the chroma block is not divided.

As described above, according to the division structure of the chroma block on the basis of the first embodiment, only division bits of the chroma block are included in the division information of the chroma block only when the luma block is divided, BT, TT, etc.) and the dividing direction, it is possible to reduce the total number of bits for representing the division structure of the chroma block, thereby improving the efficiency of image encoding and decoding have.

Example 2

The second embodiment of the present invention corresponds to a method of generating one or more candidates that correspond to the division structure of chroma blocks based on the division structure of luma blocks and determining one of the candidates as the actual structure of the chroma block .

FIG. 8 is a diagram for explaining an embodiment for generating or setting a division structure for each depth of a luma block as a division structure candidate of a chroma block, and FIG. 11 is a diagram for explaining an example in which a QT division structure of a luma block is generated as a QT division structure candidate of a chroma block And FIG.

First, the embodiment shown in FIG. 8 will be described as follows. 8 (a) shows a divided structure of luma blocks, and Figs. 8 (b) to (f) show candidates corresponding to depth division structures of luma blocks, and index values for respective candidates are represented.

As shown in Fig. 8, the structure corresponding to the depth 0 (Fig. 8 (b)) and the structure corresponding to the depth 1 (Fig. 8 (c) ), A structure corresponding to depth 2 (FIG. 8 (d)), a structure corresponding to depth 3 (FIG. 8 (e)), and a structure corresponding to depth 4 (FIG. Can be generated as candidates for the partition structure.

When the image coding apparatus signals the index value of any one of these candidates (the candidate corresponding to the actual division structure of the chroma block) to the image decoding apparatus as the division information of the chroma block, the image decoding apparatus is based on the division structure of the luma block The candidate represented by the division information (index value) of the chroma block among the one or more candidates can be determined as the division structure of the chroma block.

For example, if the image encoding apparatus signals 110 to the chrominance block by dividing the index value 110 of the candidate corresponding to the depth 2 into the image decoding apparatus, the image decoding apparatus (decoding unit 410) The candidate (Fig. 8 (d)) can be determined by the division structure of the chroma block (takes a total of 3 bits).

As another example, when the image encoding apparatus signals 1111, which is the index value of the candidate corresponding to the depth 4, as the division information of the chroma block, the decoding unit 410 receives the candidate 1111 of the candidates )) Can be determined by the division structure of the chroma block (takes a total of 4 bits).

Next, the embodiment shown in FIG. 11 will be described as follows. 11 (a) shows a QT division structure of a luma block, and Figs. 11 (b) to 11 (e) show division structure candidates based on a QT division structure of a luma block.

As shown in Fig. 11, on the basis of the divided structure of luma blocks (Fig. 11 (a)), a common QT partition structure (Fig. 11 (b)), a final QT segmentation structure (Fig. 11 (c)) which is the structure having the greatest depth among the QT segmentation structures of luma blocks, and a QT segmentation (Fig. 11 (d) and (e)) and the like can be generated as candidates for the actual division structure of the chroma block.

11 has a structure in which both the common QT division structure, the final QT division structure and the partial QT division structure are different, but according to the QT division structure of the luma block, two or more of the three division structures may have the same structure Therefore, in such a case, candidates having the same structure are preferably merged into a single candidate.

In this state, as described with reference to FIG. 8, the image encoding apparatus decodes the index value of any one of the candidates (the candidate corresponding to the actual division structure of the chroma block) as the division information of the chroma block, The image decoding apparatus can determine the candidate represented by the division information of the chroma block among the one or more candidates based on the QT division structure of the luma block as a division structure of the chroma block.

As described above, according to the second embodiment, when the image encoding apparatus signals the division information of the chroma block and the image decoding apparatus determines the division structure of the chroma block, as compared with the case where the division of the chroma block is independently implemented, It is possible to implement the signaling and division structure determination, and the efficiency can be improved.

Although the method of setting the division structure for the depth of the luma block as a candidate and the method of setting the QT division structure of the luma block as a candidate through the second embodiment have been described, If the division structure can be created and the actual division structure of the chroma block can be represented through the specification of any one of the generated candidates, various methods other than the above-described method can be applied as the method of setting the candidate.

In addition, in the case of generating candidates based on the QT partition structure of the luma block, the BT partition structure or the TT partition structure of the chroma block can be determined by further applying the embodiments of the present invention described above or below.

Example 3

Fig. 9 is a diagram illustrating a QT division structure of a chroma block based on a QT division structure of a luma block (same as a QT division structure of a luma block), and a second division structure of a chroma block And FIG.

The third embodiment corresponds to the embodiment in which the chroma block and the luma block share the QT partition structure, and the subsequent partition (second partition) is performed independently of each other.

Here, the second segment means a QT segmentation and a segmentation in a different manner, and the second segmentation may include at least one of BT segmentation and TT segmentation. Hereinafter, the third embodiment will be described with an example in which the second division corresponds to the BT division.

In this embodiment, the division information of the chroma block, which is encoded by the image coding apparatus and is signaled to the video decoding apparatus, indicates whether the chroma block corresponding to the second division structure is divided by BT and the BT division direction.

For example, in the case of the BTTT division, the division information of the chroma block may include at least one of whether the chroma block is divided into the BTTT, the division direction, and the division type (BT or TT).

9 (a) shows a divided structure of luma blocks. Fig. 9 (b) shows a QT divided structure of a luma block or a QT divided structure of a chroma block Shows an example of a second division structure (BT division) of two chroma blocks, and Figs. 9 (d) and 9 (f) show a tree structure of chroma blocks for each BT division example.

9 (a), the QT division structure of the luma block has the structure as shown in Fig. 9 (b), so that the image decoding apparatus can obtain the QT division structure shown in Fig. 9 (b) It is determined by the QT division structure of the chroma block.

That is, since the division information of the chroma block does not contain any information about the QT division of the chroma block, the QT division structure of the luma block is applied as the QT division structure of the chroma block.

In the state where the QT division structure of the chroma block is determined, the video decoding apparatus determines the BT division structure of the chroma block based on the division information of the chroma block indicating whether the chroma block is divided into BT and the BT division direction, .

For example, when the chroma block is BT divided as shown in Fig. 9C (dotted line in Fig. 9C), the chroma block division information is not divided for the leaf node in which the chroma block is not BT divided 0 "), and the chroma block is divided horizontally (" 10 "

As another example, when the chroma block is BT divided as shown in Fig. 9E (dotted line in Fig. 9E), the chroma block division information is not divided for the leaf node in which the chroma block is not BT divided Quot; 11 ") for a node where the chroma block is divided in the horizontal direction, and a vertical direction (" 11 " ) (A total of 19 bits is required).

Thus, if the image encoding apparatus encodes the division information according to the third embodiment and the image decoding apparatus is configured to determine the division structure of the chroma block based on the division information according to the third embodiment and the division structure of the luma block, It is possible to reduce the number of bits required for encoding and signaling the QT division information of the block, thereby improving the efficiency of encoding and decoding.

Although the third embodiment has been described focusing on the method in which the BT division structure of the chroma block is determined based on the division information of the chroma block, a process in which the TT division structure of the chroma block is determined based on the division information of the chroma block is additionally performed And the process of determining the TT division structure of the chroma block may be performed in place of the process of determining the BT division structure.

Example 4

The fourth embodiment is the same as the third embodiment in terms of applying the QT division structure of the luma block as the QT division structure of the chroma block, but differs from the third embodiment in terms of the method of expressing the second division structure of the chroma block.

Here, as in the case of the third embodiment, the second partition means a QT division and a division in a different manner, and the second division may include at least one of a BT division and a TT division. Hereinafter, the fourth embodiment will be described with an example in which the second division corresponds to the BT division.

Specifically, in the third embodiment, whether or not the second division (BT division) of the chroma block is performed irrespective of whether or not the BT division of the luma block is performed (the division information indicates whether the chroma block is divided by BT and the BT division direction) In the case of Embodiment 4, the chroma block can be divided into the second division (BT) only when the luma block is divided (the division information indicates whether or not the chroma block performed when the luma block is divided by BT is divided).

10A shows a divided structure of luma blocks, FIG. 10B shows a QT divided structure of a luma block or a QT divided structure of a chroma block (shared structure), FIG. 10 (c) and (e) show an example of a second division (BT division) of two chroma blocks, and Figs. 10 (d) and 10 (f) show a tree structure of chroma blocks for each BT division example.

10 (a), the QT partitioning structure of the luma block has the structure as shown in FIG. 10 (b), so that the video decoding apparatus has the QT partitioning structure shown in FIG. 10 (b) It is determined by the QT division structure of the chroma block.

That is, since the division information of the chroma block does not include any information for the QT division of the chroma block, the video decoding apparatus applies the QT division structure of the luma block as the QT division structure of the chroma block.

In a state where the QT division structure of the chroma block is determined, the video decoding apparatus determines the BT division structure of the chroma block based on the division information indicating whether or not the chroma block is divided by BT, thereby determining the entire division structure of the chroma block.

For example, when the chroma block is BT divided as shown in FIG. 10C (dotted line in FIG. 10C), the chroma block division information does not show any information for the leaf node in which the luma block is not BT divided , The video decoding apparatus determines that the chroma block is not divided by BT.

Further, since the division information of the chroma block indicates that the chroma block is divided (" 1 ") with respect to the node divided in the horizontal direction, the video decoding apparatus determines that the chroma block is divided for the node.

 Further, since the division information of the chroma block indicates that the chroma block is not divided (" 0 ") with respect to the node not divided, the video decoding apparatus determines that the chroma block is not divided for the node.

As another example, when the chroma block is BT-divided as shown in Fig. 10E (dotted line in Fig. 10 (e)), the chroma block division information does not show any information for the leaf node in which the luma block is not BT divided , And the chroma block is divided ("1") for the vertically divided node and the vertically divided node (a total of 4 bits is required).

Thus, if the image encoding apparatus encodes the division information according to the fourth embodiment and the image decoding apparatus is configured to determine the division structure of the chroma block based on the division information according to the fourth embodiment and the division structure of the luma block, It is possible to reduce the number of bits required for encoding and signaling the QT division information of the block, thereby improving the efficiency of encoding and decoding.

Example 5

In the second embodiment described above, when determining one of the one or more candidates based on the QT division structure of the luma block as a QT division structure of a chroma block, embodiments of the present invention described above or described below are further applied, It is possible to determine a partition structure or a TT partition structure.

The fifth embodiment described below corresponds to a method of determining the second division structure of the chroma block on the premise that any one of the one or more candidates is determined as the QT division structure of the chroma block.

Here, the second division is the same as that described in the third and fourth embodiments, and the fifth embodiment will be described with reference to an example in which the second division corresponds to the BT division.

In order to determine the QT division structure of the chroma block and the second division (BT division) structure through a separate method (embodiment), the division information of the chroma block includes one of the one or more division structure candidates based on the QT division structure of the luma block First dividing information indicating one and second dividing information indicating a second dividing (BT dividing direction and BT dividing direction) of the chroma block.

In the case of the BTTT division, the second division information of the chroma block may include information on at least one of the BTTT division of the chroma block, the division direction, and the division type (BT or TT).

When the division information of the chroma block including the first division information and the second division information is signaled from the image encoding apparatus, the decoding unit 410 outputs the QT division structure of the chroma block and the BT Respectively.

First, in the case of the QT division structure of the chroma block, the decoding unit 410 determines the candidate represented by the first division information among the one or more division structure candidates based on the QT division structure of the luma block as the QT division structure of the chroma block.

Next, in the case of the BT division structure of the chroma block, the decoding unit 410 determines the BT division structure of the chroma block based on the second division information. For example, If the chroma block indicates that the BT block is not segmented (" 0 "), the chroma block is determined not to be BT segmented, and if the chroma block is BT divided horizontally (" 10 & And determines that the chroma block is BT divided in the vertical direction (" 11 ") when the chroma block is divided in the vertical direction.

Example 6

FIGS. 12 to 14 are diagrams for explaining Embodiment 6 in which a division structure of a chroma block is determined using a flag indicating whether or not the division structure of a luma block and the division structure of a chroma block are the same.

12, when a flag (equaltoLuma_flag) indicating whether the division structure of the luma block and the division structure of the chroma block are the same is signaled from the image encoding apparatus, the decoding unit 410 decodes this flag from the bit stream (S1210), and grasps the contents indicated by this flag (S1220).

("1"), the decoding unit 410 applies the division structure of the luma block to the division structure of the chroma block (S1230). If the flag indicates that the division structure of the chroma block is identical to the division structure of the chroma block And determines the division structure of the block. In this case, the division information (chroma_tree) of the chroma block is not separately signaled from the image encoding apparatus to the image decoding apparatus.

Alternatively, when the corresponding flag indicates a difference between the division structure of the luma block and the division structure of the chroma block (" 0 "), the decoding unit 410 decodes the division information of the chroma block signaled from the image encoding device S1240), and determines the division structure of the chroma block based on the decoded information.

Thus, if the division structure of the chroma block is determined by using the flag indicating the same as the division structure of the luma and the chroma block, if the division structures of the luma and the chroma block are mutually the same, Is not required, the decoding and coding efficiency is improved.

According to the embodiment, in the case where the chroma block and the luma block share division information only for a certain portion, the method described in Embodiment 6 can be applied to a flag indicating whether the divided structures of the luma and the chroma block are entirely the same and / And may be implemented in such a manner that a block division structure is determined using a flag indicating the same.

That is, in the embodiment described below, it is possible to perform a separate dividing process for the part (the shared division structure) sharing the division information and the part independently dividing (the second division) have. In the second division referred to in this embodiment, one or more of the BT division and the TT division may be included. In the following, the sixth embodiment will be described with reference to an example in which the second division corresponds to the BT division.

(EqualtoLuma_flag) indicating that the divided structures of the luma and chroma blocks are equal to each other, a first flag (equaltoLumaQT_flag) which is a flag indicating that the QT division structure is the same only, and / 2 flag (equaltoLumaBT_flag) may exist.

Figs. 13 and 14 show two embodiments of a method of separately performing the division structure of the chroma block for each of the QT division structure and the second division structure.

13, when the flag (equaltoLuma_flag) is signaled from the image encoding apparatus, the decoding unit 410 decodes the flag from the bit stream (S1310) and grasps the content indicated by this flag (S1320 ).

("1"), the decryption unit 410 applies the divided structure of the luma block to the divided structure of the chroma block (S1330) And determines the division structure of the block. In this case, the division information (chroma_tree) of the chroma block is not separately signaled from the image encoding apparatus to the image decoding apparatus.

Alternatively, when the corresponding flag indicates a difference between the division structure of the luma block and the division structure of the chroma block (" 0 "), the decoding unit 410 decodes the first The flag is decoded (S1340), and the contents indicated by the first flag are grasped (S1350).

When the first flag indicates that the first flag is not identical (" 0 "), this means that the luma block and the chroma block are different from each other in the QT and BT structures. Therefore, the decoding unit 410 decodes the chroma block Information (S1360), and determines the division structure of the chroma block based on the decoded information. In this case, the division information of the chroma block includes both the QT of the chroma block and the information (chroma_tree (QTBT)) of the BT division structure.

When the first flag indicates the same (" 1 "), this means that only the QT partition structure is identical to the luma block and the chroma block (S1370), the BT division information (chroma_tree (BT)) of the chroma block signaled from the image coding apparatus is decoded (S1380), and based on the decoded BT division information, the chroma block The BT partition structure of FIG.

14, when the flag (equaltoLuma_flag) is signaled from the image encoding apparatus, the decoding unit 410 decodes the flag from the bitstream (S1410) and grasps the content indicated by this flag S1420).

In the embodiment represented in FIG. 14, the flag (equaltoLuma_flag) corresponds to a flag indicating whether at least one of the QT and BT segment structures of the luma block and the chroma block are the same.

("0"), the decoding unit 410 decodes the division information (chroma_tree) of the chroma block signaled from the image encoding apparatus (S1430 ), And determines the division structure of the chroma block based on the decoded information.

Alternatively, if the flag indicates that at least one of the QT split structure and the BT split structure is identical (" 1 "), the decoding unit 410 extracts a first flag (S1440), and grasps the contents indicated by the first flag (S1450).

(&Quot; 0 ") indicating that the first flag is not the same (not only the QT division but also the BT division), this means that the luma block and the chroma block are the same in both QT and BT structure, 410) applies the division structure of the luma block to the division structure of the chroma block (S1460) to determine the division structure of the chroma block. In this case, the division information (chroma_tree) of the chroma block is not separately signaled from the image encoding apparatus to the image decoding apparatus.

(&Quot; 1 ") indicating that the luma block and the chroma block are the same only in the QT partition structure (only the BT partition structure is different) The unit 410 decides the QT division structure of the luma block as the QT division structure of the chroma block S1470 and decodes the BT division information chroma_tree BT of the chroma block signaled from the image coding unit S1480, The BT division structure of the chroma block is determined based on the divided BT division information.

Although the method of using the same between divided structures of luma and chroma blocks is described as being performed independently from the embodiments 1 to 5 with respect to the embodiment 6, the method of embodiment 6 according to the embodiment merges with the embodiments 1 to 5 Can be performed.

In a case where this is performed in a merged manner, there are some changes in Embodiments 1 to 5. For example, in the case where the flags are the same (the luma block and the chroma block have the same division structure), the decoding unit 410 determines the division structure of the luma block as a division structure of the chroma block, The chroma block division structure determination method described in 5 may not be performed. That is, the decoding unit 410 can be configured to perform the method of the embodiments 1 to 5 only when the flag indicates that the flags are not the same.

When the luma block and the chroma block are identical to each other only in the QT segmentation structure, the decoding unit 410 outputs the QT segmentation structure of the luma block to the QT segmentation structure of the chroma block, And determines the second division structure of the chroma block based on the second division information of the chroma block signaled from the video coding sequence.

If the luma block and the chroma block are different from each other only in the BT partition structure in the case where the sixth embodiment and the fourth embodiment are performed in combination, the decoding unit 410 outputs the QT division structure of the luma block to the QT division structure of the chroma block , Decodes the BT division information (chroma_tree (BT)) of the chroma block signaled from the image encoding apparatus, and determines the BT division structure of the chroma block based on the decoded BT division information. At this time, the BT division information includes only information indicating whether or not to be divided.

When the sixth embodiment and the second embodiment are performed in combination, if the division structure of the luma block and the division structure of the chroma block are different, the decoding unit 410 outputs the division structure candidate of the chroma block signaled from the image coding apparatus And determines the division structure of the chroma block based on the decoded information. At this time, one or more candidate partition structures are set based on the partition structure of the luma block. Since the partition structure of the luma block is different from the partition structure of the chroma block, the same partition structure as the luma block is excluded from the candidates.

When the sixth embodiment and the fifth embodiment are performed in combination, if the luma block and the chroma block are (partially) the same in the QT division structure, the decoding unit 410 decodes one or more Decodes the first division information indicating any one of the division structure candidates, and determines the QT division structure of the chroma block based on the decoded information. Further, the BT division information (chroma_tree (BT)) of the chroma block signaled from the image coding apparatus is decoded, and the BT division structure of the chroma block is determined based on the decoded BT division information.

The foregoing description is merely illustrative of the technical idea of the present embodiment, and various modifications and changes may be made to those skilled in the art without departing from the essential characteristics of the embodiments. Therefore, the present embodiments are to be construed as illustrative rather than restrictive, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of the present embodiment should be construed according to the following claims, and all technical ideas within the scope of equivalents thereof should be construed as being included in the scope of the present invention.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is related to Korean Patent Application No. 10-2018-0001722 filed on January 5, 2018, which is incorporated herein by reference in its entirety, and Korean Patent Application filed on September 5, 2018 No. 10-2018-0106104.

Claims (14)

1. A method of restoring a chroma block corresponding to a coding tree unit (CTU) in an image decoding apparatus,

   Decoding the partition information of the luma block corresponding to the CTU and the partition information of the chroma block from the bitstream; And

   And reconstructing the chroma block based on division information of the luma block and division information of the chroma block.
2. The method according to claim 1,

   The division information of the chroma block indicates whether or not the chroma block is divided when the luma block is divided,

   The step of restoring the chroma block comprises:

   Wherein a node of a chroma block corresponding to a node partitioned in the partition structure of the luma block is not partitioned by a partition of the chroma block corresponding to a node not divided in the partition structure of the luma block, Determining whether or not to partition the chroma block, and restoring the chroma block,

   And the division information of the chroma block corresponding to the non-divided node is not decoded from the bit stream.
3. The method according to claim 1,

   Wherein the division information of the chroma block represents one of one or more division structure candidates based on the division structure of the luma block,

   The step of restoring the chroma block comprises:

   Determining a candidate represented by the division information of the chroma block among the one or more division structure candidates as a division structure of the chroma block, and restoring the chroma block.
4. The method of claim 3,

   Wherein the partitioned structure candidates are partitioned structures of the luma blocks at respective depths.
5. The method according to claim 1,

   The division information of the chroma block indicates whether the chroma block is divided into a second division, which is a division of a quad tree (QT) division, and the second division direction,

   The step of restoring the chroma block comprises:

   Wherein the QT division structure of the luma block is determined as the QT division structure of the chroma block and the second division structure of the chroma block is determined based on the division information of the chroma block to restore the chroma block. A method of restoring a block.
6. The method according to claim 1,

   The division information of the chroma block indicates whether or not the chroma block is divided into a second division in the case where the luma block is divided into a second division which is a division of the QT division,

   The step of restoring the chroma block comprises:

   A QT partition structure of the luma block is determined as a QT partition structure of the chroma block, a node of a chroma block corresponding to a node not divided in the second partition structure of the luma block is not divided into a second partition, The node of the chroma block corresponding to the divided node in the second division structure determines whether to divide the second block based on the division information of the chroma block to restore the chroma block,

   And the division information of the chroma block corresponding to the non-divided node is not decoded from the bit stream.
7. The method according to claim 1,

   Wherein the division information of the chroma block is divided into first division information indicating one of one or more division structure candidates based on a QT division structure of the luma block and second division information indicating whether the chroma block is divided into a second division that is a division different from QT division, And second division information indicating a second division direction,

   The step of restoring the chroma block comprises:

   Determining a candidate represented by the first division information among the one or more division structure candidates as a QT division structure of the chroma block and determining a second division structure of the chroma block based on the second division information, And reconstructing the chroma block.
8. A video decoding apparatus comprising:

   Decodes division information of a luma block corresponding to a coding tree unit (CTU) and division information of a chroma block corresponding to the CTU from a bitstream, and uses division information of the luma block A decoding unit for determining a division structure of the luma block and determining a division structure of the chroma block based on the division structure of the luma block and the division information of the chroma block; And

   And an image reconstructor for reconstructing an image based on the division structure of the luma block and the division structure of the chroma block.
9. 9. The method of claim 8,

   The division information of the chroma block indicates whether or not the chroma block is divided when the luma block is divided,

   Wherein the decoding unit comprises:

   Wherein a node of a chroma block corresponding to a node partitioned in the partition structure of the luma block is not partitioned by a partition of the chroma block corresponding to a node not divided in the partition structure of the luma block, Decide whether to split,

   And the division information of the chroma block corresponding to the non-divided node is not decoded from the bit stream.
10. 9. The method of claim 8,

    Wherein the division information of the chroma block represents one of one or more division structure candidates based on the division structure of the luma block,

    Wherein the decoding unit comprises:

    And determines a candidate represented by the division information of the chroma block among the one or more division structure candidates as a division structure of the chroma block.
11. 11. The method of claim 10,

    Wherein the divided structure candidates are partitioned structures of the luma blocks at respective depths.
12. 9. The method of claim 8,

    The division information of the chroma block indicates whether the chroma block is divided into a second division, which is a division of a quad tree (QT) division, and the second division direction,

    Wherein the decoding unit comprises:

    Wherein the QT division structure of the luma block is determined as the QT division structure of the chroma block and the second division structure of the chroma block is determined on the basis of the division information of the chroma block.
13. 9. The method of claim 8,

    The division information of the chroma block indicates whether or not the chroma block is divided into a second division in the case where the luma block is divided into a second division which is a division of the QT division,

    Wherein the decoding unit comprises:

    A QT partition structure of the luma block is determined as a QT partition structure of the chroma block, a node of a chroma block corresponding to a node not divided in the second partition structure of the luma block is not divided into a second partition, The node of the chroma block corresponding to the divided node in the second division structure determines whether to divide the second division based on the division information of the chroma block,

    And the division information of the chroma block corresponding to the non-divided node is not decoded from the bit stream.
14. 9. The method of claim 8,

    Wherein the division information of the chroma block is divided into first division information indicating one of one or more division structure candidates based on a QT division structure of the luma block and second division information indicating whether the chroma block is divided into a second division that is a division different from QT division, And second division information indicating a second division direction,

    Wherein the decoding unit comprises:

    Determines a candidate represented by the first division information among the one or more division structure candidates as a QT division structure of the chroma block and determines a second division structure of the chroma block based on the second division information Image decoding apparatus.

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