WO2017065490A1

WO2017065490A1 - Method for encoding/decoding image, and apparatus therefor

Info

Publication number: WO2017065490A1
Application number: PCT/KR2016/011419
Authority: WO
Inventors: 손은용; 허진; 남정학; 박승욱; 예세훈
Original assignee: 엘지전자(주)
Priority date: 2015-10-13
Filing date: 2016-10-12
Publication date: 2017-04-20

Abstract

The present invention discloses a method for encoding/decoding an image and an apparatus therefor. Specifically, a method for a decoding apparatus decoding an image may comprise the steps of: parsing a quantisation unit (QU) split flag indicating whether the QU is split from a current block; deriving a split format of the QU if the QU split flag is activated; and deriving a quantisation parameter (QP) for each split QU.

Description

Image encoding / decoding method and apparatus therefor

The present invention relates to a still image or moving image processing method, and more particularly, to a method for encoding / decoding a still image or moving image using a quantization unit (or block) structure and an apparatus supporting the same.

Compression coding refers to a series of signal processing techniques for transmitting digitized information through a communication line or for storing in a form suitable for a storage medium. Media such as an image, an image, an audio, and the like may be a target of compression encoding. In particular, a technique of performing compression encoding on an image is called video image compression.

Next-generation video content will be characterized by high spatial resolution, high frame rate and high dimensionality of scene representation. Processing such content would result in a tremendous increase in terms of memory storage, memory access rate, and processing power.

Accordingly, there is a need to design coding tools for more efficiently processing next generation video content.

An object of the present invention is to propose a method of defining or dividing a quantization unit, which is a basic block structure, when quantizing / dequantizing an image.

An object of the present invention is to propose a method for signaling a partition structure of a quantization unit.

The technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the following description. Could be.

According to an aspect of the present invention, a method of decoding a video by a decoding apparatus includes parsing a QU division flag indicating whether a quantization unit (QU) is split from a current block, and activating the QU division flag. In this case, the method may include deriving a split form of the QU and deriving a quantization parameter (QP) for each divided QU.

An aspect of the present invention provides a decoding apparatus for decoding an image, comprising: a QU division flag parser for parsing a QU division flag indicating whether a quantization unit (QU) is split from a current block, the QU division flag When is activated, it may include a QU division type derivation unit for deriving the division form of the QU and a QP derivation unit for deriving a quantization parameter (QP) for each divided QU.

Preferably, the QU may be divided in a quad-tree form from the current block.

Preferably, the QU may be divided according to a QU division type selected from a plurality of predefined QU division types.

Preferably, the partitioned form of the QU may be derived by parsing information on the QU partitioned mode indicating the partitioned form of the QU.

Preferably, the split form of the QU may be derived according to an intra prediction mode applied to the current block.

Preferably, when the intra prediction mode is in a vertical direction, a split form may be determined as 2N × N or 2N × nU.

Preferably, when the intra prediction mode is in a horizontal direction, a split form may be determined as N × 2N or nL × 2N.

Preferably, the QP difference value is parsed, and the QP may be derived by summing a QP predictor derived from a QP of a neighboring QU and the parsed QP difference value.

Preferably, the neighboring QU may be a QU neighboring the top of the QU or a QU neighboring to the left of the QU.

According to an embodiment of the present invention, quantization can be flexibly performed by defining a basic unit for performing quantization / dequantization.

In addition, according to an embodiment of the present invention, by defining a basic unit for performing quantization / dequantization, higher coding efficiency may be achieved by performing quantization / dequantization suitable for each situation.

The effects obtainable in the present invention are not limited to the above-mentioned effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. .

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, included as part of the detailed description in order to provide a thorough understanding of the present invention, provide embodiments of the present invention and together with the description, describe the technical features of the present invention.

1 is a schematic block diagram of an encoder in which encoding of a still image or video signal is performed according to an embodiment to which the present invention is applied.

2 is a schematic block diagram of a decoder in which encoding of a still image or video signal is performed according to an embodiment to which the present invention is applied.

3 is a diagram for describing a partition structure of a coding unit that may be applied to the present invention.

4 is a diagram for explaining a prediction unit applicable to the present invention.

5 is a diagram illustrating a structure in which a prediction unit, a transform unit, and a quantization unit are divided from a coding unit according to an embodiment of the present invention.

6 is a diagram illustrating a structure in which a quantization unit is divided from a coding unit according to an embodiment of the present invention.

7 is a diagram illustrating a structure in which a quantization unit is divided from a transform unit according to an embodiment of the present invention.

8 is a diagram illustrating an image decoding method according to an embodiment of the present invention.

9 is a diagram illustrating a quantization unit divider according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, one of ordinary skill in the art appreciates that the present invention may be practiced without these specific details.

In some instances, well-known structures and devices may be omitted or shown in block diagram form centering on the core functions of the structures and devices in order to avoid obscuring the concepts of the present invention.

In addition, the terminology used in the present invention was selected as a general term widely used as possible now, in a specific case will be described using terms arbitrarily selected by the applicant. In such a case, since the meaning is clearly described in the detailed description of the part, it should not be interpreted simply by the name of the term used in the description of the present invention, and it should be understood that the meaning of the term should be interpreted. .

Specific terms used in the following description are provided to help the understanding of the present invention, and the use of such specific terms may be changed to other forms without departing from the technical spirit of the present invention. For example, signals, data, samples, pictures, frames, blocks, etc. may be appropriately replaced and interpreted in each coding process.

Hereinafter, the term 'block' or 'unit' refers to a unit in which a process of encoding / decoding such as prediction, transformation, and / or quantization is performed, and may be configured as a multidimensional array of samples (or pixels, pixels).

'Block' or 'unit' may mean a multi-dimensional array of samples for luma components, or may mean a multi-dimensional array of samples for chroma components. In addition, the multi-dimensional arrangement of the sample for the luma component and the multi-dimensional arrangement of the sample for the chroma component may also be included.

For example, 'block' or 'unit' refers to a coding block (CB) that represents an array of samples to be encoded / decoded, and a coding tree block composed of a plurality of coding blocks (CTB). Block), Prediction Block (PB) (or Prediction Unit (PU)), which means an array of samples to which the same prediction is applied, and Transform Block (TB :), which is an array of samples to which the same transformation is applied. It may be interpreted as meaning including all of a transform block (or a transform unit (TU)).

Also, unless stated otherwise in this specification, a 'block' or 'unit' is a syntax structure used in encoding / decoding an array of samples for a luma component and / or a chroma component. can be interpreted to include a sturcture. Here, the syntax structure refers to zero or more syntax elements existing in the bitstream in a specific order, and the syntax element refers to an element of data represented in the bitstream.

For example, a 'block' or 'unit' includes a coding unit (CU) including a coding block (CB) and a syntax structure used for encoding the coding block (CB), and a plurality of coding units. A prediction unit (PU), a transform block (TB), including a coding tree unit (CU), a prediction block (PB), and a syntax structure used for prediction of the prediction block (PB); It may be interpreted as meaning including all transform units (TUs) including a syntax structure used for transformation of the corresponding transform block TB.

In addition, in the present specification, the 'block' or 'unit' is not necessarily limited to an array of square or rectangular samples (or pixels or pixels), and polygonal samples having three or more vertices (or pixels or pixels). It can also mean an array of. In this case, it may also be referred to as a polygon block or a polygon unit.

Referring to FIG. 1, the encoder 100 may include an image divider 110, a subtractor 115, a transform unit 120, a quantizer 130, an inverse quantizer 140, an inverse transform unit 150, and a filtering unit. 160, a decoded picture buffer (DPB) 170, a predictor 180, and an entropy encoder 190. The predictor 180 may include an inter predictor 181 and an intra predictor 182.

The image divider 110 divides an input video signal (or a picture or a frame) input to the encoder 100 into one or more blocks.

The subtractor 115 outputs a predicted signal (or a predicted block) output from the predictor 180 (that is, the inter predictor 181 or the intra predictor 182) in the input image signal. ) To generate a residual signal (or differential block). The generated difference signal (or difference block) is transmitted to the converter 120.

The transform unit 120 may convert a differential signal (or a differential block) into a transform scheme (eg, a discrete cosine transform (DCT), a discrete sine transform (DST), a graph-based transform (GBT), and a karhunen-loeve transform (KLT)). Etc.) to generate transform coefficients. In this case, the transform unit 120 may generate transform coefficients by performing a transform using a transform mode determined according to the prediction mode applied to the difference block and the size of the difference block.

The quantization unit 130 quantizes the transform coefficients and transmits the transform coefficients to the entropy encoding unit 190, and the entropy encoding unit 190 entropy codes the quantized signals and outputs them as bit streams.

Meanwhile, the quantized signal output from the quantization unit 130 may be used to generate a prediction signal. For example, the quantized signal may recover the differential signal by applying inverse quantization and inverse transformation through an inverse quantization unit 140 and an inverse transformation unit 150 in a loop. A reconstructed signal (or a reconstruction block) may be generated by adding the reconstructed difference signal to a prediction signal output from the inter predictor 181 or the intra predictor 182.

Meanwhile, in the compression process as described above, adjacent blocks are quantized by different quantization parameters, thereby causing deterioration of the block boundary. This phenomenon is called blocking artifacts, which is one of the important factors in evaluating image quality. In order to reduce such deterioration, a filtering process may be performed. Through this filtering process, the image quality can be improved by removing the blocking degradation and reducing the error of the current picture.

The filtering unit 160 applies filtering to the reconstruction signal and outputs it to the reproduction apparatus or transmits the decoded picture buffer to the decoding picture buffer 170. The filtered signal transmitted to the decoded picture buffer 170 may be used as the reference picture in the inter prediction unit 181. As such, by using the filtered picture as a reference picture in the inter prediction mode, not only image quality but also encoding efficiency may be improved.

The decoded picture buffer 170 may store the filtered picture for use as a reference picture in the inter prediction unit 181.

The inter prediction unit 181 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to a reconstructed picture.

Here, since the reference picture used to perform the prediction is a transformed signal that has been quantized and dequantized in units of blocks at the time of encoding / decoding in the previous time, blocking artifacts or ringing artifacts may exist. have.

Accordingly, the inter prediction unit 181 may interpolate the signals between pixels in sub-pixel units by applying a lowpass filter to solve performance degradation due to discontinuity or quantization of such signals. Herein, the sub-pixels mean virtual pixels generated by applying an interpolation filter, and the integer pixels mean actual pixels existing in the reconstructed picture. As the interpolation method, linear interpolation, bi-linear interpolation, wiener filter, or the like may be applied.

The interpolation filter may be applied to a reconstructed picture to improve the precision of prediction. For example, the inter prediction unit 181 may generate an interpolation pixel by applying an interpolation filter to an integer pixel and perform prediction using an interpolated block composed of interpolated pixels.

The intra predictor 182 predicts the current block by referring to samples in the vicinity of the block to which the current encoding is to be performed. The intra prediction unit 182 may perform the following process to perform intra prediction. First, reference samples necessary for generating a prediction signal may be prepared. The predicted signal (predicted block) may be generated using the prepared reference sample. Then, the prediction mode is encoded. In this case, the reference sample may be prepared through reference sample padding and / or reference sample filtering. Since the reference sample has been predicted and reconstructed, there may be a quantization error. Accordingly, the reference sample filtering process may be performed for each prediction mode used for intra prediction to reduce such an error.

The predicted signal (or predicted block) generated by the inter predictor 181 or the intra predictor 182 is used to generate a reconstruction signal (or reconstruction block) or a differential signal (or differential). Block).

2, the decoder 200 includes an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an adder 235, a filtering unit 240, and a decoded picture buffer (DPB). Buffer Unit (250), the prediction unit 260 may be configured. The predictor 260 may include an inter predictor 261 and an intra predictor 262.

The reconstructed video signal output through the decoder 200 may be reproduced through the reproducing apparatus.

The decoder 200 receives a signal (ie, a bit stream) output from the encoder 100 of FIG. 1, and the received signal is entropy decoded through the entropy decoding unit 210.

The inverse quantization unit 220 obtains a transform coefficient from the entropy decoded signal using the quantization step size information.

The inverse transform unit 230 applies an inverse transform scheme to inverse transform the transform coefficients to obtain a residual signal (or a differential block).

The adder 235 outputs the obtained difference signal (or difference block) from the predictor 260 (that is, the predicted signal (or prediction) output from the predictor 260 (that is, the inter predictor 261 or the intra predictor 262). By adding to the generated block), a reconstructed signal (or a restored block) is generated.

The filtering unit 240 applies filtering to the reconstructed signal (or the reconstructed block) and outputs the filtering to the reproduction device or transmits the decoded picture buffer unit 250 to the reproduction device. The filtered signal transmitted to the decoded picture buffer unit 250 may be used as a reference picture in the inter predictor 261.

In the present specification, the embodiments described by the filtering unit 160, the inter prediction unit 181, and the intra prediction unit 182 of the encoder 100 are respectively the filtering unit 240, the inter prediction unit 261, and the decoder of the decoder. The same may be applied to the intra predictor 262.

Block splitting structure

In general, a still image or video compression technique (eg, HEVC) uses a block-based image compression method. The block-based image compression method is a method of processing an image by dividing the image into specific block units, and may reduce memory usage and calculation amount.

The encoder splits one image (or picture) into units of a coding tree unit (CTU) in a rectangular shape. In addition, one CTU is sequentially encoded according to a raster scan order.

In HEVC, the size of the CTU may be set to any one of 64 × 64, 32 × 32, and 16 × 16. The encoder may select and use the size of the CTU according to the resolution of the input video or the characteristics of the input video. The CTU includes a coding tree block (CTB) for luma components and a CTB for two chroma components corresponding thereto.

One CTU may be divided into a quad-tree structure. That is, one CTU has a square shape and is divided into four units having a half horizontal size and a half vertical size to generate a coding unit (CU). have. This partitioning of the quad-tree structure can be performed recursively. That is, a CU is hierarchically divided into quad-tree structures from one CTU.

CU refers to a basic unit of coding in which an input image is processed, for example, intra / inter prediction is performed. The CU includes a coding block (CB) for a luma component and a CB for two chroma components corresponding thereto. In HEVC, the size of a CU may be set to any one of 64 × 64, 32 × 32, 16 × 16, and 8 × 8.

Referring to FIG. 3, the root node of the quad-tree is associated with the CTU. The quad-tree is split until it reaches a leaf node, which corresponds to a CU.

More specifically, the CTU corresponds to a root node and has a smallest depth (ie, depth = 0). The CTU may not be divided according to the characteristics of the input image. In this case, the CTU corresponds to a CU.

The CTU may be divided into quad tree shapes, resulting in lower nodes having a depth of 1 (depth = 1). In addition, a node that is no longer divided (ie, a leaf node) in a lower node having a depth of 1 corresponds to a CU. For example, in FIG. 3 (b), CU (a), CU (b), and CU (j) corresponding to nodes a, b, and j are divided once in the CTU and have a depth of one.

At least one of the nodes having a depth of 1 may be split into a quad tree again, resulting in lower nodes having a depth of 1 (ie, depth = 2). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 2 corresponds to a CU. For example, in FIG. 3 (b), CU (c), CU (h) and CU (i) corresponding to nodes c, h and i are divided twice in the CTU and have a depth of two.

In addition, at least one of the nodes having a depth of 2 may be divided into quad tree shapes, resulting in lower nodes having a depth of 3 (ie, depth = 3). And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of 3 corresponds to a CU. For example, in FIG. 3 (b), CU (d), CU (e), CU (f), and CU (g) corresponding to nodes d, e, f, and g are divided three times in the CTU, Has depth.

In the encoder, the maximum size or the minimum size of the CU may be determined according to characteristics (eg, resolution) of the video image or in consideration of encoding efficiency. Information about this or information capable of deriving the information may be included in the bitstream. A CU having a maximum size may be referred to as a largest coding unit (LCU), and a CU having a minimum size may be referred to as a smallest coding unit (SCU).

In addition, a CU having a tree structure may be hierarchically divided with predetermined maximum depth information (or maximum level information). Each partitioned CU may have depth information. Since the depth information indicates the number and / or degree of division of the CU, the depth information may include information about the size of the CU.

Since the LCU is divided into quad tree shapes, the size of the SCU can be obtained by using the size and maximum depth information of the LCU. Or conversely, using the size of the SCU and the maximum depth information of the tree, the size of the LCU can be obtained.

For one CU, information indicating whether the corresponding CU is split (for example, a split CU flag split_cu_flag) may be transmitted to the decoder. This split mode is included in all CUs except the SCU. For example, if the flag indicating whether to split or not is '1', the CU is divided into 4 CUs again. If the flag indicating whether to split or not is '0', the CU is not divided further. Processing may be performed.

As described above, a CU is a basic unit of coding in which intra prediction or inter prediction is performed. HEVC divides a CU into prediction units (PUs) in order to code an input image more effectively.

The PU is a basic unit for generating a prediction block, and may generate different prediction blocks in PU units within one CU. However, PUs belonging to one CU are not mixed with intra prediction and inter prediction, and PUs belonging to one CU are coded by the same prediction method (ie, intra prediction or inter prediction).

The PU is not divided into quad-tree structures, but is divided once in a predetermined form in one CU. This will be described with reference to the drawings below.

The PU is divided differently according to whether an intra prediction mode or an inter prediction mode is used as a coding mode of a CU to which the PU belongs.

FIG. 4A illustrates a PU when an intra prediction mode is used, and FIG. 4B illustrates a PU when an inter prediction mode is used.

Referring to FIG. 4 (a), assuming that a size of one CU is 2N × 2N (N = 4,8,16,32), one CU has two types (ie, 2N × 2N or N). XN).

Here, when divided into 2N × 2N type PU, it means that only one PU exists in one CU.

On the other hand, when divided into N × N type PU, one CU is divided into four PUs, and different prediction blocks are generated for each PU unit. However, the division of the PU may be performed only when the size of the CB for the luminance component of the CU is the minimum size (that is, the CU is the SCU).

Referring to FIG. 4 (b), assuming that a size of one CU is 2N × 2N (N = 4,8,16,32), one CU has 8 PU types (ie, 2N × 2N). , N × N, 2N × N, N × 2N, nL × 2N, nR × 2N, 2N × nU, 2N × nD).

Similar to intra prediction, PU partitioning in the form of N × N may be performed only when the size of the CB for the luminance component of the CU is the minimum size (that is, the CU is the SCU).

In inter prediction, 2N × N splitting in the horizontal direction and N × 2N splitting in the vertical direction are supported.

In addition, it supports PU partitions of nL × 2N, nR × 2N, 2N × nU, and 2N × nD types, which are Asymmetric Motion Partition (AMP). Here, 'n' means a 1/4 value of 2N. However, AMP cannot be used when the CU to which the PU belongs is a CU of the minimum size.

In order to efficiently encode an input image within one CTU, an optimal partitioning structure of a coding unit (CU), a prediction unit (PU), and a transformation unit (TU) is subjected to the following process to perform a minimum rate-distortion. It can be determined based on the value. For example, looking at the optimal CU partitioning process in 64 × 64 CTU, rate-distortion cost can be calculated while partitioning from a 64 × 64 CU to an 8 × 8 CU. The specific process is as follows.

1) The partition structure of the optimal PU and TU that generates the minimum rate-distortion value is determined by performing inter / intra prediction, transform / quantization, inverse quantization / inverse transform, and entropy encoding for a 64 × 64 CU.

2) Divide the 64 × 64 CU into four 32 × 32 CUs and determine the optimal PU and TU partitioning structure that generates the minimum rate-distortion value for each 32 × 32 CU.

3) The 32 × 32 CU is subdivided into four 16 × 16 CUs, and a partition structure of an optimal PU and TU that generates a minimum rate-distortion value for each 16 × 16 CU is determined.

4) Subdivide the 16 × 16 CU into four 8 × 8 CUs and determine the optimal PU and TU partitioning structure that generates the minimum rate-distortion value for each 8 × 8 CU.

5) 16 × 16 blocks by comparing the sum of the rate-distortion values of the 16 × 16 CUs calculated in 3) above with the rate-distortion values of the four 8 × 8 CUs calculated in 4) above. Determine the partition structure of the optimal CU within. This process is similarly performed for the remaining three 16 × 16 CUs.

6) 32 × 32 block by comparing the sum of the rate-distortion values of the 32 × 32 CUs calculated in 2) above with the rate-distortion values of the four 16 × 16 CUs obtained in 5) above. Determine the partition structure of the optimal CU within. Do this for the remaining three 32x32 CUs.

7) Finally, compare the sum of the rate-distortion values of the 64 × 64 CUs calculated in step 1) with the rate-distortion values of the four 32 × 32 CUs obtained in step 6). The partition structure of the optimal CU is determined within the x64 block.

In the intra prediction mode, a prediction mode is selected in units of PUs, and prediction and reconstruction are performed in units of actual TUs for the selected prediction mode.

TU means a basic unit in which actual prediction and reconstruction are performed. The TU includes a transform block (TB) for a luma component and a TB for two chroma components corresponding thereto.

In the example of FIG. 3, as one CTU is divided into quad-tree structures to generate CUs, the TUs are hierarchically divided into quad-tree structures from one CU to be coded.

Since the TU is divided into quad-tree structures, the TU divided from the CU can be further divided into smaller lower TUs. In HEVC, the size of the TU may be set to any one of 32 × 32, 16 × 16, 8 × 8, and 4 × 4.

Referring again to FIG. 3, it is assumed that a root node of the quad-tree is associated with a CU. The quad-tree is split until it reaches a leaf node, which corresponds to a TU.

In more detail, a CU corresponds to a root node and has a smallest depth (that is, depth = 0). The CU may not be divided according to the characteristics of the input image. In this case, the CU corresponds to a TU.

The CU may be divided into quad tree shapes, resulting in lower nodes having a depth of 1 (depth = 1). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 1 corresponds to a TU. For example, in FIG. 3B, TU (a), TU (b), and TU (j) corresponding to nodes a, b, and j are divided once in a CU and have a depth of 1. FIG.

At least one of the nodes having a depth of 1 may be split into a quad tree again, resulting in lower nodes having a depth of 1 (ie, depth = 2). In addition, a node (ie, a leaf node) that is no longer divided in a lower node having a depth of 2 corresponds to a TU. For example, in FIG. 3B, TU (c), TU (h), and TU (i) corresponding to nodes c, h, and i are divided twice in a CU and have a depth of two.

In addition, at least one of the nodes having a depth of 2 may be divided into quad tree shapes, resulting in lower nodes having a depth of 3 (ie, depth = 3). And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of 3 corresponds to a CU. For example, in FIG. 3 (b), TU (d), TU (e), TU (f), and TU (g) corresponding to nodes d, e, f, and g are divided three times in a CU. Has depth.

A TU having a tree structure may be hierarchically divided with predetermined maximum depth information (or maximum level information). Each divided TU may have depth information. Since the depth information indicates the number and / or degree of division of the TU, it may include information about the size of the TU.

For one TU, information indicating whether the corresponding TU is split (for example, split TU flag split_transform_flag) may be delivered to the decoder. This partitioning information is included in all TUs except the smallest TU. For example, if the value of the flag indicating whether to split is '1', the corresponding TU is divided into four TUs again. If the value of the flag indicating whether to split is '0', the corresponding TU is no longer divided.

Quantization Unit (QU) Structure

In the current video codec, various blocks are designed to be flexibly divided, so that the most suitable block division structure is selected under a characteristic of a video and a limited bit rate condition. Examples of such blocks include a coding unit (CU) that is a basic unit of coding, a prediction unit (PU) that is a basic unit of prediction, and a transform unit (TU), which is a basic unit of transform.

Examples of the basic unit of the various blocks used in the video codec and the structure that can be divided are shown in Tables 1 and 2 below.

Table 1 illustrates a block structure in intra prediction (ie, intra prediction).

Table 2 illustrates a block structure in inter prediction (ie, inter prediction).

The maximum size of the CU illustrated in Tables 1 and 2 is assumed to be 64x64 and the maximum depth of the CU is 4. Accordingly, a CU may have four sizes of 64 × 64, 32 × 32, 16 × 16, and 8 × 8, and PUs and TUs that can be selected for each CU size are shown in Tables 1 and 2. In this case, it is assumed that the maximum / minimum size of the TU is 32 × 32/4 × 4 and the maximum depth of the TU is 3.

As such, there is no independent block structure for quantization and inverse quantization, and the same quantization and inverse quantization calculation process is performed with the same block size as the TU.

Accordingly, the present invention defines a quantization unit (QU) structure that allows flexible quantization in addition to the existing blocks, and performs quantization / dequantization suitable for each situation using the defined QU. Suggest how to. Thus, higher coding efficiency can be achieved.

An embodiment of the present invention proposes a definition and division method for a quantization unit (QU), which is a basic unit for performing quantization and inverse quantization operations.

The QU according to the present invention may be defined as a unit (region) for performing the same quantization or inverse quantization operation and / or a unit (region) in which quantization or inverse quantization is performed using the same quantization step size. .

First, a method of dividing a QU (that is, a division structure) will be described.

A) How to split a QU based on a CU (ie, splitting a QU from a CU)

According to an embodiment of the present invention, the QU may be divided into quad-trees based on the CU. This will be described with reference to the drawings below.

FIG. 5A illustrates a structure in which CUs of 1 to 13 are divided from 64 × 64 CTUs.

FIG. 5 (b) illustrates a structure in which a PU is divided from each CU partitioned from the CTU illustrated in FIG. 5 (a).

In FIG. 5B, CU3, CU5, and CU13 are divided into PUs of various types. CU3 is divided into 3_P1 and 3_P2 in a 2N × N divided form, CU5 is divided into 5_P1 and 5_P2 in an N × 2N divided form, and CU13 is divided into 13_P1 and 13_P2 in an N × 2N divided form. In FIG. 5B, the remaining CUs that are not divided use the same size PU as the CU.

FIG. 5 (c) illustrates a structure in which a TU is divided from each CU partitioned from the CTU illustrated in FIG. 5 (a).

In FIG. 5C, CU3, CU5, CU11, and CU13 are divided into TUs having various depths. CU3 is divided into 3_T1, 3_T2, 3_T3, 3_T4 at depth 1, CU5 is divided into 5_T1, 5_T9, 5_T10 at depth 1, 5_T2, 5_T7, 5_T8 at depth 2, 5_T3, 5_T4 at depth 3, 5_T5, 5_T6, CU11 is divided into 11_T1, 11_T2, 11_T3, and 11_T4 of depth 1, and CU13 represents a state divided into 13_T1, 13_T2, 13_T3, and 13_T4 of depth 1. As in the case of FIG. 5 (b), CUs that are not divided use TUs having the same size as the CU.

FIG. 5 (d) illustrates a partition structure of QUs from each CU partitioned from the CTU illustrated in FIG. 5 (a) according to one embodiment of the present invention. The QU may be divided from the CU in a quad-tree form as in the conventional CU and TU partitioning scheme.

In FIG. 5 (d), CU3, CU5, and CU6 are divided into QUs having various depths. CU3 is divided into 3_Q1, 3_Q2, 3_Q3, 3_Q4 at depth 1, CU5 is divided into 5_Q1, 5_Q6, 5_Q7 at depth 1, 5_Q2, 5_Q3, 5_Q4, 5_Q5 at depth 2, CU6 is 6_Q1, 6_Q2, at depth 1 The state divided into 6_Q7, 6_Q3, 6_Q4, 6_Q5, and 6_Q6 of depth 2 is shown.

For example, the encoder may determine the optimal partition structure of the QU based on a minimum rate-distortion value as in the CU / TU partitioning scheme described above.

According to another embodiment of the present invention, the QU may be split into a new form that is not previously defined based on the CU.

For example, the partitioning structure of the QU may be partitioned into a rectangle (that is, a binary-tree) rather than a quad-tree, or various reflections reflecting characteristics of a prediction direction (according to an intra prediction mode). A form of partitioning scheme may be used. This will be described with reference to the drawings below.

In FIG. 6, N represents the size of the width and height of the CU.

As shown in Figs. 6 (a) to 6 (f), splitting forms such as 2N × 2N, N × N, N × 2N, 2N × N, nL × 2N, and 2N × nU according to existing PU splitting schemes (or QU can be split from the CU by using the split mode). In this case, nR × 2N and 2N in the partition mode of the conventional PU are reflected by reflecting the tendency of 2-dimensional (2D) transformation in which non-zero transform coefficients are concentrated toward a low frequency. XnD may not be used.

6 (a) to 6 (f) illustrate a rectangular partition structure (for example, N × 2N, 2N × N, nL × 2N, 2N × nU) according to a conventional PU partitioning scheme. However, the present invention is not limited thereto. That is, the QU may be divided from the CU in the form of a binary tree, without being limited to a rectangular partition structure according to a conventional PU partitioning scheme.

The QU may be split from the CU in a split form (or split mode) as shown in FIGS. 6 (g) to 6 (l). Fig. 6 (g) is divided into N / 4 × N / 4 sized QUs and the rest, and Fig. 6 (h) is divided into N / 2 × N / 2 sized QUs and the rest, and Fig. 6 (i) The case of dividing into 3N / 4x3N / 4 sized QU and the rest is illustrated. 6 (j) to 6 (l) are (0, N / 2-1) and (N) assuming that the coordinates of the top-left sample of the N × N CU are (0,0). CU is divided into two QUs based on a line segment connecting / 2-1,0), and the CU is divided into two QUs based on a line segment connecting (0, N-1) and (N-1,0). An example in which a CU is divided into two QUs based on a line segment connecting (N / 2-1, N) and (N, N / 2-1) is divided.

6 (l) to 6 (l) illustrate a split form reflecting a tendency of 2-dimensional (2D) transformation in which non-zero transform coefficients are concentrated toward a low frequency. Doing. According to the degree to which the non-zero transform coefficients are driven toward the low frequency, the division form of any one of FIGS. 6 (g) to 6 (i) may be determined by the encoder. Similarly, the division form of any one of FIGS. 6 (j) to 6 (l) may be determined by the encoder.

In addition, the split form of the QU may be determined according to a distribution characteristic of a non-zero transform coefficient of the transform coefficient according to the intra prediction direction. For example, if a vertical in-picture prediction in which non-zero transform coefficients are concentrated in the top row of a block is used for a CU or TU, FIG. 6 (d) or FIG. A division mode such as 6 (f) may be selected. In addition, when the intra-picture prediction in the horizontal direction in which a non-zero transform coefficient is concentrated in the column on the left side of the block is used for the CU or TU, FIG. 6 (c) or FIG. 6 ( The division mode as e) may be selected.

In addition, in the case of using an intra prediction method in which non-zero transform coefficients are evenly distributed in a low frequency part, the most suitable division of FIGS. 6 (g) to 6 (l) is used. The mode can be selected. In this case, the most suitable split mode among FIGS. 6 (g) to 6 (l) may be selected by the encoder, or between the prediction directions in each screen and the split modes according to FIGS. 6 (g) to 6 (l). Correspondence relationships may be predefined with certain rules. As such, when the corresponding relationship between the specific intra prediction mode and the splitting mode of the QU is predefined, there is an advantage that no separate signaling for the QU splitting mode from the encoder to the decoder is required.

All or part of the division structure of the various QUs illustrated in FIG. 6 correspond to division types of the QU (division modes of the QU), and the QU may be divided from the CU by any one division structure determined by the encoder. have. Also, as described above, when a corresponding relationship between a specific intra prediction mode and a split mode of the QU is predefined, the decoder may derive a split form of the QU split from the current CU according to the intra prediction mode.

B) How to split a QU based on a TU (i.e., split a QU from a TU)

FIG. 7 illustrates a structure in which a QU is divided from each of the TUs divided as shown in FIG. 5C, and the shaded portion in FIG. 7 illustrates a QU divided based on the TUs.

In FIG. 7, TU1, 3_T2, TU4, 5_T9, and TU12 are divided into various types of QUs. TU1 is divided into 1_Q1 and 1_Q2 in the form of 2N × N division, 3_T2 is divided into 3_T2Q1 and 3_T2Q2 in the form of 2N × N division, TU4 is divided into 4_Q1 and 4_Q2 in the form of 2N × NU division, and 5_T9 is nL × 2N. The divided form is divided into 5_T9Q1 and 5_T9Q2, and TU12 exemplifies a state divided into 12_Q1, 12_Q2, 12_Q3, and 12_Q4 in an N × N divided form (or quad-tree form).

The difference from the QU partitioning proposed in FIG. 5 (d) is that since FIG. 5 (d) partitions the QU based on a CU, the QU can be partitioned regardless of the TU partition structure, whereas the QU partition is proposed in FIG. The TU-based QU partitioning splits the QU based on each partitioned TU, causing a limitation that the QU cannot be larger than the TU.

As such, even when splitting a QU based on a TU, all or part of the splitting structure of the QU proposed in FIG. 6 may be applied.

Meanwhile, in the above-described embodiment, only a method of dividing a QU based on a CU or a TU is illustrated, but the present invention is not limited thereto, and the QU may be divided based on the CTU. Also in this case, a quad-tree shape may be applied as shown in FIG. 5 and a partitioned shape as shown in FIG. 6 may also be applied.

Next, a method of decoding (or signaling to a decoder) information related to QU splitting at the time of splitting the QU in the decoder as described above will be described.

First, when a QU is split based on a CU as in A) above, a method of decoding QU split information in a decoder is as follows.

Referring to FIG. 8, the decoder parses a QU Split Flag for a current block (S801).

Here, the QU Split Flag may indicate whether the QU is split from a reference block (eg, CTU, CU, TU, etc.) that is a reference (ie, started) of the QU split. That is, it may be indicated whether the current block is divided into QUs.

The QU Split Flag may be transmitted in syntax for the reference block.

If the QU Split Flag value is 1 (ie, when the QU Split Flag is activated), the decoder derives the QU Split Type (S802).

On the other hand, when the value of the QU Split Flag is 0 (that is, when the QU Split Flag is deactivated), the decoder may perform Step S803 without performing Step S802.

Here, when a QU Split Mode (QU Split Mode) indicating a split type (or type) of the QU is explicitly transmitted from the encoder to the decoder, the decoder may parse the QU Split Mode.

For example, all or part of the split form of the QU illustrated in FIG. 6 is predefined as the QU split modes, and the QU split mode selected by the encoder among the predefined QU split modes is the split mode (QU Split Mode). ) May be sent.

In contrast, a QU Split Mode indicating a split type (or type) of a QU may not be explicitly transmitted from the encoder to the decoder. In this case, the decoder may determine the QU division type of the current block according to the intra prediction mode (that is, the prediction direction) applied to the current block. That is, the split form of the QU may be determined according to a distribution characteristic of a non-zero transform coefficient of the transform coefficient according to the intra prediction direction.

For example, if a vertical in-picture prediction is used for the current block (CU or TU), where non-zero transform coefficients are concentrated in the top row of the block, the decoder may The division mode as shown in FIG. 6D or 6F may be selected. In addition, if a horizontal intra prediction is used for the current block (CU or TU) in which a non-zero transform coefficient is concentrated in a column on the left side of the block, the decoder is described with reference to FIG. 6. (c) or the division mode shown in FIG. 6 (e) can be selected.

In addition, in the case of using an intra prediction method in which non-zero transform coefficients are evenly distributed in a low frequency part, the most suitable division of FIGS. 6 (g) to 6 (l) is used. The mode may be selected by the decoder. In this case, the correspondence between the prediction direction in each screen and the split form according to FIGS. 6 (g) to 6 (l) may be predefined with a predetermined rule, and the decoder may be defined in the screen according to the predefined correspondence. The QU division type may be determined according to the prediction direction. The decoder derives a quantization parameter (QP) for each QU (S803).

The quantization parameter refers to a variable used in the decoding process for scaling of transform coefficient levels.

The decoder may parse the quantization parameter difference value QP_Δ for each QU and add the parsed QP_Δ and the quantization parameter predictor QP_pred to derive a QP.

When parsing the QP_Δ for the current QU, the QP values of neighboring QUs (eg, top neighbors, left neighbors, etc.) may be used as the QP_pred for the current QU.

In this case, the neighbor QU used as QP_pred may be selected by the encoder and information about the neighbor QU used (ie, whether it is an upper neighbor QU or a left neighbor QU) may be transmitted to the decoder.

If the QP value of the neighboring QU is not available, the QP value of the current picture may be used as QP_pred. After parsing QP_Δ, the QP value of the current QU may be set as in Equation 1 below.

Quantization and inverse quantization in each QU are performed using QP_QU derived from Equation (1).

Referring to FIG. 9, the quantization unit divider implements the functions, processes, and / or methods proposed in FIGS. 5 to 8. In addition, the quantization unit divider may be implemented as a decoder device in combination with all or a part of the components of the decoder illustrated in FIG. 2.

In detail, the quantization unit divider may include a QU split flag parser 901, a QU split form derivator 902, and a QP derivator 903.

The QU Split Flag parser 901 parses a QU Split Flag for the current block.

As described above, the QU Split Flag may indicate whether the QU is split from a reference block (eg, CTU, CU, TU, etc.) that is the basis of (i.e., starting from) the division of the QU. Can be. That is, it may be indicated whether the current block is divided into QUs.

The QU Split Flag may be transmitted in syntax for the reference block.

If the QU Split Flag value is 1 (ie, when the QU Split Flag is activated), the QU Split Type Derivation Unit 902 derives the QU Split Type.

On the other hand, when the value of the QU Split Flag is 0 (that is, when the QU Split Flag is deactivated), the QU Split Type Derivation Unit 902 may not derive the QU Split Type. .

As described above, when the QU Split Mode indicating the split type (or type) of the QU is explicitly transmitted from the encoder to the decoder, the QU split type derivation unit 902 may perform the QU split mode (QU). Split Mode) can be parsed. For example, all or part of the split form of the QU illustrated in FIG. 6 is predefined as the QU split modes, and the QU split mode selected by the encoder among the predefined QU split modes is the split mode (QU Split Mode). ) May be sent.

In contrast, a QU Split Mode indicating a split type (or type) of a QU may not be explicitly transmitted from the encoder to the decoder. In this case, the QU division type deriving unit 902 may determine the QU division type of the current block according to the intra prediction mode (that is, the prediction direction) applied to the current block. That is, the split form of the QU may be determined according to a distribution characteristic of a non-zero transform coefficient of the transform coefficient according to the intra prediction direction.

For example, if a vertical in-screen prediction in which a non-zero transform coefficient is concentrated in the top row of a block is used for the current block (CU or TU), the QU splitting form The derivation unit 902 may select a split mode as illustrated in FIG. 6D or 6F. In addition, when a non-zero transform coefficient in the horizontal direction in which a non-zero transform coefficient is concentrated in a column on the left side of a block is used in the current block (CU or TU), a QU division type derivation unit 902 may select a split mode as shown in FIG. 6 (c) or FIG. 6 (e).

In addition, in the case of using an intra prediction method in which non-zero transform coefficients are evenly distributed in a low frequency part, the most suitable division of FIGS. 6 (g) to 6 (l) is used. The mode may be selected by the QU division type derivation unit 902. In this case, the correspondence relationship between the prediction direction in each screen and the split mode according to FIGS. 6 (g) to 6 (l) may be defined in advance with a predetermined rule, and the QU division type derivation unit 902 is predefined. The QU splitting shape may be determined according to the prediction direction in the screen according to the correspondence.

The QP derivation unit 903 derives a quantization parameter (QP) for each QU.

In other words, QP_Δ may be parsed for each QU, and QP_Δ and quantization parameter predictor QP_pred may be summed as shown in Equation 1 to derive QP for each QU.

Here, the QPs of neighboring QUs (eg, top neighbors, left neighbors, QU, etc.) may be used as the QP predictor QP_pred for the current QU.

In this case, information about the neighbor QU used as QP_pred (that is, whether the upper neighbor QU, the left neighbor QU, etc.) may be transmitted from the encoder. In this case, the QP derivation unit 903 may parse information about the neighbor QU. Can be.

If the QP value of the neighboring QU is not available, the QP value of the current picture may be used as QP_pred.

The QP value determined by the above method is transferred to the inverse quantization unit, and the inverse quantization unit may perform inverse quantization using the QP value.

Meanwhile, in FIGS. 8 and 9, a method of splitting a QU from any one of a CU, a TU, and a CTU (that is, a reference block) using a QU division mode determined by an encoder among a plurality of predefined QU division modes (FIG. 6). However, as described above, when the QU is split from a reference block into a recursive structure such as a quad-tree form (or a binary-tree form) as shown in FIG. 5, the partitioning of the existing CU / TU is performed. The same may be applied. That is, the decoder (particularly, the image splitter) may determine whether to split the reference block into QUs having a depth of 1 according to the QU Split Flag value of the reference block. If the reference block is divided into QUs having a depth of 1, whether or not the reference block is divided into QUs having a depth of 2 may be determined according to a QU split flag value for each QU having a depth of 1. Similarly, it is possible to determine whether to divide the QU having a depth of 3 according to the QU Split Flag value for each QU of the divided depth 2. This process may be repeated until the maximum depth value (or minimum QU size value) for the QU split is reached.

The embodiments described above are the components and features of the present invention are combined in a predetermined form. Each component or feature is to be considered optional unless stated otherwise. Each component or feature may be embodied in a form that is not combined with other components or features. It is also possible to combine some of the components and / or features to form an embodiment of the invention. The order of the operations described in the embodiments of the present invention may be changed. Some components or features of one embodiment may be included in another embodiment or may be replaced with corresponding components or features of another embodiment. It is obvious that the claims may be combined to form an embodiment by combining claims that do not have an explicit citation relationship in the claims or as new claims by post-application correction.

Embodiments according to the present invention may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In the case of a hardware implementation, an embodiment of the present invention may include one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), FPGAs ( field programmable gate arrays), processors, controllers, microcontrollers, microprocessors, and the like.

In the case of implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, function, etc. that performs the functions or operations described above. The software code may be stored in memory and driven by the processor. The memory may be located inside or outside the processor, and may exchange data with the processor by various known means.

It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential features of the present invention. Accordingly, the above detailed description should not be construed as limiting in all aspects and should be considered as illustrative. The scope of the invention should be determined by reasonable interpretation of the appended claims, and all changes within the equivalent scope of the invention are included in the scope of the invention.

As mentioned above, preferred embodiments of the present invention are disclosed for purposes of illustration, and those skilled in the art can improve and change various other embodiments within the spirit and technical scope of the present invention disclosed in the appended claims below. , Replacement or addition would be possible.

Claims

In the decoding apparatus decodes an image,

Parsing a QU division flag indicating whether a quantization unit (QU) is split from a current block;

Deriving a division type of the QU when the QU division flag is activated; And

And deriving a quantization parameter (QP) for each of the divided QUs.
The method of claim 1,

The QU is divided into quad-trees from the current block.
The method of claim 1,

And the QU is divided according to a QU partition type selected from a plurality of predefined QU partition types.
The method of claim 3,

And a split type of the QU is derived by parsing information about a split mode of the QU indicating the split type of the QU.
The method of claim 3,

And a split form of the QU is derived according to an intra prediction mode applied to the current block.
The method of claim 5,

When the intra prediction mode is in the vertical direction, a split form is determined by 2N × N or 2N × nU.
The method of claim 5,

When the intra prediction mode is the horizontal direction, the decoding method is determined to be divided into N × 2N or nL × 2N.
The method of claim 1, wherein the deriving of the QP comprises:

Parsing the QP difference value,

And the QP is derived by summing a QP predictor derived from a QP of a neighboring QU and the parsed QP difference value.
The method of claim 8,

The neighboring QU is a QU neighboring the top of the QU or a QU neighboring to the left of the QU.
In the decoding apparatus for decoding an image,

A QU division flag parsing unit for parsing a QU division flag indicating whether a quantization unit (QU) is split from a current block;

A QU division type derivation unit for deriving a division type of the QU when the QU division flag is activated; And

And a QP derivation unit for deriving a quantization parameter (QP) for each of the divided QUs.