WO2014007514A1

WO2014007514A1 - Method for decoding image and apparatus using same

Info

Publication number: WO2014007514A1
Application number: PCT/KR2013/005857
Authority: WO
Inventors: 김정선; 박준영; 김철근; 전병문; 헨드리헨드리
Original assignee: 엘지전자 주식회사
Priority date: 2012-07-02
Filing date: 2013-07-02
Publication date: 2014-01-09

Abstract

A method for decoding an image, according to the present invention, comprises the steps of: generating a reconstructed value with respect to a luminance component of a current block; and predicting the color difference of the current block on the basis of the reconstructed value of the luminance component, wherein a prediction value of the color difference is derived from arithmetic calculation of the reconstructed value of the luminance component and predetermined parameters, and wherein the parameters can be set for each image division unit. As a result, provided are an intra-prediction method of a color difference block, which can increase image coding and decoding efficiency, and an apparatus using same.

Description

Image decoding method and apparatus using same

TECHNICAL FIELD The present invention relates to video compression techniques, and more particularly, to an intra prediction method of a color difference block using luminance samples, and an apparatus using the method.

Recently, the demand for high resolution and high quality images is increasing in various applications. As an image has a high resolution and high quality, the amount of information on the image also increases.

As information volume increases, devices with various performances and networks with various environments are emerging. With the emergence of devices of varying performance and networks of different environments, the same content is available in different qualities.

In detail, as the video quality of the terminal device can be supported and the network environment is diversified, in general, video of general quality may be used in one environment, but higher quality video may be used in another environment. .

For example, a consumer who purchases video content on a mobile terminal can view the same video content on a larger screen and at a higher resolution through a large display in the home.

In recent years, as broadcasts with high definition (HD) resolution have been serviced, many users are already accustomed to high-definition and high-definition video.Ultra High Definition (UHD) has more than four times the resolution of HDTV with HDTV. I am also interested in the services of the company.

Therefore, in order to provide various video services required by users in various environments according to the quality, based on a high-efficiency encoding / decoding method for high-capacity video, the quality of the image, for example, the image quality, the resolution of the image, the size of the image, It is necessary to provide scalability in the frame rate of video and the like. In addition, various image processing methods associated with such scalability should be discussed.

An object of the present invention is to provide an intra prediction method of a chrominance block and an apparatus using the same that can increase image coding and decoding efficiency.

One embodiment of the present invention is to provide a method for predicting a sample value of a color difference signal for each transform unit in intra prediction and an apparatus using the same.

Another object of the present invention is to provide a method for predicting a sample value of a color difference signal for each coding unit in intra prediction, and an apparatus using the same.

Another object of the present invention is to provide a method for predicting a sample value of a color difference signal for each coding tree block in intra prediction, and an apparatus using the same.

An image decoding method according to an embodiment of the present invention includes generating a reconstruction value for a luminance component of a current block, and predicting a color difference component of the current block based on the reconstruction value of the luminance component, The predicted value of the color difference component may be derived by a reconstruction value of the luminance component and an arithmetic operation of a predetermined parameter, and the parameter may be set for each division unit of an image.

In this case, the parameter may be set for each coding unit.

Meanwhile, the parameter may be set for each transform unit, and the parameter may be set for each coding tree block.

The predicted value of the color difference component may be calculated as follows.

(Lrecon: restored value of luma component, α, β: parameter)

An image decoding apparatus according to another embodiment of the present invention includes a prediction unit that predicts a color difference component of the current block based on a reconstruction value of a luminance component of the current block, wherein the predicted value of the color difference component is a reconstruction of the luminance component. Derived by arithmetic operation of a value and a predetermined parameter, the parameter may be set for each division unit of an image.

According to an embodiment of the present invention, an intra prediction method of a chrominance block capable of increasing image coding and decoding efficiency and an apparatus using the same are provided.

An embodiment of the present invention provides a method for predicting a sample value of a color difference signal for each transform unit in intra prediction, and an apparatus using the same.

Another embodiment of the present invention provides a method for predicting a sample value of a color difference signal for each coding unit in intra prediction, and an apparatus using the same.

Another embodiment of the present invention provides a method for predicting a sample value of a color difference signal for each coding tree block in intra prediction, and an apparatus using the same.

1 is a block diagram schematically illustrating a video encoding apparatus according to an embodiment of the present invention.

2 is a block diagram schematically illustrating a video decoding apparatus according to an embodiment of the present invention.

3 is a diagram illustrating an example of an intra prediction mode.

4 is a diagram for describing a prediction value when a current block is predicted in a planar mode.

5 is a diagram illustrating sample values of luminance components existing in and around a current block according to an exemplary embodiment of the present invention.

6 is a control flowchart for explaining an image decoding method according to the present invention.

As the present invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the invention to the specific embodiments. The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the spirit of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as "comprise" or "have" are intended to indicate that there is a feature, number, step, action, component, part, or combination thereof described on the specification, and one or more other features. It is to be understood that the present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

On the other hand, each of the components in the drawings described in the present invention are shown independently for the convenience of description of the different characteristic functions in the video encoding apparatus / decoding apparatus, each component is a separate hardware or separate software It does not mean that it is implemented. For example, two or more of each configuration may be combined to form one configuration, or one configuration may be divided into a plurality of configurations. Embodiments in which each configuration is integrated and / or separated are also included in the scope of the present invention without departing from the spirit of the present invention.

Hereinafter, with reference to the accompanying drawings, it will be described in detail a preferred embodiment of the present invention. Hereinafter, the same reference numerals are used for the same components in the drawings, and redundant description of the same components is omitted.

1 is a block diagram schematically illustrating a video encoding apparatus according to an embodiment of the present invention. The video encoding / decoding method or apparatus may be implemented by an extension of a general video encoding / decoding method or apparatus that does not provide scalability, and the block diagram of FIG. 1 is based on a scalable video encoding apparatus. An embodiment of a video encoding apparatus that may be represented.

Referring to FIG. 1, the encoding apparatus 100 may include a picture divider 105, a predictor 110, a transformer 115, a quantizer 120, a reordering unit 125, an entropy encoding unit 130, An inverse quantization unit 135, an inverse transform unit 140, a filter unit 145, and a memory 150 are provided.

The picture dividing unit 105 may divide the input picture into at least one processing unit block. In this case, the block as the processing unit may be a prediction unit (hereinafter referred to as a PU), a transform unit (hereinafter referred to as a TU), or a coding unit (hereinafter referred to as "CU"). It may be called.

The processing unit blocks divided by the picture divider 105 may have a quad-tree structure.

The predictor 110 includes an inter predictor for performing inter prediction and an intra predictor for performing intra prediction, as described below. The prediction unit 110 generates a prediction block by performing prediction on the processing unit of the picture in the picture division unit 105. The processing unit of the picture in the prediction unit 110 may be a CU, a TU, or a PU. In addition, the prediction unit 110 may determine whether the prediction performed on the processing unit is inter prediction or intra prediction, and determine specific contents (eg, prediction mode, etc.) of each prediction method. In this case, the processing unit in which the prediction is performed may differ from the processing unit in which specific contents of the prediction method and the prediction method are determined. For example, the method of prediction and the prediction mode may be determined in units of PUs, and the prediction may be performed in units of TUs.

Through inter prediction, a prediction block may be generated by performing prediction based on information of at least one picture of a previous picture and / or a subsequent picture of the current picture. In addition, through intra prediction, a prediction block may be generated by performing prediction based on pixel information in a current picture.

As a method of inter prediction, a skip mode, a merge mode, a motion vector prediction (MVP), and the like can be used. In inter prediction, a reference picture may be selected for a PU and a reference block corresponding to the PU may be selected. The reference block may be selected in integer pixel units. Subsequently, a prediction block is generated in which a residual signal with the current PU is minimized and the size of the motion vector is also minimized.

The prediction block may be generated in integer sample units, or may be generated in sub-pixel units such as 1/2 pixel unit or 1/4 pixel unit. In this case, the motion vector may also be expressed in units of integer pixels or less.

Information such as an index, a motion vector predictor, and a residual signal of a reference picture selected through inter prediction is entropy encoded and delivered to the decoding apparatus. When the skip mode is applied, the residual may be used as the reconstructed block, and thus the residual may not be generated, transformed, quantized, or transmitted.

When performing intra prediction, a prediction mode may be determined in units of PUs, and prediction may be performed in units of PUs. In addition, a prediction mode may be determined in units of PUs, and intra prediction may be performed in units of TUs.

In intra prediction, the prediction mode may have 33 directional prediction modes and at least two non-directional modes. The non-directional mode may include a DC prediction mode and a planner mode (Planar mode).

In intra prediction, a prediction block may be generated after applying a filter to a reference sample. In this case, whether to apply the filter to the reference sample may be determined according to the intra prediction mode and / or the size of the current block.

The PU may be a block of various sizes / types, for example, in the case of inter prediction, the PU may be a 2N × 2N block, a 2N × N block, an N × 2N block, an N × N block (N is an integer), or the like. In the case of intra prediction, the PU may be a 2N × 2N block or an N × N block (where N is an integer). In this case, the PU of the N × N block size may be set to apply only in a specific case. For example, the NxN block size PU may be used only for the minimum size CU or only for intra prediction. In addition to the above-described PUs, PUs such as N × mN blocks, mN × N blocks, 2N × mN blocks, or mN × 2N blocks (m <1) may be further defined and used.

The residual value (the residual block or the residual signal) between the generated prediction block and the original block is input to the converter 115. In addition, the prediction mode information, the motion vector information, etc. used for the prediction are encoded by the entropy encoding unit 130 together with the residual value and transmitted to the decoding apparatus.

The transform unit 115 performs transform on the residual block in units of transform blocks and generates transform coefficients.

The transform block is a rectangular block of samples to which the same transform is applied. The transform block can be a transform unit (TU) and can have a quad tree structure.

The transformer 115 may perform the transformation according to the prediction mode applied to the residual block and the size of the block.

For example, if intra prediction is applied to a residual block and the block is a 4x4 residual array, the residual block is transformed using a discrete sine transform (DST), otherwise the residual block is transformed into a discrete cosine transform (DCT). Can be converted using.

The transform unit 115 may generate a transform block of transform coefficients by the transform.

The quantization unit 120 may generate quantized transform coefficients by quantizing the residual values transformed by the transform unit 115, that is, the transform coefficients. The value calculated by the quantization unit 120 is provided to the inverse quantization unit 135 and the reordering unit 125.

The reordering unit 125 rearranges the quantized transform coefficients provided from the quantization unit 120. By rearranging the quantized transform coefficients, the encoding efficiency of the entropy encoding unit 130 may be increased.

The reordering unit 125 may rearrange the quantized transform coefficients in the form of a 2D block into a 1D vector form through a coefficient scanning method.

The entropy encoding unit 130 may perform entropy encoding on the quantized transform coefficients rearranged by the reordering unit 125. Entropy encoding may include, for example, encoding methods such as Exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC). The entropy encoding unit 130 may include quantized transform coefficient information, block type information, prediction mode information, partition unit information, PU information, transmission unit information, and motion vector of the CUs received from the reordering unit 125 and the prediction unit 110. Various information such as information, reference picture information, interpolation information of a block, and filtering information may be encoded.

In addition, if necessary, the entropy encoding unit 130 may apply a constant change to a parameter set or syntax to be transmitted.

The inverse quantizer 135 inversely quantizes the quantized values (quantized transform coefficients) in the quantizer 120, and the inverse transformer 140 inversely transforms the inverse quantized values in the inverse quantizer 135.

The reconstructed block may be generated by combining the residual values generated by the inverse quantizer 135 and the inverse transform unit 140 and the prediction blocks predicted by the prediction unit 110.

In FIG. 1, it is described that a reconstructed block is generated by adding a residual block and a prediction block through an adder. In this case, the adder may be viewed as a separate unit (restore block generation unit) for generating a reconstruction block.

The filter unit 145 may apply a deblocking filter, an adaptive loop filter (ALF), and a sample adaptive offset (SAO) to the reconstructed picture.

The deblocking filter may remove distortion generated at the boundary between blocks in the reconstructed picture. The adaptive loop filter (ALF) may perform filtering based on a value obtained by comparing the reconstructed image with the original image after the block is filtered through the deblocking filter. ALF may be performed only when high efficiency is applied. The SAO restores the offset difference from the original image on a pixel-by-pixel basis to the residual block to which the deblocking filter is applied, and is applied in the form of a band offset and an edge offset.

Meanwhile, the filter unit 145 may not apply filtering to the reconstructed block used for inter prediction.

The memory 150 may store the reconstructed block or the picture calculated by the filter unit 145. The reconstructed block or picture stored in the memory 150 may be provided to the predictor 110 that performs inter prediction.

2 is a block diagram schematically illustrating a video decoding apparatus according to an embodiment of the present invention. As described above with reference to FIG. 1, the scalable video encoding / decoding method or apparatus may be implemented by extension of a general video encoding / decoding method or apparatus that does not provide scalability, and the block diagram of FIG. 2 shows scalable video decoding. Represents an embodiment of a video decoding apparatus that may be the basis of the apparatus

Referring to FIG. 2, the video decoding apparatus 200 includes an entropy decoding unit 210, a reordering unit 215, an inverse quantization unit 220, an inverse transform unit 225, a prediction unit 230, and a filter unit 235. Memory 240 may be included.

When an image bitstream is input in the video encoding apparatus, the input bitstream may be decoded according to a procedure in which image information is processed in the video encoding apparatus.

For example, when variable length coding (VLC, hereinafter called 'VLC') is used to perform entropy encoding in the video encoding apparatus, the entropy decoding unit 210 also uses the VLC. Entropy decoding may be performed by implementing the same VLC table as the table. In addition, when CABAC is used to perform entropy encoding in the video encoding apparatus, the entropy decoding unit 210 may correspondingly perform entropy decoding using CABAC.

Information for generating the prediction block among the information decoded by the entropy decoding unit 210 is provided to the predictor 230, and a residual value where entropy decoding is performed by the entropy decoding unit 210, that is, a quantized transform coefficient It may be input to the reordering unit 215.

The reordering unit 215 may reorder the information of the bitstream entropy decoded by the entropy decoding unit 210, that is, the quantized transform coefficients, based on the reordering method in the encoding apparatus.

The reordering unit 215 may reorder the coefficients expressed in the form of a one-dimensional vector by restoring the coefficients in the form of a two-dimensional block. The reordering unit 215 may generate an array of coefficients (quantized transform coefficients) in the form of a 2D block by scanning coefficients based on the prediction mode applied to the current block (transform block) and the size of the transform block.

The inverse quantization unit 220 may perform inverse quantization based on the quantization parameter provided by the encoding apparatus and the coefficient values of the rearranged block.

The inverse transform unit 225 may perform inverse DCT and / or inverse DST on the DCT and the DST performed by the transform unit of the encoding apparatus with respect to the quantization result performed by the video encoding apparatus.

The inverse transformation may be performed based on a transmission unit determined by the encoding apparatus or a division unit of an image. The DCT and / or DST in the encoding unit of the encoding apparatus may be selectively performed according to a plurality of pieces of information, such as a prediction method, a size and a prediction direction of the current block, and the inverse transformer 225 of the decoding apparatus may be Inverse transformation may be performed based on the performed transformation information.

The prediction unit 230 may generate the prediction block based on the prediction block generation related information provided by the entropy decoding unit 210 and the previously decoded block and / or picture information provided by the memory 240.

When the prediction mode for the current PU is an intra prediction mode, intra prediction for generating a prediction block based on pixel information in the current picture may be performed.

When the prediction mode for the current PU is an inter prediction mode, inter prediction on the current PU may be performed based on information included in at least one of a previous picture or a subsequent picture of the current picture. In this case, motion information required for inter prediction of the current PU provided by the video encoding apparatus, for example, a motion vector, a reference picture index, and the like, may be derived by checking a skip flag, a merge flag, and the like received from the encoding apparatus.

The reconstruction block may be generated using the prediction block generated by the predictor 230 and the residual block provided by the inverse transform unit 225. In FIG. 2, it is described that the reconstructed block is generated by combining the prediction block and the residual block in the adder. In this case, the adder may be viewed as a separate unit (restore block generation unit) for generating a reconstruction block.

When the skip mode is applied, the residual is not transmitted and the prediction block may be a reconstruction block.

The reconstructed block and / or picture may be provided to the filter unit 235. The filter unit 235 may apply deblocking filtering, sample adaptive offset (SAO), and / or ALF to the reconstructed block and / or picture.

The memory 240 may store the reconstructed picture or block to use as a reference picture or reference block and provide the reconstructed picture to the output unit.

Entropy decoding unit 210, reordering unit 215, inverse quantization unit 220, inverse transform unit 225, predictor 230, filter unit 235, and memory 240 included in the decoding device 200. ) Components directly related to the decoding of an image, for example, an entropy decoding unit 210, a reordering unit 215, an inverse quantization unit 220, an inverse transform unit 225, a prediction unit 230, and a filter unit ( 235) and the like may be distinguished from other components by a decoder or a decoder.

In addition, the decoding apparatus 200 may further include a parsing unit (not shown) which parses information related to an encoded image included in the bitstream. The parsing unit may include the entropy decoding unit 210 or may be included in the entropy decoding unit 210. Such a parser may also be implemented as one component of the decoder.

3 is a diagram illustrating an example of an intra prediction mode, and Table 1 shows a mapping relationship between intra prediction modes for 35 intra prediction mode numbers. Hereinafter, for convenience of description, a block composed of a luminance component of the current block may be expressed as a luminance block and a block composed of a color difference component as a color difference component.

The intra prediction mode may include 33 directional prediction modes and two non-directional modes. As shown in FIG. 3, the directional mode includes the intra prediction mode 34 in the clockwise direction starting from the second intra prediction mode in the lower left direction. The prediction mode 35 may represent an intra mode for the color difference component.

Table 1

Planar mode Intra_Planar and DC mode Intra_DC, which are non-directional modes, may be allocated to

intra prediction modes

0 and 1, respectively.

In DC mode, a single fixed value, for example, the average value of surrounding reconstructed pixel values is used as a prediction value, and in Planer mode, vertical interpolation and horizontal use are performed using vertically adjacent pixel values of the current block and horizontally adjacent pixel values. Directional interpolation is performed, and their average value is used as the predicted value.

The directional mode Intra_Angular refers to modes indicating a corresponding direction at an angle between a reference pixel located in a predetermined direction and a current pixel, and may include a horizontal mode and a vertical mode. In the vertical mode, vertically adjacent pixel values of the current block may be used as prediction values of the current block, and in the horizontal mode, horizontally adjacent pixel values may be used as prediction values of the current block.

The intra prediction mode for the current block may be transmitted with a value indicating the mode itself, but may be derived from information about candidate intra prediction modes that are likely to be the intra prediction mode of the current block.

The prediction unit generates a prediction value for the luma component of the current block by using the intra prediction mode for the current block derived as described above. Hereinafter, the generation of the predicted value for the luma component will be briefly described.

The prediction value of the current block predicted in the intra prediction mode may be generated for each TU or PU. The samples adjacent to the left side of the current block and the samples adjacent to the upper side of the current block are called reference samples, and reference values are used to generate prediction values of the current block. If a sample adjacent to the left of the current block or a sample adjacent to the upper side of the current block is not available, the samples that are not available may be replaced with the value of the predetermined sample.

If all reference samples are not available, the reference sample may be replaced with a predetermined value, for example, a median value of sample values that the image may have.

On the other hand, if the lower leftmost sample of the current block is not available, the reference samples are searched in a predetermined direction, and the sample value of the retrieved sample that is available is replaced with the left lowermost sample value.

In addition, if a sample that is not available among the samples adjacent to the left of the current block is found, the sample value of the adjacent sample, which is immediately below the sample that is not available, is replaced with a reference sample value, and the available sample among the above adjacent samples is available. If a sample that has not been found is found, the sample value of the sample existing on the right side of the sample that is not available can be replaced with the reference sample value.

Sample values adjacent to the current block may be filtered. Whether to filter and the filtering coefficient may be set differently according to the size of the transform block and the intra prediction mode. When the step difference existing in the predicted value generated after the reference sample value and the intra prediction is applied through filtering, discontinuity that may occur at the block boundary may be reduced.

If the intra prediction mode of the current block is a planar mode that is a non-directional mode, the prediction value for the current block may be obtained as a linear interpolation value of a plurality of reference sample values.

4 is a diagram for describing a prediction value when a current block is predicted in a planar mode. When the size of the current block 400 is NxN (N is an integer), for convenience of explanation, the position of the upper left sample of the block is (0,0), the position of the lower left sample is (0, N-1), The position of the upper right sample may be expressed as (N-1, 0), and the position of the lower right sample may be expressed as (N-1, N-1).

If the position of the prediction sample to be generated is (x, y), the position of the reference sample a adjacent to the prediction sample in the vertical direction is (x, -1), and the position of the reference sample b adjacent to the prediction sample in the horizontal direction is (-1, y). In addition, the prediction value P (x, y) of the prediction sample may be calculated by Equation 1 using the reference sample c located at (N, -1) and the reference sample d located at (-1, N). The sample value of reference sample a is a (x, -1), the sample value of reference sample b is b (-1, y), the sample value of reference sample c is c (N, -1), the sample value of reference sample d The value is represented by d (-1, N).

If the intra prediction mode of the current block is the DC mode, the prediction value for the current block may be generated using the average value of the reference sample values. The boundary region of the prediction block, that is, the left boundary and the right boundary of the prediction block may be filtered.

If the intra prediction mode of the current block is the directional mode, the prediction value may be generated using the reference sample value present in the direction of the prediction mode. Reference samples may be stretched according to the direction of the prediction mode. In addition, when the prediction mode is a horizontal or vertical mode, the prediction value of the current block located at a boundary adjacent to the reference sample may be generated through arithmetic operation of specific sample values existing at a predetermined position with the reference sample. The arithmetic operation has an effect of applying filtering to the boundary region adjacent to the reference sample, thereby reducing the discontinuity between the current block and the neighboring block.

As described above, a predicted value of the luminance block including the luminance component is generated, and when the residual is added to the generated predicted value, a restored value for the luminance block of the current block is derived.

When performing intra prediction on the luminance component, all of the directional intra prediction modes of FIG. 3 can be used. However, when performing intra prediction mode on the chrominance component, intra directional intra prediction mode is used only in some directional intra prediction modes without using all of the directional intra prediction modes. You can make predictions.

Intra prediction for the chrominance component includes diagonal intra prediction mode (intra prediction mode No. 34), vertical intra prediction mode (intra prediction mode No. 26), horizontal intra prediction mode (intra prediction mode No. 10), and DC intra prediction mode. And LM (Luma Estimated Mode) for predicting the color difference component based on the restored value of the restored luminance component, which is an intra prediction mode that is different from the luminance component, and the Luma Directed Mode (DM) which is the same intra prediction mode as the luminance component. Can be used for intra prediction of the chrominance component.

Table 2 below shows the mapping between intra prediction mode and intra prediction mode numbers when LM, vertical intra prediction mode (VER), horizontal intra prediction mode (HOR), DC intra prediction mode, and DM are used as intra prediction methods of the chrominance block. This table shows the relationship.

TABLE 2

Referring to Table 2, intra_chroma_pred_type 0 is Luma Estimated Mode (LM), intra_chroma_pred_type 1 is vertical intra prediction mode (VER), intra_chroma_pred_type 2 is horizontal intra prediction mode (HOR), intra_chroma_pred_type is 3 DC intra prediction mode, and intra_chroma_pred_type May perform intra prediction on a chrominance block in a DM (Luma Directed Mode).

When the luminance block and the chrominance block have the same intra prediction mode, since the intra prediction mode of the chrominance block can be represented by DM, the intra prediction number is mapped to one of the intra prediction modes of the above five chrominance blocks. There is no need to do it. For example, if the intra prediction mode of the luminance block is the vertical intra prediction mode (Ver) and the chrominance block is the vertical intra prediction mode (Ver), intra_chroma_pred_type 1 because the intra prediction mode of the chrominance block may be represented by DM having intra_chroma_pred_type 4. There is no need to map intra prediction modes. In the same manner, when the intra prediction mode of the luminance block is the vertical intra prediction mode VER, the horizontal intra prediction mode HOR, or the DC intra prediction mode DC, four intra_chroma_pred_types may indicate the intra prediction mode of the chrominance block.

If the intra prediction mode of the luminance block is the vertical intra prediction mode, the horizontal intra prediction mode, or the remaining directional intra prediction mode other than the DC mode, the intra prediction mode of the DM mode and the luminance block is not overlapped since the DM mode and the intra prediction mode of the luminance block do not overlap. Unlike when the modes overlap, intra prediction may be performed on the color difference block using five intra_chroma_pred_types.

The intra prediction mode used to perform intra prediction of the chrominance block and the mapping relationship between the intra prediction mode and the intra prediction mode number may be changed to arbitrary. In addition, the color difference block may determine whether to use the LM in the encoding and decoding steps, and Table 2 may vary depending on whether the LM is used.

For example, as shown in Table 3 below, an oblique intra prediction mode (intra prediction mode 34) and a planar mode may be used for the intra prediction mode of the chrominance block.

Table 3 below shows the relationship between the intra prediction mode of the luminance block and the intra prediction mode of the chrominance block when performing intra prediction on the chrominance block using the LM mode.

TABLE 3

Referring to Table 3, planar mode if intra_chroma_pred_mode is 0, vertical intra prediction mode if intra_chroma_pred_mode is 1, horizontal intra prediction mode if intra_chroma_pred_mode is 2, DC mode if intra_chroma_pred_mode is 3, LM if intra_chroma_pred_mode is 4, and LM if intra_chroma_pred_mode is 4 Intra prediction modes of the chrominance block may be mapped.

As shown in Table 3, if the intra prediction mode of the luminance block is the planar mode, the vertical mode, the horizontal mode, or the DC mode, the intra prediction mode that can be represented by the DM among intra_chroma_pred_mode can be represented by the intra prediction mode of the chrominance block. Instead, a diagonal intra mode (intra prediction mode 34) may be included instead.

Due to the DM, an unnecessary intra prediction mode is set to an oblique intra prediction mode (intra prediction mode 34). However, other intra prediction modes instead of intra prediction mode 34, for example, intra prediction mode 18 or intra prediction mode 2 may also be used as the intra prediction mode 34 and such embodiments are also included in the scope of the present invention. .

Information about which intra prediction mode is used as an alternative intra prediction mode can be expressed by a schedo code as shown in the table below.

Table 4

Referring to Table 4, when the intra prediction mode of the chrominance block and the intra prediction mode of the luminance block are the same, an intra prediction mode to substitute the intra prediction mode of the same chrominance block as the intra prediction mode of the luminance block is referred to as substitute_mode. Intra prediction may be performed on the chrominance block.

According to an embodiment of the present invention, when LM is used to predict a chrominance component, that is, a restored value of a luminance component may be used, and a predicted value of a chrominance sample may be generated using a linear equation such as Equation 2.

In this case, (x, y) may indicate a location in one TU. In other words, the current block may be a TU. Cpred means a predicted value of the sample, and Lrecon indicates a reconstructed value of the luma sample.

[alpha] and [beta] are constant parameters obtained by using the restored value of the luma sample around the current block and the restored value of the chrominance sample.

The decoding process in the case of obtaining α and β in units of TU is as follows.

First, (xB, yB) located at the top left sample of the current prediction block for the top left sample of the current picture and the neighboring samples p [x, y] where x, y =? 1..2 * nS? 1 are input values. In this case, the variable nS is the size of the prediction block.

The predicted value predSamples [x, y] for x, y = 0..nS? 1 is output as the output value with respect to the input value.

The current decoding process is applied when the intra prediction mode is 35 in FIG. 3, and the predicted value predSamples [x, y] of the color difference component is derived as follows.

The variable k3 and the sample array pY 'are derived by Equation 3.

As described above, through Equation 3, new first variables k3 and pY 'may be calculated using linear interpolation.

By using the first variable calculated in Equation 3 above, the second variables L, C, LL, LC, and k2 for calculating α and β as shown in Equation 4 below may be calculated.

Α and β may be calculated using the method of Equation 5 based on the second variable calculated through Equation 4.

A color difference block sample using LM may be calculated based on Equation 6 below using α and β calculated based on Equation 5 above.

5 illustrates luminance sample values present in and around a current block according to an embodiment of the present invention.

Referring to FIG. 5, the first sample 500 represents a subsampled luminance sample used to calculate the parameters α and β. The second sample 510 represents a sample of linear interpolation of the subsampled luminance samples used to calculate the parameters α and β.

Third sample 520 represents the subsampled luminance sample used to calculate the chrominance sample. Fourth sample 530 represents a subsampled luminance sample with linear interpolation used to calculate the chrominance sample.

That is, in the method of predicting color difference samples using luminance samples according to an embodiment of the present invention, the linear color interpolation is performed by considering the difference between the distance between the luminance samples and the color difference samples, rather than using the luminance samples when predicting the color difference samples. The interpolated luminance sample value can be calculated and used.

By using this linear interpolation method, more accurate prediction can be performed when the color difference signal is predicted in an image format having different positions of the color difference signal and the luminance signal. The luminance sample value may be calculated by performing linear interpolation based on the phase and sampling rate difference between the luminance sample and the chrominance sample, and this embodiment may also be included in the scope of the present invention.

On the other hand, the parameters α and β may be set for each unit in which color difference samples are predicted, that is, for each TU, or the same α and β may be used in all TUs calculated for each CU and included in one CU.

Computing α and β in one CU can reduce the number of times of access to the memory storing luminance samples and chrominance samples, compared to calculating α and β for each TU. Can be reduced.

In addition, since the size of the accessed memory is limited, the load can be reduced in terms of hardware implementation. In addition, since the number of samples used when calculating α and β increases compared to calculating α and β for each TU, α and β of a more general meaning can be obtained. Therefore, in the case of obtaining α and β for each CU, there is a difference in the average difference in the bit rate as shown in Table 5 compared to obtaining for each TU.

Table 5

The decoding process for obtaining α and β in CU units is as follows.

First, (xB, yB) located at the top left sample of the current coding block for the top left sample of the current picture and the surrounding sample p [x, y] where x, y =? 1..2 * nS? 1 are input values. In this case, the variable nS is the size of the coding block.

The variable k3 and the sample array pY 'are derived by Equation 7.

As described above, a new first variable (k3, pY ′) may be calculated by using linear interpolation as described above.

By using the first variable calculated in Equation 7 above, the second variables L, C, LL, LC, and k2 for calculating α and β as shown in Equation 8 below may be calculated.

Α and β may be calculated using the method of Equation 9 based on the second variable calculated through Equation 8.

Color difference block samples using LM may be calculated based on Equation 10 below using α and β calculated based on Equation 9 above.

On the other hand, according to another embodiment of the present invention, α and β can be obtained using the LCU (largest coding unit) surrounding. That is, if the largest coding block is expressed differently, α and β may be calculated for each coding tree block (CTB) unit. In this case, α and β need to be calculated only once for each LCU unit, which is more effective for hardware implementation. The decoding process for this is as follows.

First, (xB, yB) located at the top left sample of the current coding tree block for the top left sample of the current picture and the surrounding samples p [x, y] where x, y =? 1..2 * nS? 1 are input values. In this case, the variable nS is the size of the coding tree block.

The variable k3 and the sample array pY 'are derived by Equation 11.

As described above, through Equation 11, new first variables k3 and pY 'may be calculated using linear interpolation.

By using the first variable calculated in Equation 11 above, the second variables L, C, LL, LC, and k2 for calculating α and β as shown in Equation 12 below may be calculated.

Α and β may be calculated using the method of Equation 13 based on the second variable calculated through Equation 12.

Color difference block samples using LM may be calculated based on Equation 14 below using α and β calculated based on Equation 13 above.

FIG. 6 is a diagram for explaining another image decoding method according to an embodiment of the present invention, specifically, a prediction method when LM is applied to a color difference component.

First, the prediction unit generates a reconstruction value for the luminance component of the current block (S610).

In the luminance block including the luminance component, a prediction value is derived using a mode most suitable for prediction of the luminance block among intra prediction modes including a directional mode and a non-directional mode, and the prediction value is added to the residual to restore the luminance component.

Then, the color difference component is predicted based on an arithmetic operation of a parameter set for each division unit of the image based on the restored value of the luminance component of the current block (S620).

In this case, the division unit of the image may be a coding unit, a transformation unit, or may be set for each coding tree block. Alternatively, the parameters α and β may be calculated for each prediction unit.

As the unit of the block in which the parameters α and β are calculated increases, the amount of image information to be coded and decoded decreases, thereby increasing coding and decoding efficiency of the image.

In the exemplary system described above, the methods are described based on a flowchart as a series of steps or blocks, but the invention is not limited to the order of steps, and certain steps may occur in a different order or concurrently with other steps than those described above. Can be. In addition, since the above-described embodiments may include examples of various aspects, a combination of each embodiment should also be understood as an embodiment of the present invention. Accordingly, the invention is intended to embrace all other replacements, modifications and variations that fall within the scope of the following claims.

Claims

Generating a reconstruction value for the luminance component of the current block;

Predicting a color difference component of the current block based on a reconstruction value of the luminance component,

The predicted value of the color difference component is derived by an arithmetic operation of the restored value of the luminance component and a predetermined parameter,

And the parameter is set for each division unit of an image.
The method of claim 1,

And the parameter is set for each coding unit.
The method of claim 1,

And the parameter is set for each transform unit.
The method of claim 1,

And the parameter is set for each coding tree block.
The method according to any one of claims 1 to 4,

The prediction value of the chrominance component is calculated as in the following equation.

(Lrecon: restored value of luma component, α, β: parameter)
A predicting unit predicting a color difference component of the current block based on a reconstruction value of the luminance component of the current block;

The predicted value of the color difference component is derived by an arithmetic operation of the restored value of the luminance component and a predetermined parameter,

And the parameter is set for each division unit of an image.
The method of claim 6,

And the parameter is set for each coding unit.
The method of claim 6,

And the parameter is set for each conversion unit.
The method of claim 6,

And the parameter is set for each coding tree block.
The method according to any one of claims 6 to 9,

The prediction value of the chrominance component is calculated as follows.

(Lrecon: restored value of luma component, α, β: parameter)