HK1215641B - Method and apparatus for image decoding - Google Patents

Description

Method and apparatus for image decoding

This application is a divisional application of the invention patent application with application number 201280047319.9, international application number PCT/KR2012/003078, application date April 20, 2012, and invention name “Method and device for image decoding”.

Technical Field

The present invention relates to an image decoding method and an image decoding device, and more particularly, to a method and a device for creating an intra-frame prediction block of each sub-block of a current block and creating a reconstructed block using size information of a prediction block.

Background Art

Image data must be encoded to efficiently store or transmit it. MPEG-1, MPEG-2, MPEG-4, H.264/MPEG-4 AVC (Advanced Video Coding), and the like are known as technologies for encoding image data. In these technologies, a picture is divided into macroblocks, and whether intra-frame coding or inter-frame coding should be performed is determined for each macroblock. The macroblocks are then encoded using the specified coding method.

In H.264, the latest image compression technology, intra-frame prediction is performed to enhance the efficiency of intra-frame coding. That is, instead of encoding the current block with reference to a reference picture, a prediction block is created using pixel values spatially adjacent to the current block to be encoded. Specifically, using the adjacent pixel values, an intra-frame prediction mode with minimal distortion is selected by comparison with the original macroblock. The selected intra-frame prediction mode and the adjacent pixel values are then used to create a prediction block for the current block to be encoded. A residual block is created, containing the difference signal between the current block and the prediction block, and the residual block is transformed, quantized, and entropy encoded. The intra-frame prediction mode used to create the prediction block is also encoded.

However, in H.264, the intra-frame prediction mode of the current block is encoded regardless of the directionality of the intra-frame prediction mode of the left block and the upper block of the current block, so there is a problem of low coding efficiency. When the number of intra-frame prediction modes increases to enhance the coding efficiency of the residual block, an intra-frame prediction coding method with higher efficiency than the intra-frame prediction mode coding method of H.264 is needed.

In H.264, using the intra prediction mode of the current block only creates a reconstructed block of the same size as the current block. Therefore, as the size of the current block increases, there is a problem in that the residual signal between the predicted block and the original block increases and coding efficiency decreases. Therefore, a new intra prediction encoding/decoding method for creating a predicted block more similar to the original block is needed.

Summary of the Invention

Technical issues

An object of the present invention is to provide a method and apparatus capable of enhancing image compression efficiency by reducing the number of bits required to encode the intra-frame prediction mode of the current block by utilizing the intra-frame prediction modes of the left block and the upper block of the current block.

Another object of the present invention is to provide a method and apparatus capable of enhancing image compression efficiency by creating a prediction block more similar to the current block using an intra prediction mode of the current block to reduce the number of bits required for encoding the residual block.

Solutions to the problem

According to one aspect of the present invention, an image decoding method is provided, which includes: reconstructing an intra-frame prediction mode group indicator and a prediction mode index of a current block; constructing a first group (MPM group) using the valid intra-frame prediction modes of the left block and the upper block of the current block; when the intra-frame prediction mode group indicator indicates the first group, determining the intra-frame prediction mode corresponding to the prediction mode index in the first group as the intra-frame prediction mode of the current block, and when the intra-frame prediction mode group indicator indicates the second group, determining the intra-frame prediction mode corresponding to the prediction mode index in the second group; and creating a prediction block, wherein the size of the prediction block is determined according to transform size information.

In the image decoding method, a first group may include three intra-frame prediction modes. When only one of the intra-frame prediction modes of the left block and the upper block of the current block is valid, two intra-frame prediction modes may be added to the first group. The two added intra-frame prediction modes may be determined based on the valid intra-frame prediction mode.

Beneficial effects

The image decoding method according to the present invention includes: reconstructing an intra-frame prediction mode group indicator and a prediction mode index of a current block; constructing a first group (MPM group) using valid intra-frame prediction modes of a left block and an upper block of the current block; when the intra-frame prediction mode group indicator indicates the first group, determining the intra-frame prediction mode corresponding to the prediction mode index in the first group as the intra-frame prediction mode of the current block, and when the intra-frame prediction mode group indicator indicates the second group, determining the intra-frame prediction mode corresponding to the prediction mode index in the second group; and creating a prediction block. The size of the prediction block is determined according to transform size information.

Therefore, by making the first group include a mode that is highly likely to be equal to the prediction mode of the current block based on the valid intra prediction modes of the left block and the upper block of the current block, and determining the intra prediction mode group indicator and the prediction mode index to be encoded using the first group, the amount of information of the intra prediction mode to be encoded can be reduced. By creating a prediction block similar to the current block, the number of bits of the residual block to be encoded/decoded can be reduced, thereby enhancing encoding and decoding efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a moving picture encoding apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a moving picture decoding apparatus according to another embodiment of the present invention.

FIG. 3 is a diagram illustrating a method of creating an intra prediction block in a moving picture decoding apparatus according to one embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating an intra prediction mode according to one embodiment of the present invention.

FIG. 5 is a block diagram illustrating the intra prediction block creating unit 300 according to one embodiment of the present invention.

FIG6 is a diagram illustrating a reconstruction block creation process according to one embodiment of the present invention.

FIG. 7 is a diagram illustrating a reconstruction block creation process according to another embodiment of the present invention.

DETAILED DESCRIPTION

Below, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention may be modified in various forms and may have various embodiments. These embodiments are not intended to limit the present invention, but should be understood as including all modifications, equivalents, and alternatives that fall within the spirit and technical scope of the present invention. In the description of the present invention with reference to the accompanying drawings, the same reference numerals represent the same components.

The motion picture encoding device and the motion picture decoding device according to the present invention can be user terminals such as personal computers, notebook PCs, personal digital assistants, portable multimedia players, smart phones, wireless communication terminals, and TVs, or servers providing services. The motion picture encoding device and the motion picture decoding device can be devices having the following components: a communication device such as a communication modem for communicating with various devices or a wireless or wired communication network, a memory for storing various programs and data for encoding and decoding images, and a microprocessor for executing the programs to perform operations and controls.

FIG. 1 is a block diagram illustrating a moving picture encoding apparatus according to an embodiment of the present invention.

According to an embodiment of the present invention, the motion picture encoding device 100 includes an intra-frame prediction module 110, an inter-frame prediction module 120, a transform and quantization module 130, an entropy encoding module 140, an inverse quantization and inverse transform module 150, a post-processing module 160, a picture buffer 170, a subtraction module, and an addition module.

The intra prediction module 110 creates an intra prediction block using reconstructed pixels of a picture or slice to which the current block belongs. The intra prediction module 110 selects one of a predetermined number of intra prediction modes according to the size of the current block to be predictively encoded, and creates a prediction block according to the selected intra prediction mode.

The inter prediction module 120 performs a motion estimation operation using a reference picture stored in the picture buffer 170 and determines a reference picture index and a motion vector for the motion estimation operation. The inter prediction module 120 then creates an intra prediction block of the current block using the reference picture index and the motion vector.

The transform and quantization module 130 transforms and quantizes the residual block of the prediction block created by the intra prediction module 110 or the inter prediction module 120. Transformation is performed using a one-dimensional transform matrix in the horizontal and vertical directions. The residual block used for intra prediction is transformed using a transform matrix determined according to the size of the transform block (i.e., the size of the residual block) and the intra prediction mode. The residual block used for inter prediction is transformed using a predetermined transform matrix.

The transform and quantization module 130 quantizes the transform block using a quantization step size. The quantization step size may be changed per coding unit equal to or larger than a predetermined size.

The quantized transform block is provided to the inverse quantization and inverse transform module 150 and the entropy encoding module 140 .

The inverse quantization and inverse transform module 150 inversely quantizes the quantized transform block and inversely transforms the inverse quantized transform block to reconstruct the residual block. The addition module adds the residual block reconstructed by the inverse quantization and inverse transform module 150 to the prediction block from the intra prediction module 110 or the inter prediction module 120 to create a reconstructed block.

The post-processing module 160 is used to improve the image quality of the reconstructed picture, and includes a deblocking filter module 161 , an offset module 162 , and a loop filter module 163 .

The deblocking filter module 161 adaptively applies a deblocking filter to the boundaries between prediction blocks and transform blocks. The boundaries can be limited to boundaries of an 8x8 grid. The deblocking filter module 161 determines the boundaries to be filtered, determines the boundary strength of the boundaries, and, when the boundary strength is greater than 0, determines whether a deblocking filter should be applied to the boundaries. If it is determined that the boundaries should be filtered, the deblocking filter module 161 selects a filter to be applied to the boundaries and filters the boundaries using the selected filter.

The offset module 162 determines whether an offset should be applied by picture or slice to reduce distortion between pixels in an image subjected to the deblocking filter module and the corresponding original pixels. Alternatively, the slice is divided into a plurality of offset regions, and an offset type can be determined for each offset region. The offset type can include a predetermined number of edge offset types and band offset types. When the offset type is an edge offset type, the edge type to which each pixel belongs is determined and the corresponding offset is applied. The edge type is determined based on the distribution of the values of two pixels adjacent to the current pixel.

The loop filter module 163 adaptively loop filters the reconstructed image based on the comparison result between the reconstructed image that has undergone the offset module 162 and the original image. Whether loop filtering should be performed on the reconstructed image is determined for each coding unit. The size and coefficients of the loop filter to be applied can be changed for each coding unit. Information indicating whether adaptive loop filtering should be applied for each coding unit can be included in each slice header. In the case of chroma signals, whether adaptive loop filtering should be applied for each picture can be determined. Therefore, information indicating whether chroma components are filtered can be included in a slice header or a picture header.

The picture buffer 170 receives the post-processed image data from the post-processing module 160 and reconstructs and stores the image in units of pictures. The picture may be an image in units of frames or an image in units of fields.

The entropy encoding module 140 performs entropy encoding on the quantized coefficient information quantized by the transform and quantization module 130, the intra prediction information received from the intra prediction module 110, the motion information received from the inter prediction module 120, etc. The entropy encoding module 140 includes a scanning module 145 for transforming the coefficients of the quantized transform block into one-dimensional quantized coefficients.

The scanning module 145 determines the scanning type used to transform the coefficients of the quantized transform block into one-dimensional quantized coefficients. The scanning type may vary depending on the directional intra prediction mode and the size of the transform block. The quantized coefficients are scanned in a backward direction.

When the quantized transform block is larger than a predetermined size, the transform coefficients are divided into a plurality of sub-blocks and scanned. The scan type applied to the transform coefficients of these sub-blocks is the same. The scan type applied to the sub-blocks may be a zigzag scan, or may be the same scan type as the scan type applied to the transform coefficients of the sub-blocks.

FIG. 2 is a block diagram illustrating a moving picture decoding apparatus 200 according to an embodiment of the present invention.

The motion picture decoding device 200 according to an embodiment of the present invention includes an entropy decoding module 210, an inverse quantization module 220, an inverse transform module 230, an intra-frame prediction module 240, an inter-frame prediction module 250, a post-processing module 260, a picture buffer 270 and an addition module 280.

The entropy decoding module 210 decodes the received bitstream and divides the bitstream into intra-frame prediction information, inter-frame prediction information, quantization coefficient information, etc. The entropy decoding module 210 provides the decoded intra-frame prediction information to the intra-frame prediction module 240 and provides the decoded inter-frame prediction information to the inter-frame prediction module 250. The entropy decoding module 210 includes an inverse scanning module 215 for inversely scanning the decoded quantization coefficient information.

The inverse scanning module 215 converts the quantized coefficient information into a two-dimensional quantized block. One of multiple scan types is selected for the conversion. The scan type can vary depending on the directional intra prediction mode and the size of the transform block. The quantized coefficients are scanned in a backward direction. When the quantized transform block is larger than a predetermined size, the transform coefficients are divided into a plurality of sub-blocks and scanned. The scan type applied to the transform coefficients of these sub-blocks is the same. The scan type applied to the sub-blocks can be a zigzag scan or can be the same scan type as the scan type applied to the transform coefficients of the sub-blocks.

The inverse quantization module 220 determines a quantization step size prediction factor for the current coding unit and adds the determined quantization step size prediction factor to the received residual quantization step size to reconstruct the quantization step size for the current coding unit. The inverse quantization module 220 inverse quantizes the quantized block using the quantization step size and the inverse quantization matrix. The quantization matrix is determined based on the size of the quantized block and the prediction mode. Specifically, the quantization matrix is selected based on at least one of the prediction mode for the current block and the intra-frame prediction mode for a quantized block of a predetermined size.

The inverse transform module 230 inversely transforms the inverse quantized transform block to reconstruct a residual block. An inverse transform matrix to be applied to the inverse quantized block may be determined according to a prediction mode and an intra prediction mode.

The addition module 280 adds the prediction block created by the intra prediction module 240 or the inter prediction module 250 and the residual block reconstructed by the inverse transform module 230 to create a reconstructed block.

The intra prediction module 240 reconstructs the intra prediction mode of the current block based on the intra prediction information received from the entropy decoding module 210. Then, the intra prediction module 240 creates a prediction block according to the reconstructed intra prediction mode.

The inter-frame prediction module 250 reconstructs the reference picture index and motion vector based on the inter-frame prediction information received from the entropy decoding module 210. The inter-frame prediction module 250 then uses the reference picture index and motion vector to create a prediction block for the current block. When motion compensation with decimal prediction is applied, the selected interpolation filter is applied to create the prediction block.

The operation of the post-processing module 260 is the same as the operation of the post-processing module 160 shown in FIG. 1 , and thus will not be described again.

The picture buffer 270 stores the decoded image post-processed by the post-processing module 260 in units of pictures.

FIG. 3 is a diagram illustrating a method of creating an intra prediction block according to an embodiment of the present invention.

First, intra prediction information from a received bitstream is entropy decoded ( S110 ).

The intra prediction information includes an intra prediction mode group indicator and a prediction mode index. The intra prediction mode group indicator indicates whether the intra prediction mode of the current block belongs to an MPM group or a group other than the MPM group. The prediction mode index is information indicating a specific intra prediction mode in the intra prediction mode group indicated by the intra prediction mode group indicator.

The intra-frame prediction mode group indicator can be received in the form of an unsigned integer. In this case, the intra-frame prediction mode group indicator can be used without entropy decoding. Alternatively, the intra-frame prediction mode group indicator can be adaptively entropy encoded according to the type of the current slice. For example, the intra-frame prediction mode group indicator can be entropy encoded using the context determined according to the slice type. Therefore, the intra-frame prediction mode group indicator can be decoded using the context determined according to the type of the current slice. The entropy encoding method of the prediction mode index varies depending on whether the intra-frame prediction mode belongs to the MPM group. Therefore, different methods are used to entropy decode the prediction mode index. Specifically, when the intra-frame prediction mode group indicator indicates that the intra-frame prediction mode of the current block belongs to the MPM group, the prediction mode index is binarized in a truncated exponential Golomb code manner or a truncated unary code manner, and then entropy encoded. Therefore, after the binary information is obtained by performing entropy decoding, the prediction mode index is reconstructed using the above-mentioned method. When the intra prediction mode group indicator indicates that the intra prediction mode of the current block does not belong to the MPM group, the prediction mode index may be binarized to a fixed length. Therefore, after obtaining binary information by performing entropy decoding, the prediction mode index may be reconstructed.

Then, an MPM group is created using the intra prediction modes of blocks adjacent to the current block, and then the intra prediction mode of the current block is reconstructed using the MPM group (S120). The MPM group includes three intra prediction modes. This will be described with reference to FIG4. FIG4 is a diagram illustrating intra prediction modes according to an embodiment of the present invention.

(1) When the intra prediction modes of the upper block and the left block of the current block both exist and are different from each other, the MPM group includes these two intra prediction modes and one additional intra prediction mode.

When one of the two intra-frame prediction modes is DC mode and the other is not planar mode, the additional intra-frame prediction mode may be planar mode. Similarly, when one of the two intra-frame prediction modes is planar mode and the other is not DC mode, the additional intra-frame prediction mode may be DC mode.

When the two intra prediction modes are the DC mode and the planar mode, the additional intra prediction mode may be the vertical mode or the horizontal mode.

When the two intra prediction modes are neither the DC mode nor the planar mode, the additional intra prediction mode may be an intra prediction mode having directionality between the two intra prediction modes, the DC mode, or the planar mode.

(2) When the intra prediction modes of the upper block and the left block of the current block both exist and are equal to each other, the MPM group includes the intra prediction mode and two additional intra prediction modes.

When the intra prediction mode is neither DC mode nor planar mode, two additional intra prediction modes are set as two intra prediction modes adjacent to the intra prediction mode. When the intra prediction mode is DC mode, the two additional intra prediction modes may be planar mode and vertical mode.

(3) When only one intra prediction mode exists among the intra prediction modes of the upper block and the left block of the current block, the MPM group includes the intra prediction mode and two additional intra prediction modes. The two additional intra prediction modes are determined according to the intra prediction mode.

(4) When the intra prediction mode of the upper block and the left block of the current block does not exist, the MPM group includes the DC mode, the planar mode, and the vertical mode.

When the intra prediction mode group indicator indicates the MPM group, the intra prediction mode indicated by the prediction mode index is selected from the MPM group, and the selected intra prediction mode is determined as the intra prediction mode of the current block. The intra prediction mode group indicator may be flag information indicating whether the intra prediction mode of the current block belongs to the MPM group or to a group other than the MPM group.

When the intra-frame prediction mode group indicator does not indicate the MPM group, the intra-frame prediction module 240 determines the intra-frame prediction mode indicated by the prediction mode index among the intra-frame prediction modes other than the intra-frame prediction modes belonging to the MPM group (hereinafter referred to as the residual intra-frame prediction mode) as the intra-frame prediction mode of the current block. The prediction mode index assigned to the residual intra-frame prediction mode varies according to the configuration of the MPM group. That is, the decoded prediction mode index indicates the index of the residual intra-frame prediction mode rearranged according to the configuration of the MPM group. Therefore, the intra-frame prediction module 240 selects the intra-frame prediction mode of the current block from the residual intra-frame prediction mode according to the decoded prediction mode index and the intra-frame prediction mode belonging to the MPM group.

Specifically, the residual intra prediction modes of the current block are rearranged in mode number order, and the intra prediction mode corresponding to the received prediction mode index is selected as the intra prediction mode of the current block. In this case, the residual intra prediction modes may be rearranged, but the intra prediction mode of the current block may be determined by comparing the intra prediction mode numbers belonging to the MPM group with the intra prediction mode index of the current block.

This method can be applied to the case where mode number 2 is assigned to the DC mode among the non-directional modes, mode number 34 is assigned to the planar mode, and directional mode numbers are assigned to other modes. However, since the probability of selecting the planar mode and the DC mode as the intra-frame prediction mode of the current block is higher than the probability of selecting the other directional modes as the intra-frame prediction mode of the current block, a small mode number (for example, mode code 0) is assigned to the planar mode and the above-mentioned method can be applied. In this case, the mode numbers of other lower-ranked modes are increased by 1.

Alternatively, the lowest index may be assigned to the non-directional mode. For example, when the intra-frame prediction mode of the current block is the planar mode and the residual intra-frame prediction mode includes the planar mode, the intra-frame prediction mode index may include 0. For example, when the residual intra-frame prediction mode includes the planar mode and the DC mode, the intra-frame prediction mode corresponding to the prediction mode index in a state where the planar mode, the DC mode, and the directional mode are arranged in this order may be set as the intra-frame prediction mode of the current block. For example, mode number 0 and mode number 1 may be assigned to the planar mode and the DC mode, respectively, or mode number 0 and mode number 1 may be assigned to the DC mode and the planar mode, respectively. In this case, the intra-frame prediction mode index of the current block may be compared with the intra-frame prediction mode number belonging to the MPM group to determine the intra-frame prediction mode of the current block.

Then, the size of the prediction block is determined using information indicating the transform size of the current block (S130).

When the size of the prediction block is equal to the size of the current block, the prediction block is created using the intra prediction mode of the current block and the reference pixels of the current block. Reference pixels are pixels reconstructed or created before the current block.

When the size of the prediction block is smaller than the size of the current block, that is, when the current block can be divided into multiple subblocks and intra prediction is performed on the subblocks, the same intra prediction mode (that is, the intra prediction mode of the current block) is used to create the prediction blocks of each subblock. The prediction blocks of the second subblock or the subblocks following the second subblock in decoding order are created using the reconstructed pixels of the previous subblock. Therefore, after the prediction blocks, residual blocks, and reconstructed blocks are created in units of subblocks, the prediction blocks of the next subblock are created.

Then, it is determined whether all reference pixels of the block corresponding to the size of the prediction block are valid (S140). Reference pixels are pixels that have been previously decoded and reconstructed. When it is determined that at least one of the reference pixels is invalid, a reference pixel is created (S150).

Specifically, when it is determined that none of the reference pixels are valid, the reference pixel value is replaced by a value of 2 ^{L - 1.} Here, L represents the number of bits representing the grayscale of the luminance component.

When valid reference pixels exist only in one direction with respect to the position of the invalid reference pixel, the nearest reference pixel among the valid reference pixels is copied to create the reference pixel.

When valid reference pixels exist in two directions relative to the position of an invalid reference pixel, a reference pixel located closest in a predetermined direction may be copied, or two closest reference pixels in two directions may be averaged to create a reference pixel.

Then, it is determined whether the reference pixels should be filtered (S160). The reference pixels are adaptively filtered according to the reconstructed intra prediction mode and the size of the prediction block.

When the intra prediction mode is DC mode, the reference pixels are not filtered. When the intra prediction mode is vertical mode and horizontal mode, the intra prediction module 240 also does not filter the reference pixels. However, when the intra prediction mode is a directional mode other than vertical mode and horizontal mode, the reference pixels are adaptively filtered according to the intra prediction mode and the size of the prediction block. When the size of the prediction block is 4x4, in order to reduce complexity, the reference pixels are not filtered regardless of the intra prediction mode. Filtering is used to smooth the changes in pixel values between reference pixels and a low-pass filter is used. The low-pass filter can be [1, 2, 1] as a 3-tap filter or [1, 2, 4, 2, 1] as a 5-tap filter. When the size of the prediction block is in the range of 8x8 to 32x32, the reference pixels are filtered according to more intra prediction modes as the prediction block size increases.

Then, a prediction block is created according to the intra prediction mode (S180). Reference pixels used for the prediction block may be pixels adaptively filtered according to the size of the prediction block and the intra prediction mode.

In DC mode, the average value of the N upper reference pixels located at (x=0, ..., N-1, y=-1), the M left reference pixels located at (x=-1, y=0, ..., M-1), and the center pixel located at (x=-1, y=-1) can be determined as the predicted pixel of the prediction block. However, the predicted pixel adjacent to the reference pixel can be created using the average value and a weighted average of the reference pixels adjacent to the predicted pixel. In planar mode, the predicted pixel can be created in the same manner as in DC mode.

In vertical mode, reference pixels located in the vertical direction are set as predicted pixels. However, a predicted pixel adjacent to the left reference pixel can be created by using the change between the reference pixels located in the vertical direction and the left reference pixel. The change refers to the change between the center reference pixel and the left reference pixel adjacent to the predicted pixel. In horizontal mode, predicted pixels can be created in the same manner as in vertical mode, except for the direction.

5 is a block diagram illustrating an intra-frame prediction block creation unit 300 according to an embodiment of the present invention. The intra-frame prediction block creation unit 300 according to the present invention includes a parsing module 310, a prediction mode decoding module 320, a prediction block size determination module 330, a reference pixel validity determination module 340, a reference pixel creation module 350, a reference pixel filtering module 360, and a prediction block creation module 370.

The parsing module 310 performs entropy decoding on the received bitstream to obtain intra-frame prediction information and transform block size information.

The intra-frame prediction information includes an intra-frame prediction mode group indicator and a prediction mode index. The intra-frame prediction mode group indicator indicates to which of the MPM group and the group other than the MPM group the intra-frame prediction mode of the current block belongs. The prediction mode index is information indicating a specific intra-frame prediction mode in the prediction mode group indicated by the intra-frame prediction mode group indicator. The method for entropy decoding the intra-frame prediction information is the same as in step S110 of Figure 3.

The transform block size information includes information indicating the transform block size and at least one flag (split_transform_flag) transmitted from the encoder.

The prediction mode decoding module 320 creates an MPM group using the intra prediction modes of blocks adjacent to the current block, and reconstructs the intra prediction mode of the current block using the MPM group and the entropy-decoded intra prediction information. The MPM group includes three intra prediction modes.

(1) When the intra prediction modes of the upper block and the left block of the current block both exist and are different from each other, the MPM group includes these two intra prediction modes and one additional intra prediction mode.

When one of the two intra-frame prediction modes is DC mode and the other is not planar mode, the additional intra-frame prediction mode may be planar mode. Similarly, when one of the two intra-frame prediction modes is planar mode and the other is not DC mode, the additional intra-frame prediction mode may be DC mode.

When the two intra prediction modes are the DC mode and the planar mode, the additional intra prediction mode may be the vertical mode or the horizontal mode.

When the two intra prediction modes are neither the DC mode nor the planar mode, the additional intra prediction mode may be an intra prediction mode having directionality between the two intra prediction modes, the DC mode, or the planar mode.

(2) When the intra prediction modes of the upper block and the left block of the current block both exist and are equal to each other, the MPM group includes the intra prediction mode and two additional intra prediction modes.

When the intra prediction mode is neither DC mode nor planar mode, two additional intra prediction modes are set as two intra prediction modes adjacent to the intra prediction mode. When the intra prediction mode is DC mode, the two additional intra prediction modes may be planar mode and vertical mode.

(3) When only one intra prediction mode exists among the intra prediction modes of the upper block and the left block of the current block, the MPM group includes the intra prediction mode and two additional intra prediction modes. The two additional intra prediction modes are determined according to the intra prediction mode.

(4) When the intra prediction mode of the upper block and the left block of the current block does not exist, the MPM group includes the DC mode, the planar mode, and the vertical mode.

When the intra prediction mode group indicator indicates the MPM group, the intra prediction mode indicated by the prediction mode index is selected from the MPM group, and the selected intra prediction mode is determined as the intra prediction mode of the current block. The intra prediction mode group indicator may be flag information indicating whether the intra prediction mode of the current block belongs to the MPM group or to a group other than the MPM group.

When the intra-frame prediction mode group indicator does not indicate the MPM group, the intra-frame prediction module 240 determines the intra-frame prediction mode indicated by the prediction mode index among the intra-frame prediction modes other than the intra-frame prediction modes belonging to the MPM group (hereinafter referred to as the residual intra-frame prediction mode) as the intra-frame prediction mode of the current block. The prediction mode index assigned to the residual intra-frame prediction mode varies according to the configuration of the MPM group. That is, the decoded prediction mode index indicates the index of the residual intra-frame prediction mode rearranged according to the configuration of the MPM group. Therefore, the intra-frame prediction module 240 selects the intra-frame prediction mode of the current block from the residual intra-frame prediction mode according to the decoded prediction mode index and the intra-frame prediction mode belonging to the MPM group.

Specifically, the residual intra prediction modes of the current block are rearranged in mode number order, and the intra prediction mode corresponding to the received prediction mode index is selected as the intra prediction mode of the current block. In this case, the residual intra prediction modes may be rearranged, but the intra prediction mode of the current block may be determined by comparing the intra prediction mode numbers belonging to the MPM group with the intra prediction mode index of the current block.

The MPM group construction method can be applied to a case where the mode number 2 is assigned to the DC mode among the non-directional modes, the mode number 34 is assigned to the planar mode, and the directional mode numbers are assigned to other modes. However, since the probability of selecting the planar mode and the DC mode as the intra-frame prediction mode of the current block is higher than the probability of selecting the other directional modes as the intra-frame prediction mode of the current block, a small mode number (for example, mode code 0) is assigned to the planar mode and the above-mentioned method can be applied. In this case, the mode numbers of other lower-ranked modes are increased by 1.

Alternatively, the lowest index may be assigned to the non-directional mode. For example, when the intra-frame prediction mode of the current block is the planar mode and the residual intra-frame prediction mode includes the planar mode, the intra-frame prediction mode index may include 0. For example, when the residual intra-frame prediction mode includes the planar mode and the DC mode, the intra-frame prediction mode corresponding to the prediction mode index in a state where the planar mode, the DC mode and the directional mode are arranged in this order may be set as the intra-frame prediction mode of the current block. For example, mode number 0 and mode number 1 may be assigned to the planar mode and the DC mode, respectively, or mode number 0 and mode number 1 may be assigned to the DC mode and the planar mode, respectively. In this case, the intra-frame prediction mode index of the current block may be compared with the intra-frame prediction mode number belonging to the MPM group to determine the intra-frame prediction mode of the current block.

The prediction block size determination module 330 determines the size of the prediction block of the current block using the block transform size. The size of the prediction block may have the size of the current block or the size of a subblock of the current block.

When the size of the prediction block is equal to the size of the current block, the prediction block is created using the intra prediction mode of the current block and the reference pixels of the current block. Reference pixels are pixels reconstructed or created before the current block.

When the size of the prediction block is smaller than the size of the current block, that is, when the current block can be divided into multiple subblocks and intra prediction is performed on the subblocks, the same intra prediction mode (that is, the intra prediction mode of the current block) is used to create the prediction blocks of each subblock. The prediction blocks of the second subblock or the subblocks following the second subblock in decoding order are created using the reconstructed pixels of the previous subblock. Therefore, after the prediction blocks, residual blocks, and reconstructed blocks are created in units of subblocks, the prediction blocks of the next subblock are created.

Specific steps of the above operation will be described later.

Next, the reference pixel validity determination module 340 determines whether all reference pixels of a block corresponding to the size of the prediction block are valid. Reference pixels are pixels that have been previously decoded and reconstructed.

When it is determined that at least one of the reference pixels is invalid, the reference pixel validity determination module 340 creates a reference pixel.

Specifically, when it is determined that none of the reference pixels are valid, the reference pixel value is replaced by a value of 2 ^{L - 1.} Here, L represents the number of bits representing the grayscale of the luminance component.

When valid reference pixels exist only in one direction with respect to the position of the invalid reference pixel, the nearest reference pixel among the valid reference pixels is copied to create the reference pixel.

When valid reference pixels exist in two directions relative to the position of an invalid reference pixel, a reference pixel located closest in a predetermined direction may be copied, or two closest reference pixels in two directions may be averaged to create a reference pixel.

The reference pixel filtering module 360 determines whether the reference pixel should be filtered. The reference pixel is adaptively filtered according to the reconstructed intra prediction mode and the size of the prediction block.

When the intra prediction mode is DC mode, the reference pixels are not filtered. When the intra prediction mode is vertical mode and horizontal mode, the intra prediction module 240 also does not filter the reference pixels. However, when the intra prediction mode is a directional mode other than vertical mode and horizontal mode, the reference pixels are adaptively filtered according to the intra prediction mode and the size of the prediction block. When the size of the prediction block is 4x4, in order to reduce complexity, the reference pixels are not filtered regardless of the intra prediction mode. Filtering is used to smooth the changes in pixel values between reference pixels and a low-pass filter is used. The low-pass filter can be [1, 2, 1] as a 3-tap filter or [1, 2, 4, 2, 1] as a 5-tap filter. When the size of the prediction block is in the range of 8x8 to 32x32, the reference pixels are filtered according to more intra prediction modes as the prediction block size increases.

The prediction block creation module 370 creates a prediction block according to the intra prediction mode. Reference pixels used for the prediction block may be pixels adaptively filtered according to the size of the prediction block and the intra prediction mode.

In DC mode, the average value of the N upper reference pixels located at (x=0, ..., N-1, y=-1), the M left reference pixels located at (x=-1, y=0, ..., M-1), and the center pixel located at (x=-1, y=-1) can be determined as the predicted pixel of the prediction block. However, the predicted pixel adjacent to the reference pixel can be created using the average value and a weighted average of the reference pixels adjacent to the predicted pixel. In planar mode, the predicted pixel can be created in the same manner as in DC mode.

In vertical mode, reference pixels located in the vertical direction are set as predicted pixels. However, each predicted pixel adjacent to the left reference pixel can be created using the change between the reference pixels located in the vertical direction and the left reference pixel. The change refers to the change between the center reference pixel and the left reference pixel adjacent to the predicted pixel. In horizontal mode, predicted pixels can be created in the same manner as in vertical mode, except for the direction.

Fig. 6 is a diagram illustrating a reconstructed block creation process according to an embodiment of the present invention. Fig. 6 illustrates a reconstructed block creation process when the size of a prediction block is equal to that of a current block.

First, the intra prediction mode of the current block is reconstructed (S210). The intra prediction mode is reconstructed using the same method described in step S120 of FIG. 3 .

Then, it is determined whether all reference pixels at predetermined positions of the current block are valid (S220). When at least one of the reference pixels at the predetermined positions is invalid, an invalid reference pixel is created at the corresponding position (S230). The method of creating the reference pixel is the same as that in step S150 of Figure 3. Here, the size of the prediction block is equal to the size of the current block.

Subsequently, it is determined whether the reference pixels of the current block should be filtered (S240). Whether the reference pixels should be filtered is determined based on the reconstructed intra prediction mode and the size of the current block. Whether the reference pixels should be filtered is determined using the method described in step S170 of Figure 3. When it is determined that the reference pixels of the current block should be filtered, the reference pixels of the current block are filtered (S250).

Then, a prediction block of the current block is created using the reconstructed intra prediction mode (S260). A residual block is created using the reconstructed intra prediction mode (S270). The size of the residual block is equal to the size of the current block.

Finally, the prediction block of the current block and the residual block of the current block are added to create a reconstructed block (S280).

FIG7 is a diagram illustrating a reconstructed block creation process according to another embodiment of the present invention. FIG7 illustrates a reconstructed block creation process of a current block when the size of a prediction block is smaller than that of the current block.

First, the intra prediction mode of the current block is reconstructed (S310). The intra prediction mode is reconstructed using the same method as that in step S120 of FIG. 3 .

A subblock of the current block to be decoded is determined (S320).

Then, it is determined whether all reference pixels at predetermined positions of the sub-block are valid (S330). When at least one of the reference pixels at the predetermined positions is invalid, an invalid reference pixel is created at the corresponding position (S340). The method of creating the reference pixel is the same as that in step S150 of FIG. 3. Here, the size of the prediction block is equal to the size of the current block.

Subsequently, it is determined whether the reference pixels of the sub-block should be filtered (S350). Whether the reference pixels should be filtered is determined based on the reconstructed intra prediction mode and the size of the current block. Whether the reference pixels should be filtered is determined using the method described in step S170 of Figure 3. If it is determined that the reference pixels of the sub-block should be filtered, the reference pixels of the sub-block are filtered (S360).

Then, a prediction block of the sub-block is created using the reconstructed intra prediction mode (S370). A residual block of the sub-block is created using the reconstructed intra prediction mode (S380).

Subsequently, the prediction block of the current block and the residual block of the current block are added to create a reconstructed block (S390).

Then, it is determined whether the subblock is the last subblock of the current block (S395). When the subblock is not the last subblock of the current block, the process from determining the next subblock in the encoding order (S320) to creating a prediction block for the subblock (S390) is repeated. In this case, subblocks subsequent to the first subblock in the decoding order use some pixels of the reconstructed previous subblock as reference pixels for the subblock. Since invalid reference pixels always exist in subblocks subsequent to the first subblock, the process of step S340 can be performed without performing the process of step S330. When the subblocks are of the same size, reference pixel filtering can be performed on the first subblock, but reference pixel filtering can be omitted for subsequent subblocks.

While the present invention has been described with reference to the embodiments, it will be apparent to those skilled in the art that the present invention can be modified and changed in various forms without departing from the spirit and scope of the invention as described in the appended claims.

Claims (9)

1. A method for decoding an image, the method comprising the following steps: Generate residual blocks; Reconstruct the intra-prediction mode group indicator and prediction mode index of the current block, wherein the intra-prediction mode group indicator indicates which intra-prediction mode group in the first group and the second group the intra-prediction mode of the current block belongs to, and the prediction mode index specifies the intra-prediction mode in the intra-prediction mode group indicated by the intra-prediction mode group indicator. The first group, comprising three intra-prediction modes, is constructed using the valid intra-prediction modes of the left and upper blocks of the current block. When the intra-prediction mode group indicator points to the first group, the intra-prediction mode in the first group corresponding to the prediction mode index is determined as the intra-prediction mode of the current block; and when the intra-prediction mode group indicator points to the second group, the intra-prediction mode in the second group corresponding to the prediction mode index is determined as the intra-prediction mode of the current block. A prediction block is generated based on the determined intra-prediction mode of the current block; Reconstruction blocks are generated using residual blocks and prediction blocks; Perform deblocking filtering on the reconstructed image. Specifically, the residual block is generated by inverse quantization of the quantized block obtained from the converted quantization coefficient information, and by inverse transformation of the inverse quantized block. When only one of the intra-prediction modes of the left block and the upper block of the current block exists, two intra-prediction modes are added to the first group, wherein the two intra-prediction modes are different from the existing only intra-prediction mode, and the two intra-prediction modes are determined based on the existing only intra-prediction mode. Wherein, when the intra-prediction mode of the left block and the intra-prediction mode of the upper block are not equal to each other and the intra-prediction mode of the left block and the intra-prediction mode of the upper block are DC mode and planar mode, the first group includes the intra-prediction mode of the left block, the intra-prediction mode of the upper block and the vertical mode. 2. The method according to claim 1, wherein the quantization block is dequantized using a quantization step size obtained based on a quantization step size prediction factor and a residual quantization step size. 3. The method according to claim 1, wherein when the size of the current block is equal to the size of the prediction block, a prediction block is generated based on a determined intra-frame prediction mode, and when the size of the current block is greater than the size of the prediction block, multiple prediction blocks are generated based on the determined intra-frame prediction mode. 4. The method according to claim 1, wherein when the intra-prediction mode of the left block and the intra-prediction mode of the upper block are not equal to each other and the intra-prediction mode of the left block and the intra-prediction mode of the upper block are neither DC mode nor planar mode, the first group includes the intra-prediction mode of the left block, the intra-prediction mode of the upper block, and the planar mode. 5. The method according to claim 1, wherein when the intra-prediction mode of the left block of the current block and the intra-prediction mode of the upper block are not present, the first group includes a planar mode, a DC mode and a vertical mode. 6. The method of claim 1, wherein the DC mode and the planar mode have lower mode numbers than other intra-frame prediction modes. 7. The method of claim 6, wherein mode number 0 is assigned to the planar mode. 8. The method of claim 6, wherein mode number 0 is assigned to the planar mode and mode number 1 is assigned to the DC mode. 9. The method of claim 1, wherein the size of the prediction block is determined based on the transformation size information.