JP6796405B2 - Intra Predictor, Image Coding Device, Image Decoding Device and Program - Google Patents

Description

The present invention relates to an intra-predictor, an image coding device, an image decoding device and a program.

As one of the typical video compression methods, MPEG-H HEVC / ITU-TH. 265 method (Moving Picture Expert Group-H ISO / IEC 23008 High Efficiency Video Coding / International Telecommunication Union-Telecommunication Transmission Union-Telecommunication System In the HEVC method, a group of intra-prediction mode candidates called MPM (Most Probable Mode) is used in intra-prediction.

The HEVC MPM is configured for each target block of intra-prediction, and includes three intra-prediction mode candidates (hereinafter, referred to as intra-prediction mode candidates). As a general rule, the 0th of the 3 intra prediction mode candidates in the MPM is the intra prediction mode of the reference block to the left of the target block (hereinafter referred to as the left block), and the 1st is the reference block above the target block (hereinafter, right). Intra-prediction mode (called block), the second is defined as planar mode. However, even in exceptions such as when the left block or upper block cannot be used, or when the intra prediction mode of the left block and the intra prediction mode of the right block match, the intra prediction mode candidates in the MPM and their ranking are determined. ing.

Further, in the HEVC method, the coded stream of the intra prediction mode has (a) a flag indicating whether or not the intra prediction mode is used in the MPM, and (b) whether or not the 0th intra prediction mode candidate in the MPM is used. And (c) a flag indicating whether or not the first intra prediction mode candidate in the MPM is used. In the coded stream, the flags (a)-(c) are arranged in that order. The flag (b) and the flag (c) exist only when the value of the flag (a) is true, and the flag (c) exists only when the value of the flag (b) is false. Therefore, when the intra prediction mode candidate having the 0th rank in the MPM among the intra prediction mode candidates in the MPM is used, the flags (a) and (b) are included in the coded stream. That is, the amount of information in the intra prediction mode is only 2 bits. On the other hand, when the first or second intra-prediction mode candidate is used, the flags included in the coded stream include flags (a), flags (b), and flags (c) in the coded stream. included. That is, a 3-bit amount of information is required for coding in the intra prediction mode. Therefore, when the intra prediction mode is encoded using the MPM, the rank of the intra prediction mode candidate in the MPM (hereinafter referred to as the MPM rank) affects the amount of information generated in the coded stream generated by the coding. ..

Japanese Unexamined Patent Publication No. 2013-081100

In the conventional coding method, for example, the HEVC method or the coding process described in Patent Document 1, the order of MPM is determined based on the spatial positional relationship of the intra prediction mode candidates. Therefore, the conventional coding method has a problem that the coding efficiency of the intra prediction mode is not necessarily high.

The present invention has been made in view of the above points, and an object of the present invention is to provide an intra predictor, an image coding device, an image decoding device, and a program capable of improving the coding efficiency of the intra prediction mode. ..

The present invention has been made to solve the above-mentioned problems, and [1] One aspect of the present invention is the reference block obtained by using the intra prediction mode defined for the reference blocks around the target block. When the error index acquisition unit that acquires the error index between the prediction image and the input image of the reference block and the intra prediction mode defined for the reference block are used as the intra prediction mode candidates of the target block, the intra prediction mode A ranking determination unit that determines the ranking of candidates in ascending order of the error index, and the ranking determination unit provides the error index larger than the threshold value of the predetermined error index as the intra prediction mode candidate of the reference block. The intra-prediction device is characterized in that the rank of the intra-prediction mode is lower than the rank of a predetermined intra-prediction mode candidate .
According to the configuration of [1], there is a high possibility that the representative intra prediction mode that gives the prediction image having the smallest error amount with the input image matches the intra prediction mode candidate having a high rank. Therefore, the number of intra prediction mode candidates related to the coding of the intra prediction mode, that is, the number of coding bits thereof can be reduced. Further, the rank of the intra prediction mode candidate of the intra prediction mode related to the reference pixel that gives the error index larger than the threshold value of the predetermined error index is lower than that of the predetermined intra prediction mode candidate. Therefore, in the coding of the intra prediction mode, the intra prediction mode candidate having a smaller error index is preferentially used. Therefore, the number of intra prediction mode candidates used in coding the intra prediction mode related to the target block, that is, the number of coding bits can be reduced as compared with the intra prediction mode related to the reference block alone.

[2] One aspect of the present invention is the above-mentioned intra-predictor, and the error index acquisition unit sets the error index to a value within the range of the error index related to the intra-prediction mode defined for each of the reference blocks. It is characterized in that the threshold value of is variably set.
According to the configuration of [2] , in the coding of the intra prediction mode, a predetermined intra prediction mode candidate having a smaller error index than the intra prediction mode related to the reference pixel is prioritized based on the threshold value of the determined error index. Whether or not to use it is determined. Therefore, the number of coding bits of the intra prediction mode is smaller than that of the reference block that always gives an error index larger than the threshold value of the constant error index and the rank of the intra prediction mode is lower than the rank of the predetermined intra prediction mode candidate. can do.

[3] One aspect of the present invention is the above-mentioned intra-predictor, and the error index acquisition unit obtains the quantization conversion coefficient of the prediction residual based on the prediction image as the error index by variable length coding. It is characterized by acquiring the number of bits of the bit string to be obtained.
According to the configuration of [3] , the order of the intra-prediction mode candidates is determined in ascending order of the number of bits of the prediction residual bit string based on the intra-prediction image generated by using each intra-prediction mode candidate. Since there is a high possibility that the intra prediction mode candidate having a small number of bits as the size of the prediction residual matches the intra prediction mode related to the target block, the number of coding bits related to the coding of the intra prediction mode is reduced. be able to.

[4] One aspect of the present invention is the above-mentioned intra-predictor, and the error index acquisition unit uses a non-zero value among the quantization conversion coefficients of the prediction residual based on the prediction image as the error index. It is characterized by acquiring the number of non-zero conversion coefficients having.
According to the configuration of [4] , the ranking of the intra prediction mode candidates is the intra prediction mode candidate having a small number of non-zero conversion coefficients of the prediction residual based on the intra prediction image generated by using each intra prediction mode candidate. However, there is a high possibility that it matches the intra prediction mode related to the target block. Therefore, the number of coding bits related to the coding in the intra prediction mode can be reduced.

[5] One aspect of the present invention is the above-mentioned intra-predictor, and the error index acquisition unit acquires the sum of squares of the quantization conversion coefficients of the prediction residuals based on the prediction image as the error index.
According to the configuration of [5] , the rank of the intra prediction mode candidates is the intra prediction mode in which the sum of squares of the quantization conversion coefficients of the prediction residuals based on the intra prediction image generated by each intra prediction mode candidate is small. The candidate is more likely to match the intra-prediction mode for the target block. Therefore, the number of coding bits related to the coding in the intra prediction mode can be reduced.

[6] One aspect of the present invention is the above-mentioned intra-predictor, and the error index acquisition unit acquires the absolute value sum of the quantization conversion coefficient of the prediction residual based on the prediction image as the error index. ..
According to the configuration of [6] , the rank of the intra prediction mode candidates is the intra prediction in which the sum of the absolute values of the quantization conversion coefficients of the prediction residuals based on the intra prediction image generated by each intra prediction mode candidate is small. There is a high possibility that the mode candidate matches the intra prediction mode related to the target block. Therefore, the number of coding bits related to the coding in the intra prediction mode can be reduced.

[7] One aspect of the present invention acquires an error index between the predicted image of the reference block obtained by using the intra prediction mode defined for the reference block around the target block and the input image of the reference block. An error index acquisition unit and a ranking determination unit that determines the rank of the intra prediction mode candidate in ascending order of the error index when the intra prediction mode defined for the reference block is used as the intra prediction mode candidate of the target block. A prediction obtained by including an intra-prediction device or an intra-prediction device according to any one of [1] to [6] and performing intra-prediction using each predetermined intra-prediction mode with reference to the decoded image of the reference block. based on the error amount between the input image of the image and the target block, selects one of intra prediction modes, and determines the candidate determination section of the same rank of the intra prediction mode candidate and the intra prediction mode selected, determination It is an image coding apparatus including a coding unit for encoding information on the ranking of the intra-prediction mode candidates .
According to the configuration of [7] , the rank of the intra-prediction mode candidate which is the same as the intra-prediction mode selected based on the amount of error between the predicted image and the input image is high. Since the number of intra-prediction mode candidates related to coding can be reduced, the number of coding bits in the intra-prediction mode can be reduced.

[8] One aspect of the present invention acquires an error index between the predicted image of the reference block obtained by using the intra prediction mode defined for the reference block around the target block and the input image of the reference block. When the error index acquisition unit and the intra prediction mode defined for the reference block are used as the intra prediction mode candidates of the target block, the order determination unit that determines the rank of the intra prediction mode candidates in the ascending order of the error index. an intra predictor, or any intra-predictor of [1] [6] includes, a decoding unit for decoding the rank information of the intra prediction mode candidate, wherein the intra prediction mode candidate decoded Position The image decoding device is characterized in that a decoded image of the target block is generated using the predicted image of the reference block obtained by using the same intra prediction mode as the above .
According to the configuration of [8] , among the intra-prediction mode candidates ordered in ascending order of the error index, a decoded image is generated using a prediction image based on the same intra-prediction mode as the input-order intra-prediction mode candidate. Will be done. Therefore, the higher-order encoded intra-prediction mode can be decoded with a smaller number of coding bits.

[9] One aspect of the present invention provides an error index between the predicted image of the reference block and the input image of the reference block obtained by using the intra prediction mode defined for the reference block around the target block on the computer. When the error index acquisition unit to be acquired and the intra prediction mode defined for the reference block are used as the intra prediction mode candidates of the target block, the rank determination unit that determines the rank of the intra prediction mode candidates in the ascending order of the error index. , And the ranking determination unit determines the ranking of the intra prediction mode of the reference block that gives the error index larger than the threshold of the predetermined error index as the intra prediction mode candidate, and the ranking of the predetermined intra prediction mode candidate. It is a program to function as an intra-predictor , which is characterized by being lower than .
According to the configuration of [9] , there is a high possibility that the representative intra prediction mode that gives the prediction image having the smallest error amount from the input image matches the intra prediction mode candidate having a high ranking. Therefore, the number of intra prediction mode candidates related to the coding of the intra prediction mode, that is, the number of coding bits thereof can be reduced.

According to the present invention, the coding efficiency of the intra prediction mode can be improved.

It is a schematic block diagram which shows the structure of the image coding apparatus which concerns on 1st Embodiment. It is a schematic block diagram which shows the structure of the image decoding apparatus which concerns on 1st Embodiment. It is a schematic block diagram which shows the structure of the intra prediction part which concerns on 1st Embodiment. It is a figure which shows the example of the target block and the reference block. It is a figure which shows the example of the intra prediction mode. It is a schematic block diagram which shows the structure of the intra prediction part which concerns on 1st Embodiment. It is a flowchart which shows the MPM determination process which concerns on 1st Embodiment. It is a flowchart which shows the MPM determination process which concerns on 2nd Embodiment.

(First Embodiment)
Hereinafter, the first embodiment of the present invention will be described with reference to the drawings.
The configuration of the image coding device 1 according to the present embodiment will be described. FIG. 1 is a schematic block diagram showing a configuration of an image coding device 1 according to the present embodiment. In the example shown in FIG. 1, the image coding device 1 applies the intra prediction unit 110 (described later) as an example of the intra predictor according to the present embodiment to the image coding device that performs the image coding process according to the HEVC method. It will be done.

The image coding device 1 includes a subtraction unit 101, an orthogonal conversion unit 102, a quantization unit 103, a variable length coding unit 104, an inverse quantization unit 105, an inverse orthogonal conversion unit 106, an addition unit 107, a loop filter 108, and a reference memory. It includes 109, an intra prediction unit 110, an inter prediction unit 111, and a selection unit 112.

The input image for each picture is input to the subtraction unit 101 from the outside of the image coding device 1. A picture means one image and is also called a frame. A prediction image in each target block of interest as a processing target at that time is input to the subtraction unit 101 from the selection unit 112. Hereinafter, the predicted image in the target block is referred to as a predicted image block. The target block is an area having a predetermined size formed by dividing a target picture, which is a picture of interest as a processing target at that time. The size of the block is called the block size. The block size is, for example, any one of 32 × 32 pixels, 16 × 16 pixels, 8 × 8 pixels, and 4 × 4 pixels. The subtraction unit 101 subtracts the predicted image block from the input image block which is the input image in the target block for each block to generate a residual image block, and outputs the generated residual image block to the orthogonal conversion unit 102.

The orthogonal conversion unit 102 performs orthogonal conversion on the residual image block input from the subtraction unit 101 to calculate the orthogonal conversion coefficient, and outputs the calculated orthogonal conversion coefficient to the quantization unit 103. As the orthogonal transform, for example, a discrete cosine transform (DCT: Discrete Cosine Transform) or a discrete sine transform (DST: Discrete Sine transform) is used.

The quantization unit 103 quantizes the orthogonal conversion coefficient input from the orthogonal conversion unit 102 using a predetermined quantization parameter, and calculates the quantization orthogonal conversion coefficient. The quantization unit 103 outputs a code indicating the calculated quantization orthogonal conversion coefficient to the variable length coding unit 104 and the inverse quantization unit 105.

The variable-length coding unit 104 aggregates the code of the quantization orthogonal conversion coefficient input from the quantization unit 103, the representative intra prediction mode information input from the intra prediction unit 110, and the motion information input from the inter prediction unit 111. To generate a data string. The variable-length coding unit 104 performs variable-length coding processing on the generated data string. The variable length coding process is also called entropy coding. The variable length coding process compresses the amount of information of various input data. As the variable length coding process, for example, a context-adaptive binary arithmetic coding method (CABAC: Adapt-based Adaptive Binary Analysis Coding) is used. The variable-length coding unit 104 outputs the coded data composed of the bit strings obtained by performing the variable-length coding process to the outside of the image coding device 1. The prediction mode information from the intra prediction unit 110 will be described later.

The inverse quantization unit 105 calculates the inverse quantization orthogonal conversion coefficient by performing inverse quantization on the code of the quantization orthogonal conversion coefficient input from the quantization unit 103 using a predetermined quantization parameter. The inverse quantization unit 105 outputs the calculated inverse quantization orthogonal conversion coefficient to the inverse orthogonal conversion unit 106.

The inverse orthogonal conversion unit 106 performs inverse orthogonal conversion on the inverse quantization orthogonal conversion coefficient input from the inverse quantization unit 105, and generates a decoding residual image block which is a decoding residual image in the target block. The inverse orthogonal transformation is an inverse operation with the orthogonal transformation performed by the orthogonal transforming unit 102. The inverse orthogonal conversion unit 106 outputs the decoding residual image block to the addition unit 107.

The addition unit 107 adds the decoding residual image block to the prediction image block of the target block input from the selection unit 112 to generate a decoded image block which is a decoded image in the target block. The addition unit 107 outputs the generated decoded image block to the loop filter 108 and the intra prediction unit 110. The decoded image block corresponds to the decoded image block obtained by performing the encoding process.

The loop filter 108 includes an element filter that performs a predetermined filter process on the decoded image block input from the addition unit 107, and sequentially stores the decoded image block obtained by the filtering process in the reference memory 109. Therefore, the reference memory 109 stores the decoded image of the picture at each time point within a predetermined time from the target picture, including the target picture.
The loop filter 108 includes a deblocking filter (DF: Deblocking Filter) and a pixel adaptive offset (SAO: Single Adaptive Offset) as element filters.

A decoded image block is input from the addition unit 107 to the intra prediction unit 110 for each block. The intra prediction unit 110 refers to the decoded image in the reference block around the target picture, and performs intra prediction for each of the plurality of predetermined intra prediction modes. As the reference block, a decoded image block that has been decoded within a predetermined range from the target block is used. The intra prediction unit 110 performs intra prediction for each intra prediction mode and generates a prediction image in the target block. Hereinafter, the predicted image in the target block is referred to as an intra prediction block. The intra prediction unit 110 outputs the generated intra prediction block to the selection unit 112.

The intra prediction unit 110 calculates the amount of error between the intra prediction image block and the input image block generated for each intra prediction mode. The intra prediction unit 110 defines the intra prediction mode with the smallest amount of error acquired from the plurality of intra prediction modes as the representative intra prediction mode. Further, the intra prediction unit 110 acquires the representative intra prediction mode defined for the reference block and its error index. Here, the error index is an index value indicating the magnitude of the residual, which is the difference between the input image block and the predicted image block in the reference block. The error index is distinguished from the amount of error indicating the magnitude of the residual, which is the difference between the input image block and the predicted image block in the target block. When the representative intra prediction mode defined for the reference block is used as the intra prediction mode candidate of the target block, the intra prediction unit 110 determines the order of the intra prediction mode candidates in ascending order of the error index. Then, the intra prediction unit 110 determines whether or not there is an intra prediction mode candidate that matches the representative intra prediction mode related to the target block. The intra prediction mode 110 generates representative intra prediction mode information including information on the rank of the intra prediction mode candidates determined to match, and outputs the generated representative intra prediction mode information to the variable length coding unit 104. The configuration of the intra prediction unit 110 will be described later.

The inter-prediction unit 111 determines the motion vector of the target block of the input image input from the outside by performing block matching with reference to the decoded image of the picture within a predetermined period stored in the reference memory 109. The pictures within a predetermined period include pictures that are past and future than the target picture. In block matching, the inter-prediction unit 111 calculates the amount of error of the inter-prediction image block obtained from the target block based on the decoded image within a predetermined range with respect to the input image block for each displacement amount. The inter-prediction unit 111 determines the displacement amount with the smallest error amount as a motion vector. The inter-prediction unit 111 performs motion compensation on the decoded image in the reference block using the determined motion vector, and generates a prediction image in the target block. Hereinafter, the predicted image in the target block is referred to as an inter-predicted image block. The inter-prediction unit 111 outputs the generated inter-prediction image block to the selection unit 112. The inter-prediction unit 111 outputs motion information including information of a predetermined motion vector to the variable-length coding unit 104. The motion vector defined for the target block may be equal to the motion vector defined for any of the encoded reference blocks. In that case, the inter-prediction unit 111 identifies a reference block that gives a motion vector equal to the motion vector defined for the target block. Then, the inter-prediction unit 111 includes the position information of the specified reference block in the motion information instead of the motion vector information.

The selection unit 112 switches between the intra prediction image block input from the intra prediction unit 110 and the inter prediction image block input from the inter prediction unit 111, and transfers the prediction image block selected by the switching to the subtraction unit 101 and the addition unit 107. Output.

Next, the configuration of the image decoding device 2 according to the present embodiment will be described. FIG. 2 is a schematic block diagram showing the configuration of the image decoding device 2 according to the present embodiment. In the example shown in FIG. 1, the image decoding device 2 is formed by applying the intra prediction unit 207 (described later) to the image decoding device according to the HEVC method as another example of the intra predictor according to the present embodiment.

The image decoding device 2 includes a variable length decoding unit 201, an inverse quantization unit 202, an inverse orthogonal conversion unit 203, an addition unit 204, a loop filter 205, a reference memory 206, an intra prediction unit 207, and an inter prediction unit 208. Orthogonal. The processing performed by the inverse quantization unit 202, the inverse orthogonal conversion unit 203, and the addition unit 204 is the same as that of the inverse quantization unit 105, the inverse orthogonal conversion unit 106, and the addition unit 107 of the image coding apparatus 1, respectively.

The coded data generated by the image coding device 1 is input to the variable length decoding unit 201 from the outside of the image decoding device 2. The variable-length decoding unit 201 performs variable-length decoding of the bit string included in the input coded data to acquire the code of the quantization orthogonal conversion coefficient for each block, the representative intra prediction mode information, and the motion information. The variable-length decoding method is a variable-length decoding method corresponding to the variable-length coding method used for generating coded data. The variable-length decoding unit 201 outputs the code of the quantization orthogonal conversion coefficient, the representative intra prediction mode information, and the motion information related to each target block to the inverse quantization unit 202, the intra prediction unit 207, and the inter prediction unit 208, respectively.

In the inverse quantization unit 202, the sign of the quantization orthogonal conversion coefficient of the target block input from the variable length decoding unit 201 is inversely quantized using a predetermined quantization parameter, and the inverse quantization orthogonal conversion coefficient is calculated. To do. The inverse quantization unit 202 outputs the calculated inverse quantization orthogonal conversion coefficient to the inverse orthogonal conversion unit 203.

The inverse orthogonal conversion unit 203 performs inverse orthogonal conversion on the inverse quantization orthogonal conversion coefficient of the target block input from the inverse quantization unit 202 to generate a decoding residual image block. The inverse orthogonal conversion unit 203 outputs the decoding residual image block to the addition unit 204.

The intra prediction image block related to the target block is input to the addition unit 204, or the inter prediction image block related to the target block is input from the inter prediction unit 208 as the prediction image block. The addition unit 204 adds the prediction image block and the decoding residual image block input from the inverse orthogonal conversion unit 203 to generate the decoding image block. The addition unit 204 outputs the generated decoded image block to the outside of the loop filter 205 and the image decoding device 2.

The loop filter 205 includes DF and SAO as element filters for performing predetermined filtering on the decoded image block related to the target block input from the addition unit 204, and sequentially transfers the decoded image blocks obtained by the filtering to the reference memory 206. Remember. Therefore, the reference memory 206 stores the decoded images of the pictures at each time point including the target picture.
The variable length decoding unit 201 inputs the representative intra prediction mode information of the target block to the intra prediction unit 207, and the addition unit 204 inputs the decoded image block.

The intra prediction unit 207 acquires a representative intra prediction mode defined for the decoded reference block within a predetermined range from the target picture and an error index thereof. The intra prediction unit 207
When the representative intra prediction mode defined for the reference block is used as the intra prediction mode candidate of the target block, the order of the intra prediction mode candidates is determined in ascending order of the error index.
Among the intra-prediction mode candidates, the intra-prediction unit 207 identifies the intra-prediction mode candidates in the order indicated by the representative intra-prediction mode information input from the variable-length decoding unit 201 as the representative intra-prediction mode. The intra prediction unit 207 refers to the decoded image block of the reference block input from the addition unit 204, performs intra prediction using the specified intra prediction mode, and outputs the intra prediction image block generated to the addition unit 204. .. The configuration of the intra prediction unit 207 will be described later.

Motion information related to the target block is input to the inter-prediction unit 208 from the variable-length decoding unit 201. When the input motion information indicates a motion vector, the inter-prediction unit 208 uses the motion vector to perform motion compensation for the decoded image within a predetermined range from the target block. When the input motion information indicates the position information of the reference block, the inter-prediction unit 208 performs motion compensation using the motion vector of the reference block specified by the position information. The inter-prediction unit 208 outputs the inter-prediction image block obtained by performing motion compensation to the addition unit 204.

(Intra Prediction Department)
Next, the configuration of the intra prediction unit 110 of the image coding device 1 according to the present embodiment will be described. FIG. 3 is a schematic block diagram showing the configuration of the intra prediction unit 110 according to the present embodiment. The intra prediction unit 110 includes an intra predictor 1100 and a candidate determination unit 1107. The intra-predictor 1100 includes an intra-prediction image generation unit 1101, a subtraction unit 1102, an MPM determination unit 1103, and an intra-prediction mode information generation unit 1106.

The intra-prediction image generation unit 1101 performs intra-prediction for each of a plurality of predetermined intra-prediction modes with reference to the decoded image in the reference block within a predetermined range from the target block. In the HEVC method, the block that is the processing unit of the intra prediction corresponds to the processing unit (TU: Transfer Unit; conversion unit) in which the orthogonal transformation and the inverse orthogonal transformation are performed. The number of intra prediction modes is 35 modes. A specific example of the intra prediction mode will be described later. The intra prediction image generation unit 1101 outputs the intra prediction image block obtained by the intra prediction to the subtraction unit 1102 and the candidate determination unit 1107.

The subtraction unit 1102 subtracts the intra prediction image block input from the intra prediction image generation unit 1101 from the input image block to generate a residual image block for each intra prediction mode. The subtraction unit 1102 outputs the generated residual image block to the candidate determination unit 1107.

The MPM determination unit 1103 determines the MPM and the rank in the MPM based on the error index of each decoded reference block and the representative intra prediction mode. The MPM is a group consisting of intra-prediction mode candidates, which is a part of a plurality of intra-prediction modes. The ranking in MPM is the ranking of intra-prediction mode candidates belonging to MPM. In the HEVC method, when determining the MPM as shown in FIG. 4, the block A to the left of the target block T and the block B above the target block are used. The MPM determination unit 1103 includes an error index acquisition unit 1104 and an MPM ranking determination unit 1105.

The error index acquisition unit 1104 acquires the error indexes Da and Db of the reference blocks A and B from the storage unit (not shown) included in the intra-predictor 1100. Reference blocks A and B are blocks to the left of and above the target block T. The error index is an index value that is presumed to show a tendency similar to the amount of error of the residual image related to the target block. In the present embodiment, an index value that quantitatively represents the magnitude of the residual in each decoded reference block adjacent to the target block is used as an error index. Since the error index is derived from the decoded residual image block, it can be acquired by either the intra prediction unit 110 of the image coding device 1 or the intra prediction unit 207 of the image decoding device 2. The residual image block is obtained by performing intra-prediction for each of the reference blocks A and B using the representative intra-prediction modes Pa and Pb. The representative intra prediction mode that gives the smallest error index in the reference block is presumed to be a highly reliable intra prediction mode that can more faithfully reproduce the input image even in the target block adjacent to the reference block. Because.

As the error index, for example, any of the following index values may be used. (1) The number of bits of the bit string obtained by variable-length coding the code indicating the quantization orthogonal conversion coefficient, (2) The number of non-zero conversion coefficients having a value other than zero among the quantization orthogonal conversion coefficients, (3) DC component of quantized quadrature conversion coefficient, (4) sum of squares of quantized quadrature conversion coefficient, (5) sum of absolute values of quantized quadrature conversion coefficient. In the image coding apparatus 1, (1) and (2) can be obtained from the variable length coding unit 104 and the inverse quantization unit 105, respectively. (3)-(5) can all be obtained from the inverse orthogonal conversion unit 106.

Regarding (1), when the orthogonal conversion coefficient becomes smaller, the quantization orthogonal conversion coefficient becomes 0 or an integer value close to 0, and the bits that take a value of 0 by binarization are the main. Therefore, it is presumed that the smaller the value of (1), the smaller the prediction residual in the target block due to the same intra prediction mode as the representative intra prediction mode.
Regarding (2), when the orthogonal conversion coefficient to be encoded becomes smaller, the quantization orthogonal conversion coefficient becomes 0 or an integer value close to 0. Therefore, it is presumed that the smaller the value of (2) to be counted, the smaller the predicted residual in the target block.
(3) corresponds to the average value of the pixel values for each pixel in the target block. It is presumed that the smaller the value of (3), the smaller the prediction residual in the target block by the same intra prediction mode as the representative intra prediction mode.
(4) corresponds to the sum of the squared values of the quantized orthogonal conversion coefficients for each spatial frequency in the target block. It is presumed that the smaller the value of (4), the smaller the prediction residual in the target block by the same intra prediction mode as the representative intra prediction mode.
(5) corresponds to the sum of the absolute values of the quantization orthogonal conversion coefficients for each spatial frequency in the target block. The smaller the value of (5), the smaller the prediction residual in the target block by the same intra prediction mode as the representative intra prediction mode.

The error index acquisition unit 1104 uses as an error index the amount of error acquired by the candidate determination unit 1107 in the process of determining the representative intra prediction mode for the past target block, that is, the reference block, or the index value acquired in the process of calculating the error amount. It may be adopted. Therefore, it is not necessary to add a process for calculating the error index related to the representative intra prediction mode of the reference block. Regarding the representative intra prediction mode of the invalid block, the error index acquisition unit 1104 defines a value Z sufficiently larger than the maximum value of the error index that can be calculated as the error index related to the representative intra prediction mode. For example, when the number of non-zero conversion coefficients is used as the error index, the error index acquisition unit 1104 may set the error amount Z to be twice the number of pixels in the block size. The error index acquisition unit 1104 outputs the error indexes Da and Db of the representative intra prediction modes Pa and Pb to the MPM ranking determination unit 1105, respectively.

The MPM ranking determination unit 1105 is based on the error index Da related to the representative intra prediction mode Pa of the block A, the error index Db related to the representative intra prediction mode Pb of the block B, and a predetermined default MPM input from the error index acquisition unit 1104. Then, the intra-prediction mode candidates belonging to MPM and their rankings are determined. The default MPM is an intra prediction mode candidate set when the reference block is invalid, and a predetermined intra prediction mode candidate set as an intra prediction mode candidate other than the intra prediction mode of the reference block. The MPM ranking determination unit 1105 outputs information on the determined intra-prediction mode candidates and their rankings to the intra-prediction mode information generation unit 1106.

In the HEVC method, any or a combination of planar prediction, direct current (direct current) prediction and vertical reference is used as the default MPM. The intra prediction mode candidate set when the reference block is invalid is DC prediction. Invalid reference block means that there is no representative intra prediction mode available for the reference block. Invalid reference block means, for example, that the reference block does not exist in the target picture, that the reference block is not encoded in the intra-prediction, that it has not been decoded, and that the reference block belongs to the slice to which the target block belongs. It corresponds to the fact that it exists in a slice different from the above, that the reference block exists in a tile different from the tile to which the target block belongs, or that it is encoded using the I_PCM mode as a predetermined special coding mode. .. The I_PCM mode is a coding mode in which a code indicating a signal value for each pixel is output as it is.

When the representative intra prediction mode Pa and the representative intra prediction mode Pb are different and the error index Da is equal to or less than the error index Db, the MPM ranking determination unit 1105 sets the 0th intra prediction mode candidate of the MPM as the representative intra prediction mode Pa. The first intra-prediction candidate of MPM is defined as the representative intra-prediction mode Pb. Further, when the representative intra prediction mode Pa and the representative intra prediction mode Pb are different and the error index Da is larger than the error index Db, the MPM ranking determination unit 1105 selects the 0th intra prediction mode candidate of the MPM as the representative intra. The prediction mode Pb is defined, and the first intra prediction mode candidate of MPM is defined as the representative intra prediction mode Pa.

When the representative intra prediction mode Pa and the representative intra prediction mode Pb are different, the MPM ranking determination unit 1105 determines the second intra prediction mode candidate of the MPM by using the same procedure as the HEVC method. That is, when neither the representative intra prediction mode Pa nor the representative intra prediction mode Pb is a planar prediction, the MPM ranking determination unit 1105 determines the planar prediction as the second intra prediction mode candidate of the MPM. Further, when neither the representative intra prediction mode Pa nor the representative intra prediction mode Pb is DC prediction, the MPM ranking determination unit 1105 determines the planar prediction as the second intra prediction mode candidate of MPM. Otherwise, the MPM ranking determination unit 1105 defines the second intra-prediction candidate of the MPM as a vertical reference.

When the representative intra prediction mode Pa and the representative intra prediction mode Pb are equal, the MPM ranking determination unit 1105 determines each intra prediction mode candidate of the MPM by using the same procedure as the HEVC method. That is, when the representative intra prediction mode Pa and the representative intra prediction mode Pb are planar prediction or DC prediction, the MPM ranking determination unit 1105 predicts the 0th, 1st, and 2nd intra prediction mode candidates of MPM, respectively. , DC prediction, vertical reference. Further, when the representative intra prediction mode Pa and the representative intra prediction mode Pb are directional predictions, the MPM ranking determination unit 1105 selects the 0th, 1st, and 2nd intra prediction mode candidates of the MPM as representative intra prediction modes, respectively. Intra prediction mode in the same reference direction as Pa and Pb, representative intra prediction mode Pa and Pb are adjacent to the reference direction in the counterclockwise direction, and representative intra prediction modes Pa and Pb are adjacent to the reference direction in the clockwise direction. Determined as an intra prediction mode candidate.

For an invalid reference block, the MPM ranking determination unit 1105 defines the representative intra prediction mode as DC prediction, as in the HEVC method. That is, when the block A is invalid, the MPM ranking determination unit 1105 determines DC prediction as the representative intra prediction mode Pa. When the block B is invalid, the MPM ranking determination unit 1105 determines DC prediction as the representative intra prediction mode Pb.

The intra prediction mode information generation unit 1106 generates intra prediction mode candidates of MPM input from the MPM ranking determination unit 1105 and intra prediction mode information indicating their ranks. The intra prediction mode information generation unit 1106 outputs the generated intra prediction mode information to the candidate determination unit 1107.

Intra prediction mode information is input to the candidate determination unit 1107 from the intra prediction mode information generation unit 1106, and the intra prediction image block from the intra prediction image generation unit 1101 and the residual image block from the subtraction unit 1102 for each intra prediction mode. Is entered. The candidate determination unit 1107 calculates an error amount indicating the magnitude of the residual for each intra prediction mode, and defines the intra prediction mode having the smallest calculated error amount as the representative intra prediction mode of the target block. The candidate determination unit 1107 may perform rate-distortion optimization, for example, when determining the representative intra prediction mode. Rate-distortion optimization is the sum of the squares or absolute values of the quantization conversion coefficients of the residual image block as the error amount J, and the sum of the multiplication values of the bit length of the quantization conversion coefficients and the predetermined constant λ. This is a method for determining the minimum intra prediction mode. The constant λ is determined based on the quantization parameters. As the quantization parameter, the quantization width of the orthogonal conversion coefficient is used. The candidate determination unit 1107 outputs the prediction image block related to the determined representative intra prediction mode to the subtraction unit 101 via the selection unit 112. Further, the candidate determination unit 1107 stores the index of the target block in association with the determined representative intra prediction mode and the error index in the storage unit (not shown) of the intra predictor 1100. The stored intra prediction mode and error index are used as the intra prediction mode and error index of the reference block after the target block is changed.

The candidate determination unit 1107 determines whether or not the predetermined representative intra prediction mode is included in the intra prediction mode candidates of the MPM indicated by the intra prediction mode information input from the intra prediction mode information generation unit 1106. When determining that it is included, the candidate determination unit 1107 identifies the rank of the intra-prediction mode candidate that is the same as the representative intra-prediction mode. Then, the candidate determination unit 1107 uses an intra prediction flag indicating that the representative intra prediction mode is included in the intra prediction mode candidates of the MPM and a bit string indicating a number of the specified rank as the representative intra prediction mode information with a variable length. Output to the coding unit 104.

When it is determined that the representative intra prediction mode is not included in the MPM intra prediction mode candidates, the candidate determination unit 1107 sorts the mode numbers of the MPM intra prediction mode candidates in ascending order. An integer value from 0 to 34 is set as a mode number for identifying each of the plurality of intra prediction modes. The candidate determination unit 1107 reduces the mode number of the representative intra prediction mode by 1 when the mode number of the representative intra prediction mode is equal to or higher than the mode number of the 0th intra prediction mode candidate of MPM. When the mode number of the reduced representative intra prediction mode is equal to or higher than the mode number of the first intra prediction mode candidate of MPM, the candidate determination unit 1107 further reduces the mode number of the representative intra prediction mode by one. When the mode number of the representative intra prediction mode further reduced is equal to or higher than the mode number of the second intra prediction mode candidate of the MPM, the candidate determination unit 1107 further reduces the mode number of the representative intra prediction mode by one. Then, the candidate determination unit 1107 determines the mode number of the representative intra prediction mode as the adoption prediction mode number. By these procedures, the adoption prediction mode number is represented as a value of any of 0 to 31, so that the amount of information is within 5 bits. The candidate determination unit 1107 uses a bit string indicating an intra prediction flag indicating that the representative intra prediction mode is not included in the MPM intra prediction mode candidates and an adoption prediction mode number as representative intra prediction mode information, and a variable length coding unit 104. Output to.

In the HEVC method, when the number of the intra-prediction mode candidate of MPM is subjected to the variable length coding process, the cut rice binarization (TR: Truncated Rice Binarization) method is adopted in the binarization process. The TR method is a method of binarizing an input value which is an integer of any of 0 to the maximum value cMax. The TR method is a method of sequentially arranging the values of "1" at the beginning by the number equal to the input value and adding "0" to the end to convert the value into a bit string. However, when the input value is equal to the maximum value cMax, the trailing "0" is omitted. Therefore, when the input value is any value from 0 to cMax-1, the smaller the input value, the smaller the code amount of the bit string generated by the TR method. When the number of MPM intra-prediction mode candidates is 3, the maximum value cMax is 2, so the number of the intra-prediction mode candidate as an input value is represented by any of 0 to 2.

In the conventional HEVC method, when the representative intra prediction mode Pa and the representative intra prediction mode Pb are different, the representative intra prediction modes Pa and Pb are defined as the first and second intra prediction mode candidates of the MPM regardless of the error index. It was.
On the other hand, the MPM ranking determination unit 1105 according to the present embodiment defines the representative intra prediction mode for each reference block having a smaller error index as an intra prediction mode candidate having a higher rank (fewer number values). Therefore, there is a high possibility that the representative intra prediction mode of the target block that gives the predicted image block having the smallest error amount from the input image matches the intra prediction mode candidate having a high rank. That is, since the number of the intra prediction mode candidate that matches the representative intra prediction mode becomes small, the code amount after binarization becomes small. The representative intra prediction mode is used in the image decoding device 2 to generate a predicted image in the target block.

(Intra prediction mode)
Next, an example of the intra prediction mode will be described. In the HEVC method, there are 35 types of intra prediction modes. Each of the intra prediction modes has a mode number of 0 to 34. The 35 types of intra prediction modes are classified into directional prediction, DC prediction and planar prediction. Directional prediction is an intra that calculates the pixel value of each pixel in the target block by interpolating the pixel values of the pixels in the direction closest to each of the left and right sides of the line segment in the reference direction among the predetermined reference pixels in the reference block. Prediction mode. As shown in FIG. 5A, the reference directions are 33 directions from diagonally lower left to diagonally upper right, and any mode number from 2 to 34 is assigned clockwise in ascending order in each direction. The reference direction is denser as it is closer to the horizontal direction, and is sparsely distributed as it is closer to the diagonal direction of the target block. In the example shown in FIG. 5A, the starting point of the arrow indicating the reference direction is set at the center of the target block.

The DC prediction is an intra prediction mode in which the average value of the pixel values between predetermined reference pixels in the reference block is calculated as the pixel value of each pixel in the target block Bk. In the example shown in FIG. 5B, the reference pixels referred to in the DC prediction are 2N reference pixels indicated by black circles. N indicates the number of pixels corresponding to the length of one side of the target block. As the reference pixel, the pixel closest to the upper side and the left side of the target block in the reference block is used. However, in a block having a block size of 32 × 32 pixels or less, as an exception process, the pixel values of the pixels in the top row and the leftmost column of the target block are updated with the average value with the pixel values of the nearest reference pixels. To. The shaded area Ep in FIG. 5B indicates the top row and the leftmost column of the target block. The mode number for DC prediction is 1.

The planar prediction is an intra prediction mode in which the pixel values of each of the four reference pixels are interpolated to calculate the pixel value of each pixel in the target block. As reference pixels, the pixels (x, y) in the target block are diagonally above (-1, y) and above (x, -1), diagonally above the target pixel (N, -1), and diagonally below the left (-). 1, N) pixels are used. (X, y) and the like indicate the coordinate values of each pixel whose origin is the position of the pixel at the upper left corner of the target block. The four reference pixels used for calculating the pixel value of the target pixel are indicated by black circles surrounded by a dashed line circle in FIG. 5B.

(Intra Prediction Department)
Next, the configuration of the intra prediction unit 207 of the image decoding device 2 according to the present embodiment will be described. FIG. 6 is a schematic block diagram showing the configuration of the intra prediction unit 207 according to the present embodiment. The intra prediction unit 207 includes an intra predictor 2070. The intra-predictor 2070 includes an MPM determination unit 2071, an intra-prediction mode information generation unit 2074, a candidate determination unit 2075, and an intra-prediction image generation unit 2076.

Similar to the MPM determination unit 1103, the MPM determination unit 2071 determines the MPM and the MPM internal ranking based on the error index of each decoded reference block and the representative intra prediction mode. The MPM determination unit 2071 includes an error index acquisition unit 2072 and an MPM ranking determination unit 2073.

The error index acquisition unit 2072 acquires the representative intra prediction modes Pa and Pb of the reference blocks A and B adjacent to the target block and the error indexes Da and Db from the storage unit (not shown) provided in the intra predictor 2070. .. The error index acquisition unit 2072 acquires an error index of the same type as the error index acquired by the error index acquisition unit 1104. Regarding the representative intra prediction mode of the invalid reference block, the error index acquisition unit 2072 sets the value Z to a value Z sufficiently larger than the maximum value of the error index that can be calculated as the error index, similarly to the error index acquisition unit 1104. Determine. The error index acquisition unit 2072 outputs the representative intra prediction modes Pa and Pb and their respective error indexes Da and Db to the MPM ranking determination unit 2073.

The MPM ranking determination unit 2073 uses the same method as the MPM ranking determination unit 1105 to obtain the error index Da related to the representative intra prediction mode Pa, the error index Db related to the representative intra prediction mode Pb, and the predetermined error index Db input from the error index acquisition unit 2072. Based on the default MPM of, the intra-prediction mode candidates belonging to the MPM and their ranking are determined. The MPM ranking determination unit 2073 outputs information on the determined intra-prediction mode candidates and their rankings to the intra-prediction mode information generation unit 2074.

Here, when the representative intra prediction mode Pa and the representative intra prediction mode Pb are different and the error index Da is equal to or less than the error index Db, the MPM ranking determination unit 2073 predicts the representative intra prediction as the 0th intra prediction mode candidate of the MPM. The mode Pa is set, and the representative intra prediction mode Pb is set as the first intra prediction candidate of MPM. Further, when the representative intra prediction mode Pa and the representative intra prediction mode Pb are different and the error index Da is larger than the error index Db, the MPM ranking determination unit 2073 is the representative intra as the 0th intra prediction mode candidate of the MPM. The prediction mode Pb is set, and the representative intra prediction mode Pa is set as the first intra prediction mode candidate of MPM.

When the representative intra prediction mode Pa and the representative intra prediction mode Pb are different, the MPM ranking determination unit 2073 determines the second intra prediction mode candidate of the MPM by using the same procedure as the HEVC method. Further, even when the representative intra prediction mode Pa and the representative intra prediction mode Pb are equal, the MPM ranking determination unit 2073 determines each intra prediction mode candidate of the MPM by using the same procedure as the HEVC method. For invalid reference blocks, the MPM ranking determination unit 2073 defines the representative intra prediction mode as DC prediction.

The intra prediction mode information generation unit 2074 generates the intra prediction mode candidates of the MPM input from the MPM ranking determination unit 2073 and the intra prediction mode information indicating their ranks. The intra prediction mode information generation unit 2074 outputs the generated intra prediction mode information to the candidate determination unit 2075.

In the candidate determination unit 2075, the variable length decoding unit 201 inputs the intra prediction flag and the number of the candidate rank of the intra prediction mode or the bit string indicating the number of the adoption prediction mode as the representative intra prediction mode information of the target block. Further, the candidate determination unit 2075 is input with the intra prediction mode information from the MPM ranking determination unit 2073. The candidate determination unit 2075 determines whether or not the representative intra prediction mode is included in the MPM intra prediction mode candidates based on the intra prediction flag. When determining that it is included, the candidate determination unit 2075 selects an intra prediction mode candidate in the order indicated by the number included in the representative intra prediction mode information from the MPM indicated by the intra prediction mode information. The candidate determination unit 2075 determines that the selected intra prediction mode candidate is the representative intra prediction mode of the target block.

When it is determined that the representative intra prediction mode is not included in the MPM intra prediction mode candidates, the candidate determination unit 2075 sorts the mode numbers of the MPM intra prediction mode candidates in ascending order. The candidate determination unit 2075 increments the adoption prediction mode number by 1 when the adoption prediction mode number included in the intra prediction mode information is equal to or greater than the mode number of the 0th intra prediction mode candidate of MPM. The candidate determination unit 2075 further increases the recruitment prediction mode number by 1 when the increased recruitment prediction mode number is equal to or higher than the mode number of the first intra prediction mode candidate of MPM. When the hiring prediction mode number further increased is equal to or higher than the mode number of the second intra prediction mode candidate of MPM, the candidate determination unit 2075 further increases the hiring prediction mode number by one. Then, the candidate determination unit 2075 defines the adoption prediction mode number as the representative intra prediction mode of the target block. The candidate determination unit 2075 outputs the information of the determined representative intra prediction mode to the intra prediction image generation unit 2076.

The intra prediction image generation unit 2076 refers to the decoded image in the reference block, and performs intra prediction using the representative intra prediction mode indicated by the information input from the candidate determination unit 2075. The intra prediction image generation unit 2076 outputs the intra prediction image block generated by the intra prediction to the addition unit 204.

(MPM determination process)
Next, the MPM determination process according to the present embodiment will be described. FIG. 7 is a flowchart showing the MPM determination process according to the present embodiment. In the following description, the case where the MPM determination unit 1103 is the main operator of the MPM determination process is taken as an example, but the MPM determination unit 2071 also performs the process shown in FIG.

(Step S101) The error index acquisition unit 1104 of the MPM determination unit 1103 determines whether or not the block A is invalid. When it is determined that the property is invalid (YES in step S101), the process proceeds to step S102. When it is determined to be valid (step S101 NO), the error index acquisition unit 1104 acquires the error index related to the representative intra prediction mode Pa of the block A, and proceeds to the process of step S103.
(Step S102) The MPM ranking determination unit 1105 defines the representative intra prediction mode Pa of the block A as DC prediction. The error index acquisition unit 1104 defines the error index Da of the block A as the maximum value Z of the error index. After that, the process proceeds to step S103.

(Step S103) The error index acquisition unit 1104 determines whether or not the block B is invalid. When it is determined that the property is invalid (YES in step S103), the process proceeds to step S104. When it is determined to be valid (step S103 NO), the error index acquisition unit 1104 acquires the error index related to the representative intra prediction mode Pb of the block B, and proceeds to the process of step S105.
(Step S104) The MPM ranking determination unit 1105 defines the representative intra prediction mode Pb of the block B as DC prediction. The error index acquisition unit 1104 defines the error index Db of the block B as the maximum value Z of the error index. After that, the process proceeds to step S105.

(Step S105) The MPM ranking determination unit 1105 determines whether or not the representative intra prediction mode Pa of the block A and the representative intra prediction mode Pb of the block B are different. When it is determined that they are different (YES in step S105), the process proceeds to step S106. When it is determined that they do not differ (step S105 NO), the process proceeds to step S109.

(Step S106) The MPM ranking determination unit 1105 determines whether or not the error index Da related to the representative intra prediction mode Pa is equal to or less than the error index Db related to the representative intra prediction mode Pb. When it is determined that the error index Da is equal to or less than the error index Db (YES in step S106), the process proceeds to step S107. When it is determined that the error index Da is larger than the error index Db (step S106 NO), the process proceeds to step S108.

(Step S107) The MPM ranking determination unit 1105 determines the representative intra prediction mode Pa as the 0th intra prediction mode candidate MPM [0] of the MPM, and sets the representative intra prediction as the 1st intra prediction mode candidate MPM [1] of the MPM. Defined in mode Pb. The MPM ranking determination unit 1105 determines the second intra prediction mode candidate MPM [2] of the MPM by performing the same processing as in the HEVC method. After that, the process of FIG. 7 is completed.

(Step S108) The MPM ranking determination unit 1105 determines the representative intra prediction mode Pb as the 0th intra prediction mode candidate MPM [0] of the MPM, and sets the representative intra prediction mode as the 1st intra prediction mode candidate MPM [1] of the MPM. Set to mode Pa. The MPM ranking determination unit 1105 determines the second intra prediction mode candidate MPM [2] of the MPM by performing the same processing as in the HEVC method. After that, the process of FIG. 7 is completed.

(Step S109) The MPM ranking determination unit 1105 determines the intra-prediction mode candidates MPM [0] to MPM [2] from the 0th to the 2nd MPM by performing the same processing as in the HEVC method. After that, the process of FIG. 7 is completed.

As described above, the image coding device 1 and the image decoding device 2 according to the present embodiment include the intra predictors 1100 and 2070. The intra predictors 1100 and 2070 include error index acquisition units 1104 and 2072 and MPM ranking determination units 1105 and 2073. The error index acquisition units 1104 and 2072 acquire the error index of the predicted image obtained by the intra prediction using the representative intra prediction mode of the reference block around the target block. When the representative intra prediction mode of the reference block is adopted as the intra prediction mode candidate, the MPM ranking determination units 1105 and 2073 determine the rank of the intra prediction mode candidate in ascending order of the error index.
According to this configuration, there is a high possibility that the representative intra prediction mode that gives the intra prediction image having the smallest amount of error with the input image block matches the intra prediction mode candidate having a high rank. The number of intra prediction mode candidates related to the coding of the representative intra prediction mode, that is, the number of coding bits thereof can be reduced.

Further, the error index acquisition units 1104 and 2072 acquire the number of bits of the bit string obtained by variable-length coding the data of the quantization conversion coefficient of the prediction residual based on the intra prediction image as the error index.
According to this configuration, the order of the intra prediction mode candidates is determined in ascending order of the number of bits of the prediction residual bit string based on the intra prediction image generated by using each intra prediction mode candidate. It is highly possible that the intra-prediction mode candidate having a small number of bits as the size of the prediction residual matches the representative intra-prediction mode related to the target block. Therefore, the number of coding bits related to the coding in the representative intra prediction mode can be reduced.

Further, the error index acquisition units 1104 and 2072 acquire the number of non-zero conversion coefficients having a value other than zero among the quantization conversion coefficients of the prediction residual based on the intra prediction image as the error index.
According to this configuration, the ranking of the intra prediction mode candidates is the target of the intra prediction mode candidates having a small number of non-zero conversion coefficients of the prediction residual based on the intra prediction image generated by using each intra prediction mode candidate. There is a high possibility that it matches the representative intra prediction mode related to the block. Therefore, the number of coding bits related to the coding in the representative intra prediction mode can be reduced.

Further, the error index acquisition units 1104 and 2072 acquire the sum of squares of the quantization conversion coefficients of the prediction residuals based on the intra prediction image as the error index.
According to this configuration, the rank of the intra prediction mode candidates is such that the intra prediction mode candidates having a small sum of squares of the quantization conversion coefficients of the prediction residuals based on the intra prediction images generated by using each intra prediction mode candidates are There is a high possibility that it matches the representative intra prediction mode related to the target block. Therefore, the number of coding bits related to the coding in the representative intra prediction mode can be reduced.

Further, the error index acquisition units 1104 and 2072 acquire the absolute value sum of the quantization conversion coefficients of the prediction residuals based on the intra prediction image as an error index.
According to this configuration, the rank of the intra prediction mode candidates is such that the intra prediction mode candidates having a small sum of the absolute values of the quantization conversion coefficients of the prediction residuals based on the intra prediction images generated by using each intra prediction mode candidates are , There is a high possibility that it matches the representative intra prediction mode related to the target block. Therefore, the number of coding bits related to the coding in the representative intra prediction mode can be reduced.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to the drawings. The same components as those in the above-described embodiment are designated by the same reference numerals and the description thereof will be incorporated. The intra-predictors 1100 and 2070 according to the present embodiment execute an intra-prediction mode candidate group determination process that is a generalization of the MPM determination process based on the HEVC method shown in FIG. The image coding device 1 and the image decoding device 2 according to the present embodiment according to the present embodiment include the intra-predictors 1100 and 2070, respectively. Also in this embodiment, the group consisting of intra-prediction mode candidates is called MPM.

(MPM determination process)
Next, the MPM determination process according to the present embodiment will be described. In the example described below, the number of reference blocks is generalized to M. M is an integer greater than or equal to 2. Moreover, the number of MPM is represented as N. There are 0th to N-1th ranks of MPM. N is an integer greater than M. The representative intra prediction modes of M reference blocks 1 to M are represented by P [1] to P [M]. For example, the reference block related to the representative intra prediction mode used as MPM is not limited to the block above or to the left of the target block as in the HEVC method, and for example, the upper left block and the upper right block of the target block are further used. May be done. The error index for each of the reference blocks 1 to M is represented by D [1] to D [M]. The default MPM is represented as L [1], L [2], .... However, the representative intra prediction mode set for the invalid reference block is represented by X.

FIG. 8 is a flowchart showing the MPM determination process according to the present embodiment. In the following description, the case where the MPM determination unit 1103 is the main body of operation is taken as an example, but the MPM determination unit 2071 can also be the main body of operation. Further, in the process shown in FIG. 8, the threshold value D [0] is introduced. The threshold value D [0] is an adjustment parameter for adjusting the degree to which the default MPM is selected.

(Step S121) The error index acquisition unit 1104 of the MPM determination unit 1103 is set to the same intra prediction mode as each of the representative intra prediction modes P [1] to P [M] of the valid reference blocks among the M reference blocks. Obtain the relevant error index. Further, the error index acquisition unit 1104 determines the error index as the maximum value Z for each of the invalid reference blocks among the M reference blocks. After that, the process proceeds to step S122.

(Step S122) The MPM ranking determination unit 1105 sorts the M error indexes D [1] to D [M] and the threshold value D [0] in ascending order. The threshold values D [0] obtained by sorting and the indexes of the reference blocks 1 to M are represented as i [0] to i [M]. The indexes i [0] to i [M] take an integer value of 1 or more. The list of indexes i [0] to i [M] shows the sorted reference blocks and the rank of the thresholds. However, the MPM ranking determination unit 1105 defines the representative intra prediction mode of invalid reference blocks as the intra prediction mode X, and sets the index of those reference blocks to 0. After that, the process proceeds to step S123.

(Step S123) The MPM ranking determination unit 1105 sets the initial values of the indexes x and y to 0 and 1, respectively. The index x is an index for counting the rank of the representative intra prediction mode of the reference block adopted as the intra prediction mode candidate of the MPM in the ascending order of the error index. The index y is an index for counting the rank of the default MPM adopted as the intra prediction mode candidate of the MPM. After that, the process proceeds to step S124.

(Step S124) The MPM ranking determination unit 1105 determines whether or not the x-th index i [x] is 0. By this determination, it is determined whether or not the xth reference block is invalid. When it is determined to be 0 (YES in step S124), the process proceeds to step S129. When it is determined that the value is not 0 (NO in step S124), the process proceeds to step S125.

(Step S125) The MPM ranking determination unit 1105 determines whether or not the MPM includes the i [x] th intra prediction mode P [i [x]]. When it is determined that it is included (YES in step S125), the process proceeds to step S128. When it is determined that it is not included (step S125 NO), the process proceeds to step S126.

(Step S126) The MPM ranking determination unit 1105 adds the i [x] th intra prediction mode P [i [x]] to the MPM. The rank of the added intra prediction mode P [i [x]] in the MPM is the lowest at that time. Therefore, the order of the intra prediction mode P [i [x]] added to the MPM for the first time is 0th. After that, the process proceeds to step S127.
(Step S127) The MPM ranking determination unit 1105 adds 1 to the value of the index x (increment). After that, the process proceeds to step S128.
(Step S128) The MPM ranking determination unit 1105 determines whether the total number of intra prediction modes included in the MPM is less than N. When it is determined that the value is less than N (YES in step S128), the process proceeds to step S124. When it is determined that the value is N or more (step S128 NO), the process shown in FIG. 8 is terminated.

(Step S129) The MPM ranking determination unit 1105 determines whether or not the y-th default MPM L [y] is included in the MPM. When it is determined that it is included (YES in step S129), the process proceeds to step S128. When it is determined that it is not included (step S129 NO), the process proceeds to step S130.

(Step S130) The MPM ranking determination unit 1105 adds the y-th default MPM L [y] to the MPM. The rank of the added default MPM L [y] in the MPM is the lowest at that time. After that, the process proceeds to step S131.
(Step S131) The MPM ranking determination unit 1105 adds 1 to the value of the index y (increment). After that, the process proceeds to step S128.

In step S122, the error indexes may be equal to each other for the representative intra prediction modes of some two or more reference blocks out of M. Among reference blocks having the same error index, the error index acquisition unit 1104 determines, for example, the order of each reference block in the ascending order of the index of the reference block. The error index acquisition unit 1104 may further acquire other types of error indexes, and may determine the rank of each reference block in ascending order of the acquired error indexes.

Further, in step S122, the representative intra prediction modes of two or more (for example, K) reference blocks of a part of M may be the same as each other. When the representative intra prediction modes that are the same as each other are non-directional predictions, the error index acquisition unit 1104 does not have to set the ranking for each K-1 reference block. In that case, the MPM ranking determination unit 1105 adds K-1 default MPMs to the MPM in the process of step S130. Further, the representative intra prediction modes that are the same as each other may be directional prediction. In that case, the error index acquisition unit 1104 sets the K-1 intra-prediction mode related to the directional prediction as the K-1 intra-prediction mode candidate in order from the reference direction of the representative intra-prediction mode to the nearest reference direction. You may choose. Further, the error index acquisition unit 1104 may also set the intra prediction mode related to the direction prediction of K-1 pieces as the target of sorting based on the error index. As a result, the intra prediction mode in which the error index is larger than the threshold value D [0] is excluded from the MPM as the unreliable intra prediction mode.

According to the process shown in FIG. 8, the smaller the threshold value D [0], the higher the degree to which the default MPM is selected as the MPM intra-prediction mode candidate. For example, when the threshold D [0] is set to Z + 1 as a value larger than the maximum value Z, the representative intra prediction mode of the reference block takes precedence over the default MPM. When the threshold value D [0] is set to a value within the range where it may appear as an error index, the default MPM may be selected in preference to the representative intra prediction mode of the reference block.

Therefore, the error index acquisition unit 1104 may set the threshold value D [0] variably based on the range of the error index related to the reference block. For example, the error index acquisition unit 1104 sets the threshold value D [0] to a value within the range of the error index related to each reference block. In that case, the default MPM is preferentially selected over the intra-prediction mode which is the same as the representative intra-prediction mode of the reference block related to the error index larger than the threshold value D [0]. This is because such an intra prediction mode is considered to be less reliable than the default MPM because the error index indicating the magnitude of the prediction residual is high.

When there is a default MPM that gives an error index smaller than the error index related to any reference block, the error index acquisition unit 1104 sets the threshold value D [0] to the error index related to the default MPM and the error related to the reference block. It may be set to a value between the index and the index. In that case, the default MPM related to the error index smaller than the threshold value D [0] is preferentially selected as the MPM than the representative intra prediction mode of the reference block related to the error index larger than the threshold value D [0]. ..

In setting the threshold value D [0], the error index acquisition unit 1104 may use the number of bits of the bit string obtained by variable-length coding a code indicating the quantization orthogonal conversion coefficient as the error index. Therefore, an MPM for encoding the intra prediction mode with a smaller amount of code is selected.
The error index acquisition unit 1104 does not necessarily have to change the threshold value D [0] for each picture. The setting cycle of the threshold value D [0] may be a longer cycle, for example, a predetermined number of pictures, an access unit, or a sequence. Therefore, the processing amount is smaller than that when the threshold value D [0] is changed for each picture.
The error index acquisition unit 1104 outputs the set threshold value D [0] to the variable length coding unit 104.

The variable-length coding unit 104 includes the threshold D [0] in the header of the bitstream forming the coded data (for example, the header of the SPS [Sequence Parameter Set] of the HEVC method, the header of the PPS [Picture Parameter Set], etc.).
The variable-length decoding unit 201 extracts the threshold value D [0] from the header of the bit stream forming the input encoded data, and outputs the extracted threshold value D [0] to the error index acquisition unit 2072. The error index acquisition unit 2072 acquires the threshold value D [0] input from the variable length decoding unit 201, and the error index acquisition unit 2072 and the MPM ranking determination unit 2073 acquire the threshold value [0] input from the variable length decoding unit 201. ] Is used for the processing of FIG.

In the processing shown in FIG. 8, M = 2, N = 3, threshold value D [0] = Z + 1, intra prediction mode X related to the invalid reference block is DC prediction, default MPM L [1], L [2]. , L [3] are planar prediction, DC prediction, and vertical reference, respectively, and the process is the same as that shown in FIG. 7 based on the HEVC method.

As described above, in the intra-predictors 1100 and 2070 according to the present embodiment, the MPM ranking determination units 1105 and 2073 give an error index larger than the threshold of a predetermined error index as an intra-prediction mode candidate. It is characterized in that the ranking of the representative intra prediction mode of the above is lower than the ranking of a predetermined default MPM.
With this configuration, the rank of the intra-prediction mode candidate of the representative intra-prediction mode for the reference pixel giving the error index larger than the threshold of the predetermined error index is lower than the predetermined default MPM. Therefore, in the coding of the representative intra prediction mode, the default MPM having a smaller error index is preferentially used as the intra prediction mode candidate. Therefore, the number of intra prediction mode candidates used in coding the representative intra prediction mode related to the target block, that is, the number of coding bits can be reduced as compared with only the representative intra prediction mode related to the reference block.

Further, the error index acquisition units 1104 and 2072 determine the threshold value of the error index based on the range of the error index related to each intra prediction mode candidate.
With this configuration, in the coding of the representative intra prediction mode, it is determined whether or not to preferentially use the default MPM having a smaller error index than the representative intra prediction mode related to the reference pixel based on the threshold value of the determined error index. Will be done. Therefore, the number of coding bits of the representative intra prediction mode is smaller than that of the representative intra prediction mode of the reference block that always gives an error index larger than the threshold of the constant error index lower than the rank of the predetermined default MPM. can do.

Although one embodiment of the present invention has been described in detail with reference to the drawings, the specific configuration is not limited to the above, and various design changes and the like are made without departing from the gist of the present invention. It is possible to do.

For example, the image coding device 1, the image decoding device 2, and the intra-predictor according to the above-described embodiment may be implemented independently, or the image coding device 1 and the image decoding device 2 including the intra-predictor It may be implemented as an image processing system including.

A part of the above-mentioned image coding device 1 or image decoding device 2 and a part of the intra predictors configured as the intra predictors 1100 and 2070 may be realized by a computer. In that case, the program for realizing this control function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read by the computer system and executed. The "computer system" referred to here is a computer system built in the image coding device 1, the image decoding device 2, or the intra-predictors 1100 and 2070, and includes hardware such as an OS and peripheral devices. .. Further, the "computer-readable recording medium" refers to a storage device such as a flexible disk, a magneto-optical disk, a portable medium such as a ROM or a CD-ROM, or a hard disk built in a computer system. Furthermore, a "computer-readable recording medium" is a medium that dynamically holds a program for a short period of time, such as a communication line when a program is transmitted via a network such as the Internet or a communication line such as a telephone line. In that case, a program may be held for a certain period of time, such as a volatile memory inside a computer system serving as a server or a client. Further, the above-mentioned program may be a program for realizing a part of the above-mentioned functions, and may be a program for realizing the above-mentioned functions in combination with a program already recorded in the computer system.
Further, a part or all of the image coding device 1, the image decoding device 2, or the intra-predictors 1100 and 2070 in the above-described embodiment may be realized as an integrated circuit such as an LSI (Large Scale Integration). Each functional block of the image coding device 1, the image decoding device 2, or the intra-predictors 1100 and 2070 may be individually converted into a processor, or a part or all of them may be integrated into a processor. Further, the method of making an integrated circuit is not limited to LSI, and may be realized by a dedicated circuit or a general-purpose processor. Further, when an integrated circuit technology that replaces an LSI appears due to advances in semiconductor technology, an integrated circuit based on this technology may be used.

1 ... image coding device, 2 ... image decoding device, 101 ... subtraction unit, 102 ... orthogonal conversion unit, 103 ... quantization unit, 104 ... variable length coding unit, 105 ... inverse quantization unit, 106 ... inverse orthogonal conversion Unit, 107 ... Addition unit, 108 ... Loop filter, 109 ... Reference memory, 110 ... Intra prediction unit, 111 ... Inter prediction unit, 112 ... Selection unit, 201 ... Variable length decoding unit, 202 ... Inverse quantization unit, 203 ... Inverse orthogonal conversion unit, 204 ... Addition unit, 205 ... Loop filter, 206 ... Reference memory, 207 ... Intra prediction unit, 208 ... Inter prediction unit, 1100 ... Intra predictor, 1101 ... Intra prediction image generation unit, 1102 ... Subtraction unit , 1103 ... MPM determination unit, 1104 ... Error index acquisition unit, 1105 ... MPM ranking determination unit, 1106 ... Intra prediction mode information generation unit, 1107 ... Candidate determination unit, 2070 ... Intra predictor, 2071 ... MPM determination unit, 2072 ... Error index acquisition unit, 2073 ... MPM ranking determination unit, 2074 ... Intra prediction mode information generation unit, 2075 ... Candidate judgment unit, 2076 ... Intra prediction image generation unit

Claims (9)

An error index acquisition unit that acquires an error index between the predicted image of the reference block obtained by using the intra prediction mode defined for the reference block around the target block and the input image of the reference block .
When the intra prediction mode defined for the reference block is used as the intra prediction mode candidate of the target block, a ranking determination unit for determining the rank of the intra prediction mode candidate in ascending order of the error index is provided .
The ranking determination unit lowers the rank of the intra prediction mode of the reference block that gives the error index larger than the threshold value of the predetermined error index as the intra prediction mode candidate to the rank of the predetermined intra prediction mode candidate. An intra-predictor that features that . The intra prediction according to claim 1 , wherein the error index acquisition unit variably sets a threshold value of the error index to a value within the range of the error index related to the intra prediction mode defined for each of the reference blocks. vessel. The first or claim is characterized in that the error index acquisition unit acquires the number of bits of a bit string obtained by variable-length coding the quantization conversion coefficient of the prediction residual based on the prediction image as the error index. 2. The intra-predictor according to 2 . Claim 1 or claim 1, wherein the error index acquisition unit acquires, as the error index, the number of non-zero conversion coefficients having a value other than zero among the quantization conversion coefficients of the prediction residual based on the prediction image. The intra-predictor according to claim 2 . The intra-predictor according to claim 1 or 2 , wherein the error index acquisition unit acquires the sum of squares of the quantization conversion coefficients of the prediction residual based on the prediction image as the error index. The intra-predictor according to claim 1 or 2 , wherein the error index acquisition unit acquires the sum of the absolute values of the quantization conversion coefficients of the prediction residuals based on the prediction image as the error index. An error index acquisition unit that acquires an error index between the predicted image of the reference block obtained by using the intra prediction mode defined for the reference block around the target block and the input image of the reference block, and the target block. When the intra prediction mode defined for the reference block is used as the intra prediction mode candidate, the intra predictor including the rank determination unit for determining the rank of the intra prediction mode candidate in the ascending order of the error index, or claim 1. The intra-predictor according to any one of claim 6 and
One of the intra prediction modes is selected based on the amount of error between the predicted image obtained by performing intra prediction using each predetermined intra prediction mode with reference to the decoded image of the reference block and the input image of the target block. Selected,
A candidate determination unit that determines the ranking of the intra prediction mode candidates that are the same as the selected intra prediction mode,
An encoding unit that encodes the information on the ranking of the determined intra-prediction mode candidates,
An image coding device comprising. An error index acquisition unit that acquires an error index between the predicted image of the reference block obtained by using the intra prediction mode defined for the reference block around the target block and the input image of the reference block, and the target block. When the intra prediction mode defined for the reference block is used as the intra prediction mode candidate, the intra predictor including the rank determination unit for determining the rank of the intra prediction mode candidate in the ascending order of the error index, or claim 1. The intra-predictor according to any one of claim 6 and
It is equipped with a decoding unit that decodes the ranking information of the intra prediction mode candidates .
Image decoding apparatus and generates a decoded image of the current block using the predictive image of the reference block obtained by using the intra-prediction mode candidate same intra prediction mode and the decoded order. An error index acquisition unit that acquires an error index between the predicted image of the reference block and the input image of the reference block obtained by using the intra prediction mode defined for the reference block around the target block on the computer.
When the intra prediction mode defined for the reference block is used as the intra prediction mode candidate of the target block, a ranking determination unit for determining the rank of the intra prediction mode candidate in ascending order of the error index is provided .
The ranking determination unit lowers the rank of the intra prediction mode of the reference block that gives the error index larger than the threshold value of the predetermined error index as the intra prediction mode candidate to the rank of the predetermined intra prediction mode candidate. it, program for functioning as an intra predictor according to claim.

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