JP2013232960A - Image decoder, image decoding method, and image decoding program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image decoding technique that further improves encoding efficiency by reducing a generated code amount of an intra-prediction mode.SOLUTION: An image decoder comprises: an intra-frame prediction mode storage unit; a prediction direction difference derivation unit that derives a prediction direction difference indicating a degree of difference between prediction directions of intra-frame prediction modes; a preferential prediction mode list creation unit that creates a list of preferential prediction modes to be candidates for an intra-frame prediction mode of a decoding object block on the basis of the intra-frame prediction modes of a plurality of reference blocks used for derivation of the prediction direction difference and the prediction direction difference; a preferential prediction mode determination flag decoding unit; a preferential prediction mode decoding unit; and a non-preferential prediction mode decoding unit. The preferential prediction mode list creation unit sets, as a preferential prediction mode, at least one of an intra-frame prediction mode having a prediction direction adjacent to the prediction directions of the intra-frame prediction modes of the reference blocks and an average value mode in addition to the same intra-frame prediction modes as the intra-frame prediction modes of the reference blocks.

Description

  The present invention relates to an image decoding technique, and more particularly to an intra-screen decoding technique.

MPEG-4 AVC, which is an international standard for moving picture coding, employs a method called intra prediction as a method of intra-screen coding that completes processing within one screen. Intra prediction is to create a predicted image of a processing target block by duplicating a decoded sample value adjacent to the processing target block in a designated prediction direction. MPEG-4 AV
In C, nine types of prediction directions shown in FIGS. 1A and 1B are defined, and an appropriate prediction direction is designated by transmitting the mode number of the intra prediction mode indicating the prediction direction in each block. Take the configuration.

The predicted image quality can be improved by extending the number of defined prediction directions. FIG.
The symbol 201 in FIG. 2 shows definition examples of 17 types of prediction directions, and the code 202 in FIG. 2B shows definition examples of 34 types of prediction directions. However, an increase in the number of definitions in the prediction direction leads to an increase in the amount of transmission information in the intra prediction mode. As the number of prediction direction definitions increases, the proportion of the intra prediction mode in the total amount of generated codes increases, so the need for an efficient transmission method increases.

Patent Document 1 describes means for reducing the code amount of the intra prediction mode by reducing the total number of intra prediction modes to be transmitted. The method of Patent Literature 1 scans the intra prediction modes of a plurality of blocks for a predetermined integration unit, and when all the intra prediction modes in the integration unit are the same, one intra prediction mode for each integration unit. Is transmitted to reduce the in-screen prediction mode to be transmitted.

JP 2009-246975 A

ISO / IEC 14496-10 Information technology-Coding of audio-visual objects-Part 10: Advanced Video Coding

In general, in intra prediction, an intra prediction mode occurrence probability model that assumes that the same intra prediction mode as the intra prediction mode of the block adjacent to the encoding target block of the image is likely to be selected in the encoding target block. The intra prediction mode is encoded on the assumption of In reality, however, the intra-prediction mode generation distribution varies from block to block, so that encoding with the same probability model set for all blocks always achieves more efficient encoding of the intra-prediction mode. difficult.

Since the method of Patent Document 1 does not consider the occurrence frequency of the intra-screen prediction mode of each block, the above problem is still not solved.

The present invention has been made in view of such circumstances, and an object thereof is to provide an image decoding technique capable of reducing the amount of generated codes in the intra prediction mode and further improving the encoding efficiency.

In order to solve the above-described problem, information for specifying the intra prediction mode is decoded in block units from the encoded stream, and the image signal is decoded using the decoded information for specifying the intra prediction mode. An image decoding apparatus, wherein an intra-screen prediction mode storage unit that stores an intra-screen prediction mode of a decoded block, and intra-screen prediction modes of a plurality of reference blocks used for intra-screen prediction processing of a decoding target block A prediction direction difference deriving unit for deriving a prediction direction difference indicating the degree of difference in the prediction direction of the obtained in-screen prediction mode, and a plurality of reference blocks used for deriving the prediction direction difference. Priority prediction that creates a list of priority prediction modes that are candidates for the intra prediction mode of the decoding target block based on the intra prediction mode and the prediction direction difference A command list creation unit, a priority prediction mode determination flag decoding unit that decodes information indicating whether the intra prediction mode of the decoding target block is a priority prediction mode, and the intra prediction mode of the decoding target block is a priority prediction mode. And a priority prediction mode decoding unit that decodes information for specifying an intra-screen prediction mode of the decoding target block based on the created list, and an intra-screen prediction mode of the decoding target block is a non-priority prediction mode. A non-priority prediction mode decoding unit that decodes information for specifying an intra-screen prediction mode of the decoding target block based on the list, and the priority prediction mode list creation unit includes the reference block In addition to the same intra prediction mode as the intra prediction mode, the prediction direction of the intra prediction mode of the reference block To provide an image decoding apparatus which is characterized in that at least one priority prediction modes of the adjacent intra prediction mode and average mode with prediction direction.
In addition, in order to solve the above-described problem, the information for specifying the intra prediction mode is decoded in block units from the encoded stream, and the image signal is converted using the decoded information for specifying the intra prediction mode. An image decoding method for decoding, referring to a memory storing an intra-screen prediction mode of a decoded block, obtaining intra-screen prediction modes of a plurality of reference blocks used for intra-screen prediction processing of a decoding target block, A step of deriving a prediction direction difference indicating the degree of difference in the prediction direction of the acquired intra-screen prediction mode, and an intra-screen prediction mode of a plurality of reference blocks used for deriving the prediction direction difference and the prediction direction difference Creating a list of priority prediction modes that are candidates for the intra prediction mode of the decoding target block; and an intra prediction mode of the decoding target block. Decoding information indicating whether or not is a priority prediction mode, and when the intra prediction mode of the decoding target block is a priority prediction mode, the intra prediction mode of the decoding target block based on the created list And information for specifying the intra prediction mode of the decoding target block based on the list when the intra prediction mode of the decoding target block is a non-priority prediction mode. In the non-priority prediction mode decoding step, and in the step of creating the list of priority prediction modes, in addition to the same intra prediction mode as the intra prediction mode of the reference block, the intra prediction of the reference block Of the in-screen prediction mode and the average value mode that have a prediction direction adjacent to the mode prediction direction, To provide an image decoding method characterized by one of Kutomo priority prediction mode.

It should be noted that any combination of the above-described constituent elements and a conversion of the expression of the present invention between a method, an apparatus, a system, a recording medium, a computer program, etc. are also effective as an aspect of the present invention.

According to the present invention, it is possible to reduce the amount of generated codes in the intra prediction mode and improve the encoding efficiency.

It is a figure explaining the prediction direction of 9 intra prediction modes. It is a figure explaining the prediction direction of the intra prediction mode of 17 patterns and 34 patterns. It is a figure explaining the encoding tree for encoding 9 patterns of intra prediction modes. It is a figure explaining the encoding syntax for transmitting intra prediction mode according to the encoding tree of FIG. It is a block diagram which shows the structure of the image coding apparatus for performing the encoding method of the intra prediction mode which concerns on embodiment. It is a block diagram which shows the detailed structure of the 1st Example of the intra prediction mode encoding part of FIG. It is a flowchart explaining the intra prediction mode encoding procedure by the intra prediction mode encoding part of FIG. It is a block diagram which shows the structure of the image decoding apparatus for performing the decoding method of the intra prediction mode which concerns on embodiment. It is a block diagram which shows the detailed structure of the 1st Example of the intra prediction mode decoding part of FIG. It is a flowchart explaining the intra prediction mode decoding procedure by the intra prediction mode decoding part of FIG. It is a flowchart explaining the procedure which determines the reference intra prediction mode in embodiment. It is a flowchart explaining the procedure which calculates the prediction direction difference in embodiment. It is a figure explaining the table which matched prediction mode and prediction direction number. It is a figure explaining the table which matched the priority prediction mode and the encoding tree with the prediction direction difference. It is a figure explaining the conversion process of the prediction direction of intra prediction mode. It is a conceptual diagram explaining the priority prediction mode determined according to a prediction direction difference. It is a figure which shows the example of the encoding tree used by embodiment. It is a figure explaining the block configuration of an image, and a reference block. It is a figure explaining the relationship between a process target block and a reference block. It is a block diagram which shows the detailed structure of the 2nd Example of the intra prediction mode encoding part of FIG. It is a flowchart explaining the intra prediction mode encoding procedure by the intra prediction mode encoding part of FIG. It is a block diagram which shows the detailed structure of the 2nd Example of the intra prediction mode decoding part of FIG. It is a flowchart explaining the intra prediction mode decoding procedure by the intra prediction mode decoding part of FIG. It is a flowchart explaining the operation | movement which decodes the priority prediction mode in a 2nd Example. It is a flowchart explaining the operation | movement which calculates the priority prediction mode determination flag and priority prediction mode index in a 2nd Example. It is a flowchart explaining the operation | movement which calculates the non-priority prediction mode index in a 2nd Example. It is a flowchart explaining the operation | movement which encodes the non-priority prediction mode index in a 2nd Example. It is a flowchart explaining the operation | movement which decodes the non-priority prediction mode index in a 2nd Example. It is a flowchart explaining the operation | movement which calculates the object prediction mode in a 2nd Example. It is a flowchart explaining the operation | movement which calculates the prediction direction difference in a 2nd Example. It is a flowchart explaining the operation | movement which produces the priority prediction mode list | wrist in a 2nd Example. It is a figure explaining the correspondence of the prediction direction difference of a 2nd Example, and a priority prediction mode list | wrist. It is a flowchart explaining the operation | movement which encodes the priority prediction mode in a 2nd Example. It is a figure for demonstrating the encoding tree / decoding tree for encoding / decoding the priority prediction mode in a 2nd Example.

  First, a technique that is a premise of the embodiment of the present invention will be described.

In the following description, “processing target block” refers to an encoding target block in the case of encoding processing by an image encoding device, and refers to a decoding target block in the case of decoding processing by an image decoding device. is there. “Processed block” refers to a decoded block that has been encoded in the case of encoding processing by the image encoding device, and a decoded block in the case of decoding processing by the image decoding device. is there. Hereinafter, unless otherwise noted, this meaning is used.

[Encoding tree]
FIG. 3 is a diagram illustrating a coding tree for coding the nine patterns of intra prediction modes in FIG. The transmission method in the intra prediction mode in MPEG-4 AVC is shown in FIG.
). In the figure, an internal node (circle) is assigned a code, and a leaf (square) is assigned an intra prediction mode number. Of the leaves, reference numeral 302 denotes a priority prediction mode. The priority prediction mode will be described later. For example, “1” is assigned to the priority prediction mode, and code “0111” is assigned to mode 7.

FIG. 4 is a diagram illustrating an encoding syntax for transmitting an intra prediction mode according to the encoding tree of FIG. The prev_intra_pred_flag shown in FIGS. 4A and 4B is a syntax element that specifies whether the mode is the priority prediction mode, rem_intra_pred_mode.
Is a syntax element that represents a mode number. When decoding, first 1-bit prev_intra_
read pred_flag from the encoded sequence, prev_intra_pred_fl
When ag is 1, the intra prediction mode is set as the priority prediction mode, and the process proceeds to the next syntax.
Otherwise, 3-bit prev_intra_pred_flag is read, and the intra prediction mode is set as the prediction mode indicated by rem_intra_pred_mode.

In order to encode the 17 patterns of intra prediction modes in FIG. 2, reference numeral 3 in FIG.
A similar transmission method can be used according to the coding tree indicated by 03.

[Priority prediction mode]
In order to determine the priority prediction mode, an already processed adjacent block adjacent to the processing target block is referred to. The already processed adjacent block is adjacent to the left side of the processing target block and located on the uppermost side (referred to as “reference block A”), and adjacent to the upper side of the processing target block and positioned on the leftmost side (see “Reference”). It is referred to as “Block B”.

An example of the processed adjacent reference block will be described with reference to FIG. All blocks (reference numerals 1802 to 1811) spatially located on the upper side and / or left side of the processing target block 1801 in the figure are already processed, and blocks that are not (reference numerals 1812 to 1815).
Is unprocessed. The block adjacent to the left side of the processing target block 1801 is the block 180.
7 and block 1809, of which block 1807 located on the upper side is referred to as reference block A. The block adjacent to the upper side of the processing target block 1801 is only the block 1803, and this block 1803 is referred to as a reference block B.

Each of the intra prediction mode numbers of the reference block A and the reference block B is modeIdx.
When A, modeIdxB, the index m of the priority prediction mode of the processing target block
pmIdx is represented by the following formula.
mpmIdx = min (modeIdxA, modeIdxB)
That is, the priority prediction mode matches one of the intra prediction modes of the reference block.

[Relationship between priority prediction mode and coding tree]
The coding tree in FIG. 3 is such that a 1-bit code is assigned to the priority prediction mode, and a 1 + 3 = 4 bit code is uniformly assigned to the other modes, and follows the following probability model.
p (mpm) ≧ 0.5, where mpm represents the priority mode.
p (m) = 0.0625 = (1−p (mpm)) / 8, where m ≠ mpm

However, taking statistics of the frequency of occurrence of the actual intra prediction mode, the occurrence probability of the priority prediction mode is an average p (mpm) = 0.2, and the coding tree in FIG. It cannot be said that it is in conformity with.

The expected value bits _fact of the average generated code amount according to the statistical probability of occurrence of the priority prediction mode when the coding tree of FIG. 3 is used is bits _fact = 3.4 (= 0.2 × 1 + 0.8 × 4).
) (Bit). On the other hand, when the expected value bits _opt of the average generated code amount when an optimal coding tree is designed on the assumption that p (mpm) = 0.2, bits _opt = 3.2 (
3), it can be seen from the estimation of the average generated code amount that the coding tree in FIG. 3 is not optimal.

The above method sets a probability model based on the global occurrence probability of an intra prediction mode that matches the intra prediction mode of an adjacent block. Actually, each block has a different distribution of intra prediction modes, and an appropriate probability model can be set for each block. However, in the above method, since the same probability model is always set for all blocks, it cannot be said that efficient coding is performed.

If only setting an appropriate probability model is considered, for example, it is possible to acquire intra prediction modes of all reference blocks and set different appropriate probability models for all combinations thereof. . However, since the number of combinations of a plurality of prediction modes increases exponentially with respect to the number of prediction modes, such a method is not a realistic solution from the viewpoint of complicated processing.

By the way, intra prediction replicates a decoded sample value in a designated prediction direction, and all frequency components along the prediction direction are lost, while frequency components in a direction orthogonal to the prediction direction are preserved. Therefore, for an image having a characteristic frequency component, it is appropriate to select an intra prediction mode that can reflect the frequency component as much as possible. In other words, it can be said that there is a correlation between the image and the intra prediction mode, and the following properties can be used when selecting the intra prediction mode.

(1) Intra prediction with a close prediction direction derives a similar prediction image. When the target image is accompanied by a minute change, the intra prediction mode that expresses the minute change is appropriate, and the probability of occurrence of the intra prediction mode that is close in direction is high.

(2) When the prediction directions of intra prediction of adjacent blocks are close to each other, it is considered that the image correlation between adjacent blocks is high. At that time, the image of the processing target block is likely to have a high correlation with the adjacent block, and the prediction direction of the intra prediction of the processing target block is particularly likely to be concentrated in the peripheral direction of the prediction direction of the adjacent block.

(3) When the prediction directions of intra prediction of adjacent blocks are far from each other, it is considered that the image correlation between adjacent blocks is low. At that time, the correlation between the image of the processing target block and the adjacent block tends to be lower than the case where the prediction directions of the intra prediction of the adjacent block are close to each other, and the prediction direction of the intra prediction of the processing target block is It becomes difficult to concentrate in the peripheral direction of the predicted direction.

In this embodiment, in view of this property, by preparing a plurality of coding trees according to the probability model and switching the probability model according to the prediction direction difference of the intra prediction mode of the reference block, the coding tree is obtained. In addition to limiting the complexity of the encoding process accompanying the increase, the amount of generated code in the intra prediction mode can be reduced, and the encoding efficiency can be improved.

[Encoding device]
A preferred image encoding apparatus for carrying out the present invention will be described with reference to the drawings. FIG. 5 is a block diagram showing a configuration of the image coding apparatus according to the embodiment. The image coding apparatus according to the embodiment includes a subtracting unit 501, an orthogonal transform / quantization unit 502, an inverse quantization / inverse transform unit 503, an adder 504, a decoded image memory 505, and an intra prediction unit 506. , Texture information encoding unit 507, intra prediction mode encoding unit 508, and intra prediction mode selection unit 509.
With. Since the embodiment of the present invention focuses on intra-screen prediction, components related to inter-screen prediction are not shown and description thereof is omitted.

The intra prediction mode selection unit 509 selects an optimal intra prediction mode for each block of the image, and provides the selected intra prediction mode to the intra prediction unit 506 and the intra prediction mode encoding unit 508.

The intra prediction mode encoding unit 508 performs variable length encoding on the input intra prediction mode, and outputs an intra prediction mode bitstream. Intra prediction mode encoding unit 50
The detailed configuration and operation of 8 will be described later.

The intra prediction unit 506 generates an intra prediction image using the input intra prediction mode and the decoded image of the adjacent block stored in the decoded image memory 505, and provides the generated intra prediction image to the subtraction unit 501.

The subtraction unit 501 generates a difference image by subtracting the intra prediction image from the original image to be encoded, and provides the generated difference signal to the orthogonal transform / quantization unit 502.

The orthogonal transform / quantization unit 502 generates texture information by performing orthogonal transform / quantization on the difference image, and supplies the generated texture information to the inverse quantization / inverse transform unit 503 and the texture information encoding unit 507.

The texture information encoding unit 507 performs entropy encoding on the texture information and outputs a texture information bit stream.

The inverse quantization / inverse transform unit 503 generates a decoded differential signal by performing inverse quantization / inverse orthogonal transform on the texture information received from the orthogonal transform / quantization unit 502, and adds the generated decoded differential signal to the adder 504. To give.

The adding unit 504 generates a decoded image by adding the intra-predicted image and the decoded difference signal, and stores the generated decoded image in the decoded image memory 505.

[Decoding device]
A preferred image decoding apparatus for implementing the present invention will be described with reference to the drawings. FIG. 8 is a block diagram showing the configuration of the image decoding apparatus according to the embodiment. The image decoding apparatus according to the embodiment
A texture information decoding unit 801, an inverse quantization / inverse transform unit 802, an intra prediction mode decoding unit 803, an adding unit 804, a decoded image memory 805, and an intra prediction unit 806 are provided. Since the embodiment of the present invention focuses on intra-screen prediction, constituent elements related to inter-screen prediction are not shown and description thereof is omitted.

The decoding process of the image decoding apparatus in FIG. 8 corresponds to the decoding process provided in the image encoding apparatus in FIG. 5, so the inverse quantization / inverse conversion unit 802 and the addition unit 804 in FIG. 8. Each configuration of the decoded image memory 805 and the intra prediction unit 806 includes each of the inverse quantization / inverse transform unit 503, the addition unit 504, the decoded image memory 505, and the intra prediction unit 506 of the image encoding device in FIG. And corresponding functions.

The intra prediction mode decoding unit 803 entropy decodes the input intra prediction mode bitstream to generate an intra prediction mode, and provides the generated intra prediction mode to the intra prediction unit 806. The detailed configuration and operation of the intra prediction mode decoding unit 803 will be described later.

The intra prediction unit 806 generates an intra prediction image using the input intra prediction mode and the decoded image of the adjacent block stored in the decoded image memory 805, and provides the generated intra prediction image to the addition unit 804.

The texture information decoding unit 801 generates texture information by entropy decoding the texture information. The generated texture information is given to the inverse quantization / inverse transform unit 802.

The inverse quantization / inverse transform unit 802 performs inverse quantization / inverse orthogonal transform on the texture information received from the texture information decoding unit 801 to generate a decoded differential signal, and provides the generated decoded differential signal to the adding unit 804 .

The addition unit 804 generates a decoded image by adding the intra predicted image and the decoded differential signal, stores the generated decoded image in the decoded image memory 805, and outputs the decoded image.

The intra prediction mode encoding and decoding processing according to the embodiment of the present invention is performed by the intra prediction mode encoding unit 508 of the video encoding device in FIG. 5 and the intra prediction mode decoding unit 803 of the video decoding device in FIG. To be implemented. Hereinafter, details of the intra prediction mode encoding and decoding processing according to the embodiment will be described.

[Encoding block]
In the embodiment, as shown in FIG. 18, the screen is hierarchically divided into rectangular blocks, and each block is sequentially processed in a predetermined processing order. Each block to be divided is called a coding block. A block 1817 in FIG. 18 is a maximum unit of division in the embodiment, and this is called a maximum encoding block. A block 1816 in FIG. 18 is a minimum unit of division in the embodiment, and this is called a minimum coding block. In the following description, the minimum encoding block is 4 × 4 pixels and the maximum encoding block is 16 × 16 pixels.

[Predicted block]
Of the encoded blocks, a unit for performing intra prediction is called a prediction block. The prediction block has a size of not less than the minimum coding block and not more than the maximum coding block. FIG.
Now blocks 1802, 1803 and 1804 are 16 × 16 blocks, block 18
05, 1810, 1811, and 1801 are 8 × 8 blocks, blocks 1806, 18
07, 1808, and 1809 are 4 × 4 blocks. Blocks 1812, 1813, 18
14 and 1815 are unprocessed blocks, and the encoding block size is not fixed. In the encoding procedure, an optimal prediction block size is determined and the prediction block size is transmitted.
In the decoding procedure, the predicted block size is obtained from the bit stream. Hereinafter, the prediction block is described as a processing unit.

[Predicted block size and intra prediction mode]
The intra prediction mode configuration is switched according to the size of the prediction block. The 4 × 4 block defines 17 patterns of intra prediction modes indicated by reference numeral 201 in FIG.
For the × 8 block and the 16 × 16 block, 34 patterns of intra prediction modes indicated by reference numeral 202 in FIG. 2B are defined. This is because, even if an intra prediction mode having an excessive number of patterns is defined for a prediction block of a small size, quality improvement sufficient for an increase in the amount of generated codes cannot be obtained.

[Reference block and reference intra prediction mode]
The reference block is a block A that is adjacent to the left side of the processing target block and located on the uppermost side, and a block B that is adjacent to the upper side of the processing target block and is positioned on the leftmost side. The prediction mode of block A is refModeA, and the prediction mode of block B is refModeB. The intra prediction mode of each reference block is referred to as “reference intra prediction mode”. The reference intra prediction mode when no reference block exists is set to a DC prediction mode (also referred to as “average value mode”).

(First embodiment)
[Encoding procedure]
The 1st Example of the encoding method of the intra prediction mode which concerns on embodiment of this invention is demonstrated. FIG. 6 is a block diagram of a detailed configuration of the first exemplary embodiment of the intra prediction mode encoding unit 508 of FIG. The intra prediction mode encoding unit 508 according to the first embodiment includes an intra prediction mode memory 601, a reference mode determination unit 602, a priority mode determination unit 603, and a prediction direction difference calculation unit 6.
04, a coding tree selection unit 605, and a variable length coding unit 606. Hereinafter, the encoding procedure in the intra prediction mode will be described with reference to the flowchart of FIG.

The intra prediction mode memory 601 stores and stores the intra prediction mode input from the intra prediction mode selection unit 509.

The reference mode determination unit 602 acquires the intra prediction mode of the encoded reference block adjacent to the encoding target block from the intra prediction mode memory 601 and determines the reference intra prediction mode (step S701). Details of the reference intra prediction mode determination procedure will be described later.

The prediction direction difference calculation unit 604 acquires the reference intra prediction mode from the reference mode determination unit 602, and calculates the prediction direction difference (step S702). Details of the prediction direction difference calculation procedure will be described later.

The priority mode determination unit 603 acquires the prediction direction difference from the prediction direction difference calculation unit 604, and acquires the reference intra prediction mode from the reference mode determination unit 602. Priority mode determination unit 603
Determines the priority mode based on the obtained prediction direction difference and the reference intra prediction mode (step S703). Details of the priority mode determination procedure will be described later.

The coding tree selection unit 605 acquires the prediction direction difference from the prediction direction difference calculation unit 604, and determines a coding tree according to the prediction direction difference (step S704). Details of the coding tree selection procedure will be described later.

The variable length coding unit 606 receives the intra prediction mode input of the current block, obtains the priority mode from the priority mode determination unit 603, and obtains the coding tree from the coding tree selection unit 605. The variable length coding unit 606 performs variable length coding on the intra prediction mode of the block to be coded using the priority mode and the coding tree (step S705). Variable length encoding unit 6
In 06, the generated bit sequence is output, and the series of intra prediction mode encoding processing ends.

[Reference Intra Prediction Mode Determination Procedure]
The details of the reference intra prediction mode determination procedure in step S701 in FIG. 7 will be described with reference to the flowchart in FIG.

The reference mode determination unit 602 in FIG. 6 acquires the intra prediction mode of the reference block adjacent to the coding target block from the intra prediction mode memory 601.

The number of patterns of the intra prediction mode of the reference block A adjacent to the left of the encoding target block and the intra prediction mode of the encoding target block are compared (step S1101).

When the number of intra prediction mode patterns of the encoding target block is equal to or greater than the number of intra prediction mode patterns of the reference block A, the intra prediction mode of the reference block A is set to “reference mode A” as it is (step S1102).

When the number of intra prediction mode patterns of the block to be encoded is smaller than the number of intra prediction mode patterns of the reference block A, reference mode conversion is performed (step S1103).
. The conversion of the reference mode will be described later.

The number of patterns of the intra prediction mode of the reference block B adjacent on the encoding target block and the intra prediction mode of the encoding target block are compared (step S1104).

When the number of intra prediction mode patterns of the block to be encoded is equal to or greater than the number of intra prediction mode patterns of the reference block B, the intra prediction mode of the reference block B is set to “reference mode B” as it is (step S1105).

When the number of intra prediction mode patterns of the encoding target block is smaller than the number of intra prediction mode patterns of the reference block B, reference mode conversion is performed (step S1106).
.

Reference mode conversion will be described. As shown in FIG. 19, the reference mode conversion is performed in comparison with the encoding target block 1901 in the reference block A 1902 and the reference block B.
This is a case where the size of 1901 is large. Here, the encoding target block 1901 is a 4 × 4 pixel block, and the reference block A 1902 and the reference block B 1903 are 8 × 8 pixel blocks. The encoding target block 1901 is indicated by 17 shown by reference numeral 201 in FIG.
A pattern intra prediction mode is defined. For the reference block A 1902 and the reference block B 1903, 34 patterns of intra prediction modes indicated by reference numeral 202 in FIG. 2B are defined.

In this way, when the number of intra prediction mode patterns of the encoding target block is smaller than the number of patterns of the intra prediction mode of the reference block, the intra prediction mode of the reference block is encoded when the intra prediction mode of the encoding target block is encoded. It is necessary to degenerate the pattern into an intra prediction mode pattern of the encoding target block in some form. The reference mode conversion is a process of converting an intra prediction mode of a reference block having a large number of patterns into an intra prediction mode of an encoding target block having a small number of patterns.

With reference to FIG. 15, the conversion process of intra prediction mode is demonstrated. Reference numeral 1501 in FIG.
Shows the prediction direction of the 34 patterns of intra prediction modes as a point on the horizontal axis. Reference numeral 1502 in FIG. 15 indicates the prediction direction of the 17 patterns of intra prediction modes as a point on the horizontal axis. The position of the point indicated by the broken line 1502 in FIG. 15 is a prediction direction that is included in the 34 pattern intra prediction mode but not included in the 17 pattern intra prediction mode.

In the 34 patterns of intra prediction modes indicated by reference numeral 1501, it is assumed that the prediction direction of reference mode A is the position indicated by reference numeral 1503, and the prediction direction of reference mode B is the position indicated by reference numeral 1504. At this time, in the 17 patterns of intra prediction modes indicated by reference numeral 1502, there is no prediction mode at points corresponding to the prediction directions of the reference modes A and B (points represented by broken lines). Accordingly, in the 17 patterns of intra prediction modes, points (reference numerals 1505 and 1506) adjacent to the points corresponding to the prediction directions of the reference modes A and B are selected as the converted prediction modes instead.

[Prediction direction difference calculation procedure]
Details of the prediction direction difference calculation procedure in step S702 of FIG. 7 will be described with reference to the flowchart of FIG.

The prediction direction difference calculation unit 604 acquires the reference modes A and B from the reference mode determination unit 602,
Reference mode A and reference mode B are compared (step S1201).

When both the reference modes A and B are in the average value mode, a special value −2 indicating that both are in the average value mode is set as the prediction direction difference (step S1202).

When one of the reference modes A and B is the average value mode, a special value -1 indicating that one of the reference modes A and B is the average value mode is set (step S1203).

When both the reference modes A and B are not the average value mode, the prediction direction number is determined by referring to the table in which the prediction mode and the prediction direction number in FIG.

FIG. 13 is a table in which intra prediction mode numbers are associated with prediction direction numbers. As shown in FIG. 1 and FIG. 2, the intra prediction mode numbers are not assigned in the order of azimuth, and cannot be used for calculating the prediction direction difference as they are. Therefore, a table is prepared in which the prediction mode number is associated with the prediction direction number assigned in the direction of the direction. When calculating the prediction direction difference, the prediction direction number is used.

Prediction direction numbers D and A for the reference mode A and the reference mode B are assumed to be dirA and dirB, respectively.
IfDir is calculated (step S1204).
DiffDir = min (16−abs (dirA−dirB), abs (dirA−
dirB))
Here, abs () is a function for calculating the absolute value of the argument, and min () is a function for selecting the minimum value of the two arguments.

Next, the prediction direction difference DiffDir is compared with a predetermined upper limit value. If the prediction direction difference DiffDir exceeds the upper limit value, the prediction direction difference DiffDir is replaced with the upper limit value (step S12).
05). For example, in the case of 17 patterns of intra prediction modes, the maximum value of the prediction direction difference is 8
The upper limit value is set to 4, for example, and when the prediction direction difference exceeds 4, it is rounded to 4.

[Priority mode determination procedure]
Details of the priority mode determination procedure in step S703 of FIG. 7 will be described.

The priority mode determination unit 603 acquires the prediction direction difference from the prediction direction difference calculation unit 604, and acquires the reference mode from the reference mode determination unit 602. The priority mode determination unit 603 refers to the table in which the prediction direction difference illustrated in FIG. 14 is associated with the priority prediction mode and the coding tree, and determines the number of priority modes and the mode number of the priority mode.

FIG. 14 associates the number of priority modes, the priority mode number, and the coding tree number with the prediction direction difference. FIG. 14 shows a case where 17 patterns of intra prediction modes are used, and the prediction direction difference takes values from 0 to 8 and special values −1 and −2. As the prediction direction difference increases, the number of priority modes increases. In order to increase the number of priority modes, in addition to reference modes A and B, modes adjacent to reference modes A and B and average value modes are used as priority modes.

  The priority mode for each prediction direction difference will be described below using the example of FIG.

[When the prediction direction difference is 0]
This is a case where both the reference mode A and the reference mode B are not the average value mode and the prediction directions match. From the table shown in FIG. 14, in this case, the number of priority modes is three, and the first priority mode is the same as the reference mode A and the reference mode B, the second priority mode, and the third priority mode are the first. The prediction mode is set in the direction adjacent to the priority mode. Reference numeral 1601 in FIG.
These are conceptual diagrams of priority modes when the prediction direction difference is zero. The prediction direction of the first priority mode is point 1602, the prediction direction of the second priority mode is point 1603, and the prediction direction of the third priority mode is point 1604, which are adjacent to each other.

[When the prediction direction difference is 1]
This is a case where the prediction direction of the reference mode A and the prediction direction of the reference mode B are adjacent to each other.
From the table shown in FIG. 14, in this case, the number of priority modes is four, the first priority mode is the reference mode A, the second priority mode is the reference mode B, the third priority mode, and the fourth priority mode are the first. The prediction mode is set to a direction adjacent to the first priority mode and the second priority mode. Reference numeral 1605 in FIG. 16 is a conceptual diagram of the priority mode when the prediction direction difference is 1. The first priority mode is code 1
606, the second priority mode is represented by reference numeral 1607, the third priority mode is represented by reference numeral 1608, and the fourth priority mode is represented by reference numeral 1609.

[When the prediction direction difference is 2]
This is a case where there is another prediction direction between the prediction direction of the reference mode A and the prediction direction of the reference mode B. From the table shown in FIG. 14, in this case, the number of priority modes is five.
The priority mode is the reference mode A, the second priority mode is the reference mode B, the third priority mode is the prediction mode indicating the prediction direction sandwiched between the first priority mode and the second priority mode, the fourth priority mode, and the fifth priority mode. Are the prediction modes in the prediction direction that are adjacent to the first priority mode and the second priority mode and are not in the third priority mode, respectively. Reference numeral 1610 in FIG. 16 is a conceptual diagram of the priority mode when the prediction direction difference is 2. The first priority mode is code 1611, the second priority mode is code 1612, the third priority mode is code 1613, the fourth priority mode is code 1614, and the fifth priority mode is code 161.
5 are prediction directions represented by 5 respectively.

[When the prediction direction difference is 3 or more]
Since the same procedure is followed, the description when the prediction direction difference is 3 or more is omitted.

[When prediction direction difference is -2 (when both reference modes are in average value mode)]
This is a case where both the reference mode A and the reference mode B are in the average value mode. From the table shown in FIG. 14, in this case, the number of priority modes is one, and the first priority mode is the average value mode.

[When the prediction direction difference is -1 (when only one of the reference modes is the average value mode)]
This is a case where only one of the reference mode A and the reference mode B is the average value mode. FIG.
From this table, the number of priority modes is four in this case. The first priority mode is reference mode A, the second priority mode is reference mode B, the third priority mode, and the fourth priority mode are first priority. The prediction mode in the direction adjacent to the prediction direction of the prediction mode that is not the average value mode among the mode and the second priority mode is used.

[Encoding tree selection procedure]
Details of the coding tree selection procedure in step S704 of FIG. 7 will be described. Here, 17 patterns of intra prediction modes are taken as an example.

The coding tree selection unit 605 acquires the prediction direction difference from the prediction direction difference calculation unit 604. FIG.
The coding tree is selected with reference to the table shown in FIG. In the table of FIG. 14, the number of the coding tree is associated with the number of priority modes.

FIG. 17 is an example of a coding tree. Reference numeral 1701 denotes an encoding tree having an encoding tree number 0 (referred to as “encoding tree 0”), reference numeral 1702 an encoding tree having an encoding tree number 1 (referred to as “encoding tree 1”), and reference numeral 1703 an encoding tree. This is a coding tree of number 2 (referred to as “coding tree 2”). Other coding trees are omitted. Each coding tree is classified into either a leaf indicating the priority mode or a leaf indicating the prediction mode number, and the priority mode determined in the priority mode determination procedure is adaptively assigned to the leaf indicating the priority mode. . On the other hand, for the leaf indicating the prediction mode number, the prediction mode number of the prediction mode to be processed is fixedly assigned (after conversion excluding the priority mode). In any coding tree, a code having a shorter code length than the other prediction modes is assigned to the priority mode. Therefore, if the intra prediction mode of the block to be encoded corresponds to any priority mode, the generated code amount is Get smaller.

  Hereinafter, the selection of a coding tree according to the prediction direction difference will be described with reference to FIG.

[When the prediction direction difference is 0]
Referring to FIG. 14, coding tree 0 is selected. In the coding tree 0, three leaves are set as the priority mode. From the priority mode determination procedure, the first priority mode is the same mode as the reference mode A and the reference mode B, the second priority mode, and the third priority mode are prediction modes in the prediction direction adjacent to the first priority mode, respectively.

[When the prediction direction difference is 1]
Referring to FIG. 14, the coding tree 1 is selected. In the coding tree 1, four leaves are set as the priority mode. From the priority mode determination procedure, the first priority mode is the reference mode A, the second priority mode is the reference mode B, the third priority mode, the fourth priority mode is the first priority mode, and the second priority mode is an adjacent prediction mode. It is.

[When the prediction direction difference is 2]
Referring to FIG. 14, the coding tree 2 is selected. In the coding tree 2, five leaves are set as the priority mode. From the priority mode determination procedure, the first priority mode is the reference mode A, the second priority mode is the reference mode B, the third priority mode is the prediction mode indicating the prediction direction between the first priority mode and the second priority mode, The 4-priority mode and the fifth-priority mode are prediction modes in a prediction direction that are adjacent to the first-priority mode and the second-priority mode and are not the third-priority mode, respectively.

[When the prediction direction difference is 3 or more]
The description is omitted to follow the same procedure.

[When the prediction direction difference is -2]
Referring to FIG. 14, the coding tree 9 is selected.

[When the prediction direction difference is -1]
Referring to FIG. 14, the coding tree 10 is selected.

As described above, in the image coding apparatus according to the present embodiment, in addition to the intra prediction mode of the reference block, the prediction mode indicating the direction adjacent to the prediction direction of the intra prediction mode of the reference block is set to the priority prediction mode. Used as A plurality of coding trees having different numbers of priority prediction modes are prepared in advance, and the coding trees are switched based on the difference in the prediction direction of the prediction mode of the reference block in the screen. The encoding efficiency of the prediction mode can be improved.

A coding tree having a smaller number of priority prediction modes is selected as the prediction direction difference is smaller, and a block to be coded using a coding tree in which a code string having a shorter code length than the other prediction modes is assigned to the priority prediction mode. Intra prediction mode is encoded.

When a plurality of reference blocks indicate prediction directions close to each other, the image correlation between the reference blocks is high, and there is a high possibility that the image correlation between the processing target block and the reference block is high. At that time, the prediction direction of the processing target block tends to concentrate particularly in the vicinity of the prediction direction of the reference block. on the other hand,
When the prediction directions indicated by the plurality of reference blocks are separated from each other, the image correlation between the reference blocks is low, and there is a high possibility that the image correlation between the processing target block and the reference block is low. At that time, the degree of concentration of the prediction direction of the processing target block in the vicinity of the prediction direction of the reference block is lower than that in the case where a plurality of reference blocks indicate the near prediction direction.

In the intra prediction mode encoding process according to the present embodiment, the prediction mode occurrence probability of each processing target block is determined by adopting a configuration in which the encoding tree is switched based on the prediction direction difference between the prediction modes of a plurality of reference blocks. It is possible to accurately estimate and reduce the amount of generated codes in the intra prediction mode.

[Decryption procedure]
A first example of the decoding method in the intra prediction mode according to the embodiment of the present invention will be described.
FIG. 9 is a block diagram of a detailed configuration of the first exemplary embodiment of the intra prediction mode decoding unit 803 in FIG. The intra prediction mode decoding unit 803 according to the first embodiment includes an intra prediction mode memory 901, a reference mode determination unit 902, a priority mode determination unit 903, a prediction direction difference calculation unit 904,
A decoding tree selection unit 905 and a variable length decoding unit 906 are provided.

The intra prediction mode decoding process in the intra prediction mode decoding unit 803 of FIG.
9 corresponds to the intra-prediction mode encoding process in the intra-prediction mode encoding unit 508, so that the intra-prediction mode memory 901, the reference mode determination unit 902, the priority mode determination unit 903, and the prediction direction difference calculation unit 904 in FIG. , And the decoding tree selection unit 905 are:
Intra prediction mode memory 601, reference mode determination unit 602, priority mode determination unit 6 in FIG. 6
03, the prediction direction difference calculation unit 604, and the coding tree selection unit 605 have functions corresponding respectively.

Hereinafter, the decoding procedure in the intra prediction mode will be described with reference to the flowchart of FIG.

The reference mode determination unit 902 acquires the intra prediction mode of the decoded reference block adjacent to the decoding target block from the intra prediction mode memory 901, and determines the reference intra prediction mode (step S1001). The reference intra prediction mode determination procedure follows the procedure shown in the flowchart of FIG. 11 in the same manner as the reference intra prediction mode determination procedure in the reference mode determination unit 602 of FIG.

The prediction direction difference calculation unit 904 acquires the reference intra prediction mode from the reference mode determination unit 902, and calculates the prediction direction difference (step S1002). Since the prediction direction difference calculation procedure follows the procedure shown in the flowchart of FIG. 12 in the same manner as the prediction direction calculation procedure in the prediction direction difference calculation unit 604 of FIG. 6, detailed description thereof is omitted.

The priority mode determination unit 903 acquires the prediction direction difference from the prediction direction difference calculation unit 904, and acquires the reference intra prediction mode from the reference mode determination unit 902. Priority mode determination unit 903
Determines the priority mode based on the obtained prediction direction difference and the reference intra prediction mode (step S1003). The priority mode determination procedure follows the same procedure as the priority mode determination procedure in the priority mode determination unit 603 in FIG.

The decoding tree selection unit 905 acquires the prediction direction difference from the prediction direction difference calculation unit 904, and determines a decoding tree according to the prediction direction difference (step S1004). The decoding tree selection procedure follows the same procedure as the coding tree selection procedure in the coding tree selection unit 605 in FIG. However, “encoding tree” is read as “decoding tree”.

The variable length decoding unit 906 receives an intra prediction mode bitstream, acquires a priority mode from the priority mode determination unit 903, and acquires a decoding tree from the decoding tree selection unit 905. The variable length decoding unit 906 performs variable length decoding using the priority mode and the decoding tree for the intra prediction mode of the decoding target block (step S1005). The variable length decoding unit 906 stores the decoded intra prediction mode in the intra prediction mode memory 901 and outputs it to the outside, and ends the series of intra prediction mode decoding processes.

(Second embodiment)
[Encoding procedure]
A second example of the intra prediction mode encoding method according to the embodiment of the present invention will be described. FIG. 20 is a block diagram of a detailed configuration of the second example of the intra prediction mode encoding unit 508 of FIG. The intra prediction mode encoding unit 508 according to the second embodiment includes an intra prediction mode memory 2601, a priority prediction mode list creation unit 2602, a priority prediction mode determination flag calculation unit 2603, a priority prediction mode determination flag encoding unit 2604, and priority prediction. Mode index calculation unit 2605, priority prediction mode index encoding unit 2606, non-priority prediction mode index calculation unit 2607, non-priority prediction mode index encoding unit 2608
, A priority prediction mode determination unit 2609, and a prediction direction difference calculation unit 2610. Hereinafter, FIG.
The encoding procedure in the intra prediction mode will be described with reference to the flowchart of FIG.

The prediction direction difference calculation unit 2610 acquires the intra prediction modes refModeA and refModeB of the adjacent blocks from the intra prediction mode memory 2601 and calculates the prediction direction difference diffDir between refModeA and refModeB (step S2701). Details of the prediction direction difference calculation procedure will be described later.

The priority prediction mode list creation unit 2602 receives the prediction direction difference d from the prediction direction difference calculation unit 2610.
IfDir is acquired, and the intra prediction modes refModeA and refModeB of the adjacent blocks are acquired from the intra prediction mode memory 2601. The priority prediction mode list creation unit 2602 creates a priority prediction mode list mpMlist based on diffDir, refModeA, and refModeB, and a priority prediction mode list size mpmlists.
size is determined (step S2702). Details of the procedure for creating the priority prediction mode list will be described later. Further, the target intra prediction mode is stored in the intra prediction mode memory 2601.

The priority prediction mode determination flag calculation unit 2603 acquires the target prediction mode and the priority prediction mode list mpmList, and calculates a priority prediction mode determination flag ppmFlag. Also,
The priority prediction mode index calculation unit 2605 generates a priority prediction mode index mpmIn.
Dex is calculated (step S2703), and the priority prediction mode determination flag encoding unit 2604 encodes the priority prediction mode determination flag ppmFlag (step S2704). Details of the priority prediction mode determination flag and the priority prediction mode index calculation procedure will be described later.

The priority prediction mode determination unit 2609 determines a priority prediction mode determination flag ppmFlag (step S2705).

When the priority prediction mode determination flag mpmFlag is true, the priority prediction mode index encoding unit 2606 encodes the priority prediction mode index mpmIndex (step S2706) and ends the process. Details of the priority prediction mode index encoding procedure will be described later.

When the priority prediction mode determination flag mpmFlag is false, the non-priority prediction mode index calculation unit 2607 selects the non-priority prediction mode index remModeInd.
ex is calculated (step S2707), and the non-priority prediction mode index encoding unit 2608 is calculated.
Performs encoding of the calculated non-priority prediction mode remModeIndex (step S2).
708). Details of the non-priority prediction mode index calculation procedure and the non-priority prediction mode encoding procedure will be described later.

[Prediction direction difference calculation procedure]
Details of the prediction direction difference calculation procedure in step S2701 of FIG. 21 will be described with reference to the flowchart of FIG.

The prediction direction difference calculation unit 2610 acquires the intra prediction modes refModeA and refModeB of adjacent blocks from the intra prediction mode memory 2601. refModeA
And refModeB are both determined to be 2 (average value mode) (step S3201).

When the values of refModeA and refModeB are both 2 (average value mode)
The prediction direction difference diffDir is set to −2 (step S3202). -2 is refMo
It is a special value indicating that the values of deA and refModeB are both 2 (average value mode).

At least one of the values of refModeA and refModeB is 2 (average value mode)
If not, it is further determined whether either refModeA or refModeB is 2 (average value mode) (step S3203). refModeA and refMo
When either deB is 2 (average value mode), the prediction direction difference diffDir is set to -1.
Is set (S3205). -1 is a special value indicating that one of the values of refModeA and refModeB is 2 (average value mode).

When the values of refModeA and refModeB are not both 2 (average value mode)
The difference between the prediction directions of refModeA and refModeB is calculated (step S3204).
. When the target block is a 4 × 4 block, the prediction direction number is determined by referring to the table of FIG. The prediction direction difference diffDir is calculated by the following formula, where the prediction direction number of refModeA is dirA and the prediction direction number of refModeB is dirB.
DiffDir = min (16−abs (dirA−dirB), abs (dirA−
dirB))
When the target block is 8x8, 16x16 block, as in the case of 4x4 block,
The prediction direction number is determined by this method, and the prediction direction difference diffDir is calculated by the following calculation formula.
DiffDir = min (33−abs (dirA−dirB), abs (dirA−
dirB))

[Procedure for creating a priority prediction mode list]
Details of the priority prediction mode list creation procedure in step S2702 of FIG. 21 will be described with reference to the flowchart of FIG.

The priority prediction mode list creation unit 2602 receives the prediction direction difference d from the prediction direction difference calculation unit 2610.
iffDir is acquired, intra prediction modes refModeA and refModeB of adjacent blocks are acquired from the intra prediction mode memory 2601, and the value of the prediction direction difference diffDir is determined (step S3901).

When the value of the prediction direction difference diffDir is −2, that is, refModeA and refMo
When both deBs are in the average value mode (step S3902), mpmlistsize is set to 2 as indicated by reference numeral 4001 in FIG. Furthermore, mpmList [0] is ref
ModeA and mpmList [1] are set to 0 (vertical prediction mode). mpmList
[1] is set to 0 (vertical prediction mode) because the occurrence frequency of the vertical prediction mode is high on average and, on average, the average value mode is set to only the average value mode as the priority prediction mode. The frequency of occurrence is not high.

When the value of the prediction direction difference diffDir is -1, that is, refModeA and refMo
When either one of the deBs is in the average value mode (step S3903), reference numeral 4 in FIG.
As shown in 002, mpmlistsize is set to 4. In addition, mpmList [0
] For min (refModeA, refModeB), and mpmList [1] for max (
refModeA, refModeB), mpmList [2] is left (dirMo
de), mpmList [3] is set to right (dirMode). However, dir
Mode is the prediction mode that is not the average mode of refModeA and refModeB, left (dirMode) is the prediction mode adjacent to the left direction of dirMode, and right (dirMode) is the prediction mode adjacent to the right direction of dirMode. .

When the value of the prediction direction difference diffDir is 0 (step S3904), that is, ref
When ModeA and refModeB are the same and both are in the average value mode, as shown by reference numeral 4003 in FIG. In addition, mpm
List [0] is refModeA, mpmList [1] is left (refMode
A), mpMlist [2] is set to right (refModeA). However, ref
t (refModeA) is a prediction mode adjacent to the left direction of refModeA, right
t (refModeA) is a prediction mode adjacent to the right direction of refModeA.

When the value of the prediction direction difference diffDir is 1 (step S3905), that is, ref
When ModeA and refModeB are adjacent, as indicated by reference numeral 4004 in FIG.
Set mpmlistsize to 4. Furthermore, mpmList [0] is changed to min (refM
odeA, refModeB), mpmList [1] is set to max (refModeA, r
efModeB), mpmList [2] is left (leftMode), mpmLi
Let st [3] be right (rightMode). However, leftMode is r
Of efModeA and refModeB, the prediction mode in the left direction, rightMode is r
The right prediction mode of efModeA and refModeB is set to left (left
Mode) is a prediction mode adjacent to the left direction of leftMode, right (right)
tMode) is a prediction mode adjacent to the right mode of rightMode.

When the value of the prediction direction difference diffDir is 2 (step S3906), reference numeral 4 in FIG.
As shown in 005, mpmlistsize is set to 5. In addition, mpmList [0
] Min (refModeA, refModeB, interMode), mpmLi
st [1] is median (refModeA, refModeB, interMode
), MpmList [2] is set to max (refModeA, refModeB, inter
Mode), mpmList [3] to left (leftMode), mpmList [
4] is set to right (rightMode). However, interMode is ref
Prediction mode sandwiched between Mode A and refMode B, median (refMode
eA, refModeB, interMode) is refModeA, refModeB
, InterMode intermediate value, leftMode is refModeA and refMode
B left prediction mode, rightMode is refModeA and refMode
B is set to the right prediction mode, and left (leftMode) is leftMode
Prediction mode, right (rightMode) adjacent to the left direction, is rightMo
The prediction mode is adjacent to the right of de.

When none of the above conditions is satisfied (step S3907), that is, when the value of the prediction direction difference diffDir is 3 or more, the block size of the target block is referred to in addition to the prediction direction difference diffDir.

When the block size is 4 × 4, as shown by reference numeral 4006 in FIG.
Let tsize be 5. Further, mpmList [0] is changed to min (refModeA, r
efModeB), mpmList [1] is set to max (refModeA, refMode).
B), mpmList [2] is changed to min (left (refModeA), right (r
efModeA)), mpmList [3] to min (left (refModeB),
right (refModeB)) and mpmList [4] are set to the average value mode, m
Sort pmList in ascending order.

When the block size is not 4 × 4, as shown by reference numeral 4007 in FIG.
Let issize be 7. Further, mpmList [0] is changed to min (refModeA
, RefModeB), mpmList [1] max (refModeA, refMo
deB), mpmList [2] to left (refModeA), mpmList [3
] To right (refModeA) and mpmList [4] to left (refMode)
eB), mpmList [5] to right (refModeB), mpmList [6
] In the average value mode, mpmList is sorted in ascending order.

In step S3907, the reason why the average value mode is added to the priority prediction mode list in addition to the prediction mode adjacent to the reference prediction mode is as follows. Step S3907
This is when the difference in prediction direction is 3 or more. When the direction difference between refModeA and refModeB is far, it is estimated that the image correlation between the reference block A and the reference block B is low. At this time, the degree of concentration in the vicinity of refModeA and refModeB decreases compared to the case where the prediction direction differences are close, and the average value mode frequency increases on average.

[Priority prediction mode determination flag, priority prediction mode index calculation procedure]
Details of the priority prediction mode determination flag and the priority prediction mode index calculation procedure in step S2703 of FIG. 21 will be described with reference to the flowchart of FIG.

In this procedure, the process proceeds by scanning mpmlist in ascending order. The priority prediction mode determination flag calculation unit 2603 and the priority prediction mode index calculation unit 2605 include a priority prediction mode determination flag mpmFlag and a priority prediction mode index mpmIn.
dex is initialized with false and 0, respectively. Variable i for scanning mpmList
Is initialized with 0 (step S3201).

If the variable i is less than mpmListSize (step S3202), that is, if all elements of mpmList have not been scanned yet, mpmList [i] and c
The urrModeIndex is compared (step S3203). When mpmList [i] and currModeIndex are equal, it indicates that the target prediction mode is equal to the i-th element of the priority prediction mode list, and mpmFlag is set to true and mpmIndex is set to i.
(Step S3204), and the process proceeds to step S2704 in FIG. mpm
If List [i] and currModeIndex are different, i is incremented by 1 (step S3205), and scanning is continued.

In step S3202, when the variable i is greater than or equal to mpmListSize, that is, when all elements of mpmList have been scanned, the priority prediction mode determination flag and priority prediction mode index calculation procedure ends, and the process proceeds to step S2704 in FIG. move on.
This indicates that the target prediction mode is not included in the priority prediction mode list, and mpmFl
The ag and mpmIndex are not reset. That is, mpmFlag = false
, MpmIndex = 0.

[Priority prediction mode index encoding procedure]
FIG. 33 shows details of the priority prediction mode index encoding procedure in step S2706 of FIG.
This will be described with reference to the flowchart of FIG.

The priority prediction mode index encoding unit 2606 is a priority prediction mode list creation unit 260.
2. Obtain mpmlist and mpmlistsize from 2.

The priority prediction mode index encoding unit 2606 selects an encoding tree to be used for encoding the priority prediction mode index mpmIndex based on mpmlistsize (step S4101).

FIG. 34 shows a coding tree as a selection candidate. Reference numeral 4201 in FIG. 34 denotes mpmlistsi.
This is an encoding tree when ze is 2. In FIG. 34, reference numeral 4202 indicates that mpmlistsize is 3.
This is the coding tree for. Reference numeral 4203 in FIG. 34 is an encoding tree when mpmlistsize is 4. A reference numeral 4204 in FIG. 34 is an encoding tree when mpmlistsize is 5. A reference numeral 4205 in FIG. 34 is an encoding tree when mpmlistsize is 7.
In the priority prediction mode coding tree, the priority of the priority prediction mode is high (mode number is small).
A code string having a shorter code length is assigned.

The priority prediction mode index encoding unit 2606 uses the selected encoding tree to perform mpm
Index encoding is performed (step S4102), and the process ends.

[Non-priority prediction mode index calculation procedure]
Details of the non-priority prediction mode index calculation procedure in step S2707 of FIG.
This will be described with reference to the flowchart of FIG.

In this procedure, the process proceeds by scanning mpmList in descending order of the index. The non-priority prediction mode index calculation unit 2607 initializes the non-priority prediction mode index remModeIndex with the target prediction mode currModeIndex,
A variable i for scanning mpmList is initialized with mpmListSize-1 (step S3301), and mpmList is sorted in ascending order of values (step S3302).

If the variable i is greater than or equal to 0 (step S3303), that is, if all the elements of mpmList have not been scanned yet, remModeIndex and mpmList [i
] Are compared (step S3304). remModeIndex is mpmList [i
], 1 is subtracted from the value of remModeIndex (step S3305).
). 1 is subtracted from the value of the variable i (step S3306), and scanning is continued.

In step S3303, when the variable i is less than 0, that is, when all the elements of mpmList have been scanned, the non-priority prediction mode index calculation procedure is terminated,
The process proceeds to step S2708 in FIG.

[Non-priority prediction mode index encoding procedure]
Details of the non-priority prediction mode index encoding procedure in step S2708 of FIG. 21 are shown in FIG.
This will be described with reference to the flowchart of FIG.

The non-priority prediction mode index encoding unit 2608 determines the target block size (
Step S3401).

When the target block is a 4 × 4 block, the non-priority prediction mode index encoding unit 2
In step S <b> 3402, the value of remModeIndex is compared with the value of mpmListSize−1. When remModeIndex is smaller than mpmListSize-1, remModeIndex is encoded with 3 bits, and the process ends (step S3403). If not, ie remModeIndex is mpmLis
When tSize-1 or more, remModeIndex is set to mpmListSize.
-1 is added (step S3404), and the upper 3 bits of remModeIndex are encoded (step S3405). Further, the least significant bit of remModeIndex is encoded (step S3406), and the process is terminated.

When the target block is a 4 × 4 block, 17 patterns of intra prediction are defined. In the present embodiment, the number mpmListSize of the priority prediction mode is at least 2.
Therefore, the non-priority prediction mode index remModeIndex is converted to any value of [0, 14]. Therefore, the non-priority prediction mode index remMode
When fixed-length encoding is performed on an Index, conversion to a 4-bit code word is performed. However, as the number of priority prediction modes mpmListSize increases, the number of candidates that the non-priority prediction mode index can take decreases, so all remModeInde
Expressing x with 4 bits is redundant. Therefore, in this procedure, remModeIn
Dex is converted into a 3-bit or 4-bit codeword, and variable length coding is performed.

When the target block is an 8 × 8 block or a 16 × 16 block, the non-priority prediction mode index encoding unit 2608 performs remModeIndex and mpmListSi.
The values of ze-1 are compared (step S3407). remModeIndex is mpmL
If it is smaller than istSize-2, remModeIndex is encoded with 4 bits, and the process ends (step S3408). If not, ie remMod
When eIndex is greater than or equal to mpmListSize-2, remModeInde
Add mpmListSize-2 to x (step S3409), remModeInd
The upper 4 bits of ex are encoded (step S3410). In addition, remModeIn
The least significant 1 bit of dex is encoded (step S3411), and the process ends.

When the target block is an 8 × 8 block or a 16 × 16 block, 34 patterns of intra prediction are defined. In this embodiment, the number of priority prediction modes mpmLis
Since tSize is at least 2, the non-priority prediction mode index remModeI
ndex is converted to any value of [0, 31]. Therefore, when fixed-length encoding is performed on the non-priority prediction mode index remModeIndex, conversion to a 5-bit codeword is performed. All remMods as if they were 4x4 blocks
Expressing eIndex with 5 bits is redundant. Therefore, in this procedure, remM
The odeIndex is converted into a 4-bit or 5-bit codeword, and variable length coding is performed.

[Decryption procedure]
A second example of the intra prediction mode decoding method according to the embodiment of the present invention will be described. FIG. 22 is a block diagram of a detailed configuration of the second example of the intra prediction mode decoding unit 803 of FIG. The intra prediction mode decoding unit 803 according to the second embodiment includes an intra prediction mode memory 2901, a priority prediction mode list creation unit 2902, a priority prediction mode determination flag decoding unit 2903, a priority prediction mode index decoding unit 2904, and a priority prediction mode calculation unit. 2905, non-priority prediction mode index decoding unit 2906, non-priority prediction mode calculation unit 29
07 and a prediction direction difference calculation unit 2908.

Since the intra prediction mode decoding process in the intra prediction mode decoding unit 803 in FIG. 22 corresponds to the intra prediction mode encoding process in the intra prediction mode encoding unit 508 in FIG. 20, the intra prediction mode memory 2901 in FIG. The priority prediction mode list creation unit 2902 and the prediction direction difference calculation unit 2908 are configured by the intra prediction mode memory 2601, the priority prediction mode list creation unit 2602, and the prediction direction difference calculation unit 26 shown in FIG.
Each of the ten components has the same function.

Hereinafter, the decoding procedure in the intra prediction mode will be described with reference to the flowchart of FIG.

The prediction direction difference calculation unit 2908 acquires the intra prediction modes refModeA and refModeB of the adjacent blocks from the intra prediction mode memory 2901, and predicts the prediction direction difference diff.
Dir is calculated (step S3001). The prediction direction difference calculation procedure follows the procedure shown in FIG. 30 as in the case of the prediction direction difference calculation unit 2610 in FIG.

The priority prediction mode list creation unit 2902 acquires the intra prediction modes refModeA and refModeB of adjacent blocks from the intra prediction mode memory 2901, and acquires the prediction direction difference diffDir from the prediction direction difference calculation unit 2908. The priority prediction mode list creation unit 2902 acquires the obtained refModeA, refModeB, and diffDir.
The priority prediction mode list mpmList is created based on the above, and the size mpmListSize of the priority prediction mode list is determined (step S3002). Similar to the priority prediction mode list creation procedure in the priority prediction mode list creation unit 2602 of FIG. 20, the priority prediction mode list creation procedure follows the procedure shown in the flowchart of FIG.

The priority prediction mode determination flag decoding unit 2903 reads 1 bit from the encoded sequence, decodes the priority prediction mode determination flag ppmFlag (step S3003), and determines the value of the priority prediction mode determination flag ppmFlag (step S3004).

If the priority prediction mode determination flag mpmFlag is true, the priority prediction mode index decoding unit 2904 decodes the priority prediction mode index mpmIndex (step S3005). Details of the priority prediction mode index decoding procedure will be described later. Further, the priority prediction mode calculation unit 2905 calculates mpmI of the priority prediction mode list mpmlist.
The ndex-th element mpmList [mpmIndex] is used as the target prediction mode currMo.
It is deIndex (step S3006). Target prediction mode currModeInd
ex is stored in the intra prediction mode memory 2901, and the process ends.

When the priority prediction mode determination flag mpmFlag is false, the non-priority prediction mode index decoding unit 2906 performs the non-priority prediction mode index remModeInd.
ex is decoded (step S3007), and the non-priority prediction mode calculation unit 2907 calculates the calculated r
The target prediction mode currModeIndex is calculated based on emModeIndex (
Step S3008). The target prediction mode currModeIndex is stored in the intra prediction mode memory 2901, and the process ends. The decoding procedure of the non-priority prediction mode index and the target prediction mode calculation procedure will be described later.

[Priority prediction mode index decoding procedure]
Details of the priority prediction mode index decoding procedure in step S3005 of FIG. 23 will be described with reference to the flowchart of FIG.

The priority prediction mode index decoding unit 2904 is a priority prediction mode list creation unit 2902.
To obtain mpmlist and mpmlistsize.

The priority prediction mode index decoding unit 2904 selects a decoding tree to be used for decoding the priority prediction mode index mpmIndex based on mpmlistsize (step S3101).

FIG. 34 shows a decoding tree as a selection candidate. Reference numeral 4201 in FIG. 34 denotes mpmlistsiz
This is a decoding tree when e is 2. Reference numeral 4202 in FIG. 34 denotes a decoding tree when mpmlistsize is 3. Reference numeral 4203 in FIG. 34 is a decoding tree when mpmlistsize is 4. Reference numeral 4204 in FIG. 34 is a decoding tree when mpmlistsize is 5. Reference numeral 4205 in FIG. 34 is decoding when mpmlistsize is 7.

The priority prediction mode index decoding unit 2904 uses the selected decoding tree to generate the mpmIn
Dex is decoded (step S3102), and the process ends.

[Non-priority prediction mode index decoding procedure]
Details of the non-priority prediction mode index decoding procedure in step S3007 of FIG.
This will be described with reference to the flowchart of FIG.

The non-priority prediction mode index decoding unit 2906 determines the target block size (step S3501).

When the target block is a 4 × 4 block, 3-bit fixed length decoding is performed, and remMo
deIndex is set (step S3502). The value of remModeIndex is determined (step S3503). When remModeIndex is smaller than mpmListSize-1, remModeIndex is determined and the process ends. Otherwise, remModeIndex is shifted right by 1 bit (step S3504), and 1 bit is further read from the encoded sequence and added to remModeIndex (step S3505). The value of pmListSize-1 is subtracted from remModeIndex to determine the final remModeIndex (step S3506), and the process ends.

When the target block is an 8 × 8 block or a 16 × 16 block, 4-bit fixed-length decoding is performed to obtain remModeIndex (step S3507). remMod
The value of eIndex is determined (step S3508). remModeIndex is mp
When it is smaller than mListSize-2, remModeIndex is determined and the process is terminated. Otherwise, remModeIndex is shifted right by 1 bit (step S3509), and 1 bit is read from the encoded sequence, and remModeInd
Add to ex (step S3510). remModeIndex to mpmList
Size-2 is subtracted to determine the final remModeIndex (step S3511).
), The process is terminated.

[Non-priority prediction mode calculation procedure]
Details of the prediction mode calculation procedure in step S3008 in FIG. 23 will be described with reference to the flowchart in FIG.

In this procedure, the process proceeds by scanning mpmList in ascending order of the index. The non-priority prediction mode calculation unit 2907 uses the target prediction mode currModeInde
Initialize x with the non-priority prediction mode index remModeIndex and mpmLis
A variable i for scanning t is initialized with 0 (step S3601), and mpmList is sorted in ascending order of values (step S3602).

If the variable i is less than mpmListSize (step S3603), that is, if not all elements of mpmList have been scanned yet, currModeInd.
Ex and mpmList [i] are compared (step S3604). currModeIn
If dex is greater than or equal to mpmList [i], 1 is added to the value of currModeIndex (step S3605). 1 is added to the value of the variable i (step S3606), and scanning is continued.

In step S3603, when i becomes greater than or equal to mpmListSize, that is, when all elements of mpmList have been scanned, the process ends.

The image encoding device and image decoding device of the first embodiment described above have the following operational effects.

(1) The coding tree is switched based on the prediction direction difference between the plurality of processed screen prediction modes. Since the prediction direction indicated by the intra prediction mode has a correlation with the target image, the correlation between the decoded images of a plurality of reference blocks is estimated from the prediction direction difference of the intra prediction mode, and based on the correlation Therefore, it is possible to set an appropriate probability model and reduce the amount of generated codes in the prediction mode of the target image.

(2) When the prediction direction difference is small, the correlation between decoded images of a plurality of reference blocks is high,
It is estimated that the processing target block is also highly correlated with the reference block. Since the generated intra prediction modes are concentrated around the prediction direction indicated by the mode that matches the reference prediction mode, the entire intra prediction mode is reduced by shortening the code length of the intra prediction mode indicating the prediction direction close to the prediction direction of the reference prediction mode. The amount of generated codes can be reduced.

(3) When the prediction direction difference is large, the correlation between decoded images of a plurality of reference blocks is low,
It is estimated that the correlation between the processing target block and the reference block is lower than when the prediction direction difference is close. In this case, the amount of generated codes can be reduced by reducing the deviation of the code length of each prediction mode.

(4) Since coding trees are switched based on the prediction direction difference between the screen prediction modes of a plurality of processed blocks, the number of coding trees to be created is at most half of the prediction directions that can be taken in the intra prediction mode. Compared to a configuration that considers all combinations of a plurality of prediction modes, the complexity can be greatly reduced.

  The image encoding device and the image decoding device of the second embodiment have the following operational effects.

(1) The coding tree of the priority prediction mode is switched based on the prediction direction difference between the plurality of processed screen prediction modes. Since the prediction direction indicated by the intra prediction mode has a correlation with the target image, the correlation between the decoded images of a plurality of reference blocks is estimated from the prediction direction difference of the intra prediction mode, and based on the correlation Therefore, an appropriate probability model can be set, and the amount of generated codes can be reduced.

(2) When the prediction direction difference is small, the correlation between decoded images of a plurality of reference blocks is high,
It is estimated that the processing target block is also highly correlated with the reference block. Since the generated intra prediction modes are concentrated around the prediction direction indicated by the mode that matches the reference prediction mode, the intra prediction mode indicating the prediction direction adjacent to the prediction direction of the reference prediction mode is set as the priority prediction mode. Thus, the overall generated code amount can be reduced.

(3) When the prediction direction difference is large, the correlation between decoded images of a plurality of reference blocks is low,
It is estimated that the correlation between the processing target block and the reference block is lower than when the prediction direction difference is close. In this case, the amount of generated codes can be reduced by setting the average value mode to the priority prediction mode in addition to the prediction direction adjacent to the prediction direction of the reference prediction mode.
Furthermore, in switching the coding tree in the priority prediction mode, the block size of the target block is used in addition to the prediction direction difference. If the defined intra prediction modes are different, the distribution of intra prediction modes is also different. In a configuration in which the defined intra prediction mode depends on the block size, a more appropriate coding tree can be selected by using the block size of the target block, and the generated code amount can be reduced.

(4) Since coding trees are switched based on the prediction direction difference between a plurality of processed screen prediction modes, the number of coding trees to be created is at most half of the prediction directions that can be taken in the intra-screen prediction mode. Compared to a configuration in which all combinations are implemented, the complexity can be greatly reduced.

The moving image encoded stream output from the moving image encoding apparatus of the embodiment described above has a specific data format so that it can be decoded according to the encoding method used in the embodiment. Therefore, the moving picture decoding apparatus corresponding to the moving picture encoding apparatus can decode the encoded stream of this specific data format.

When a wired or wireless network is used to exchange an encoded stream between a moving image encoding device and a moving image decoding device, the encoded stream is converted into a data format suitable for the transmission form of the communication path. It may be transmitted. In that case, a video transmission apparatus that converts the encoded stream output from the video encoding apparatus into encoded data in a data format suitable for the transmission form of the communication channel and transmits the encoded data to the network, and receives the encoded data from the network Then, a moving image receiving apparatus that restores the encoded stream and supplies the encoded stream to the moving image decoding apparatus is provided.

The moving image transmitting apparatus is a memory that buffers the encoded stream output from the moving image encoding apparatus, a packet processing unit that packetizes the encoded stream, and transmission that transmits the packetized encoded data via the network. Part. The moving image receiving apparatus generates a coded stream by packetizing the received data, a receiving unit that receives the packetized coded data via a network, a memory that buffers the received coded data, and packet processing. And a packet processing unit provided to the video decoding device.

The above processing relating to encoding and decoding can be realized as a transmission, storage, and reception device using hardware, and is stored in a ROM (Read Only Memory), a flash memory, or the like. It can also be realized by firmware or software such as a computer. The firmware program and software program can be recorded on a computer-readable recording medium, provided from a server through a wired or wireless network, or provided as a data broadcast of terrestrial or satellite digital broadcasting Is also possible.

The present invention has been described based on the embodiments. The embodiments are exemplifications, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are within the scope of the present invention. .

501 subtraction unit, 502 orthogonal transform / quantization unit, 503 inverse quantization / inverse transform unit,
504 addition unit, 505 decoded image memory, 506 intra prediction unit, 507 texture information encoding unit, 508 intra prediction mode encoding unit, 509 intra prediction mode selection unit, 601 intra prediction mode memory, 602 reference mode determination unit,
603 priority mode determination unit, 604 prediction direction difference calculation unit, 605 coding tree selection unit, 606 variable length coding unit, 801 texture information decoding unit, 802 dequantization /
Inverse transformation unit, 803 intra prediction mode decoding unit, 804 addition unit, 805 decoded image memory, 806 intra prediction unit, 901 intra prediction mode memory, 902
A reference mode determination unit, 903 a priority mode determination unit, 904 a prediction direction difference calculation unit, 9
05 decoding tree selection unit, 906 variable length decoding unit, 2601 intra prediction mode memory, 2602 priority prediction mode list creation unit, 2603 priority prediction mode determination flag calculation unit, 2604 priority prediction mode determination flag encoding unit, 2605 priority prediction mode index Calculation unit, 2606 priority prediction mode index encoding unit, 2607 non-priority prediction mode index calculation unit, 2608 non-priority prediction mode index encoding unit, 2609 priority prediction mode determination unit, 2610 prediction direction difference calculation unit, 2901
Intra prediction mode memory, 2902 priority prediction mode list creation unit, 2903 priority prediction mode determination flag decoding unit, 2904 priority prediction mode index decoding unit, 2
905 priority prediction mode calculation unit, 2906 non-priority prediction mode index decoding unit,
2907 non-priority prediction mode calculation unit, 2908 prediction direction difference calculation unit.

Claims (6)

An image decoding apparatus that decodes information for specifying an intra prediction mode in block units from an encoded stream, and decodes an image signal using information for specifying the decoded intra prediction mode,
An intra-screen prediction mode storage unit for storing the intra-screen prediction mode of the decoded block;
A prediction direction difference indicating the degree of difference in the prediction direction of the obtained prediction mode in the prediction mode obtained from the prediction mode storage unit in the prediction mode of the plurality of reference blocks used in the prediction process of the decoding target block. A prediction direction difference deriving unit for deriving
Priority prediction for creating a list of priority prediction modes that are candidates for the intra-screen prediction mode of the decoding target block based on the intra-screen prediction mode of the plurality of reference blocks used for deriving the prediction direction difference and the prediction direction difference A mode list creation section;
A priority prediction mode determination flag decoding unit that decodes information indicating whether the intra prediction mode of the decoding target block is a priority prediction mode;
When the intra prediction mode of the decoding target block is a priority prediction mode, a priority prediction mode decoding unit that decodes information for specifying the intra prediction mode of the decoding target block based on the created list,
A non-priority prediction mode decoding unit that decodes information for specifying the intra-screen prediction mode of the decoding target block based on the list when the intra-screen prediction mode of the decoding target block is a non-priority prediction mode. ,
In addition to the same intra prediction mode as the intra prediction mode of the reference block, the priority prediction mode list creation unit includes an intra prediction mode having a prediction direction adjacent to the prediction direction of the intra prediction mode of the reference block, and An image decoding apparatus characterized in that at least one of average value modes is set as a priority prediction mode.   The non-priority prediction mode decoding unit rearranges the elements included in the list in ascending order of the intra-screen prediction mode index preset in each intra-screen prediction mode, and includes the priority included in the rearranged list. When the non-priority prediction mode index obtained by scanning the prediction mode from the beginning and directly decoding and decoding from the encoded stream is equal to or higher than the intra prediction mode index indicating the priority prediction mode, 1 is set to the non-priority prediction mode index. The intra-frame prediction mode indicated by the non-priority prediction mode index subjected to the addition processing when scanning of the last element of the priority prediction mode list is completed is repeated as the intra-screen prediction mode of the decoding target block. The image decoding apparatus according to claim 1, wherein: An image decoding method that decodes information for specifying an intra prediction mode in block units from an encoded stream, and decodes an image signal using information for specifying the decoded intra prediction mode,
With reference to the memory storing the intra-screen prediction mode of the decoded block, the intra-screen prediction mode of the plurality of reference blocks used for the intra-screen prediction process of the decoding target block is acquired, and the prediction direction of the acquired intra-screen prediction mode Deriving a prediction direction difference indicating the degree of difference between,
Creating a list of priority prediction modes that are candidates for the intra prediction mode of the block to be decoded based on the intra prediction modes of the plurality of reference blocks used for deriving the prediction direction difference and the prediction direction difference; ,
Decoding information indicating whether the intra prediction mode of the decoding target block is a priority prediction mode;
Decoding the information for specifying the intra-screen prediction mode of the decoding target block based on the created list when the intra-screen prediction mode of the decoding target block is a priority prediction mode;
A non-priority prediction mode decoding step for decoding information for specifying the intra-screen prediction mode of the decoding target block based on the list when the intra-screen prediction mode of the decoding target block is a non-priority prediction mode. ,
In the step of creating the list of priority prediction modes, in addition to the same intra prediction mode as the intra prediction mode of the reference block, in the screen having a prediction direction adjacent to the prediction direction of the intra prediction mode of the reference block An image decoding method, wherein at least one of a prediction mode and an average value mode is set as a priority prediction mode.   In the non-priority prediction mode decoding step, the elements included in the list are rearranged in ascending order of the intra-screen prediction mode index preset in each intra-screen prediction mode, and the priority included in the rearranged list When the non-priority prediction mode index obtained by scanning the prediction mode from the beginning and directly decoding and decoding from the encoded stream is equal to or higher than the intra prediction mode index indicating the priority prediction mode, 1 is set to the non-priority prediction mode index. The intra-frame prediction mode indicated by the non-priority prediction mode index subjected to the addition processing when scanning of the last element of the priority prediction mode list is completed is repeated as the intra-screen prediction mode of the decoding target block. The image decoding method according to claim 3, wherein: An image decoding program that decodes information for specifying an intra prediction mode in block units from an encoded stream, and decodes an image signal using information for specifying the decoded intra prediction mode,
With reference to the memory storing the intra-screen prediction mode of the decoded block, the intra-screen prediction mode of the plurality of reference blocks used for the intra-screen prediction process of the decoding target block is acquired, and the prediction direction of the acquired intra-screen prediction mode Deriving a prediction direction difference indicating the degree of difference between,
Creating a list of priority prediction modes that are candidates for the intra prediction mode of the block to be decoded based on the intra prediction modes of the plurality of reference blocks used for deriving the prediction direction difference and the prediction direction difference; ,
Decoding information indicating whether the intra prediction mode of the decoding target block is a priority prediction mode;
Decoding the information for specifying the intra-screen prediction mode of the decoding target block based on the created list when the intra-screen prediction mode of the decoding target block is a priority prediction mode;
A non-priority prediction mode decoding step for decoding information for specifying the intra-screen prediction mode of the decoding target block based on the list when the intra-screen prediction mode of the decoding target block is a non-priority prediction mode; To run
In the step of creating the list of priority prediction modes, in addition to the same intra prediction mode as the intra prediction mode of the reference block, in the screen having a prediction direction adjacent to the prediction direction of the intra prediction mode of the reference block An image decoding program characterized in that at least one of a prediction mode and an average value mode is set as a priority prediction mode.   In the non-priority prediction mode decoding step, the elements included in the list are rearranged in ascending order of the intra-screen prediction mode index preset in each intra-screen prediction mode, and the priority included in the rearranged list When the non-priority prediction mode index obtained by scanning the prediction mode from the beginning and directly decoding and decoding from the encoded stream is equal to or higher than the intra prediction mode index indicating the priority prediction mode, 1 is set to the non-priority prediction mode index. The intra-frame prediction mode indicated by the non-priority prediction mode index subjected to the addition processing when scanning of the last element of the priority prediction mode list is completed is repeated as the intra-screen prediction mode of the decoding target block. The image decoding program according to claim 5, wherein:

Images