JP7550373B2 - Image encoding device, image encoding method, image encoding program, image decoding device, image decoding method, and image decoding program - Google Patents

Description

The present invention relates to a technology that divides an image into blocks and performs encoding and decoding on a block-by-block basis.

In image encoding and decoding, an image is divided into blocks, which are collections of a certain number of pixels, and encoding and decoding are performed on a block-by-block basis. By dividing the blocks appropriately, the coding efficiency of intra-screen prediction (intra-prediction) and inter-screen prediction (inter-prediction) is improved. In intra-prediction, the coding efficiency is also improved by predicting the secondary signal from the decoded image of the primary signal.

JP 2013-90015 A

However, predicting the secondary signal from the decoded image of the primary signal increases the amount of processing and creates dependencies between the processing of the primary signal and the processing of the secondary signal, making parallel processing difficult.

The present invention was made in light of these circumstances, and its purpose is to provide a technology that improves coding efficiency by performing block division suitable for image coding and decoding.

In order to solve the above problem, an image coding device according to one aspect of the present invention is an image coding device that divides an image into blocks and performs coding in units of the divided blocks, and includes a luminance signal block division unit that divides the luminance signal of the image into rectangles of a predetermined size to generate luminance signal blocks, a chrominance signal block division unit that divides the chrominance signal of the image into rectangles of a predetermined size to generate chrominance signal blocks, a luminance signal prediction unit that predicts the luminance signal, and a chrominance signal prediction unit that predicts the chrominance signal, wherein the luminance signal blocks and the chrominance signal blocks are divided independently in the case of intra prediction and are divided together in the case of inter prediction, and the chrominance signal prediction unit is capable of luminance-chrominance intra prediction that predicts the chrominance signal from the decoded luminance signal, and when the size of the luminance signal block is equal to or larger than a predetermined size, the use of the luminance-chrominance intra prediction is prohibited.

Another aspect of the present invention is an image coding method. This method is an image coding method for dividing an image into blocks and coding the divided blocks, and includes a luminance signal block division step of dividing the luminance signal of the image into rectangles of a predetermined size to generate luminance signal blocks, a chrominance signal block division step of dividing the chrominance signal of the image into rectangles of a predetermined size to generate chrominance signal blocks, a luminance signal prediction step of predicting the luminance signal, and a chrominance signal prediction step of predicting the chrominance signal, in which the luminance signal blocks and the chrominance signal blocks are divided independently in the case of intra prediction and are divided together in the case of inter prediction, and the chrominance signal prediction step is capable of luminance-chrominance intra prediction for predicting the chrominance signal from the decoded luminance signal, and when the size of the luminance signal block is equal to or larger than a predetermined size, the use of the luminance-chrominance intra prediction is prohibited.

Yet another aspect of the present invention is an image decoding device. This device is an image decoding device that performs decoding in units of blocks obtained by dividing an image, and includes a luminance signal block division unit that divides the luminance signal of the image into rectangles of a predetermined size to generate luminance signal blocks, a chrominance signal block division unit that divides the chrominance signal of the image into rectangles of a predetermined size to generate chrominance signal blocks, a luminance signal prediction unit that predicts the luminance signal, and a chrominance signal prediction unit that predicts the chrominance signal, wherein the luminance signal blocks and the chrominance signal blocks are divided independently in the case of intra prediction and are divided together in the case of inter prediction, and the chrominance signal prediction unit is capable of luminance-chrominance intra prediction that predicts the chrominance signal from the decoded luminance signal, and when the size of the luminance signal block is equal to or larger than a predetermined size, the use of the luminance-chrominance intra prediction is prohibited.

Yet another aspect of the present invention is an image decoding method. This method is an image decoding method for decoding an image in units of blocks obtained by dividing the image, and includes a luminance signal block division step of dividing the luminance signal of the image into rectangles of a predetermined size to generate luminance signal blocks, a chrominance signal block division step of dividing the chrominance signal of the image into rectangles of a predetermined size to generate chrominance signal blocks, a luminance signal prediction step of predicting the luminance signal, and a chrominance signal prediction step of predicting the chrominance signal, in which the luminance signal blocks and the chrominance signal blocks are divided independently in the case of intra prediction and are divided together in the case of inter prediction, and the chrominance signal prediction step is capable of luminance-chrominance intra prediction for predicting the chrominance signal from the decoded luminance signal, and when the size of the luminance signal block is equal to or larger than a predetermined size, the use of the luminance-chrominance intra prediction is prohibited.

In addition, any combination of the above components, and any transformation of the present invention into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present invention.

The present invention enables block division suitable for image encoding and decoding, improves encoding efficiency, and provides image encoding and decoding with a low processing load.

1 is a configuration diagram of an image encoding device according to a first embodiment. FIG. 1 is a configuration diagram of an image decoding device according to a first embodiment. 11 is a flowchart illustrating division into tree blocks and division within tree blocks. FIG. 13 is a diagram showing how an input image is divided into tree blocks. FIG. 1 is a diagram illustrating z-scanning. This is a diagram showing a tree block divided into four horizontally and vertically. This is a diagram showing a tree block divided horizontally into two. This is a diagram showing a tree block divided into two vertically. 11 is a flowchart for explaining the processing of each divided block when a tree block is divided into four in the horizontal and vertical directions. 11 is a flowchart illustrating processing of each divided block when a tree block is divided into two in the horizontal direction. FIG. 13 is a diagram showing how a divided block is re-divided when a tree block is divided into two horizontally. 13 is a flowchart illustrating processing of each divided block when a tree block is divided into two vertically. FIG. 13 is a diagram showing how a divided block is re-divided when a tree block is divided into two vertically. FIG. 4 illustrates an example of syntax related to block division according to the first embodiment. FIG. 1 is a diagram illustrating intra prediction. FIG. 1 is a diagram illustrating inter prediction. FIG. 2 is a diagram illustrating a color difference format. FIG. 1 is a diagram illustrating luminance and chrominance intra prediction. 13 is a flowchart illustrating luminance and chrominance intra prediction. FIG. 13 is a diagram illustrating a case where the size of a chrominance block is larger than the size of a luminance block. FIG. 13 is a diagram illustrating a case where the size of a chrominance block is smaller than the size of a luminance block. 13 is a diagram illustrating an example of the syntax of an intra color difference prediction mode. 11A and 11B are diagrams illustrating an example of limiting intra color difference prediction by replacing surrounding pixels. FIG. 13 is a diagram illustrating luminance and chrominance intra prediction depending on differences in the size of luminance blocks.

An embodiment of the present invention provides an image coding technique that divides an image into rectangular blocks and encodes and decodes the divided blocks.

(First embodiment)
An image coding device 100 and an image decoding device 200 according to a first embodiment of the present invention will be described.

Figure 1 is a configuration diagram of an image encoding device 100 according to a first embodiment. Here, Figure 1 shows only the data flow related to the image signal, and each component supplies additional information other than the image signal, such as a motion vector or a prediction mode, to the encoded bitstream generator 105 to generate the corresponding encoded data, but the data flow related to the additional information is not shown.

The block division unit 101 divides an image into coding target blocks, which are processing units for encoding, and supplies image signals in the coding target blocks to the residual signal generation unit 103. The block division unit 101 also supplies image signals of the coding target blocks to the prediction image generation unit 102 in order to evaluate the degree of matching of the prediction image.

The block division unit 101 recursively divides an image into rectangles of a predetermined size to generate blocks to be coded. The block division unit 101 includes a quartering unit that divides the target block in the recursive division into four horizontally and vertically to generate four blocks, and a bisection unit that divides the target block in the recursive division into two horizontally or vertically to generate two blocks. The detailed operation of the block division unit 101 will be described later.

The predicted image generating unit 102 performs intra-picture prediction (intra prediction) or inter-picture prediction (inter prediction) based on the prediction mode from the decoded image signal supplied from the decoded image memory 108 to generate a predicted image signal. The image signal in the block to be coded supplied from the block division unit 101 is used for evaluation of intra prediction and inter prediction. In intra prediction, a predicted image signal is generated using the image signal of the block to be coded supplied from the block division unit 101 and the image signal of the surrounding coded block adjacent to the block to be coded that exists in the same picture as the block to be coded supplied from the decoded image memory 108. In inter prediction, the image signal of the block to be coded supplied from the block division unit 101 is used as a reference picture with a coded picture stored in the decoded image memory 108 that is before or after the picture (coding picture) containing the block to be coded, and block matching evaluation such as block matching is performed between the coding picture and the reference picture to obtain a motion vector indicating the amount of motion, and motion compensation is performed from the reference picture based on this amount of motion to generate a predicted image signal. The predicted image generation unit 102 supplies the predicted image signal thus generated to the residual signal generation unit 103.

The residual signal generation unit 103 performs subtraction between the image signal to be coded and the prediction signal generated by the prediction image generation unit 102 to generate a residual signal, which is then supplied to the orthogonal transformation and quantization unit 104.

The orthogonal transform and quantization unit 104 performs orthogonal transform and quantization on the residual signal supplied from the residual signal generation unit 103, and supplies the orthogonally transformed and quantized residual signal to the coded bitstream generation unit 105 and the inverse quantization and inverse orthogonal transform unit 106.

The coded bit string generation unit 105 generates a coded bit string for the orthogonally transformed and quantized residual signal supplied from the orthogonal transformation and quantization unit 104. The coded bit string generation unit 105 also generates a corresponding coded bit string for additional information such as a motion vector, a prediction mode, and block division information.

The inverse quantization and inverse orthogonal transform unit 106 inverse quantizes and inverse orthogonally transforms the orthogonally transformed and quantized residual signal supplied from the orthogonal transform and quantization unit 104, and supplies the inverse quantized and inverse orthogonally transformed residual signal to the decoded image signal superimposition unit 107.

The decoded image signal superimposition unit 107 superimposes the predicted image signal generated by the predicted image generation unit 102 and the residual signal that has been inverse quantized and inverse orthogonal transformed by the inverse quantization and inverse orthogonal transformation unit 106 to generate a decoded image, which is stored in the decoded image memory 108. Note that the decoded image may also be subjected to a filtering process to reduce block distortion and the like caused by encoding before being stored in the decoded image memory 108.

Figure 2 is a configuration diagram of an image decoding device 200 according to the first embodiment. Here, Figure 2 shows only the data flow related to the image signal, and additional information other than the image signal, such as motion vectors and prediction modes, is supplied by the bitstream decoding unit 201 to each component for use in the corresponding processing, but the data flow related to the additional information is not shown.

The bit string decoding unit 201 decodes the supplied encoded bit string and supplies the orthogonally transformed and quantized residual signal to the block division unit 202.

The block division unit 202 determines the shape of the block to be decoded based on the decoded block division information, and supplies the orthogonally transformed and quantized residual signal of the determined block to be decoded to the inverse quantization and inverse orthogonal transformation unit 203.

The block division unit 202 recursively divides the image into rectangles of a predetermined size based on the decoded block division information to generate blocks to be decoded. The block division unit 202 includes a quartering unit that divides the target block in the recursive division into four horizontally and vertically to generate four blocks, and a bisection unit that divides the target block in the recursive division into two horizontally or vertically to generate two blocks. The detailed operation of the block division unit 202 will be described later.

The inverse quantization and inverse orthogonal transformation unit 203 performs inverse orthogonal transformation and inverse quantization on the orthogonally transformed and quantized residual signal supplied to it, thereby obtaining an inverse orthogonally transformed and inverse quantized residual signal.

The predicted image generation unit 204 generates a predicted image signal from the decoded image signal supplied from the decoded image memory 206 and supplies it to the decoded image signal superimposition unit 205.

The decoded image signal superimposition unit 205 generates a decoded image signal by superimposing the predicted image signal generated by the predicted image generation unit 204 and the residual signal that has been inverse orthogonally transformed and inverse quantized by the inverse quantization and inverse orthogonal transformation unit 203, and outputs the decoded image signal and stores it in the decoded image memory 206. Note that the decoded image may also be subjected to a filtering process to reduce block distortion and the like caused by encoding before being stored in the decoded image memory 206.

The operation of the block division unit 101 of the image encoding device 100 will now be described in detail. Figure 3 is a flowchart explaining division into tree blocks and division within a tree block.

First, the input image is divided into tree blocks of a predetermined size (S1000). For example, the tree blocks are 128 pixels x 128 pixels. However, the tree blocks are not limited to 128 pixels x 128 pixels, and any size and shape may be used as long as they are rectangular. The size and shape of the tree blocks may be fixed and determined between the encoding device and the decoding device, or may be determined by the encoding device and recorded in the encoded bit stream, and the decoding device may use the recorded block size. Figure 4 shows how the input image is divided into tree blocks. The tree blocks are encoded and decoded in raster scan order, that is, from left to right and top to bottom.

The inside of the tree block is further divided into rectangular blocks. The inside of the tree block is coded and decoded in z-scan order. Figure 5 shows the z-scan order. In z-scan, coding and decoding are performed in the order of top left, top right, bottom left, bottom right. The inside of the tree block can be divided into four or two, and a four-way division is performed both horizontally and vertically. A two-way division is performed either horizontally or vertically. Figure 6 shows a tree block divided into four horizontally and vertically. Figure 7 shows a tree block divided into two horizontally. Figure 8 shows a tree block divided into two vertically.

Referring again to FIG. 3, it is determined whether to divide the inside of the tree block into four parts horizontally and vertically (S1001).

If it is determined that the inside of the tree block should be divided into four (S1001: Yes), the inside of the tree block is divided into four (S1002), and each of the blocks divided into four horizontally and vertically is processed (S1003). The re-division process of the blocks divided into four will be described later (Figure 9).

If it is determined that the inside of the tree block should not be divided into four (S1001: No), it is determined whether or not to divide the inside of the tree block into two (S1004).

If it is determined that the inside of the tree block should be divided into two (S1004: Yes), it is determined whether the direction of division should be horizontal (S1005).

If it is determined that the direction of division is horizontal (S1005: Yes), the inside of the tree block is divided horizontally into two (S1006), and each process of the blocks divided horizontally into two is performed (S1007). The re-division process of the blocks divided horizontally into two will be described later (Figure 10).

If it is determined that the direction of division is vertical rather than horizontal (S1005: No), the inside of the tree block is divided vertically into two (S1008), and each process of the blocks divided vertically into two is performed (S1009). The re-division process of the blocks divided horizontally into two will be described later (Fig. 11).

If it is determined that the inside of the tree block is not to be divided into two (S1004: No), the block division process ends without dividing the inside of the tree block into blocks (S1010).

Next, we will use the flowchart in Figure 9 to explain how each block is processed when the tree block is divided into four blocks horizontally and vertically.

Determine whether to divide the inside of the block into four again horizontally and vertically (S1101).

If it is determined that the inside of the block should be divided into four again (S1101: Yes), the inside of the block is divided into four again (S1102), and each block divided into four horizontally and vertically is processed (S1103).

If it is determined that the interior of the block should not be divided into four again (S1101: No), it is determined whether or not to divide the interior of the block into two (S1104).

If it is determined that the block interior is to be divided into two (S1104: Yes), it is determined whether the direction of division is the horizontal direction (S1105).

If it is determined that the direction of division is the horizontal direction (S1105: Yes), the inside of the block is divided into two horizontally (S1106), and each process of the blocks divided horizontally is performed (S1107).

If it is determined that the direction of division is vertical rather than horizontal (S1105: No), the block is divided vertically into two (S1108), and each process of the blocks divided vertically into two (S1109).

If it is determined that the inside of the block is not to be divided into two (S1104: No), the block division process ends without dividing the inside of the block into blocks (S1110).

The process shown in the flowchart in Figure 9 is executed for each of the four divided blocks. The inside of each block is also coded and decoded in z-scan order.

Next, we will use the flowchart in Figure 10 to explain how each block is processed when a tree block is divided horizontally into two.

When a tree block is divided horizontally into two, each divided block first determines whether to divide the block into four horizontally and vertically (S1201).

If it is determined that the inside of the block should be divided into four (S1201: Yes), the inside of the block is divided into four (S1202), and each block divided into four horizontally and vertically is processed (S1203).

If it is determined that the interior of the block should not be divided into four (S1201: No), it is determined whether or not to divide the interior of the block into two again (S1204).

If it is determined that the block should be divided into two again (S1204: Yes), the block is divided vertically (S1205), and each process of the blocks divided vertically into two is carried out (S1206).

If it is determined that the block should not be divided into two again (S1204: No), the block division process ends without dividing the inside of the block again (S1207).

Figure 11 shows how a divided block is re-divided when a tree block is divided into two horizontally. Here, when a parent block, a tree block, is divided into two horizontally, the divided block is automatically divided into two vertically, and only vertical division is permitted. Also, when a parent block, a tree block, is divided into two, it is also possible to completely prohibit the child block from being divided into four. This makes it possible to prohibit blocks from being divided in the same direction as the parent block, preventing block division into horizontally elongated rectangles, and making encoding and decoding easier.

The process shown in the flowchart in Figure 10 is executed for each block divided horizontally into two. The inside of each divided block is also coded and decoded in the order of top to bottom.

Next, we will use the flowchart in Figure 12 to explain how each block is processed when a tree block is vertically divided into two.

When a tree block is divided vertically into two, each divided block first determines whether to divide the block into four horizontally and vertically (S1301).

If it is determined that the inside of the block should be divided into four (S1301: Yes), the inside of the block is divided into four (S1302), and each block divided into four horizontally and vertically is processed (S1303).

If it is determined that the inside of the block should not be divided into four (S1301: No), it is determined whether or not to divide the inside of the block into two again (S1304).

If it is determined that the block should be divided into two again (S1304: Yes), the block is divided horizontally (S1305), and each process of the block divided horizontally into two is performed (S1306).

If it is determined that the block should not be divided into two again (S1304: No), the block division process ends without dividing the block again (S1307).

Figure 13 shows how a divided block is re-divided when a tree block is divided into two vertically. Here, when a parent block, a tree block, is divided into two vertically, the divided block is automatically divided into two horizontally, and only horizontal division is permitted. Also, when a parent block, a tree block, is divided into two, it is also possible to completely prohibit the child block from being divided into four. This makes it possible to prohibit blocks from being divided in the same direction as the parent block, preventing block division into vertically elongated rectangles, and making encoding and decoding easier.

The process shown in the flowchart in Figure 12 is executed for each block divided vertically into two. The inside of each divided block is also coded and decoded in the order from left to right.

Note that, although we have described the redivision of the divided blocks when a tree block is divided, the parent block does not have to be a tree block. For example, if a tree block (128x128) is divided into four, and the four divided blocks (64x64) are further divided into four or in half, the above process is also applied to the division of the further divided blocks.

Next, the operation of the block division unit 202 of the image decoding device 200 will be described. Blocks are divided using the same processing procedure as the block division unit 101 of the image encoding device 100, but the block division unit 101 of the image encoding device 100 selects a block division pattern and outputs the selected block division information, whereas the block division unit 202 of the image decoding device divides blocks using block division information decoded from the encoded bit stream, and also differs in that when decoding block division information from the encoded bit stream, in a situation where re-division in the same direction is prohibited, information with no options is not transmitted in the bit stream in a syntax structure.

An example of the syntax (syntax rules for encoded bitstreams) for block division in the first embodiment is shown in FIG. 14. To divide the inside of a tree block, first, a flag (4_division_flag) indicating whether or not to divide it into four is sent and received. If it is divided into four (4_division_flag is 1), the tree block is divided into four and the process is terminated. After that, the inside of the block divided into four is again divided using the syntax shown in FIG. 14. If it is not divided into four (4_division_flag is 0), a flag (2_division_flag) indicating whether or not to divide it into two is sent and received. If it is divided into two (2_division_flag is 1), a flag (2_division_direction) indicating the direction of division into two is also sent and received. If 2_division_direction is 1, it indicates division in the vertical direction, and if 2_division_direction is 0, it indicates division in the horizontal direction. After that, the inside of the block divided into two is again divided using the syntax shown in FIG. 14. If no division is required (2_division_flag is 0), the process ends without dividing the tree block.

Here, the process of re-dividing the inside of a block that has been divided into four or two will be described. The process of re-dividing the inside of a block also uses the syntax shown in FIG. 14, but compared to dividing a tree block, the difference is that there are restrictions on the division direction when dividing into two. In other words, if a tree block is divided into two, when the inside of the block divided into two is to be re-divided, it is prohibited to divide in the same direction as the division direction in which the tree block was divided into two. This prevents the divided blocks from becoming elongated rectangles, and prevents an increase in memory bandwidth required for intra prediction and inter prediction. Preventing an increase in memory bandwidth will be described in detail later.

It is also possible to count the number of times the image has been split in two in the same direction, and restrict further splitting in the same direction if a certain number of splits has been exceeded. For example, splitting in two in the same direction is permitted up to two times, but splitting in two in the same direction is prohibited from the third time onwards.

In FIG. 14, the syntax is such that 4-division is selected as a priority, and information on whether to divide into 4 or not is transmitted and received before information on whether to divide into 2 or not. On the other hand, when 2-division is selected as a priority, it is also possible to use a syntax in which information on whether to divide into 2 or not is transmitted and received before information on whether to divide into 4 or not. This is because transmitting and receiving an event that is more likely to occur first reduces the amount of code transmitted as a bit stream. In other words, it is also possible to estimate in advance whether 4-division or 2-division is more likely to occur, and to transmit and receive the division information that is more likely to occur first. For example, by transmitting and receiving whether to prioritize 4-division or 2-division in the header information of the image, the encoding device can adaptively determine the priority number of divisions with the highest encoding efficiency, and the decoding device can divide the inside of the tree block using a syntax based on the selected priority number of divisions.

In the image encoding device 100 and the image decoding device 200, intra prediction and inter prediction are performed using the divided blocks. Both intra prediction and inter prediction involve copying pixels from memory.

An example of intra prediction is shown in FIG. 15(a) to FIG. 15(d). FIG. 15(a) and FIG. 15(b) show the prediction direction and mode number of intra prediction. In intra prediction, as shown in FIG. 15(c) and FIG. 15(d), a predicted image of a block to be encoded/decoded is generated by copying pixels from pixels that have already been encoded/decoded and are adjacent to the block to be encoded/decoded. In intra prediction, the process from predicted image generation to encoded/decoded pixel generation is repeated for each block, so the processing order is sequential for each block, and the more the inside of the block is divided into smaller parts, the heavier the overall processing load becomes. In addition, the more elongated the shape of the block is, the heavier the processing of copying pixels from memory becomes. In addition, since orthogonal transformation of residual signals is performed for encoding/decoding, the more types of rectangular sizes are required, the more types of orthogonal transformation are required, which results in an increase in the circuit scale. Therefore, when dividing the inside of a block into two, by restricting the division into two in the same direction as the division method of the parent block, the increase in memory bandwidth required for intra prediction can be prevented.

Figure 16 shows an example of inter prediction. Inter prediction generates a predicted image of a block to be coded/decoded by copying pixels from pixels included in an image that has already been coded/decoded in units of blocks. In inter prediction, when copying pixels from a reference image in units of blocks, the device is often configured to require acquisition of memory management units containing the necessary pixels. Therefore, the smaller the block is divided and the more elongated the shape of the block is, the heavier the overall processing load becomes. In addition, when performing decimal precision motion compensation using an interpolation filter on a reference image, it is necessary to copy pixels by adding several pixels to the pixels contained in the block, and the smaller the size of the block, the larger the relative ratio of the several pixels to be added becomes, and the heavier the overall processing load becomes. Therefore, when dividing the inside of a block into two, by restricting the division into two in the same direction as the division direction of the parent block, it is possible to prevent an increase in the memory bandwidth required for inter prediction.

Next, the relationship between the luminance signal and the color difference signal in intra prediction will be described. As formats between the luminance signal and the color difference signal, 4:2:0, 4:2:2, 4:4:4, and the like are conventionally known. In the 4:2:0 format shown in FIG. 17(a), the luminance signal is sampled at two pixels in both the horizontal and vertical directions, whereas the color difference signal is sampled at one pixel in both the horizontal and vertical directions. Since the human eye is more sensitive to the luminance signal than the color difference signal, the amount of information of the color difference signal is reduced more than that of the luminance signal. In the 4:2:2 format shown in FIG. 17(b), the luminance signal is sampled at two pixels in the horizontal direction, whereas the color difference signal is sampled at one pixel in the horizontal direction. In the vertical direction, the luminance signal is sampled at two pixels in the vertical direction, whereas the color difference signal is sampled at two pixels in the vertical direction. In the 4:4:4 format shown in FIG. 17(c), the luminance signal is sampled at two pixels in both the horizontal and vertical directions, whereas the color difference signal is sampled at two pixels in both the horizontal and vertical directions.

The present embodiment will be described using the 4:2:0 format, which is the most widely used format for image coding, as an example. The block division units 101 and 202 include a luminance block division unit that divides the luminance signal of an image to generate luminance blocks, and a chrominance block division unit that divides the chrominance signal of an image to generate chrominance blocks. In intra prediction, the luminance signal and the chrominance signal are divided into blocks independently. In other words, in intra prediction, the size of the luminance block and the size of the chrominance block are determined independently. In intra prediction, the luminance signal and the chrominance signal each copy pixel values from surrounding pixels, so that the prediction efficiency is improved by dividing the luminance signal and the chrominance signal into blocks independently. In contrast, in inter prediction, the luminance signal and the chrominance signal are treated together and divided into blocks. In other words, in inter prediction, the size of the luminance block and the size of the chrominance block are the same. This is because in inter prediction, it is not necessary to distinguish between luminance and chrominance in motion compensation.

The predicted image generating units 102 and 204 include a luminance signal prediction unit that predicts a luminance signal and a chrominance signal prediction unit that predicts a chrominance signal. The chrominance signal prediction unit performs luminance and chrominance intra prediction to predict the chrominance signal from encoded and decoded pixels of the luminance signal in order to improve the prediction efficiency of the chrominance signal in intra prediction. In luminance and chrominance intra prediction, the primary signal is encoded and decoded before the secondary signal, and the secondary signal is predicted using the encoded and decoded primary signal. Here, in the 4:2:0 format and the 4:2:2 format, the luminance signal has a larger amount of information than the chrominance signal, so the luminance signal is the primary signal and the chrominance signal is the secondary signal. In the 4:4:4 format, the luminance signal and the chrominance signal have the same amount of information, but it is common to use the luminance signal as the primary signal and the chrominance signal as the secondary signal to match other formats.

Figure 18 is a diagram explaining luminance and chrominance intra prediction, and Figure 19 is a flowchart explaining luminance and chrominance intra prediction.

As shown in FIG. 18, luminance/chrominance intra prediction is performed based on the degree of correlation between the coded/decoded peripheral pixels 12a, 12b of the luminance block 10 and the coded/decoded peripheral pixels 16a, 16b of the chrominance block 14. Since luminance/chrominance intra prediction predicts a chrominance signal, the peripheral pixels for which the degree of correlation is calculated are defined based on the peripheral pixels of the chrominance block to be coded/decoded. In other words, the peripheral pixels of the luminance block that are in the same positions as the peripheral pixels determined for the chrominance block are used to calculate the degree of correlation.

First, the degree of correlation between the peripheral pixels of the encoded/decoded luminance signal and the peripheral pixels of the color difference signal is calculated (S1901). Next, the encoded/decoded luminance signal of the block to be encoded/decoded is downsampled (S1902). Here, a plurality of filter types for downsampling may be prepared so that the filter type can be selected. For example, a plurality of filter types may be selected by preparing filters with different strengths, or a plurality of filter types may be selected by preparing filters with different numbers of taps. The filter type may be automatically selected using the degree of correlation between the peripheral pixels, or the filter type may be encoded/decoded and transmitted in the bit stream. In addition, if the downsampling filter for the luminance signal of the block to be encoded/decoded is not determined using the degree of correlation between the peripheral pixels, the processing of steps S1901 and S1902 may be performed in any order, and steps S1901 and S1902 can be processed in parallel.

Finally, the color difference signal is predicted from the downsampled luminance signal based on the degree of correlation between surrounding pixels (S1903). In the case of 4:2:0 format, downsampling is 1/2 in the horizontal and vertical directions. In the case of 4:2:2 format, downsampling is 1/2 in the horizontal direction and no downsampling is performed in the vertical direction. In the case of 4:4:4 format, no downsampling is performed in either the horizontal or vertical directions.

In luminance and chrominance intra prediction, prediction processing of the chrominance signal can start after the encoding/decoding of the luminance signal of the block to be encoded/decoded is completed. Therefore, the timing at which prediction processing of the chrominance block can start depends on the size of the luminance block and the size of the chrominance block.

Figures 20(a) and 20(b) are diagrams explaining luminance and chrominance intra prediction when the size of the chrominance block is larger than the size of the luminance block. The number of pixels of the first to fourth luminance blocks 20a, 20b, 20c, and 20d divided into four shown in Figure 20(a) is 16x16, and the number of pixels of the chrominance block 20e shown in Figure 20(b) is 16x16.

Here, the comparison of the sizes of the luminance block and the chrominance block is not a comparison of the number of pixels in the block, but a comparison of the area taking into account the chrominance format. That is, in the 4:2:0 format, the area occupied by the luminance block is half the area occupied by the chrominance block, so if the number of pixels in the luminance block is 16x16 and the number of pixels in the chrominance block is 16x16, the size of the luminance block is smaller. In the 4:2:0 format, if the number of pixels in the luminance block is 16x16 and the number of pixels in the chrominance block is 8x8, the area occupied by both blocks is the same, and the luminance block and the chrominance block are the same size.

To compare the sizes of luminance blocks and chrominance blocks by comparing the number of pixels in the blocks rather than the area of the blocks, the number of pixels in the chrominance blocks can be converted to the number of pixels in the luminance block using the ratio of the luminance signal to the chrominance signal in the chrominance format. In the case of the 4:2:0 format, the number of pixels in the luminance signal is twice the number of pixels in the chrominance signal, so the number of pixels in the chrominance blocks is doubled to convert to the number of pixels in the luminance block. For example, in the 4:2:0 format, if the size of the luminance block is 16x16 and the size of the chrominance block is 16x16, the converted size of the chrominance block converted to the number of pixels in the luminance block is 32x32, which shows that the size of the chrominance block is larger.

In intra prediction, after the decoding process is completed on a block-by-block basis, the prediction process for the following block can be performed. In other words, after the decoding of the first luminance block 20a is completed, the decoding of the second luminance block 20b can be performed, after the decoding of the second luminance block 20b is completed, the decoding of the third luminance block 20c can be performed, and after the decoding of the third luminance block 20c is completed, the decoding of the fourth luminance block 20d can be performed.

When the size of the chrominance block is larger than the size of the luminance block, the surrounding pixels required for the prediction process of the chrominance block 20e, both luminance pixels and chrominance pixels, exist before the decoding of the four luminance blocks 20a, 20b, 20c, and 20d, and it is possible to calculate the degree of correlation between the surrounding pixels of the luminance signal and the surrounding pixels of the chrominance signal in step S1901 of Figure 19 without waiting for the decoding of the four luminance blocks 20a, 20b, 20c, and 20d.

Next, after the first luminance block 20a is decoded, downsampling of the luminance signal is performed in step S1902 of FIG. 19. Without waiting for the completion of the decoding of the second luminance block 20b, the pixels of the chrominance block 20e corresponding to the position of the first luminance block 20a can be predicted. Similarly, after the second luminance block 20b is decoded, downsampling of the luminance signal is performed, and without waiting for the completion of the decoding of the third luminance block 20c, the pixels of the chrominance block 20e corresponding to the position of the second luminance block 20b are predicted. Furthermore, after the third luminance block 20c is decoded, downsampling of the luminance signal is performed, and without waiting for the completion of the decoding of the fourth luminance block 20d, the pixels of the chrominance block 20e corresponding to the position of the third luminance block 20c are predicted. Finally, after the fourth luminance block 20d is decoded, downsampling of the luminance signal is performed, and the pixels of the chrominance block 20e corresponding to the position of the fourth luminance block 20d are predicted.

Figures 21(a) and 21(b) are diagrams explaining luminance and chrominance intra prediction when the size of the chrominance block is smaller than the size of the luminance block. The number of pixels of the luminance block 21a shown in Figure 21(a) is 16x16, and the number of pixels of the first to fourth chrominance blocks 21b, 21c, 21d, and 21e divided into four shown in Figure 21(b) is 4x4.

In the 4:2:0 format, the area occupied by a luminance block is half the area occupied by a chrominance block, so if the number of pixels in the luminance block is 16x16 and the number of pixels in the chrominance block is 4x4, the size of the chrominance block is smaller. To compare the sizes of the luminance block and the chrominance block by comparing the number of pixels in the block rather than the area of the blocks, in the 4:2:0 format, the number of pixels in the chrominance block is doubled to convert it to the number of pixels in the luminance block. In the 4:2:0 format, if the size of the luminance block is 16x16 and the size of the chrominance block is 4x4, the converted size of the chrominance block converted to the number of pixels in the luminance block is 8x8, which shows that the size of the chrominance block is smaller.

When the size of the chrominance block is smaller than the size of the luminance block, the neighboring pixels of the first chrominance block 21b can be used before the luminance block 21a is decoded, both as luminance pixels and chrominance pixels, but the neighboring pixels of the second to fourth chrominance blocks 21c, 21d, and 21e cannot be used until the luminance block 21a is decoded. In other words, the degree of correlation between the neighboring pixels of the luminance signal and the neighboring pixels of the chrominance signal in step S1901 of FIG. 19 cannot be calculated for the second chrominance block 21c until the luminance block 21a is decoded and the first and second chrominance blocks 21b and 21c are decoded. Similarly, the degree of correlation between the neighboring pixels of the luminance signal and the neighboring pixels of the chrominance signal cannot be calculated for the third chrominance block 21d until the luminance block 21a is decoded and the first and second chrominance blocks 21b and 21c are decoded. Similarly, for the fourth chrominance block 21e, the degree of correlation between the surrounding pixels of the luminance signal and the surrounding pixels of the chrominance signal cannot be calculated for the fourth chrominance block 21e until the decoding of the luminance block 21a is complete and the decoding of the first to third chrominance blocks 21b, 21c, and 21d is complete.

In this way, when the size of a chrominance block is smaller than the size of a luminance block, performing luminance/chrominance intra prediction results in processing dependencies between the luminance block and the chrominance block, and between the chrominance blocks, making it unsuitable for parallel processing. Therefore, when the size of a chrominance block is smaller than the size of a luminance block, luminance/chrominance intra prediction is restricted. Methods for restricting luminance/chrominance intra prediction include (1) restricting by syntax, (2) replacing the intra chrominance mode, and (3) replacing surrounding pixels.

Figure 22 shows an example of the syntax for intra chrominance prediction mode. When the chrominance prediction mode number is 0, the same intra prediction mode as the luminance prediction mode is used for the chrominance prediction mode. For example, when the luminance prediction mode is a horizontal prediction mode, the chrominance prediction mode is also a horizontal prediction mode. When the chrominance prediction mode number is 1, average value mode (DC mode) is used. DC mode performs intra prediction using the average value of surrounding pixels. When the chrominance prediction mode number is 2, luminance and chrominance intra prediction mode is used.

As a method for restricting luminance and chrominance intra prediction, (1) when restricting by syntax, mode 2 is prohibited from use because it represents luminance and chrominance intra prediction. In other words, mode 2 is not transmitted, and the intra chrominance mode is selected from mode 0 and mode 1.

As a method of restricting luminance and chrominance intra prediction, (2) when replacing an intra chrominance mode, if the chrominance prediction mode number is specified as mode 2, a vertical prediction mode is used instead of the luminance and chrominance intra prediction mode. However, the prediction mode to be replaced is not limited to a vertical prediction mode, and may be another prediction mode. It is also more preferable that the luminance prediction mode of mode 0 and the mode to be replaced by mode 2 are the same, so that there is no overlap in the modes used.

When (3) replacing surrounding pixels as a method for limiting luminance and chrominance intra prediction, the surrounding pixels are replaced to calculate the degree of correlation between the surrounding pixels of the luminance signal and the surrounding pixels of the chrominance signal in step S1901 of FIG. 19. This allows the degree of correlation between the surrounding pixels to be calculated for the second to fourth chrominance blocks 21c, 21d, and 21e of FIG. 21 as well, without waiting for the decoding of the luminance block 21a.

An example of replacing peripheral pixels is shown in FIG. 23. Normally, pixels in the first chrominance block 21b are used as peripheral pixels on the left side of the second chrominance block 21c, but to use pixels in the first chrominance block 21b, it is necessary to wait for the completion of decoding of the luminance block 21a and the first chrominance block 21b. Therefore, the area 21f that can be used without waiting for the completion of decoding of the luminance block 21a is used as peripheral pixels of the second chrominance block 21c. Similarly, for the third chrominance block 21d, the area 21f that can be used without waiting for the completion of decoding of the luminance block 21a is used as peripheral pixels of the third chrominance block 21d. Similarly, for the fourth chrominance block 21e, the area 21f that can be used without waiting for the completion of decoding of the luminance block 21a is used as peripheral pixels of the fourth chrominance block 21e.

In this way, in the first embodiment, when performing luminance/chrominance intra prediction, if the size of the chrominance block is smaller than the size of the luminance block, it is possible to alleviate the processing dependency between the luminance block and the chrominance block by restricting the luminance/chrominance intra prediction. This enables parallel processing of the luminance block and the chrominance block, thereby reducing the amount of processing required for encoding and decoding.

Second Embodiment
A second embodiment of the present invention will be described below. The second embodiment differs from the first embodiment in that luminance and chrominance intra prediction is restricted by evaluating the sizes of luminance blocks and chrominance blocks independently, rather than by the size relationship between luminance blocks and chrominance blocks. Other configurations and operations are the same as those of the first embodiment.

First, restrictions based on the size of the luminance block will be described. Figures 24(a) to 24(c) show luminance and chrominance intra prediction based on different sizes of luminance blocks. As shown in Figures 24(a) and 24(b), the first luminance block 24a corresponds to the position of the first chrominance block 24e, the second luminance block 24b corresponds to the position of the second chrominance block 24f, the third luminance block 24c corresponds to the position of the third chrominance block 24g, and the fourth luminance block 24d corresponds to the position of the fourth chrominance block 24h. Furthermore, the luminance block 24i in Figure 24(c) corresponds to the positions of the first to fourth chrominance blocks 24e, 24f, 24g, and 24h in Figure 24(b).

The dependency between the luminance block and the chrominance block will be explained. When the size of the luminance block is small as shown in FIG. 24(a), once the decoding of the first luminance block 24a is complete, the first chrominance block 24e can be decoded. Once the decoding of the second luminance block 24b is complete, the second chrominance block 24f can be decoded. Once the decoding of the third luminance block 24c is complete, the third chrominance block 24g can be decoded. Once the decoding of the fourth luminance block 24d is complete, the fourth chrominance block 24h can be decoded.

On the other hand, when the size of the luminance block is large as in FIG. 24(c), the first to fourth chrominance blocks 24e, 24f, 24g, and 24h cannot be decoded until the decoding of luminance block 24i is completed.

When the division of the luma block and the division of the chroma block are determined independently, if the absolute size of the luma block is large, there is a high possibility that the size of the chroma block will be smaller than the size of the luma block. Therefore, if the absolute size of the luma block is equal to or larger than a predetermined size, the luma/chroma intra prediction of the corresponding chroma block is restricted.

Similarly, if the absolute size of the chrominance block is small, there is a high possibility that the size of the chrominance block will be smaller than the size of the luma block. Therefore, if the absolute size of the chrominance block is equal to or smaller than a predetermined size, luma-chrominance intra prediction of the chrominance block is restricted.

The method for limiting luminance and chrominance intra prediction is the same as in the first embodiment.

In this way, in the second embodiment, when the absolute size of the luma block is larger than the threshold value, or when the absolute size of the chroma block is smaller than the threshold value, luma and chroma intra prediction is restricted. This makes it possible to predict the case where the size of the chroma block will be smaller than the luma block size, thereby restricting luma and chroma intra prediction and probabilistically alleviating the processing dependency between the luma block and the chroma block.

Third Embodiment
A third embodiment of the present invention will be described. In the third embodiment, the block division unit 101 divides the chrominance block so that the size of the chrominance block is not smaller than the size of the luminance block, which is different from the first embodiment, and the other configurations and operations are the same as those of the first embodiment. When dividing the chrominance block, the block division unit 101 prohibits division such that the size of the chrominance block is smaller than the size of the luminance block. This prevents the size of the chrominance block from becoming smaller than the size of the luminance block in luminance/chrominance intra prediction, and allows parallel processing of the luminance block and the chrominance block at all times.

The image coding bit stream output by the image coding device of the embodiment described above has a specific data format so that it can be decoded according to the coding method used in the embodiment, and an image decoding device corresponding to the image coding device can decode the coded bit stream of this specific data format.

When a wired or wireless network is used to exchange coded bitstreams between an image coding device and an image decoding device, the coded bitstream may be converted into a data format suitable for the transmission mode of the communication channel and transmitted. In this case, a transmitting device is provided that converts the coded bitstream output by the image coding device into coded data in a data format suitable for the transmission mode of the communication channel and transmits it to the network, and a receiving device is provided that receives the coded data from the network, restores it to a coded bitstream, and supplies it to the image decoding device.

The transmitting device includes a memory that buffers the coded bitstream output by the image coding device, a packet processing unit that packetizes the coded bitstream, and a transmitting unit that transmits the packetized coded data via a network. The receiving device includes a receiving unit that receives the packetized coded data via a network, a memory that buffers the received coded data, and a packet processing unit that packetizes the coded data to generate a coded bitstream and provides it to the image decoding device.

It is also possible to make it into a display device by adding a display unit that displays the image decoded by the image decoding device. In that case, the display unit reads out the decoded image signal generated by the decoded image signal superimposition unit 205 and stored in the decoded image memory 206, and displays it on a screen.

It is also possible to configure an imaging device by adding an imaging unit to the configuration and inputting the captured image to the image encoding device. In this case, the imaging unit inputs the captured image signal to the block division unit 101.

The above encoding and decoding processes can of course be realized as transmission, storage, and receiving devices using hardware, but can also be realized by firmware stored in ROM (read only memory) or flash memory, or by software on a computer, etc. The firmware and software programs can be provided by recording them on a recording medium that can be read by a computer, or by providing them from a server via a wired or wireless network, or as data broadcasting via terrestrial or satellite digital broadcasting.

The present invention has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of each component and each processing process, and that such modifications are also within the scope of the present invention.

100 Image encoding device, 101 Block division unit, 102 Prediction image generation unit, 103 Residual signal generation unit, 104 Orthogonal transformation and quantization unit, 105 Encoding bit string generation unit, 106 Inverse quantization and inverse orthogonal transformation unit, 107 Decoded image signal superimposition unit, 108 Decoded image memory, 200 Image decoding device, 201 Bit string decoding unit, 202 Block division unit, 203 Inverse quantization and inverse orthogonal transformation unit, 204 Prediction image generation unit, 205 Decoded image signal superimposition unit, 206 Decoded image memory.

Claims (8)

An image coding apparatus that divides an image into blocks and codes the image in units of the divided blocks,
a luminance signal block division unit that divides a luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division unit that divides the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction unit for predicting a luminance signal;
a color difference signal prediction unit for predicting a color difference signal;
an encoding unit that encodes the prediction mode to generate a bit stream;
the luminance signal block division unit divides the luminance signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
the color difference signal block division unit divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical directions;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
The chrominance signal prediction unit is capable of luminance/chrominance intra prediction for predicting a chrominance signal from an encoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is transmitted before information indicating whether to divide into two parts.
1. An image encoding device comprising: 1. An image coding method for dividing an image into blocks and coding the divided blocks, comprising the steps of:
a luminance signal block division step of dividing the luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division step of dividing the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction step of predicting a luminance signal;
a color difference signal prediction step of predicting a color difference signal;
and an encoding step of encoding the prediction mode to generate a bitstream,
The luminance signal block division step recursively divides the luminance signal of the image into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical directions;
The color difference signal block dividing step divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
the chrominance signal prediction step is capable of luminance/chrominance intra prediction for predicting a chrominance signal from an encoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is transmitted before information indicating whether to divide into two parts.
13. An image coding method comprising: An image encoding program for dividing an image into blocks and encoding the divided blocks,
a luminance signal block division step of dividing the luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division step of dividing the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction step of predicting a luminance signal;
a color difference signal prediction step of predicting a color difference signal;
and an encoding step of encoding the prediction mode to generate a bitstream;
The luminance signal block division step divides the luminance signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
the color difference signal block dividing step divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical directions;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
the chrominance signal prediction step is capable of luminance/chrominance intra prediction for predicting a chrominance signal from an encoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is transmitted before information indicating whether to divide into two parts.
2. An image encoding program comprising: An image decoding device that decodes an image in units of blocks obtained by dividing the image,
a decoding unit for decoding the encoded bitstream to obtain a prediction mode;
a luminance signal block division unit that divides a luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division unit that divides the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction unit for predicting a luminance signal;
a color difference signal prediction unit for predicting a color difference signal,
the luminance signal block division unit divides the luminance signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
the color difference signal block division unit divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical directions;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
The chrominance signal prediction unit is capable of luminance/chrominance intra prediction for predicting a chrominance signal from a decoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is received before information indicating whether to divide into two parts.
2. An image decoding device comprising: An image decoding method for decoding an image in units of blocks obtained by dividing the image, comprising the steps of:
a decoding step for decoding the encoded bitstream to obtain a prediction mode;
a luminance signal block division step of dividing the luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division step of dividing the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction step of predicting a luminance signal;
a color difference signal prediction step of predicting a color difference signal,
The luminance signal block division step divides the luminance signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
the color difference signal block dividing step divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical directions;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
the chrominance signal prediction step is capable of luminance/chrominance intra prediction for predicting a chrominance signal from a decoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is received before information indicating whether to divide into two parts.
2. An image decoding method comprising: An image decoding program for decoding an image in units of divided blocks,
a decoding step for decoding the encoded bitstream to obtain a prediction mode;
a luminance signal block division step of dividing the luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division step of dividing the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction step of predicting a luminance signal;
a color difference signal prediction step of predicting a color difference signal;
The luminance signal block division step recursively divides the luminance signal of the image into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical directions;
The color difference signal block dividing step divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
the chrominance signal prediction step is capable of luminance/chrominance intra prediction for predicting a chrominance signal from a decoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is received before information indicating whether to divide into two parts.
2. An image decoding program comprising: A method for dividing an image into blocks, encoding the divided blocks, and storing the resulting bit stream on a recording medium, comprising the steps of:
a luminance signal block division step of dividing the luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division step of dividing the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction step of predicting a luminance signal;
a color difference signal prediction step of predicting a color difference signal;
an encoding step of encoding the prediction mode to generate a bitstream;
storing the bitstream on a recording medium ;
The luminance signal block division step divides the luminance signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
The color difference signal block dividing step divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
the chrominance signal prediction step is capable of luminance/chrominance intra prediction for predicting a chrominance signal from an encoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is transmitted before information indicating whether to divide into two parts.
A storage method comprising: A transmission method for dividing an image into blocks and transmitting a bit stream encoded in units of the divided blocks, comprising the steps of:
a luminance signal block division step of dividing the luminance signal of the image into rectangles to generate luminance signal blocks;
a color difference signal block division step of dividing the color difference signal of the image into rectangles to generate color difference signal blocks;
a luminance signal prediction step of predicting a luminance signal;
a color difference signal prediction step of predicting a color difference signal;
an encoding step of encoding the prediction mode to generate a bitstream;
a transmitting step of transmitting the bitstream ,
The luminance signal block division step divides the luminance signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical direction;
the color difference signal block dividing step divides the color difference signal of the image recursively into four blocks in the horizontal and vertical directions or into two blocks in the horizontal or vertical directions;
The luminance signal block and the chrominance signal block are divided independently in the case of intra prediction ;
the chrominance signal prediction step is capable of luminance/chrominance intra prediction for predicting a chrominance signal from an encoded luminance signal, and prohibits use of the luminance/chrominance intra prediction when a size of the luminance signal block is equal to or larger than a predetermined size;
In the case of inter prediction, the luminance signal block and the chrominance signal block are divided into blocks of the same size ,
The bit stream has a syntax structure in which information indicating whether to divide into four parts is transmitted before information indicating whether to divide into two parts.
A transmission method comprising:

Images