IMAGE DECODING METHOD, IMAGE CODING METHOD, IMAGE DECODING APPARATUS, IMAGE CODING APPARATUS, PROGRAM, AND INTEGRATED CIRCUIT

1 . An image decoding method of decoding coded image data generated by coding image data for each block according to prediction based on a prediction mode, the image decoding method comprising:

restoring a selected prediction mode used in the prediction at a time of coding, based on mode information indicating a result of estimation of the prediction mode executed at the time of coding; and

decoding a current block included in the coded image data to generate a decoded block, according to the prediction based on the selected prediction mode,

wherein the restoring includes:

determining one of prediction modes as a first estimated prediction mode;

determining, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

restoring the selected prediction mode based on the mode information, the first estimated prediction mode, and the second estimated prediction mode.

2 . The image decoding method according to claim 1 ,

wherein the mode information includes at least flag information indicating a result of comparison between the selected prediction mode and each of the first estimated prediction mode and the second estimated prediction mode, and

in the restoring included in the restoring of a selected prediction mode,

(i) the first estimated prediction mode is determined as the selected prediction mode when the flag information indicates that the selected prediction mode agrees with the first estimated prediction mode,

(ii) the second estimated prediction mode is determined as the selected prediction mode when the flag information indicates that the selected prediction mode agrees with the second estimated prediction mode, and

(iii) the selected prediction mode is restored based on information that is further included in the mode information and specifies the selected prediction mode, when the flag information indicates that the selected prediction mode disagrees with each of the first estimated prediction mode and the second estimated prediction mode.

3 . The image decoding method according to claim 1 ,

wherein one of the first estimated prediction mode and the second estimated prediction mode indicates DC/edge prediction.

4 . The image decoding method according to claim 1 ,

wherein, in the determining of a first estimated prediction mode, a prediction mode assigned an index number that is a smallest of index numbers assigned to prediction modes of previously-decoded blocks adjacent to the current block is determined as the first prediction mode.

5 . The image decoding method according to claim 1 ,

wherein, in the determining of a second estimated prediction mode,

(i) the second estimated prediction mode is determined as a DC prediction mode when the first estimated prediction mode indicates a planar mode, and

(ii) the second estimated prediction mode is determined as the planar mode when the first estimated prediction mode does not indicate the planar mode.

6 . An image coding method of coding image data for each block, the image coding method comprising:

coding a current block included in the image data, according to prediction based on a selected prediction mode selected from among predetermined prediction mode candidates;

decoding the coded current block to generate a decoded block;

determining one of prediction modes as a first estimated prediction mode;

determining, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

outputting the coded current block and mode information to be used for restoring the selected prediction mode,

wherein the outputting includes generating the mode information based on the first estimated prediction mode, the second estimated prediction mode, and the selected prediction mode.

7 . The image coding method according to claim 6 ,

wherein, in the generating of the mode information,

when the selected prediction mode agrees with one of the first estimated prediction mode and the second estimated prediction mode, flag information indicating whether the selected prediction mode agrees with the first estimated prediction mode or the second estimated prediction mode is generated as the mode information, and

when the selected prediction mode disagrees with each of the first estimated prediction mode and the second estimated prediction mode, the mode information is generated to include (i) flag information indicating that the selected prediction mode disagrees with each of the first estimated prediction mode and the second estimated prediction mode and (ii) information specifying the selected prediction mode.

8 . The image coding method according to claim 6 ,

wherein one of the first estimated prediction mode and the second estimated prediction mode indicates DC/edge prediction.

9 . The image coding method according to claim 6 ,

wherein, in the determining of a first estimated prediction mode, a prediction mode assigned an index number that is a smallest of index numbers assigned to prediction modes of previously-coded blocks adjacent to the current block is determined as the first prediction mode.

10 . The image coding method according to claim 6 ,

wherein, in the determining of a second estimated prediction mode,

(i) the second estimated prediction mode is determined as a DC prediction mode when the first estimated prediction mode indicates a planar mode, and

(ii) the second estimated prediction mode is determined as the planar mode when the first estimated prediction mode does not indicate the planar mode.

11 . An image decoding apparatus that decodes coded image data generated by coding image data for each block according to prediction based on a prediction mode, the image decoding apparatus comprising:

a restoration unit configured to restore a selected prediction mode used in the prediction at a time of coding, based on mode information indicating a result of estimation of the prediction mode executed at the time of coding; and

a decoding unit configured to decode a current block included in the coded image data to generate a decoded block, according to the prediction based on the selected prediction mode,

wherein the restoration unit includes:

a first prediction mode estimation unit configured to determine one of prediction modes as a first estimated prediction mode;

a second prediction mode estimation unit configured to determine, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

a prediction mode restoration unit configured to restore the selected prediction mode based on the mode information, the first estimated prediction mode, and the second estimated prediction mode.

12 . An image coding apparatus that codes image data for each block, the image coding apparatus comprising:

a coding unit configured to code a current block included in the image data, according to prediction based on a selected prediction mode selected from among predetermined prediction mode candidates;

a decoding unit configured to decode the coded current block to generate a decoded block;

a first prediction mode estimation unit configured to determine one of prediction modes as a first estimated prediction mode;

a second prediction mode estimation unit configured to determine, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

an output unit configured to output the coded current block and mode information to be used for restoring the selected prediction mode,

wherein the output unit includes a mode information generation unit configured to generate the mode information based on the first estimated prediction mode, the second estimated prediction mode, and the selected prediction mode.

13 . A non-transitory computer-readable recording medium for use in a computer, the recording medium having a computer program recorded thereon for decoding coded image data generated by coding image data for each block according to prediction based on a prediction mode, and the computer program causing the computer to execute:

restoring a selected prediction mode used in the prediction at a time of coding, based on mode information indicating a result of estimation of the prediction mode executed at the time of coding; and

decoding a current block included in the coded image data to generate a decoded block, according to the prediction based on the selected prediction mode,

wherein the restoring includes:

determining one of prediction modes as a first estimated prediction mode;

determining, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

restoring the selected prediction mode based on the mode information, the first estimated prediction mode, and the second estimated prediction mode.

14 . A non-transitory computer-readable recording medium for use in a computer, the recording medium having a computer program recorded thereon for coding image data for each block, and the computer program causing the computer to execute:

coding a current block included in the image data, according to prediction based on a selected prediction mode selected from among predetermined prediction mode candidates;

decoding the coded current block to generate a decoded block;

determining one of prediction modes as a first estimated prediction mode;

determining, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

outputting the coded current block and mode information to be used for restoring the selected prediction mode,

wherein the outputting includes generating the mode information based on the first estimated prediction mode, the second estimated prediction mode, and the selected prediction mode.

15 . An integrated circuit that decodes coded image data generated by coding image data for each block according to prediction based on a prediction mode, the integrated circuit comprising:

a restoration unit configured to restore a selected prediction mode used in the prediction at a time of coding, based on mode information indicating a result of estimation of the prediction mode executed at the time of coding; and

a decoding unit configured to decode a current block included in the coded image data to generate a decoded block, according to the prediction based on the selected prediction mode,

wherein the restoration unit includes:

a first prediction mode estimation unit configured to determine one of prediction modes as a first estimated prediction mode;

a second prediction mode estimation unit configured to determine, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

a prediction mode restoration unit configured to restore the selected prediction mode based on the mode information, the first estimated prediction mode, and the second estimated prediction mode.

16 . An integrated circuit that codes image data for each block, the integrated circuit comprising:

a coding unit configured to code a current block included in the image data, according to prediction based on a selected prediction mode selected from among predetermined prediction mode candidates;

a decoding unit configured to decode the coded current block to generate a decoded block;

a first prediction mode estimation unit configured to determine one of prediction modes as a first estimated prediction mode;

a second prediction mode estimation unit configured to determine, as a second estimated prediction mode, an other one of the prediction modes that is different from the first estimated prediction mode; and

an output unit configured to output the coded current block and mode information to be used for restoring the selected prediction mode,

wherein the output unit includes a mode information generation unit configured to generate the mode information based on the first estimated prediction mode, the second estimated prediction mode, and the selected prediction mode.