Image decoding method and image decoding apparatus

1. An image decoding method for decoding a coded signal, the image decoding method comprising:

decoding first reference picture information for identifying one or more first reference pictures, from a picture header included in the coded signal;

determining whether or not a second reference picture is stored in a buffer, the second reference picture being one of the one or more first reference pictures;

determining, when the second reference picture is not stored in the buffer, a picture closest in display order to the second reference picture, among pictures stored in the buffer, to be a third reference picture;

decoding a current picture from the coded signal, using the third reference picture; and

performing a random access operation to skip decoding of a picture between a picture last decoded before the random access operation and a picture to be decoded first after the random access operation,

wherein when the current picture is the picture to be decoded first after the random access operation and the second reference picture is the picture of which decoded is skipped, in the determining of a third reference picture, the picture last decoded before the random access operation or a picture which is last in display order before the random access operation, among the pictures stored in the buffer, is determined to be the third reference picture.

2. The image decoding method according to claim 1 , further comprising

generating second reference picture information which is used in common in a picture and identifies one or more fourth reference pictures, the one or more fourth reference pictures being derived by replacing, among the one or more first reference pictures, the second reference picture with the third reference picture,

wherein, in the decoding of a current picture, the third reference picture is selected from the one or more fourth reference pictures, using the second reference picture information.

3. The image decoding method according to claim 2 ,

wherein the second reference picture information is a reference picture list indicating a correspondence relationship between (i) a reference index which is included in the coded signal and indicates a reference picture to be used, and (ii) the reference picture stored in the buffer.

4. The image decoding method according to claim 1 ,

wherein, in the determining of a third reference picture, when a plurality of pictures closest in display order to the second reference picture are present, a picture closest in display order to the current picture, among the plurality of pictures, is determined to be the third reference picture.

5. The image decoding method according to claim 1 ,

wherein in the decoding of a first reference picture:

a header of a picture unit is identified using information which is included in a slice header of a current slice included in the current picture and identifies the header of the picture unit;

a header of a unit of pictures is identified using information which is included in the identified header of the picture unit and identifies the header of the unit of the pictures, the header of the unit of the pictures being the picture header; and

the first reference picture information is obtained from the identified header of the unit of the pictures.

6. The image decoding method according to claim 1 ,

wherein the first reference picture information is information indicating a difference in display order between each of the one or more first reference pictures and the current picture.

7. An image decoding apparatus which decodes a coded signal, the image decoding apparatus comprising:

processing circuitry; and

storage accessible from the processing circuitry,

wherein the processing circuitry executes, using the storage, the image decoding method according to claim 1 .

8. An image decoding apparatus which decodes a coded signal, the image decoding apparatus comprising:

a first reference picture information decoding unit configured to decode first reference picture information for identifying one or more first reference pictures which are referable by a current picture, from a picture header included in the coded signal;

a determination unit configured to determine whether or not a second reference picture is stored in a buffer, the second reference picture being one of the one or more first reference pictures;

a third reference picture determination unit configured to determine, when the second reference picture is not stored in the buffer, a picture closest in display order to the second reference picture, among pictures stored in the buffer, to be a third reference picture;

a current picture decoding unit configured to decode the current picture from the coded signal, using the third reference picture,

wherein the current picture decoding unit is further configured to perform a random access operation to skip decoding of a picture between a picture last decoded before the random access operation and a picture to be decoded first after the random access operation, and

wherein when the current picture is the picture to be decoded first after the random access operation and the second reference picture is the picture of which decoded is skipped, in the determining of a third reference picture, the third reference picture determination unit determines the picture last decoded before the random access operation or a picture which is last in display order before the random access operation, among the pictures stored in the buffer, to be the third reference picture.