Image decoding method and image decoding apparatus using candidate motion vectors

1. An image decoding method for decoding a current block, the image decoding method comprising:

deriving a first candidate for decoding the current block, the first candidate having a first motion vector that has been used to decode a first block, a first prediction direction that corresponds to the first motion vector, and a first reference picture index that identifies a first reference picture corresponding to the first motion vector;

deriving a second candidate for decoding the current block, the second candidate having a second motion vector that has been used to decode a second block, a second prediction direction that corresponds to the second motion vector, and a second reference picture index that identifies a second reference picture corresponding to the second motion vector;

decoding information included in a slice header indicative of a maximum number, wherein the slice header corresponds to a slice containing a plurality of blocks including the current block;

determining whether or not a total number of derived candidates for decoding the current block is less than the maximum number based on the decoded information from the slice header;

when the total number of derived candidates is less than the maximum number, deriving a third candidate by combining the first motion vector, the first reference picture index, and the first prediction direction of the first motion vector and the second motion vector, the second reference picture index, and the second prediction direction of the second motion vector; and

decoding the current block by using a candidate selected from candidates including the first candidate, the second candidate and the third candidate.

2. The image decoding method according to claim 1 , wherein the first block comprises one of a spatial neighboring block of the current block or a co-located block of the current block.

3. The image decoding method according to claim 1 , wherein the second block comprises one of a spatial neighboring block of the current block or a co-located block of the current block.

4. The image decoding method according to claim 1 , wherein the first block comprises a spatial neighboring block of the current block and the second block comprises a co-located block of the current block.

5. The image decoding method according to claim 1 , wherein one of the first block or the second block comprises a co-located block of the current block coded using bi-directional prediction.

6. The image decoding method according to claim 1 , wherein the selected candidate is selected by decoding a variable-length coded index, wherein the maximum length of the variable-length coded index is based on the maximum number.

7. The image decoding method according to claim 1 , further comprising decoding a coded index corresponding to the selected candidate, wherein the maximum number is used to decode the coded index from a bitstream.

8. The image decoding method according to claim 1 , wherein deriving the first candidate includes deriving a plurality of first candidates, and the image decoding method further comprises removing a candidate that is identical to the first candidate from the plurality of first candidates before determining that a total number of candidates is less than a maximum number.

9. The image decoding method according to claim 1 , wherein deriving the first candidate includes deriving a plurality of first candidates, and the image decoding method further comprises removing an unusable-for-merging candidate from the plurality of first candidates before determining that a total number of candidates is less than a maximum number.

10. An image decoding apparatus that decodes a current block, the image decoding apparatus comprising:

a memory storing executable instructions, and

a processor adapted to execute the instructions to cause the apparatus to perform operations comprising:

deriving a first candidate for decoding the current block, the first candidate having a first motion vector that has been used to decode a first block, a first prediction direction that corresponds to the first motion vector, and a first reference picture index that identifies a first reference picture corresponding to the first motion vector;

deriving a second candidate for decoding the current block, the second candidate having a second motion vector that has been used to decode a second block, a second prediction direction that corresponds to the second motion vector, and a second reference picture index that identifies a second reference picture corresponding to the second motion vector;

decoding information included in a slice header indicative of a maximum number, wherein the slice header corresponds to a slice containing a plurality of blocks including the current block;

determining whether or not a total number of derived candidates for decoding the current block is less than the maximum number based on the decoded information from the slice header;

when the total number of derived candidates is less than the maximum number, deriving a third candidate by combining the first motion vector, the first reference picture index, and the first prediction direction of the first motion vector and the second motion vector, the second reference picture index, and the second prediction direction of the second motion vector; and

decoding the current block by using a candidate selected from candidates including the first candidate, the second candidate and the third candidate.

11. The image decoding apparatus of claim 10 , wherein the first block comprises one of a spatial neighboring block of the current block or a co-located block of the current block.

12. The image decoding apparatus of claim 10 , wherein the second block comprises one of a spatial neighboring block of the current block or a co-located block of the current block.

13. The image decoding apparatus of claim 10 , wherein the first block comprises a spatial neighboring block of the current block and the second block comprises a co-located block of the current block.

14. The image decoding apparatus of claim 10 , wherein one of the first block or the second block comprises a co-located block of the current block coded using bi-directional prediction.

15. The image decoding apparatus of claim 10 , wherein the selected candidate is selected by decoding a variable-length coded index, wherein the maximum length of the variable-length coded index is based on the maximum number.

16. The image decoding apparatus of claim 10 , wherein execution of the instructions causes the apparatus to perform operations comprising decoding a coded index corresponding to the selected candidate, wherein the maximum number is used to decode the coded index from a bitstream.

17. The image decoding apparatus of claim 10 , wherein deriving the first candidate includes deriving a plurality of first candidates, and execution of the instructions causes the apparatus to perform operations further comprising removing a candidate that is identical to the first candidate from the plurality of first candidates before determining that a total number of candidates is less than a maximum number.

18. The image decoding apparatus of claim 10 , wherein deriving the first candidate includes deriving a plurality of first candidates, and execution of the instructions causes the apparatus to perform operations further comprising removing an unusable-for-merging candidate from the plurality of first candidates before determining that a total number of candidates is less than a maximum number.