Image decoding method and image decoding apparatus using candidate motion vectors
A decoding method and apparatus are provided. A 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 is derived. A 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 is derived. When a total number of candidates is less than a maximum number, a third candidate is derived by combining the first motion vector and the first reference picture index for the first prediction direction of the first candidate and the second motion vector and the second reference picture index for the second prediction direction of the second candidate.
1. An image encoding method for encoding a current block, the image encoding method comprising:
encoding information in a slice header indicative of a maximum number of merging candidates, wherein the slice header corresponds to a slice containing a plurality of blocks including the current block;
deriving a plurality of merging candidates for encoding the current block from a corresponding plurality of blocks that are neighbor or co-located blocks of the current block, each of the plurality of merging candidates having a motion vector for encoding a corresponding one of the plurality of blocks, a prediction direction that corresponds to the corresponding one of the plurality of blocks, and a reference picture index that identifies a reference picture corresponding to the corresponding one of the plurality of blocks;
determining whether a number of the plurality of merging candidates is less than the maximum number of merging candidates;
when it is determined that the number of the plurality of merging candidates is less than the maximum number of merging candidates, deriving at least one combined candidate for bi-directional prediction, by:
assigning, the motion vector and the reference picture index of a first merging candidate of the plurality of merging candidates, to a first prediction direction of the at least one combined candidate, and
assigning, the motion vector and the reference picture index of a second merging candidate of the plurality of merging candidates, to a second prediction direction of the at least one combined candidate;
selecting a candidate from the plurality of merging candidates and the at least one combined candidate;
encoding an index corresponding to the selected candidate, wherein the maximum number of merging candidates is used to encode the index; and
encoding the current block by using the selected candidate.
2. The image encoding method according to claim 1 , wherein the prediction direction of the first merging candidate is different from the prediction direction of the second merging candidate.
3. The image encoding method according to claim 1 , wherein deriving the plurality of merging candidates includes removing any derived merging candidate that is identical to another derived merging candidate from the plurality of merging candidates before determining whether the number of the plurality of merging candidates is less than the maximum number of merging candidates.
4. The image encoding method according to claim 1 , wherein deriving the plurality of merging candidates includes removing an unusable-for-merging candidate from the plurality of merging candidates before determining whether the number of the plurality of merging candidates is less than the maximum number of merging candidates.
5. An image encoding apparatus that decodes a current block, the image encoding apparatus comprising:
a processor;
a memory storing instructions, which when executed by the processor cause the apparatus to:
encode information in a slice header indicative of a maximum number of merging candidates, wherein the slice header corresponds to a slice containing a plurality of blocks including the current block;
derive a plurality of merging candidates for encoding the current block from a corresponding plurality of blocks that are neighbor or co-located blocks of the current block, each of the plurality of merging candidates having a motion vector for encoding a corresponding one of the plurality of blocks, a prediction direction that corresponds to the corresponding one of the plurality of blocks, and a reference picture index that identifies a reference picture corresponding to the corresponding one of the plurality of blocks;
determine whether a number of the plurality of merging candidates is less than the maximum number of merging candidates;
when it is determined that the number of the plurality of merging candidates is less than the maximum number of merging candidates, derive at least one combined candidate for bi-directional prediction, by:
assign, the motion vector and the reference picture index of a first merging candidate of the plurality of merging candidates, to a first prediction direction of the at least one combined candidate, and
assign, the motion vector and the reference picture index of a second merging candidate of the plurality of merging candidates, to a second prediction direction of the at least one combined candidate;
select a candidate from the plurality of merging candidates and the at least one combined candidate;
encode an index corresponding to the selected candidate, wherein the maximum number of merging candidates is used to encode the index; and
encode the current block by using the selected candidate.
6. The image encoding apparatus according to claim 5 , wherein the prediction direction of the first merging candidate is different from the prediction direction of the second merging candidate.
7. The image encoding apparatus according to claim 5 , wherein deriving the plurality of merging candidates includes removing any derived merging candidate that is identical to another derived merging candidate from the plurality of merging candidates before determining whether the number of the plurality of merging candidates is less than the maximum number of merging candidates.
8. The image encoding apparatus according to claim 5 , wherein deriving the plurality of merging candidates includes removing an unusable-for-merging candidate from the plurality of merging candidates before determining whether the number of the plurality of merging candidates is less than the maximum number of merging candidates.