Image coding method, image decoding method, image coding apparatus, and image decoding apparatus
An image coding method includes: deriving a candidate for a motion vector predictor from a co-located motion vector; adding the candidate to a list; selecting the motion vector predictor from the list; and coding a current block and coding a current motion vector, wherein the deriving includes: deriving the candidate by a first derivation scheme in the case of determining that each of a current reference picture and a co-located reference picture is a long-term reference picture; and deriving the candidate by a second derivation scheme in the case of determining that each of the current reference picture and the co-located reference picture is a short-term reference picture.
1. A motion vector predictor deriving method of deriving a motion vector predictor for each block among blocks of pictures, the motion vector predictor deriving method comprising:
decoding classification information of pictures from a coded stream;
deriving a candidate for a motion vector predictor to be used in coding of a motion vector for a current block to be coded, from a motion vector of a collocated block included in a collocated picture, the collocated picture being different from a current picture that includes the current block; and
adding the derived candidate to a list of candidates,
wherein a reference picture of the current block is different from the collocated picture, and
wherein the deriving includes:
determining, based on the classification information, whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the collocated block is a long-term reference picture or a short-term reference picture;
deriving the candidate by directly setting the motion vector of the collocated block as the candidate without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the reference picture of the collocated block is a long-term reference picture; and
deriving the candidate from the motion vector of the collocated block by scaling the motion vector based on a temporal distance in the case of determining that each of the reference picture of the current block and the reference picture of the collocated block is a short-term reference picture.
2. A motion vector predictor deriving apparatus that derives a motion vector predictor for each block among blocks of pictures, the motion vector predictor deriving apparatus comprising:
a decoding unit configured to decode classification information of pictures from a coded stream;
a deriving unit configured to derive a candidate for a motion vector predictor to be used in coding of a motion vector for a current block to be coded, from a motion vector of a collocated block included in a collocated picture, the collocated picture being different from a current picture that includes the current block; and
an adding unit configured to add the derived candidate to a list of candidates,
wherein a reference picture of the current block is different from the collocated picture, and
the deriving unit is configured to:
determine, based on the classification information, whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the collocated block is a long-term reference picture or a short-term reference picture;
derive the candidate by directly setting the motion vector of the collocated block as the candidate without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the reference picture of the collocated block is a long-term reference picture; and
derive the candidate from the motion vector of the collocated block by scaling the motion vector based on a temporal distance in the case of determining that each of the reference picture of the current block and the reference picture of the collocated block is a short-term reference picture.
3. A motion vector predictor deriving apparatus that derives a motion vector predictor for each block among blocks of pictures, the motion vector predictor deriving apparatus comprising:
at least one processor; and
storage accessible from the at least one processor,
wherein the at least one processor is configured, using the storage, to perform operations comprising:
decoding classification information of pictures from a coded stream;
deriving a candidate for a motion vector predictor to be used in coding of a motion vector for a current block to be coded, from a motion vector of a collocated block included in a collocated picture, the collocated picture being different from a current picture that includes the current block; and
adding the derived candidate to a list of candidates,
wherein a reference picture of the current block is different from the collocated picture, and
the deriving includes:
determining, based on the classification information, whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the collocated block is a long-term reference picture or a short-term reference picture;
deriving the candidate by directly setting from the motion vector of the collocated block as the candidate without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the reference picture of the collocated block is a long-term reference picture; and
deriving the candidate from the motion vector of the collocated block by scaling the motion vector based on a temporal distance in the case of determining that each of the reference picture of the current block and the reference picture of the collocated block is a short-term reference picture.