Image coding method, image decoding method, image coding apparatus, and image decoding apparatus

An image encoding method of encoding each block among blocks of pictures, the image encoding method comprising:

• deriving a bi-predictive candidate for motion vectors of a current block to be encoded, from a first motion vector of a first block included in a first picture,
• the first picture being different from a current picture that includes the current block,
• the first block being located in the first picture at (i) a position corresponding to a position bottom right adjacent to the current block or (ii) when the position bottom right is not available, a position corresponding to a position at center of the current block in the current picture;
• adding the derived bi-predictive candidate to a list of candidates;
• selecting one bi-predictive candidate that includes a second motion vector and a third motion vector from the list of candidates;
• and
• encoding the current block using (i) the second motion vector and the third motion vector and (ii) a second reference picture and a third reference picture of the current block, the second reference picture and the third reference picture respectively corresponding to the second motion vector and the third motion vector,
• wherein the deriving includes:
• determining (i) whether each of the second reference picture and the third reference picture of the current block is a long-term reference picture or a short-term reference picture, and (ii) whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture;
• deriving each of the second motion vector and the third motion vector so as to be the same as the first motion vector without scaling based on a temporal distance in the case of determining that (i) the corresponding one of the second reference picture and the third reference picture is a long-term reference picture and (ii) the first reference picture is a long-term reference picture;
• and
• deriving each of the second motion vector and the third motion vector from the first motion vector by scaling based on a temporal distance in the case of determining that (i) the corresponding one of the second reference picture and the third reference picture is a short-term reference picture and (ii) the first reference picture is a short-term reference picture.