Moving picture coding method and moving picture decoding method using a determination whether or not a reference block has two reference motion vectors that refer forward in display order with respect to a current picture

1. A moving picture coding method of coding a current block to be coded which is included in a current picture to be coded, said moving picture coding method comprising:

determining a display order between the current picture and each of a plurality of reference pictures included in at least one of a first reference picture list and a second reference picture list, the display order being determined using an index of each of the reference pictures;

determining whether or not a reference block has two reference motion vectors, each of the two reference motion vectors referring to a block included in a picture temporally preceding the current picture in the display order, the reference block being included in one of the reference pictures, the one of the reference pictures temporally preceding the current picture in the display order;

calculating, when the reference block has the two reference motion vectors, (i) a first candidate motion vector of the current block by scaling one of the two reference motion vectors and (ii) a second candidate motion vector of the current block by scaling the other one of the two reference motion vectors;

adding the first candidate motion vector and the second candidate motion vector into a candidate list;

selecting a candidate motion vector from among candidates of motion vectors included in the candidate list; and

coding (i) the current block using a current motion vector, (ii) a difference between the current motion vector and the selected candidate motion vector, and (iii) information for identifying the selected candidate motion vector.

2. The moving picture coding method according to claim 1 ,

wherein when an order of assigning an index to a picture which can be referred to by the reference picture is same for the first reference picture list and the second reference picture list, said determining determines whether the reference block has the two reference motion vectors that refer forward in the display order with respect to the current picture.

3. The moving picture coding method according to claim 1 ,

wherein in said calculating, when the reference block does not have the two reference motion vectors, a candidate motion vector of the current block is calculated by scaling, from among reference motion vectors of the reference block, a reference motion vector that refers backward in the display order with respect to the current picture.

4. The moving picture coding method according to claim 1 ,

wherein the current motion vector is a motion vector calculated by motion estimation.

5. The moving picture coding method according to claim 1 ,

wherein the information for identifying the candidate motion vector is an index, and

in said coding, when the index is coded, a bitstream having a longer code length is assigned as a value of the index increases.

6. The moving picture coding method according to claim 1 ,

wherein in said calculating, when the reference block does not have a reference motion vector, a candidate motion vector is calculated using, from among reference motion vectors of another reference block located backward in the display order with respect to the current picture, a reference motion vector that refers forward in the display order with respect to the current picture.

7. The moving picture coding method according to claim 6 ,

wherein in said calculating, when the reference block does not have the reference motion vector and when the other reference block does not have the reference motion vector that refers forward in the display order with respect to the current picture, the candidate motion vector is calculated using a reference motion vector of the other reference block that refers backward in the display order with respect to the current picture.

8. The moving picture coding method according to claim 1 ,

wherein in said calculating, in addition to the first candidate motion vector and the second candidate motion vector, a motion vector of a block adjacent to left of the current block is a third candidate motion vector, a motion vector of a block adjacent to top of the current block is a fourth candidate motion vector, and a motion vector of a block adjacent to upper right of the current block is a fifth candidate motion vector, and

a candidate motion vector having a minimum difference relative to the current motion vector is selected from among the first to fifth candidate motion vectors.

9. A moving picture coding apparatus of coding a current block to be coded which is included in a current picture to be coded, said moving picture coding apparatus comprising:

one or more processors; and

storage coupled to the one or more processors,

wherein the one or more processors is configured to:

determine a display order between the current picture and each of a plurality of reference pictures included in at least one of a first reference picture list and a second reference picture list, the display order being determined using an index of each of the reference pictures;

determine whether or not a reference block has two reference motion vectors, each of the two reference motion vectors referring to a block included in a picture temporally preceding the current picture in the display order, the reference block being included in one of the reference pictures, the one of the reference pictures temporally preceding the current picture in the display order;

calculate, when the reference block has the two reference motion vectors, (i) a first candidate motion vector of the current block by scaling one of the two reference motion vectors and (ii) a second candidate motion vector of the current block by scaling the other one of the two reference motion vectors;

add the first candidate motion vector and the second candidate motion vector into a candidate list;

select a candidate motion vector from among candidates of motion vectors included in the candidate list; and

code (i) the current block using a current motion vector, (ii) a difference between the current motion vector and the selected candidate motion vector, and (iii) information for identifying the selected candidate motion vector.