Motion vector calculation method, picture coding method, picture decoding method, motion vector calculation apparatus, and picture coding and decoding apparatus

A decoding apparatus for decoding a current block included in a current picture, the current picture being included in a coded video stream, the decoding apparatus comprising:

• a processor;
• a non-transitory memory having stored thereon executable instructions, which when executed by the processor, cause the processor to perform the following steps:
• determining a reference picture in the coded video stream, the reference picture being included in one of (i) a first reference picture group of the current block and (ii) a second reference picture group of the current block;
• selecting a reference motion vector among from one or more reference motion vectors of a reference block in the reference picture such that in situation (A) when the reference block has a first reference motion vector and a second reference motion vector that respectively correspond to the first reference picture group and the second reference picture group, (i) the first reference motion vector is selected when the reference picture is included in the second reference picture group and (ii) the second reference motion vector is selected when the reference picture is included in the first reference picture group, in situation (B) when the reference block has only one reference motion vector, the only reference motion vector is selected, and in situation (C) when the reference block has no reference motion vector, a zero reference motion vector is selected;
• deriving the motion vector of the current block using the selected one reference motion vector;
• and
• decoding the current block using the derived motion vector.

A decoding apparatus for decoding a current block included in a current picture, the current picture being included in a coded video stream, the decoding method comprising:

• processing circuitry configured to:
• determine a reference picture in the coded video stream, the reference picture being included in one of (i) a first reference picture group of the current block and (ii) a second reference picture group of the current block;
• select a reference motion vector among from one or more reference motion vectors of a reference block in the reference picture such that in situation (A) when the reference block has a first reference motion vector and a second reference motion vector that respectively correspond to the first reference picture group and the second reference picture group, (i) the first reference motion vector is selected when the reference picture is included in the second reference picture group and (ii) the second reference motion vector is selected when the reference picture is included in the first reference picture group, in situation (B) when the reference block has only one reference motion vector, the only reference motion vector is selected, and in situation (C) when the reference block has no reference motion vector, a zero reference motion vector is selected;
• derive the motion vector of the current block using the selected one reference motion vector;
• and
• decoding circuitry configured to decode the current block using the derived motion vector.