Process and apparatus for controlling compressed motion vectors

An image encoding method performed by an encoding apparatus, the method comprising:

• deriving a temporary reference motion vector for a current block based on a reference candidate block of the current block;
• deriving a reference motion vector by compressing the temporary reference motion vector based on a shifting operation;
• storing the reference motion vector;
• deriving a merge candidate list including merge candidates for the current block based on the reference motion vector;
• deriving a motion vector for the current block based on the merge candidate list;
• deriving prediction samples for the current block based on the motion vector for the current block;
• deriving merge index information of the current block based on the merge candidate list;
• and
• encoding image information including the merge index information,
• wherein a total number of bits of a binary code representing the reference motion vector is less than a total number of bits of a binary code representing the temporary reference motion vector,
• wherein the shifting operation is a right shifting operation,
• wherein the reference motion vector is derived further based on adding an offset value related to a rounding process, and
• wherein the offset value is derived based on the temporary reference motion vector.

A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:

• deriving a temporary reference motion vector for a current block based on a reference candidate block of the current block;
• deriving a reference motion vector by compressing the temporary reference motion vector based on a shifting operation;
• storing the reference motion vector;
• deriving a merge candidate list including merge candidates for the current block based on the reference motion vector;
• deriving a motion vector for the current block based on the merge candidate list;
• deriving prediction samples for the current block based on the motion vector for the current block;
• deriving merge index information of the current block based on the merge candidate list;
• and
• encoding image information including the merge index information,
• wherein a total number of bits of a binary code representing the reference motion vector is less than a total number of bits of a binary code representing the temporary reference motion vector,
• wherein the shifting operation is a right shifting operation,
• wherein the reference motion vector is derived further based on adding an offset value related to a rounding process, and
• wherein the offset value is derived based on the temporary reference motion vector.