Process and apparatus for controlling compressed motion vectors

1. An image decoding method performed by a decoding apparatus, the method comprising:

obtaining image information including merge index information from a bitstream;

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 index information and the merge candidate list;

deriving prediction samples for the current block based on the motion vector for the current block; and

generating reconstructed samples for the current block based on the prediction samples for the current block,

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.

2. The method of claim 1 , wherein the reference candidate block is a temporal neighboring block included in a reference picture of a current picture including the current block.

3. The method of claim 1 , wherein a precision value representing the temporary reference motion vector is smaller than a precision value representing the reference motion vector.

4. The method of claim 3 , wherein the precision value representing the temporary reference motion vector is 1/16, and the precision value representing the reference motion vector is ¼.

5. The method of claim 1 , wherein the reference motion vector is derived based on the following equation,

mv =( mv ′+offset)>>2

wherein mv represents the reference motion vector, mv′ represents the temporary reference motion vector, and offset represents the offset value related to the rounding process.

6. The method of claim 3 , wherein the reference motion vector is derived based on the following equation,

mv =( mv ′+offset)>> M Shift

wherein mv represents the reference motion vector, mv′ represents the temporary reference motion vector, offset represents the offset value related to the rounding process, Mshift is derived based on the precision value representing the temporary reference motion vector and the precision value representing the reference motion vector.

7. 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.

8. The method of claim 7 , wherein a precision value representing the temporary reference motion vector is smaller than a precision value representing the reference motion vector.

9. 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.