Method and apparatus for deriving motion vector

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

deriving a temporary reference motion vector for a current block included in a current picture;

deriving a reference motion vector for the current block by applying clipping to the temporary reference motion vector;

deriving a motion vector for the current block based on the reference motion vector;

deriving a predicted block for the current block based on the motion vector for the current block; and

generating a reconstructed picture for the current picture based on the predicted block for the current block,

wherein a number of bits of a binary code representing the reference motion vector is less than a number of bits of a binary code representing the temporary reference motion vector,

wherein the reference motion vector is derived based on an equation below,

mv =Clip3(−2 17 , 2 17 −1, mv ′), and

wherein the mv′ in the equation represents the temporary reference motion vector, and the mv represents the reference motion vector.

2. The image decoding method of claim 1 , wherein

the reference motion vector is derived based on a temporal neighboring block included in a reference picture of the current picture.

3. The image decoding method of claim 1 , wherein

the reference motion vector is related to an affine control point motion vector derived based on an affine model.

4. The image decoding method of claim 1 , wherein

the number of bits of the binary code representing the reference motion vector is 16 or less.

5. The image decoding method of claim 1 , wherein

the number of bits of the binary code representing the reference motion vector is 18 or less.

6. The image decoding method of claim 1 , wherein

the binary code representing the reference motion vector for the current block includes at least one bit representing a sign of the reference motion vector and at least one bit representing a size of the reference motion vector, and

the binary code representing the temporary reference motion vector for the current block includes at least one bit representing a sign of the temporary reference motion vector and at least one bit representing a size of the temporary reference motion vector.

7. The image decoding method of claim 6 , wherein

the at least one bit representing the sign of the reference motion vector and the at least one bit representing the sign of the temporary reference motion vector are the same, and

a number of at least one bit representing the size of the reference motion vector is less than a number of at least one bit representing the size of the temporary reference motion vector.

8. The image decoding method of claim 1 , wherein

the deriving of the motion vector for the current block comprises:

deriving a temporary motion vector for the current block based on the reference motion vector; and

deriving the motion vector for the current block by applying the clipping to the temporary motion vector.

9. The image decoding method of claim 8 , wherein

a number of bits of a binary code representing the motion vector for the current block is 16 or less.

10. The image decoding method of claim 8 , wherein

a number of bits of a binary code representing the motion vector for the current block is 18 or less.

11. The image decoding method of claim 8 , wherein

the binary code representing the motion vector for the current block includes at least one bit representing a sign of the motion vector and at least one bit representing a size of the motion vector,

the binary code representing the temporary motion vector for the current block includes at least one bit representing a sign of the temporary motion vector and at least one bit representing a size of the temporary motion vector,

the at least one bit representing the sign of the motion vector and the at least one bit representing the sign of the temporary motion vector are the same, and

a number of the at least one bit representing the size of the motion vector is less than a number of the at least one bit representing the size of the temporary motion vector.

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

deriving a temporary reference motion vector for a current block included in a current picture based on a reference candidate of the current block;

deriving a reference motion vector for the current block by applying clipping to the temporary reference motion vector;

deriving a motion vector for the current block based on the reference motion vector;

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

encoding image information including information on the residual samples,

wherein a number of bits of a binary code representing the reference motion vector is less than a number of bits of a binary code representing the temporary reference motion vector,

wherein the reference motion vector is derived based on an equation below,

mv =Clip3(−2 17 , 2 17 −1, mv ′), and

wherein the mv′ in the equation represents the temporary reference motion vector, and the mv represents the reference motion vector.

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

deriving a temporary reference motion vector for a current block included in a current picture based on a reference candidate of the current block;

deriving a reference motion vector for the current block by applying clipping to the temporary reference motion vector;

deriving a motion vector for the current block based on the reference motion vector;

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

encoding image information including information on the residual samples,

wherein a number of bits of a binary code representing the reference motion vector is less than a number of bits of a binary code representing the temporary reference motion vector,

wherein the reference motion vector is derived based on an equation below,

mv =Clip3(−2 17 , 2 17 −1, mv ′), and

wherein the mv′ in the equation represents the temporary reference motion vector, and the mv represents the reference motion vector.