Method and apparatus for signaling an offset in video coding for intra block copy and/or inter prediction

1. A method for video decoding in a decoder, comprising:

decoding prediction information of a current block in a current picture from a coded video bitstream, the prediction information indicating a prediction mode for reconstructing the current block being one of: an inter prediction mode and an intra block copy (IBC) mode;

selecting a base vector from a candidate list including a motion vector candidate used in the inter prediction mode, the base vector being the motion vector candidate in response to the prediction mode being the inter prediction mode;

determining an offset vector based on offset information that includes offset directions and step sizes for constructing offset vectors;

determining a motion vector of the current block based on the motion vector candidate and the offset vector in response to the prediction mode being the inter prediction mode; and

reconstructing at least one sample in the current block based on the motion vector in response to the prediction mode being the inter prediction mode, wherein an offset between the base vector and a base predictor as determined by the offset vector is along one of an x-axis direction and a y-axis direction but not along a diagonal direction, and the step sizes are biased such that one of the step sizes has a shorter codeword than another one of the step sizes.

2. The method of claim 1 , wherein

the prediction mode is the IBC mode,

the candidate list includes a block vector candidate used in the IBC mode,

the base vector is the block vector candidate, and

the method further includes determining a block vector of the current block based on the block vector candidate and the offset vector.

3. The method of claim 2 , wherein

the prediction information includes a base index that points to the block vector candidate; and

the selecting the base vector from the candidate list further includes:

determining the base vector to be the block vector candidate based on the base index.

4. The method of claim 2 , wherein

the prediction information includes an offset direction index and a distance index; and

the determining the offset vector further includes:

determining an offset direction of the offset vector based on the offset direction index and the offset directions;

determining a step size of the offset vector based on the distance index and the step sizes; and

determining the offset vector based on the offset direction and the step size of the offset vector.

5. The method of claim 2 , wherein

the reconstructing includes reconstructing the at least one sample in the current block based on the block vector of the current block, and

the offset directions are biased such that

one of the offset directions has a longer codeword than another one of the offset directions or does not have an assigned codeword.

6. The method of claim 4 , wherein the offset directions are biased such that one of the offset directions has a longer codeword than another one of the offset directions or does not have an assigned codeword.

7. The method of claim 6 , wherein the offset directions are biased toward a negative direction in one of: the x-axis direction and the y-axis direction.

8. The method of claim 4 , wherein each step size in the step sizes is equal to one of (i) one sample size and (ii) multiple sample sizes.

9. The method of claim 1 , wherein

the offset information indicates a number of offset directions and a number of step sizes used to predict the offset vector,

the candidate list includes a block vector candidate used in the IBC mode,

the base vector is the block vector candidate in response to the prediction mode being the IBC mode, and

the method further includes determining a block vector of the current block based on the block vector candidate and the offset vector in response to the prediction mode being the IBC mode.

10. The method of claim 9 , wherein the offset directions include a combined set of offset directions and the step sizes include a combined set of step sizes, and the combined set of offset directions and the combined set of step sizes are for constructing the offset vectors for the respective inter prediction mode and the IBC mode.

11. The method of claim 9 , wherein the offset directions include separate sets of offset directions or the step sizes include separate sets of step sizes for constructing the offset vectors for the respective inter prediction mode and the IBC mode.

12. The method of claim 9 , wherein

the prediction information includes a base index; and

the selecting the base vector from the candidate list further includes:

in response to the base index pointing to the block vector candidate in the candidate list, determining that the prediction mode is the IBC mode and the base vector is the block vector candidate; and

in response to the base index pointing to the motion vector candidate in the candidate list, determining that the prediction mode is the inter prediction mode and the base vector is the motion vector candidate.

13. The method of claim 2 , further comprising:

clipping the block vector to obtain a clipped block vector, a reference block pointed by the clipped block vector being shifted from the current block by one of (i) one sample size and (ii) multiple sample sizes in an x and a y direction.

14. An apparatus for video decoding, comprising processing circuitry configured to:

decode prediction information of a current block in a current picture from a coded video bitstream, the prediction information indicating a prediction mode for reconstructing the current block being one of: an inter prediction mode and an intra block copy (IBC) mode;

select a base vector from a candidate list including a motion vector candidate used in the inter prediction mode, the base vector being the motion vector candidate in response to the prediction mode being the inter prediction mode;

determine an offset vector based on offset information that includes offset directions and step sizes for constructing offset vectors;

determine a motion vector of the current block based on the motion vector candidate and the offset vector in response to the prediction mode being the inter prediction mode; and

reconstruct at least one sample in the current block based on the motion vector in response to the prediction mode being the inter prediction mode, wherein an offset between the base vector and a base predictor as determined by the offset vector is along one of an x-axis direction and a y-axis direction but not along a diagonal direction, and the step sizes are biased such that one of the step sizes has a shorter codeword than another one of the step sizes.

15. The apparatus of claim 14 , wherein

the prediction mode is the IBC mode,

the candidate list includes a block vector candidate used in the IBC mode,

the base vector is the block vector candidate, and

the processing circuitry is configured to determine a block vector of the current block based on the block vector candidate and the offset vector.

16. The apparatus of claim 15 , wherein

the prediction information includes a base index that points to the block vector candidate; and

the processing circuitry is configured to determine the base vector to be the block vector candidate based on the base index.

17. The apparatus of claim 15 , wherein

the prediction information includes an offset direction index and a distance index; and

the processing circuitry is configured to:

determine an offset direction of the offset vector based on the offset direction index and the offset directions;

determine a step size of the offset vector based on the distance index and the step sizes; and

determine the offset vector based on the offset direction and the step size of the offset vector.

18. The apparatus of claim 15 , wherein

the processing circuitry is configured to reconstruct the at least one sample in the current block based on the block vector of the current block, and

the offset directions are biased such that

one of the offset directions has a longer codeword than another one of the offset directions or does not have an assigned codeword.

19. The apparatus of claim 17 , wherein: the offset directions are biased such that one of the offset directions has a longer codeword than another one of the offset directions or does not have an assigned codeword.

20. A non-transitory computer-readable storage medium storing a program executable by at least one processor to perform:

decoding prediction information of a current block in a current picture from a coded video bitstream, the prediction information indicating a prediction mode for reconstructing the current block being one of: an inter prediction mode and an intra block copy (IBC) mode;

selecting a base vector from a candidate list including a motion vector candidate used in the inter prediction mode, the base vector being the motion vector candidate in response to the prediction mode being the inter prediction mode;

determining an offset vector based on offset information that includes offset directions and step sizes for constructing offset vectors;

determining a motion vector of the current block based on the motion vector candidate and the offset vector in response to the prediction mode being the inter prediction mode; and

reconstructing at least one sample in the current block based on the motion vector in response to the prediction mode being the inter prediction mode, wherein an offset between the base vector and a base predictor as determined by the offset vector is along one of an x-axis direction and a y-axis direction but not along a diagonal direction, and the step sizes are biased such that one of the step sizes has a shorter codeword than another one of the step sizes.