Method and apparatus for video coding

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

decoding prediction information of a block in a current picture from a coded video bitstream, the prediction information including a first index for a prediction offset associated with an affine model in an inter prediction mode, the affine model being used to transform between the block and a reference block in a reference picture that has been reconstructed;

determining a multiplier of a non-translational parameter of the affine model, the multiplier being determined according to a default value and an offset value of a first plurality of offset values determined according to the first index, the first plurality of offset values mapped to respective indexes; and

reconstructing at least a sample of the block according to the affine model.

2. The method of claim 1 , further comprising:

determining values of translational parameters of the affine model according to a motion vector.

3. The method of claim 2 , wherein

the multiplier of the affine model is a scaling factor in the affine model, and

the offset value indicates a delta value to be applied to the default value of the scaling factor in the affine model.

4. The method of claim 2 , wherein

the multiplier of the affine model is a rotation angle in the affine model, and

the offset value indicates a delta value to be applied to the default value.

5. The method of claim 1 , further comprising:

determining a first motion vector difference of the affine model according to which offset value of a second plurality of offset values is determined to be associated with a second index; and

deriving the affine model based on a predicted motion vector and the first motion vector difference.

6. The method of claim 5 , wherein

the offset value of the second plurality of offset values indicates a pixel distance of the first motion vector difference, and

the method further includes determining a direction of the first motion vector difference according to an index of another plurality of indexes.

7. The method of claim 5 , wherein

the offset value of the second plurality of offset values indicates a pixel distance of the first motion vector difference, and

the second index is associated with both a direction and the pixel distance of the first motion vector difference.

8. The method of claim 5 , further comprising:

determining a second motion vector difference of the affine model according to which of the second plurality of offset values is associated with a third index corresponding to one of the second plurality of offset values, the first and second motion vector differences being for two control points;

determining motion vectors for the two control points based on the first and second motion vector differences; and

deriving a 4-parameter affine model based on the motion vectors of the two control points.

9. The method of claim 5 , further comprising:

determining a second motion vector difference of the affine model according to which of the second plurality of offset values is associated with a third index corresponding to one of the second plurality of offset values,

determining a third motion vector difference of the affine model based on which of the second plurality of offset values is associated with a fourth index corresponding to one of the second plurality of offset values, the first, second, and third motion vector differences being for three control points;

determining motion vectors for the three control points based on the first, second, and third motion vector differences; and

deriving a 6-parameter affine model based on the motion vectors of the three control points.

10. The method of claim 1 , further comprising:

determining a first motion vector difference for a first control point based on an offset value associated with a second index corresponding to a second plurality of offset values;

predicting a second motion vector difference for a second control point based on the first motion vector difference;

decoding a prediction error to correct the second motion vector difference from the coded video bitstream;

determining a first motion vector for the first control point and a second motion vector for the second control point based on the first motion vector difference and the corrected second motion vector difference; and

deriving the affine model at least based on the first motion vector for the first control point and the second motion vector for the second control point.

11. An apparatus for video decoding, comprising:

processing circuitry configured to:

decode prediction information of a block in a current picture from a coded video bitstream, the prediction information including a first index for a prediction offset associated with an affine model in an inter prediction mode, the affine model being used to transform between the block and a reference block in a reference picture that has been reconstructed;

determining a multiplier of a non-translational parameter of the affine model, the multiplier being determined according to a default value and an offset value of a first plurality of offset values determined according to the first index, the first plurality of offset values mapped to respective indexes; and

reconstruct at least a sample of the block according to the affine model.

12. The apparatus of claim 11 , wherein

the processing circuitry is configured to determine values of translational parameters of the affine model according to a motion vector.

13. The apparatus of claim 12 , wherein

the multiplier of the affine model is a scaling factor in the affine model, and

the offset value indicates a delta value to be applied to the default value of the scaling factor in the affine model.

14. The apparatus of claim 12 , wherein

the multiplier of the affine model is a rotation angle in the affine model, and

the offset value indicates a delta value to be applied to the default value of the rotation angle in the affine model.

15. The apparatus of claim 11 , wherein the processing circuitry is configured to

determine a first motion vector difference of the affine model according to which offset value of a second plurality of offset values is determined to be associated with a second index, and

derive the affine model based on a predicted motion vector and the first motion vector difference.

16. The apparatus of claim 15 , wherein

the offset value of the second plurality of offset values indicates a pixel distance of the first motion vector difference, and

the processing circuitry is configured to determine a direction of the first motion vector difference according to an index of another plurality of indexes.

17. The apparatus of claim 15 , wherein

the offset value of the second plurality of offset values indicates a pixel distance of the first motion vector difference, and

the second index is associated with both a direction and the pixel distance of the first motion vector difference.

18. The apparatus of claim 15 , wherein the processing circuitry is configured to:

determine a second motion vector difference of the affine model according to which of the second plurality of offset values is associated with a third index corresponding to one of the second plurality of offset values, the first and second motion vector differences being for two control points;

determine motion vectors for the two control points based on the first and second motion vector differences; and

derive a 4-parameter affine model based on the motion vectors of the two control points.

19. The apparatus of claim 15 , wherein

the processing circuitry is configured to

determine a second motion vector difference of the affine model according to which of the second plurality of offset values is associated with a third index corresponding to one of the second plurality of offset values, and

determine a third motion vector difference of the affine model according to which of the second plurality of offset values is associated with a fourth index corresponding to one of the second plurality of offset values;

the first, second, and third motion vector differences being for three control points;

determine motion vectors for the three control points based on the first, second, and third motion vector differences; and

derive a 6-parameter affine model based on the motion vectors of the three control points.

20. A non-transitory computer-readable medium storing instructions which when executed by a computer for video decoding cause the computer to perform:

decoding prediction information of a block in a current picture from a coded video bitstream, the prediction information including a first index for a prediction offset associated with an affine model in an inter prediction mode, the affine model being used to transform between the block and a reference block in a reference picture that has been reconstructed;

determining a multiplier of a non-translational parameter of the affine model, the multiplier being determined according to a default value and an offset value of a plurality of offset values determined according to the first index, the plurality of offset values mapped to respective indexes; and

reconstructing at least a sample of the block according to the affine model.