Method and apparatus for video coding

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 being indicative of an affine merge model, the current block including control points;

determining a first candidate control point motion vector (CPMV) for a first control point of the control points based on one of (i) at least two CPMVs of a first neighboring block of the first control point and (ii) a translational motion vector (MV) of a second neighboring block of the first control point;

deriving affine merge candidates for the current block based on candidate CPMVs for the control points of the current block, the candidate CPMVs of the control points including the first candidate CPMV;

determining parameters of the affine merge model based on one of the affine merge candidates, the one of the affine merge candidates being defined according to the prediction information, the parameters of the affine merge model being used to transform between the current block and a reference block in a reference picture that has been reconstructed; and

reconstructing at least a sample of the current block according to the affine merge model,

wherein:

the prediction information indicates a selection of the affine merge candidates,

the method further includes defining the one of the affine merge candidates in accordance with the prediction information, and

the determining the first candidate CPMV for the first control point further comprises one of:

determining the first candidate CPMV for the first control point based on at least two CPMVs of a first affine coded block among neighboring blocks of the first control point according to a checking order established for only the first control point; and

determining the first candidate CPMV for the first control point based on a translational MV of a first available block among the neighboring blocks of the first control point according to the checking order, responsive to none of the neighboring blocks of the first control point being affine coded.

2. The method of claim 1 , further comprising:

determining a second candidate CPMV for a second control point of the control points by copying a translational MV of a temporal motion vector predictor.

3. The method of claim 1 , wherein

the control points of the current block are derived from 2 or 3 of four candidate control points of the current block,

the first candidate CPMV of the first control point of the control points being derived from the at least two CPMVs of the first neighboring block of the first control point, and

a second candidate CPMV of a second control point of the control points being derived by copying a translational MV of a temporal motion vector predictor.

4. The method of claim 1 , wherein

the control points of the current block are derived from 2 or 3 of three candidate control points of the current block, and

the first candidate CPMV of the first control point of the control points being derived from the at least two CPMVs of the first neighboring block of the first control point.

5. The method of claim 1 , wherein the candidate CPMVs for at least two of the control points of the current block are derived from CPMVs of neighboring blocks of the at least two of the control points.

6. The method of claim 1 , wherein

the first candidate CPMV of the first control point of the current block is derived from the at least two CPMVs of the first neighboring block of the first control point, and

a candidate CPMV of each remaining control point of the current block is derived from at least two CPMVs of a neighboring block of the respective control point.

7. The method of claim 1 , wherein each of the candidate CPMVs of the control points of the current block refers to the reference picture.

8. The method of claim 1 , wherein

the first neighboring block of the first control point is a largest affine coded block of neighboring affine coded blocks of the first control point, and

the determining the first candidate CPMV for the first control point further comprises determining the first candidate CPMV for the first control point based on the at least two CPMVs of the largest affine coded block of the neighboring affine coded blocks of the first control point.

9. 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 being indicative of an affine merge model, the current block including control points;

determine a first candidate control point motion vector (CPMV) for a first control point of the control points based on one of (i) at least two CPMVs of a first neighboring block of the first control point and (ii) a translational motion vector (MV) of a second neighboring block of the first control point;

derive affine merge candidates for the current block based on candidate CPMVs for the control points of the current block, the candidate CPMVs of the control points including the first candidate CPMV;

determine parameters of the affine merge model based on one of the affine merge candidates, the one of the affine merge candidates being defined according to the prediction information, the parameters of the affine merge model being used to transform between the current block and a reference block in a reference picture that has been reconstructed; and

reconstruct at least a sample of the current block according to the affine merge model,

wherein:

the prediction information indicates a selection of the affine merge candidates,

the processing circuitry is further configured to define the one of the affine merge candidates in accordance with the prediction information, and

the processing circuitry is further configured to:

determine the first candidate CPMV for the first control point based on at least two CPMVs of a first affine coded block among neighboring blocks of the first control point according to a checking order established for only the first control point and

determine the first candidate CPMV for the first control point based on a translational MV of a first available block among the neighboring blocks of the first control point according to the checking order, responsive to none of the neighboring blocks of the first control point being affine coded.

10. The apparatus of claim 9 , the processing circuitry is further configured to:

determine a second candidate CPMV for a second control point of the control points by copying a translational MV of a temporal motion vector predictor.

11. The apparatus of claim 9 , wherein:

the control points of the current block are derived from 2 or 3 of four candidate control points of the current block,

the first candidate CPMV of the first control point of the control points being derived from the at least two CPMVs of the first neighboring block of the first control point, and

a second candidate CPMV of a second control point of the control points being derived by copying a translational MV of a temporal motion vector predictor.

12. The apparatus of claim 9 , wherein:

the control points of the current block are derived from 2 or 3 of three candidate control points of the current block, and

the first candidate CPMV of the first control point of the control points being derived from the at least two CPMVs of the first neighboring block of the first control point.

13. The apparatus of claim 9 , wherein the candidate CPMVs for at least two of the control points of the current block are derived from CPMVs of neighboring blocks of the at least two of the control points.

14. The apparatus of claim 9 , wherein:

the first candidate CPMV of the first control point of the current block is derived from the at least two CPMVs of the first neighboring block of the first control point, and

a candidate CPMV of each remaining control point of the current block is derived from at least two CPMVs of a neighboring block of the respective control point.

15. The apparatus of claim 9 , wherein

the first neighboring block of the first control point is a largest affine coded block of neighboring affine coded blocks of the first control point, and

the processing circuitry is further configured to

determine the first candidate CPMV for the first control point based on the at least two CPMVs of the largest affine coded block of the neighboring affine coded blocks of the first control point.

16. 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 current block in a current picture from a coded video bitstream, the prediction information being indicative of an affine merge model, the current block including control points;

determining a first candidate control point motion vector (CPMV) for a first control point of the control points based on one of (i) at least two CPMVs of a first neighboring block of the first control point and (ii) a translational motion vector (MV) of a second neighboring block of the first control point;

deriving affine merge candidates for the current block based on candidate CPMVs for the control points of the current block, the candidate CPMVs of the control points including the first candidate CPMV;

determining parameters of the affine merge model based on one of the affine merge candidates, the one of the affine merge candidates being defined according to the prediction information, the parameters of the affine merge model being used to transform between the current block and a reference block in a reference picture that has been reconstructed; and

reconstructing at least a sample of the current block according to the affine merge mode,

wherein:

the prediction information indicates a selection of the affine merge candidates,

the method further includes defining the one of the affine merge candidates in accordance with the prediction information, and

the determining the first candidate CPMV for the first control point further comprises one of:

determining the first candidate CPMV for the first control point based on at least two CPMVs of a first affine coded block among neighboring blocks of the first control point according to a checking order established for only the first control point and

determining the first candidate CPMV for the first control point based on a translational MV of a first available block among the neighboring blocks of the first control point according to the checking order, responsive to none of the neighboring blocks of the first control point being affine coded.