Multiple merge candidates for affine motion prediction

Abstract

Different implementations are described, particularly implementations for selecting a predictor candidate from a set of multiple predictor candidates for motion compensation of a picture block based on a motion model. The motion model, may be, e.g., an affine model in a merge mode for a video content encoder or decoder. In an embodiment, a predictor candidate is selected from the set based on a motion model for each of the multiple predictor candidates, and may be based on a criterion such as, e.g., a rate distortion cost. The corresponding motion field is determined based on, e.g., one or more corresponding control point motion vectors for the block being encoded or decoded. The corresponding motion field of an embodiment to identifies motion vectors used for prediction of sub-blocks of the block being encoded or decoded.

1 . A method for video encoding, comprising:
 accessing, for a block being encoded in a picture, a set of predictor candidates having multiple predictor candidates, wherein a predictor candidate corresponds to a spatial or a temporal neighboring block that has been encoded, wherein said block is encoded in an affine merge mode;   selecting a predictor candidate from the set of predictor candidates;   obtaining, using a plurality of motion vectors associated with the selected predictor candidate from the set of predictor candidates, a set of control point motion vectors for the block;   obtaining, based on the set of control point motion vectors, a motion field based on a motion model, wherein the motion field identifies motion vectors used for prediction of all sub-blocks of the block being encoded;   encoding the block based on the motion field; and   encoding an index for the selected predictor candidate from the set of predictor candidates.   
     
     
         2 . The method of  claim 1 , wherein the motion model is an affine model. 
     
     
         3 . The method of  claim 1 , wherein the set of control point motion vectors for the block are stored separately from the motion vectors of the motion field for the block. 
     
     
         4 . The method of  claim 3 , wherein the stored motion vectors of the motion field are for motion compensation of the block. 
     
     
         5 . The method of  claim 3 , further comprising:
 accessing a second set of control point motion vectors for the selected predictor candidate, wherein the second set of control point motion vectors are stored separately from motion vectors of all sub-blocks of the selected predictor candidate, wherein the set of control point motion vectors for the block are obtained responsive to the second set of control point motion vectors for the selected predictor candidate, and wherein the motion vectors of all sub-blocks of the selected predictor candidate are for motion compensation of the selected predictor candidate.   
     
     
         6 . A method for video decoding, comprising:
 accessing, for a block being decoded in a picture, an index corresponding to a predictor candidate, wherein the predictor candidate corresponds to a spatial or a temporal neighboring block that has been decoded, and wherein said block is decoded in an affine merge mode;   obtaining, using a plurality of motion vectors associated with the predictor candidate, a set of control point motion vectors for the block being decoded;   obtaining, based on the set of control point motion vectors, a motion field based on a motion model, wherein the motion field identifies motion vectors used for prediction of all sub-blocks of the block being decoded; and   decoding the block based on the motion field.   
     
     
         7 . The method of  claim 6 , wherein the motion model is an affine model. 
     
     
         8 . The method of  claim 6 , wherein the set of control point motion vectors for the block are stored separately from the motion vectors of the motion field for the block. 
     
     
         9 . The method of  claim 8 , wherein the stored motion vectors of the motion field are for motion compensation of the block. 
     
     
         10 . The method of  claim 8 , further comprising:
 accessing a second set of control point motion vectors for the selected predictor candidate, wherein the second set of control point motion vectors are stored separately from motion vectors of all sub-blocks of the selected predictor candidate, wherein the set of control point motion vectors for the block are obtained responsive to the second set of control point motion vectors for the selected predictor candidate, and wherein the motion vectors of all sub-blocks of the selected predictor candidate are for motion compensation of the selected predictor candidate.   
     
     
         11 . An apparatus for video encoding, comprising:
 one or more processors, wherein said one or more processors are configured to:   access, for a block being encoded in a picture, a set of predictor candidates having multiple predictor candidates, wherein a predictor candidate corresponds to a spatial or a temporal neighboring block that has been encoded, wherein said block is encoded in an affine merge mode;   select a predictor candidate from the set of predictor candidates;   obtain, using a plurality of motion vectors associated with the selected predictor candidate from the set of predictor candidates, a set of control point motion vectors for the block;   obtain, based on the set of control point motion vectors, a motion field based on a motion model, wherein the motion field identifies motion vectors used for prediction of all sub-blocks of the block being encoded;   encode the block based on the motion field; and   encode an index for the selected predictor candidate from the set of predictor candidates.   
     
     
         12 . The apparatus of  claim 11 , wherein the motion model is an affine model. 
     
     
         13 . The apparatus of  claim 11 , wherein the set of control point motion vectors for the block are stored separately from the motion vectors of the motion field for the block. 
     
     
         14 . The apparatus of  claim 13 , wherein the stored motion vectors of the motion field are for motion compensation of the block. 
     
     
         15 . The apparatus of  claim 13 , wherein the one or more processors are further configured to:
 access a second set of control point motion vectors for the selected predictor candidate, wherein the second set of control point motion vectors are stored separately from motion vectors of all sub-blocks of the selected predictor candidate, wherein the set of control point motion vectors for the block are obtained responsive to the second set of control point motion vectors for the selected predictor candidate, and wherein the motion vectors of all sub-blocks of the selected predictor candidate are for motion compensation of the selected predictor candidate.   
     
     
         16 . An apparatus for video decoding, comprising:
 one or more processors, wherein the one or more processors are configured to:   access, for a block being decoded in a picture, an index corresponding to a predictor candidate, wherein the predictor candidate corresponds to a spatial or a temporal neighboring block that has been decoded, and wherein said block is decoded in an affine merge mode;   obtain, using a plurality of motion vectors associated with the predictor candidate, a set of control point motion vectors for the block being decoded;   obtain, based on the set of control point motion vectors, a motion field based on a motion model, wherein the motion field identifies motion vectors used for prediction of all sub-blocks of the block being decoded; and   decode the block based on the motion field.   
     
     
         17 . The apparatus of  claim 16 , wherein the motion model is an affine model. 
     
     
         18 . The apparatus of  claim 16 , wherein the set of control point motion vectors for the block are stored separately from the motion vectors of the motion field for the block. 
     
     
         19 . The apparatus of  claim 18 , wherein the stored motion vectors of the motion field are for motion compensation of the block. 
     
     
         20 . The apparatus of  claim 18 , wherein the one or more processors are further configured to:
 access a second set of control point motion vectors for the selected predictor candidate, wherein the second set of control point motion vectors are stored separately from motion vectors of all sub-blocks of the selected predictor candidate, wherein the set of control point motion vectors for the block are obtained responsive to the second set of control point motion vectors for the selected predictor candidate, and wherein the motion vectors of all sub-blocks of the selected predictor candidate are for motion compensation of the selected predictor candidate.