DECODER, ENCODER AND BITSTREAM FOR EFFICIENT CODING OF GLOBAL MOTION VECTORS

1 . A decoder comprising circuitry configured to:

receive a bit stream, the bitstream including a first coded picture, first information for determining a first plurality of control point motion vector predictors for affine motion compensation including a first index to a first reference picture in a reference picture list and motion vector difference information for the first plurality of control point motion vector predictors, a second coded picture, and second information for determining a second plurality of control point motion vector predictors for affine motion compensation including a second index to a second reference picture in the reference picture list and an indication there is no motion vector difference information for the second plurality of motion vector predictors;

form a first plurality of control point motion vectors for affine motion compensation for the first picture by obtaining the first plurality of motion vector predictors using the first information, the first plurality of motion vector predictors being obtained from motion vectors used in the decoding of the first reference picture, and combining the first plurality of motion vector predictors with the motion vector difference information; and

form a second plurality of control point motion vectors for affine motion compensation for the second picture by obtaining the second plurality of motion vector predictors using the second information, the second plurality of motion vector predictors being obtained from motion vectors used in the decoding of the second reference picture, there being no motion vector difference information for the second plurality of control point motion vectors as indicated in the bit stream.

2 . The decoder of claim 1 wherein the affine motion compensation is 6-parameter affine motion compensation, and wherein the plurality of control point motion vectors for the first picture comprises three control point motion vectors, and wherein the plurality of control point motion vectors for the second picture comprises three control point motion vectors.

3 . The decoder of claim 1 wherein the affine motion compensation is 4-parameter affine motion compensation, and wherein the plurality of control point motion vectors for the first picture comprises two control point motion vectors, and wherein the plurality of control point motion vectors for the second picture comprises two control point motion vectors.

4 . The decoder of claim 1 wherein the first reference picture and second reference picture are the same reference picture.

5 . A method of decoding a bitstream comprising:

receiving a bit stream, the bitstream including a first coded picture, first information for determining a first plurality of control point motion vector predictors for affine motion compensation including a first index to a first reference picture in a reference picture list and motion vector difference information for the first plurality of control point motion vector predictors, a second coded picture, and second information for determining a second plurality of control point motion vector predictors for affine motion compensation including a second index to a second reference picture in the reference picture list and an indication there is no motion vector difference information for the second plurality of motion vector predictors;

forming a first plurality of control point motion vectors for affine motion compensation for the first picture by obtaining the first plurality of motion vector predictors using the first information, the first plurality of motion vector predictors being obtained from motion vectors used in the decoding of the first reference picture, and combining the first plurality of motion vector predictors with the motion vector difference information; and

forming a second plurality of control point motion vectors for affine motion compensation for the second picture by obtaining the second plurality of motion vector predictors using the second information, the second plurality of motion vector predictors being obtained from motion vectors used in the decoding of the second reference picture, there being no motion vector difference information for the second plurality of control point motion vectors as indicated in the bit stream.

6 . The method of claim 5 wherein the affine motion compensation is 6-parameter affine motion compensation, and wherein the plurality of control point motion vectors for the first picture comprises three control point motion vectors, and wherein the plurality of control point motion vectors for the second picture comprises three control point motion vectors.

7 . The method of claim 5 wherein the affine motion compensation is 4-parameter affine motion compensation, and wherein the plurality of control point motion vectors for the first picture comprises two control point motion vectors, and wherein the plurality of control point motion vectors for the second picture comprises two control point motion vectors.

8 . The method of claim 5 wherein the first reference picture and second reference picture are the same reference picture.

9 . A method of encoding a bitstream comprising:

receiving a video signal;

generating a bitstream representing the video signal, the bitstream including a first coded picture, first information for determining a first plurality of control point motion vector predictors for affine motion compensation including a first index to a first reference picture in a reference picture list and motion vector difference information for the first plurality of control point motion vector predictors, a second coded picture, and second information for determining a second plurality of control point motion vector predictors for affine motion compensation including a second index to a second reference picture in the reference picture list and an indication there is no motion vector difference information for the second plurality of motion vector predictors;

the encoding method further comprising configuring the bitstream for decoding by a method including:

forming a first plurality of control point motion vectors for affine motion compensation for the first picture by obtaining the first plurality of motion vector predictors using the first information, the first plurality of motion vector predictors being obtained from motion vectors used in the decoding of the first reference picture, and combining the first plurality of motion vector predictors with the motion vector difference information; and

forming a second plurality of control point motion vectors for affine motion compensation for the second picture by obtaining the second plurality of motion vector predictors using the second information, the second plurality of motion vector predictors being obtained from motion vectors used in the decoding of the second reference picture, there being no motion vector difference information for the second plurality of control point motion vectors as indicated in the bit stream.

10 . The method of claim 9 wherein the affine motion compensation is 6-parameter affine motion compensation, and wherein the plurality of control point motion vectors for the first picture comprises three control point motion vectors, and wherein the plurality of control point motion vectors for the second picture comprises three control point motion vectors.

11 . The method of claim 9 wherein the affine motion compensation is 4-parameter affine motion compensation, and wherein the plurality of control point motion vectors for the first picture comprises two control point motion vectors, and wherein the plurality of control point motion vectors for the second picture comprises two control point motion vectors.

12 . The method of claim 9 wherein the first reference picture and second reference picture are the same reference picture.