Control-point based intra direction representation for intra coding
View Patent ↗Systems and methods are described for control-point based intra mode for coding a video bitstream. In an exemplary embodiment, at least two control points in a picture are selected. The control points may be, for example, points at or adjacent to two or more corners of a current block. For each of the control points, an associated intra prediction direction is identified. The intra prediction directions may be encoded in the bitstream, e.g. using differential coding. A derived intra prediction direction is interpolated based on a position of a pixel (or of a block) relative to the control points, and the derived intra prediction direction is used to predict one or more samples in the video. Different interpolation techniques, such as triangular interpolation or bilinear interpolation may be used.
1. A method comprising:
for at least a current block, identifying at least a first intra direction for a first control point and a second intra direction for a second control point;
for at least one current sample in the current block, deriving an intra prediction direction for the sample by interpolating a direction from at least the first intra direction and the second intra direction; and
predicting the at least one current sample with intra prediction using the derived intra prediction direction.
2. The method of claim 1 , wherein the derived intra prediction direction is interpolated from the first intra direction, the second intra direction, and a third intra direction for a third control point.
3. The method of claim 1 , wherein positions of at least the first control point and the second control point are signaled in a bitstream.
4. The method of claim 1 , wherein at least the first intra direction is signaled in a bitstream.
5. The method of claim 1 , wherein at least the first and second control points are selected from the group consisting of: a control point at a top-left corner of the current block, a control point at a top-right corner of the current block, a control point at a bottom-left corner of the current block, a control point at a bottom-right corner of the current block, and a custom control point signaled in a bitstream.
6. The method of claim 1 , wherein the first control point is at a top-left corner of the current block and the second control point is at a top-right corner or a bottom-left corner of the current block.
7. The method of claim 6 , wherein a position of the second control point is determined based on intra modes in a list of most probable modes for the current block, the list being determined based on intra modes used to code neighboring blocks and wherein:
the second control point is at a top-right corner of the current block if more of the most probable modes are horizontal than vertical; and
the second control point is at a bottom-left corner of the current block if more of the most probable modes are vertical than horizontal.
8. The method of claim 1 , wherein the current sample is in a current sub-block of the current block, further comprising predicting all samples of the current sub-block using the derived intra prediction direction.
9. The method of claim 8 , wherein the current block includes a plurality of sub-blocks of samples, the method further comprising:
deriving respective intra prediction directions for each of the sub-blocks by interpolating respective directions from at least the first intra direction and the second intra direction; and
predicting samples in each of the sub-blocks by intra prediction using the respective derived intra prediction direction.
10. The method of claim 1 , wherein the first and second intra directions are selected from a list of most probable modes, the list being determined based on intra modes used to code neighboring blocks.
11. The method of claim 1 , wherein at least the second intra direction is coded in a bitstream using differential coding with respect to the first intra direction.
12. The method of claim 1 , wherein the interpolating is performed using triangular interpolation.
13. The method of claim 1 , wherein the interpolating is performed using bilinear interpolation.
14. The method of claim 1 , wherein a bitstream includes a flag indicating that control-point intra coding is used for the current block.
15. The method of claim 1 , performed by a video encoder, the method further comprising determining a prediction residual for the at least one current sample and encoding the prediction residual in a bitstream.
16. The method of claim 1 , performed by a video decoder, the method further comprising decoding a prediction residual for the at least one current sample and reconstructing the sample using a predicted value of the at least one current sample and the prediction residual.
17. An apparatus comprising:
a processor configured to perform at least:
for at least a current block, identifying at least a first intra direction for a first control point and a second intra direction for a second control point;
for at least one current sample in the current block, deriving an intra prediction direction for the sample by interpolating a direction from at least the first intra direction and the second intra direction; and
predicting the at least one current sample with intra prediction using the derived intra prediction direction.
18. The apparatus of claim 17 , wherein the derived intra prediction direction is interpolated from the first intra direction, the second intra direction, and a third intra direction for a third control point.
19. The apparatus of claim 17 , wherein the first control point is at a top-left corner of the current block and the second control point is at a top-right corner or a bottom-left corner of the current block.
20. The apparatus of claim 17 , wherein the current sample is in a current sub-block of the current block, further comprising predicting all samples of the current sub-block using the derived intra prediction direction.