Cross-component planar prediction in image and video compression

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

decoding prediction information of a chroma block to be reconstructed in a current picture, the prediction information indicating that the chroma block is predicted based at least on a luma block in the current picture using a planar mode, the luma block being reconstructed and collocated with the chroma block;

identifying one or more first chroma samples in the chroma block as located at one or more respective key positions;

predicting the one or more first chroma samples in the chroma block based on one or more luma samples in the luma block; and

predicting a second chroma sample in the chroma block based on the predicted one or more first chroma samples and at least one reference chroma sample of the chroma block, the second chroma sample being different from the one or more first chroma samples, the at least one reference chroma sample being adjacent to the chroma block.

2. The method of claim 1 , wherein

the one or more first chroma samples is a bottom-right corner chroma sample in the chroma block; and

the predicting the second chroma sample includes:

predicting, based on the bottom-right corner chroma sample and the at least one reference chroma sample, at least one of (i) chroma samples in a bottom row of the chroma block or (ii) chroma samples in a right most column of the chroma block;

in response to the chroma samples in the bottom row of the chroma block being predicted, generating a vertical predictor of the second chroma sample based on a top reference chroma sample in the at least one reference chroma sample and a predicted bottom chroma sample in the predicted chroma samples in the bottom row, the second chroma sample, the top reference chroma sample, and the predicted bottom chroma sample being in a same column in the chroma block; and

in response to the chroma samples in the right most column of the chroma block being predicted, generating a horizontal predictor of the second chroma sample based on a left reference chroma sample in the at least one reference chroma sample and a predicted right chroma sample in the predicted chroma samples in the right most column, the second chroma sample, the left reference chroma sample, and the predicted right chroma sample being in a same row of the chroma block.

3. The method of claim 2 , wherein

the predicting the at least one of (i) the chroma samples in the bottom row of the chroma block or (ii) the chroma samples in the right most column of the chroma block includes predicting the chroma samples in the bottom row of the chroma block and the chroma samples in the right most column of the chroma block; and

the predicting the second chroma sample includes predicting the second chroma sample based on the vertical predictor and the horizontal predictor.

4. The method of claim 2 , wherein

a width of the chroma block is larger than a height of the chroma block or the height of the chroma block is less than a threshold;

the predicting the at least one of (i) the chroma samples in the bottom row of the chroma block or (ii) the chroma samples in the right most column of the chroma block includes predicting the chroma samples in the bottom row of the chroma block; and

the predicting the second chroma sample includes predicting the second chroma sample based on the vertical predictor.

5. The method of claim 2 , wherein

a width of the chroma block is less than a height of the chroma block or a threshold;

the predicting the at least one of (i) the chroma samples in the bottom row of the chroma block or (ii) the chroma samples in the right most column of the chroma block includes predicting the chroma samples in the right most column of the chroma block; and

the predicting the second chroma sample includes predicting the second chroma sample based on the horizontal predictor.

6. The method of claim 2 , wherein the predicting the one or more first chroma samples comprises:

predicting the bottom-right corner chroma sample in the chroma block based on at least one of a bottom-right corner luma sample in the luma block or luma samples in the luma block that are collocated with the bottom-right corner chroma sample.

7. The method of claim 1 , wherein

the one or more first chroma samples include (i) chroma samples in a bottom row of the chroma block and (ii) chroma samples in a right most column of the chroma block; and

the predicting the second chroma sample includes:

generating a vertical predictor of the second chroma sample based on a top reference chroma sample in the at least one reference chroma sample and a predicted bottom chroma sample in the predicted chroma samples in the bottom row, the second chroma sample, the top reference chroma sample, and the predicted bottom chroma sample being in a same column in the chroma block;

generating a horizontal predictor of the second chroma sample based on a left reference chroma sample in the at least one reference chroma sample and a predicted right chroma sample in the predicted chroma samples in the right most column, the second chroma sample, the left reference chroma sample, and the predicted right chroma sample being in a same row in the chroma block; and

predicting the second chroma sample based on the vertical predictor and the horizontal predictor.

8. The method of claim 1 , wherein

a width of the chroma block is larger than a height of the chroma block or the height of the chroma block is less than a threshold;

the one or more first chroma samples include chroma samples in a bottom row of the chroma block; and

the predicting the second chroma sample includes:

generating a vertical predictor of the second chroma sample based on a top reference chroma sample in the at least one reference chroma sample and a predicted bottom chroma sample in the predicted chroma samples in the bottom row, the second chroma sample, the top reference chroma sample, and the predicted bottom chroma sample being in a same column in the chroma block; and

predicting the second chroma sample based on the vertical predictor.

9. The method of claim 1 , wherein

a width of the chroma block is less than a height of the chroma block or a threshold;

the one or more first chroma samples include chroma samples in a right most column of the chroma block; and

the predicting the second chroma sample includes:

generating a horizontal predictor of the second chroma sample based on a left reference chroma sample in the at least one reference chroma sample and a predicted right chroma sample in the predicted chroma samples in the right most column, the second chroma sample, the left reference chroma sample, and the predicted right chroma sample being in a same row in the chroma block; and

predicting the second chroma sample based on the horizontal predictor.

10. The method of claim 1 , wherein the predicting the one or more first chroma samples comprises:

predicting the one or more first chroma samples in the chroma block based on the one or more luma samples in the luma block that are collocated with the one or more first chroma samples using a cross-component linear model prediction (CCLM) mode.

11. The method of claim 1 , wherein

the at least one reference chroma sample includes at least one of a top reference chroma sample that is a top neighbor of the chroma block or a left reference chroma sample that is a left neighbor of the chroma block.

12. An apparatus for video decoding, comprising:

processing circuitry configured to:

decode prediction information of a chroma block to be reconstructed in a current picture, the prediction information indicating that the chroma block is predicted based at least on a luma block in the current picture using a planar mode, the luma block being reconstructed and collocated with the chroma block;

identify one or more first chroma samples in the chroma block as located at one or more respective key positions;

predict the one or more first chroma samples in the chroma block based on one or more luma samples in the luma block; and

predict a second chroma sample in the chroma block based on the predicted one or more first chroma samples and at least one reference chroma sample of the chroma block, the second chroma sample being different from the one or more first chroma samples, the at least one reference chroma sample being adjacent to the chroma block.

13. The apparatus of claim 12 , wherein

the one or more first chroma samples is a bottom-right corner chroma sample in the chroma block; and

the processing circuitry is configured to:

predict, based on the bottom-right corner chroma sample and the at least one reference chroma sample, at least one of (i) chroma samples in a bottom row of the chroma block or (ii) chroma samples in a right most column of the chroma block;

in response to the chroma samples in the bottom row of the chroma block being predicted, generate a vertical predictor of the second chroma sample based on a top reference chroma sample in the at least one reference chroma sample and a predicted bottom chroma sample in the predicted chroma samples in the bottom row, the second chroma sample, the top reference chroma sample, and the predicted bottom chroma sample being in a same column in the chroma block; and

in response to the chroma samples in the right most column of the chroma block being predicted, generate a horizontal predictor of the second chroma sample based on a left reference chroma sample in the at least one reference chroma sample and a predicted right chroma sample in the predicted chroma samples in the right most column, the second chroma sample, the left reference chroma sample, and the predicted right chroma sample being in a same row of the chroma block.

14. The apparatus of claim 13 , wherein the processing circuitry is configured to:

predict the chroma samples in the bottom row of the chroma block and the chroma samples in the right most column of the chroma block; and

predict the second chroma sample based on the vertical predictor and the horizontal predictor.

15. The apparatus of claim 12 , wherein

the one or more first chroma samples include (i) chroma samples in a bottom row of the chroma block and (ii) chroma samples in a right most column of the chroma block; and

the processing circuitry is configured to:

generate a vertical predictor of the second chroma sample based on a top reference chroma sample in the at least one reference chroma sample and a predicted bottom chroma sample in the predicted chroma samples in the bottom row, the second chroma sample, the top reference chroma sample, and the predicted bottom chroma sample being in a same column in the chroma block;

generate a horizontal predictor of the second chroma sample based on a left reference chroma sample in the at least one reference chroma sample and a predicted right chroma sample in the predicted chroma samples in the right most column, the second chroma sample, the left reference chroma sample, and the predicted right chroma sample being in a same row in the chroma block; and

predict the second chroma sample based on the vertical predictor and the horizontal predictor.

16. The apparatus of claim 12 , wherein

a width of the chroma block is larger than a height of the chroma block or the height of the chroma block is less than a threshold;

the one or more first chroma samples include chroma samples in a bottom row of the chroma block; and

the processing circuitry is configured to:

generate a vertical predictor of the second chroma sample based on a top reference chroma sample in the at least one reference chroma sample and a predicted bottom chroma sample in the predicted chroma samples in the bottom row, the second chroma sample, the top reference chroma sample, and the predicted bottom chroma sample being in a same column in the chroma block; and

predict the second chroma sample based on the vertical predictor.

17. The apparatus of claim 12 , wherein

a width of the chroma block is less than a height of the chroma block or a threshold;

the one or more first chroma samples include chroma samples in a right most column of the chroma block; and

the processing circuitry is configured to:

generate a horizontal predictor of the second chroma sample based on a left reference chroma sample in the at least one reference chroma sample and a predicted right chroma sample in the predicted chroma samples in the right most column, the second chroma sample, the left reference chroma sample, and the predicted right chroma sample being in a same row in the chroma block; and

predict the second chroma sample based on the horizontal predictor.

18. The apparatus of claim 12 , wherein the predicting the one or more first chroma samples comprises:

predicting the one or more first chroma samples in the chroma block based on the one or more luma samples in the luma block that are collocated with the one or more first chroma samples using a cross-component linear model prediction (CCLM) mode.

19. The apparatus of claim 12 , wherein

the at least one reference chroma sample includes at least one of a top reference chroma sample that is a top neighbor of the chroma block or a left reference chroma sample that is a left neighbor of the chroma block.

20. A non-transitory computer-readable storage medium storing instructions which when executed by at least one processor cause the at least one processor to perform:

decoding prediction information of a chroma block to be reconstructed in a current picture, the prediction information indicating that the chroma block is predicted based at least on a luma block in the current picture using a planar mode, the luma block being reconstructed and collocated with the chroma block;

identifying one or more first chroma samples in the chroma block as located at one or more respective key positions;

predicting the one or more first chroma samples in the chroma block based on one or more luma samples in the luma block; and

predicting a second chroma sample in the chroma block based on the predicted one or more first chroma samples and at least one reference chroma sample of the chroma block, the second chroma sample being different from the one or more first chroma samples, the at least one reference chroma sample being adjacent to the chroma block.