Single-line cross component linear model prediction mode

1. A method of processing video data, comprising:

determining, for a conversion between a current video block of a video that is a chroma block and a bitstream of the video, a corresponding luma block for the current video block, wherein a color format for the current video block and the corresponding luma block is 4:2:0, and a cross component linear mode is applied for the current video block;

generating down-sampled inside luma samples of the corresponding luma block;

generating down-sampled above neighboring luma samples of the corresponding luma block, wherein different down-sampled filtering schemes are used to generate the down-sampled above neighboring luma samples based on a position of the corresponding luma block, and wherein in response to the position of the corresponding luma block meeting a first position rule, a first down-sampled filtering scheme is used to generate the down-sampled above neighboring luma samples, and wherein the first position rule is that a top boundary of the corresponding luma block is overlapping with a top boundary of a current luma coding tree block (CTB) including the corresponding luma block, and wherein only one above luma sample row and a horizontal 3-tap filter are used to generate the down-sampled above neighboring luma samples in the first down-sampled filtering scheme;

deriving parameters of the cross-component linear model at least based on the down-sampled above neighboring luma samples;

generating predicted chroma samples of the current video block based on the parameters of the cross-component linear model and the down-sampled inside luma samples; and

performing the conversion based on the predicted chroma samples,

wherein the horizontal 3-tap filter uses [1, 2, 1] filtering coefficients.

2. The method of claim 1 , wherein the only one above luma sample row is adjacent to the corresponding luma block.

3. The method of claim 2 , wherein the only one above luma sample row comprises above adjacent luma samples and above-right adjacent luma samples.

4. The method of claim 2 , wherein the first down-sampled filtering scheme calculates a specific down-sampled above neighboring luma sample D from selected luma samples of the only one above luma sample row as:

D =( a [2 i -1]+2* a [2 i ]+ a [2 i+ 1]+2)>>2,

wherein a [2 i -], a [2 i ] and a [2 i +1] represent the selected luma samples.

5. The method of claim 4 , wherein in a case that one of the selected luma samples is unavailable, applying a padding process in which the unavailable luma sample is assigned with another available luma sample.

6. The method of claim 5 , wherein in a case that a[2 i -1] is unavailable, the specific down-sampled above neighboring luma sample D is derived as:

D =(3* a [2 i ]+ a [2 i+ 1]+2)>>2.

7. The method of claim 1 , wherein in response to the position of the corresponding luma block not meeting the first position rule, a second down-sampled filtering scheme is used to generate the down-sampled above neighboring luma samples.

8. The method of claim 7 , wherein multiple above luma sample rows and a filter with taps more than 3 are used to generate the down-sampled above neighboring luma samples in the second down-sampled filtering scheme.

9. The method of claim 8 , wherein at least one of multiple above luma sample rows is not adjacent to the corresponding block.

10. The method of claim 7 , further comprising:

applying a third down-sampled filtering scheme to generate the down-sampled inside luma samples of the corresponding luma block, wherein the third down-sampled filtering scheme uses same filtering taps and filtering coefficients with the second down-sampled filtering scheme.

11. The method of claim 7 , further comprising:

applying a fourth down-sampled filtering scheme to generate down-sampled left neighboring luma samples of the corresponding luma block, wherein the fourth down-sampled filtering scheme uses same filtering taps and filtering coefficients with the second down-sampled filtering scheme; and

the parameters of the cross-component linear model are further derived based on the down-sampled left neighboring luma samples.

12. The method of claim 1 , wherein the first down-sampled filtering scheme corresponds to down-sampling above neighboring luma samples to lower left positions.

13. The method of claim 1 , wherein the conversion includes encoding the current video block into the bitstream.

14. The method of claim 1 , wherein the conversion includes decoding the current video block from the bitstream.

15. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:

determine, for a conversion between a current video block of a video that is a chroma block and a bitstream of the video, a corresponding luma block for the current video block, wherein a color format for the current video block and the corresponding luma block is 4:2:0, and a cross component linear mode is applied for the current video block;

generate down-sampled inside luma samples of the corresponding luma block;

generate down-sampled above neighboring luma samples of the corresponding luma block, wherein different down-sampled filtering schemes are used to generate the down-sampled above neighboring luma samples based on a position of the corresponding luma block, and wherein in response to the position of the corresponding luma block meeting a first position rule, a first down-sampled filtering scheme is used to generate the down-sampled above neighboring luma samples, and wherein the first position rule is that a top boundary of the corresponding luma block is overlapping with a top boundary of a current luma coding tree block (CTB) including the corresponding luma block, and wherein only one above luma sample row and a horizontal 3-tap filter are used to generate the down-sampled above neighboring luma samples in the first down-sampled filtering scheme;

derive parameters of the cross-component linear model at least based on the down-sampled above neighboring luma samples;

generate predicted chroma samples of the current video block based on the parameters of the cross-component linear model and the down-sampled inside luma samples; and

perform the conversion based on the predicted chroma samples,

wherein the horizontal 3-tap filter uses [1, 2, 1] filtering coefficients.

16. The apparatus of claim 15 , wherein the only one above luma sample row is adjacent to the corresponding luma block.

17. The apparatus of claim 16 , wherein the only one above luma sample row comprise the above neighboring luma samples and above-right neighboring luma samples.

18. A non-transitory computer-readable storage medium storing instructions that cause a processor to:

determine, for a conversion between a current video block of a video that is a chroma block and a bitstream of the video, a corresponding luma block for the current video block, wherein a color format for the current video block and the corresponding luma block is 4:2:0, and a cross component linear mode is applied for the current video block;

generate down-sampled inside luma samples of the corresponding luma block;

generate down-sampled above neighboring luma samples of the corresponding luma block, wherein different down-sampled filtering schemes are used to generate the down-sampled above neighboring luma samples based on a position of the corresponding luma block, and wherein in response to the position of the corresponding luma block meeting a first position rule, a first down-sampled filtering scheme is used to generate the down-sampled above neighboring luma samples, and wherein the first position rule is that a top boundary of the corresponding luma block is overlapping with a top boundary of a current luma coding tree block (CTB) including the corresponding luma block, and wherein only one above luma sample row and a horizontal 3-tap filter are used to generate the down-sampled above neighboring luma samples in the first down-sampled filtering scheme;

derive parameters of the cross-component linear model at least based on the down-sampled above neighboring luma samples;

generate predicted chroma samples of the current video block based on the parameters of the cross-component linear model and the down-sampled inside luma samples; and

perform the conversion based on the predicted chroma samples,

wherein the horizontal 3-tap filter uses [1, 2, 1] filtering coefficients.

19. The storage medium of claim 18 , wherein the only one above luma sample row is adjacent to the corresponding luma block.

20. A non-transitory computer-readable recording medium storing a bitstream which is generated by a method performed by a video processing apparatus, wherein the method comprises:

determining, for a conversion between a current video block of a video that is a chroma block and the bitstream of the video, a corresponding luma block for the current video block, wherein a color format for the current video block and the corresponding luma block is 4:2:0, and a cross component linear mode is applied for the current video block;

generating down-sampled inside luma samples of the corresponding luma block;

generating down-sampled above neighboring luma samples of the corresponding luma block, wherein different down-sampled filtering schemes are used to generate the down-sampled above neighboring luma samples based on a position of the corresponding luma block, and wherein in response to the position of the corresponding luma block meeting a first position rule, a first down-sampled filtering scheme is used to generate the down-sampled above neighboring luma samples, wherein the first position rule is that a top boundary of the corresponding luma block is overlapping with a top boundary of a current luma coding tree block (CTB) including the corresponding luma block, and wherein only one above luma sample row and a horizontal 3-tap filter are used to generate the down-sampled above neighboring luma samples in the first down-sampled filtering scheme;

deriving parameters of the cross-component linear model at least based on the down-sampled above neighboring luma samples;

generating predicted chroma samples of the current video block based on the parameters of the cross-component linear model and the down-sampled inside luma samples; and

generating the bitstream based on the predicted chroma samples,

wherein the horizontal 3-tap filter uses [1, 2, 1] filtering coefficients.