Conditions in parameter derivation for intra prediction

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, values of parameters of a cross-component linear model at least based on R chroma samples and down-sampled neighboring luma samples, wherein the R chroma samples are selected from a group of neighboring chroma samples based on a position rule and R is greater than or equal to 2, and wherein the down-sampled neighboring luma samples are generated with a down sampling process based a color format of the current video block; and

performing the conversion based on the values of parameters,

wherein at least one neighboring chroma sample of the group of neighboring chroma samples does not belong to the R chroma samples based on a size of the current video block, and wherein neighboring luma samples corresponding to the at least one neighboring chroma sample not belonging to the R chroma samples is not applied with the down sampling process;

wherein the group of neighboring chroma samples includes left-neighboring chroma samples and above-neighboring chroma samples,

wherein maximum and/or minimum of a luma component value and its related chroma component value are derived to generate the values of parameters, and the maximum and/or the minimum of the luma component value and its related chroma component value are derived based on the left-neighboring chroma samples and above-neighboring chroma samples,

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case that the left-neighboring chroma samples and above-neighboring chroma samples are unavailable, and

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL==0 and numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.

2. The method of claim 1 , wherein in the position rule, the positions of the R selected chroma samples are selected based on a first position offset value (F) and a step value (S), and wherein the F and S are derived at least based on availabilities of the group of neighboring chroma samples and the size of the current video block.

3. The method of claim 1 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL+numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.

4. The method of claim 1 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of cntL==0 and cntT==0, the cntL and the cntT indicating a number of the selected chroma samples from the left-neighboring chroma samples and a number of the selected chroma samples from the above-neighboring chroma samples, respectively.

5. The method of claim 1 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of cntL+cntT==0, the cntL and the cntT indicating a number of the selected chroma samples from the left-neighboring chroma samples and a number of the selected chroma samples from the above-neighboring chroma samples, respectively.

6. The method of claim 2 , wherein F=Floor (numSampL/2i) or F=Floor (numSampT/2i), the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively, and the Floor operation is used to obtain an integer part of a number.

7. The method of claim 6 , wherein S=Max(1, Floor (numSampL/2j)), or S=Max(1, Floor (numSampT/2j)), and the Max operation is used to obtain a maximum of multiple numbers.

8. The method of claim 6 , wherein i is equal to 2 or 3, and j is equal to 1 or 2.

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

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

11. 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, values of parameters of a cross-component linear model at least based on R chroma samples and down-sampled neighboring luma samples, wherein the R chroma samples are selected from a group of neighboring chroma samples based on a position rule and R is greater than or equal to 2, and wherein the down-sampled neighboring luma samples are generated with a down sampling process based on a color format of the current video block; and

perform the conversion based on the values of parameters,

wherein at least one neighboring chroma sample of the group of neighboring chroma samples does not belong to the R chroma samples based on a size of the current video block, and wherein neighboring luma samples corresponding to the at least one neighboring chroma sample not belonging to the R chroma samples is not applied with the down sampling process,

wherein the group of neighboring chroma samples includes left-neighboring chroma samples and above-neighboring chroma samples,

wherein maximum and/or minimum of a luma component value and its related chroma component value are derived to generate the values of parameters, and the maximum and/or the minimum of the luma component value and its related chroma component value are derived based on the left-neighboring chroma samples and above-neighboring chroma samples,

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case that the left-neighboring chroma samples and above-neighboring chroma samples are unavailable, and

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL==0 and numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.

12. The apparatus of claim 11 , wherein in the position rule, the positions of the R selected chroma samples are selected based on a first position offset value (F) and a step value (S), and wherein the F and S are derived at least based on availabilities of the group of neighboring chroma samples and the size of the current video block.

13. The apparatus of claim 11 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL+numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.

14. The apparatus of claim 11 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of cntL==0 and cntT==0, the cntL and the cntT indicating a number of the selected chroma samples from the left-neighboring chroma samples and a number of the selected chroma samples from the above-neighboring chroma samples, respectively.

15. The apparatus of claim 11 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of cntL+cntT==0, the cntL and the cntT indicating a number of the selected chroma samples from the left-neighboring chroma samples and a number of the selected chroma samples from the above-neighboring chroma samples, respectively.

16. 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, values of parameters of a cross-component linear model at least based on R chroma samples and down-sampled neighboring luma samples, wherein the R chroma samples are selected from a group of neighboring chroma samples based on a position rule and R is greater than or equal to 2, and wherein the down-sampled neighboring luma samples are generated with a down sampling process based on a color format of the current video block; and

perform the conversion based on the values of parameters,

wherein at least one neighboring chroma sample of the group of neighboring chroma samples does not belong to the R chroma samples based on a size of the current video block, and wherein neighboring luma samples corresponding to the at least one neighboring chroma sample not belonging to the R chroma samples is not applied with the down sampling process;

wherein the group of neighboring chroma samples includes left-neighboring chroma samples and above-neighboring chroma samples,

wherein maximum and/or minimum of a luma component value and its related chroma component value are derived to generate the values of parameters, and the maximum and/or the minimum of the luma component value and its related chroma component value are derived based on the left-neighboring chroma samples and above-neighboring chroma samples,

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case that the left-neighboring chroma samples and above-neighboring chroma samples are unavailable, and

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL==0 and numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.

17. The non-transitory computer-readable storage medium of claim 16 , wherein in the position rule, the positions of the R selected chroma samples are selected based on a first position offset value (F) and a step value (S), and wherein the F and S are derived at least based on availabilities of the group of neighboring chroma samples and the size of the current video block.

18. The non-transitory computer-readable storage medium of claim 16 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL+numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.

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

determining, for a current video block of the video that is a chroma block, values of parameters of a cross-component linear model at least based on R chroma samples and down-sampled neighboring luma samples, wherein the R chroma samples are selected from a group of neighboring chroma samples based on a position rule and R is greater than or equal to 2, and wherein the down-sampled neighboring luma samples are generated with a down sampling process based on a color format of the current video block; and

generating the bitstream based on the values of parameters,

wherein at least one neighboring chroma sample of the group of neighboring chroma samples does not belong to the R chroma samples based on a size of the current video block, and wherein neighboring luma samples corresponding to the at least one neighboring chroma sample not belonging to the R chroma samples is not applied with the down sampling process,

wherein the group of neighboring chroma samples includes left-neighboring chroma samples and above-neighboring chroma samples,

wherein maximum and/or minimum of a luma component value and its related chroma component value are derived to generate the values of parameters, and the maximum and/or the minimum of the luma component value and its related chroma component value are derived based on the left-neighboring chroma samples and above-neighboring chroma samples,

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case that the left-neighboring chroma samples and above-neighboring chroma samples are unavailable, and

wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL==0 and numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.

20. The non-transitory computer-readable storage medium of claim 19 , wherein the maximum and/or the minimum of the luma component value and its related chroma component value are not derived in a case of numSampL+numSampT==0, the numSampL and the numSampT indicating a number of available left-neighboring chroma samples and a number of available above-neighboring chroma samples, respectively.