Usage of templates for decoder-side intra mode derivation

1. A method of processing video data, comprising:

constructing, during a conversion between a current video block of a video and a bitstream of the video, at least one template set for the current video block from a plurality of sub-templates;

deriving at least one intra-prediction mode (IPM) based on cost calculations, wherein the cost calculations are determined based on the at least one template set and the current block;

determining, based on the at least one IPM, a final predictor of the current video block; and

performing the conversion based on the final predictor,

wherein the at least one template set is selected from the plurality of sub-templates based on a coding information for the current video block, wherein the coding information includes a block dimension of the current video block, and the at least one template set is selected from the plurality of sub-templates based on a relationship between the block dimension of the current video block and a pre-defined threshold, and wherein a BW represents a block width of the current video block and a BH represents a block height of the current video block, and

wherein a left sub-template is not selected in a case that the BW divided by the BH is greater than or equal to a first threshold.

2. The method of claim 1 , wherein the plurality of sub-templates includes at least one of the following: a left sub-template, an above sub-template, a right-above sub-template, a left-below sub-template, and a left-above sub-template.

3. The method of claim 1 , wherein the plurality of sub-templates includes non-adjacent samples of the current video block.

4. The method of claim 1 , wherein the at least one template set includes a single sub-template, and wherein the single sub-template is one of a left sub-template, an above sub-template, a right-above sub-template, a left-below sub-template, or a left-above sub-template.

5. The method of claim 1 , wherein the at least one template set includes any one of the following:

a) a left sub-template and an above sub-template;

b) a left sub-template, an above sub-template, and a left-above sub-template;

c) a left sub-template and a left-below sub-template;

d) an above sub-template and a right-above sub-template;

e) a left sub-template, a left-below sub-template, an above sub-template, and a right-above sub-template; or

f) a left sub-template, a left-below sub-template, an above sub-template, a right-above sub-template, and a left-above sub-template.

6. The method of claim 1 , wherein the coding information further includes a block shape of the current video block.

7. The method of claim 1 , wherein an above sub-template is not selected in a case that the BH divided by the BW is greater than or equal to a second threshold.

8. The method of claim 1 , wherein a dimension of one of the plurality of sub-templates is based on the at least one of the following:

a) a dimension of the current video block;

b) a block shape of the current video block;

c) a slice type of the current video block; or

d) a picture type of the current video block.

9. The method of claim 8 , wherein the dimension of one of the plurality of sub-templates is one of L1×BH, BW×L2, BW′×L2, L1×BH′ or L1×L2, where L1 is a height, L2 is a width, BW and BH represent width and height of the current video block respectively, BW′ represents the width of a right-above sub-template and BH′ represents the height of a left-below sub-template respectively, and wherein BW′ is equal to BW or BH and BH′ is equal to BH or BW, and L1 or L2 is a predefined value.

10. The method of claim 9 , wherein values of L1 and L2 depend on a slice type of the current video block or a dimension of the current video block.

11. The method of claim 9 , wherein values of L1 and L2 are related to a first syntax element presented in the bitstream.

12. The method of claim 11 , wherein constructing at least one template set from a plurality of sub-templates is further related to a second syntax element presented in the bitstream.

13. The method of claim 1 , wherein constructing at least one template set from a plurality of sub-templates is further related to a second syntax element presented in the bitstream.

14. The method of claim 13 , wherein in response to the plurality of sub-templates being unavailable, no template set is constructed and no IPM is derived.

15. The method of claim 14 , wherein the final predictor of the current video block is determined based on a predefined IPM, and wherein the predefined IPM is one of DC mode, planar mode, horizontal mode, or vertical mode.

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

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

18. 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:

construct, during a conversion between a current video block of a video and a bitstream of the video, at least one template set for the current video block from a plurality of sub-templates;

derive at least one intra-prediction mode (IPM) based on cost calculations, wherein the cost calculations are determined based on the at least one template set and the current block;

determine, based on the at least one IPM, a final predictor of the current video block; and

perform the conversion based on the final predictor,

wherein the at least one template set is selected from the plurality of sub-templates based on a coding information for the current video block, wherein the coding information includes a block dimension of the current video block, and the at least one template set is selected from the plurality of sub-templates based on a relationship between the block dimension of the current video block and a pre-defined threshold, and wherein a BW represents a block width of the current video block and a BH represents a block height of the current video block, and

wherein a left sub-template is not selected in a case that the BW divided by the BH is greater than or equal to a first threshold.

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:

constructing, during a conversion between a current video block of a video and a bitstream of the video, at least one template set for the current video block from a plurality of sub-templates;

deriving at least one intra-prediction mode (IPM) based on cost calculations, wherein the cost calculations are determined based on the at least one template set and the current block;

determining, based on the at least one IPM, a final predictor of the current video block; and

generating the bitstream based on the final predictor,

wherein the at least one template set is selected from the plurality of sub-templates based on a coding information for the current video block, wherein the coding information includes a block dimension of the current video block, and the at least one template set is selected from the plurality of sub-templates based on a relationship between the block dimension of the current video block and a pre-defined threshold, and wherein a BW represents a block width of the current video block and a BH represents a block height of the current video block, and

wherein a left sub-template is not selected in a case that the BW divided by the BH is greater than or equal to a first threshold.

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

construct, during a conversion between a current video block of a video and a bitstream of the video, at least one template set for the current video block from a plurality of sub-templates;

derive at least one intra-prediction mode (IPM) based on cost calculations, wherein the cost calculations are determined based on the at least one template set and the current block;

determine, based on the at least one IPM, a final predictor of the current video block; and

perform the conversion based on the final predictor,

wherein the at least one template set is selected from the plurality of sub-templates based on a coding information for the current video block, wherein the coding information includes a block dimension of the current video block, and the at least one template set is selected from the plurality of sub-templates based on a relationship between the block dimension of the current video block and a pre-defined threshold, and wherein a BW represents a block width of the current video block and a BH represents a block height of the current video block, and

wherein a left sub-template is not selected in a case that the BW divided by the BH is greater than or equal to a first threshold.