Implicit transform selection in video coding

A device for coding video data, the device comprising:

• a memory configured to store video blocks;
• and
• one or more processors implemented in circuitry and configured to:
• infer, for a current transform block of a current video block, a transform type from a plurality of transform types that includes one or more discrete cosine transforms (DCTs) and one or more discrete sine transforms (DSTs), wherein, to infer the transform type, the one or more processors are configured to:
• determine whether a size of the current transform block satisfies a size threshold, wherein the size of the current transform block satisfies the size threshold where the size of the current transform block is greater than or equal to 4 and less than or equal to 16;
• determine whether the current video block is partitioned using intra- subblock partitioning (ISP);
• select, responsive to determining that the size of the current transform block satisfies the size threshold and that the current video block is partitioned using ISP a particular DST of the one or more DSTs as the selected transform type, wherein to select the particular DST, the one or more processors are configured to select the particular DST regardless of an intra prediction mode selected to predict the current video block, wherein the particular DST is a DST-7;
• and
• select, responsive to determining that the size of the current transform block does not satisfy the size threshold and that the current video block is partitioned using ISP, a particular DCT of the one or more DCTs as the selected transform type, wherein particular DCT is a DCT-2;
• transform, using the selected transform type, the current transform block to obtain a block of reconstructed residual data for the video block;
• and
• reconstruct, based on the reconstructed residual data for the video block, the video block,
• wherein, to infer the transform type for the current transform block, the one or more processors are configured to infer the transform type for the current transform block responsive to determining that multiple transform selection (MTS) is enabled for the current video block,
• wherein, to determine whether the current video block is partitioned using ISP, the one or more processors are configured to determine, based on values of one or more syntax elements decoded from a video bitstream, whether the current video block is partitioned using ISP,
• wherein the size of the current transform block comprises:
• a width of the current transform block;
• and
• a height of the current transform block
• wherein, to select the transform type, the one or more processors are configured to select a transform type for horizontal use and selecting a transform type for vertical use, and wherein the one or more processors are further configured to:
• select the DST-7 as the selected transform type for horizontal use responsive to determining that the width of the current transform block satisfies a width size threshold and that the current video block is partitioned using ISP;
• and
• select the DST-7 as the selected transform type for vertical use responsive to determining that the height of the current transform block satisfies a height size threshold and that the current video block is partitioned using ISP, and wherein the width threshold equals the height threshold.

A computer-readable storage medium storing instructions that, when executed, cause one or more processors of a video coding device to:

• infer, for a current transform block of a current video block, a transform type from a plurality of transform types that includes one or more discrete cosine transforms (DCTs) and one or more discrete sine transforms (DSTs), wherein the instructions that cause the one or more processors to infer the transform type comprise instructions that cause the one or more processors to:
• determine whether a size of the current transform block satisfies a size threshold, wherein the size of the current transform block satisfies the size threshold where the size of the current transform block is greater than or equal to 4 and less than or equal to 16;
• determine whether the current video block is partitioned using intra-subblock partitioning (ISP);
• and
• select, responsive to determining that the size of the current transform block satisfies the size threshold and that the current video block is partitioned using ISP a particular DST of the one or more DSTs as the selected transform type, wherein the instructions that cause the one or more processors to select the particular DST comprise instructions that cause the one or more processors to select the particular DST regardless of an intra prediction mode selected to predict the current video block, wherein the particular DST is a DST-7;
• and
• select, responsive to determining that the size of the current transform block does not satisfy the size threshold and that the current video block is partitioned using ISP, a particular DCT of the one or more DCTs as the selected transform type, wherein particular DCT is a DCT-2;
• transform, using the selected transform type, the current transform block to obtain a block of reconstructed residual data for the video block;
• and
• reconstruct, based on the reconstructed residual data for the video block, the video block,
• wherein the instructions that cause the one or more processors to infer the transform type for the current transform block comprise instructions that cause the one or more processors to infer the transform type for the current transform block responsive to determining that multiple transform selection (MTS) is enabled for the current video block,
• wherein the instructions that cause the one or more processors to determine whether the current video block is partitioned using ISP comprise instructions that cause the one or more processors to determine, based on values of one or more syntax elements decoded from a video bitstream, whether the current video block is partitioned using ISP,
• wherein the size of the current transform block comprises:
• a width of the current transform block;
• and
• a height of the current transform block
• wherein the instructions that cause the one or more processors to select the transform type comprise instructions that cause the one or more processors to select a transform type for horizontal use and selecting a transform type for vertical use, and further comprising instructions that cause the one or more processors to:
• select the DST-7 as the selected transform type for horizontal use responsive to determining that the width of the current transform block satisfies a width size threshold and that the current video block is partitioned using ISP;
• and
• select the DST-7 as the selected transform type for vertical use responsive to determining that the height of the current transform block satisfies a height size threshold and that the current video block is partitioned using ISP, and
• wherein the width threshold equals the height threshold.