Methods and systems for signaling and performing secondary transforms

A method of video encoding performed at a computing system having memory and one or more processors, the method comprising:

• receiving video data comprising a plurality of blocks, including a first block;
• identifying a primary transform kernel type for the first block;
• when the primary transform kernel type for the first block is a two-dimensional discrete cosine transform (DCT-DCT) or a two-dimensional asymmetric discrete sine transform (ADST-ADST):
• identifying a secondary transform kernel type and a secondary transform set identifier for the first block;
• entropy encoding a syntax element indicating the secondary transform set identifier, wherein a context for entropy encoding the syntax element is determined based on an intra prediction mode of the first block, and wherein the context is configured to apply a maximum range to a most probable set symbol;
• performing a secondary transform on the first block based on the secondary transform kernel type and secondary transform set identifier;
• and
• transmitting the entropy-encoded syntax element in a video bitstream;
• and
• when the primary transform kernel type for the first block is a transform type other than DCT-DCT or ADST-ADST, forgoing performing the secondary transform on the first block.

A method of generating a video bitstream, comprising:

• generating a video bitstream, including:
• identifying a primary transform kernel type for a first block of video data;
• when the primary transform kernel type for the first block is a two-dimensional discrete cosine transform (DCT-DCT) or a two-dimensional asymmetric discrete sine transform (ADST-ADST):
• identifying a secondary transform kernel type and a secondary transform set identifier for the first block;
• entropy encoding a syntax element indicating the secondary transform set identifier, wherein a context for entropy encoding the syntax element is determined based on an intra prediction mode of the first block, and wherein the context is configured to apply a maximum range to a most probable set symbol;
• performing a secondary transform on the first block based on the secondary transform kernel type and secondary transform set identifier;
• and
• transmitting the entropy-encoded syntax element in a video bitstream;
• and
• when the primary transform kernel type for the first block is a transform type other than DCT-DCT or ADST-ADST, forgoing performing the secondary transform on the first block;
• and
• transmitting the video bitstream to a video decoding apparatus, wherein the video bitstream comprises the first entropy-encoded syntax element.