Secondary transforms for compound inter-intra prediction modes

A device for decoding a current block from a video bitstream, the device comprising a memory for storing computer instructions and a processor in communication with the memory, wherein, when the processor executes the computer instructions, the processor is configured to cause the device to:

• determine that the current block is predicted under a CIIP mode;
• generate a set of secondary transform coefficients for the current block from the video bitstream;
• determine a CIIP submode of the CIIP mode for the current block from the video bitstream among a set of CIIP submode candidates, the CIIP submode indicating an intra prediction mode among a set of intra prediction modes used under the CIIP mode for the current block, and the set of CIIP submode candidates comprises at least one of:
• an II DC PRED mode;
• an II V PRED mode;
• an II H PRED mode;
• or
• an II SMOOTH PRED mode;
• determine a transform kernel based on the intra prediction mode, to be used in an inverse separable or non-separable secondary transform;
• apply a combined inter intra secondary transform by:
• using the determined transform kernel to perform the inverse separable or non-separable secondary transform on the set of secondary transform coefficients to obtain a set of primary transform coefficients of the current block;
• and
• performing an inverse primary transform on the set of primary transform coefficients to obtain a residual block of the current block;
• and
• decode, under the CIIP mode, the current block from the residual block.

A non-transitory storage medium for storing computer readable instructions, the computer readable instructions, when executed by a processor, causing the processor to:

• receive a current block from a video bitstream;
• determine that the current block is predicted under a CIIP mode;
• generate a set of secondary transform coefficients for the current block from the video bitstream;
• determine a CIIP submode of the CIIP mode for the current block from the video bitstream among a set of CIIP submode candidates, the CIIP submode indicating an intra prediction mode among a set of intra prediction modes used under the CIIP mode for the current block, and the set of CIIP submode candidates comprises at least one of:
• an II DC PRED mode;
• an II V PRED mode;
• an II H PRED mode;
• or
• an II SMOOTH PRED mode;
• determine a transform kernel based on the intra prediction mode, to be used in an inverse separable or non-separable secondary transform;
• apply a combined inter intra secondary transform by:
• using the determined transform kernel to perform the inverse separable or non-separable secondary transform on the set of secondary transform coefficients to obtain a set of primary transform coefficients of the current block;
• and
• performing an inverse primary transform on the set of primary transform coefficients to obtain a residual block of the current block;
• and
• decode, under the CIIP mode, the current block from the residual block.