Combination of mode-dependent and fixed transform types in video coding

An apparatus for decoding video data from a video bitstream, the apparatus comprising:

• a processor configured to:
• for a block of a plurality of blocks encoded into the video bitstream using a prediction mode:
• determine a transform mode for the block using the prediction mode, wherein:
• the transform mode comprises a first transform mode responsive to the prediction mode being an inter-prediction mode,
• the transform mode comprises a second transform mode responsive to the prediction mode being an intra-prediction mode of a plurality of available intra-prediction modes,
• the first transform mode comprises one of a plurality of available first transform types, each of the plurality of available first transform types comprising respective combinations of transforms selected from the group consisting of:
• first fixed transforms, each comprising a respective fixed transformation matrix, and
• first mode-dependent, learned transforms, each comprising a respective transformation matrix that was generated iteratively using blocks predicted using the inter-prediction mode,
• the second transform mode comprises one of a plurality of available second transform types, each of the plurality of available second transform types comprising respective combinations of transforms selected from the group consisting of:
• second fixed transforms, each comprising a respective fixed transformation matrix, wherein the second fixed transforms comprise a proper subset of the first fixed transforms, and
• a second mode-dependent, learned transform comprising a transformation matrix that is generated iteratively using blocks predicted using the intra-prediction mode;
• and
• decode the block using the prediction mode and the transform mode.

An apparatus for encoding video data into a video bitstream, the apparatus comprising:

• a processor configured to, for a respective block of a plurality of blocks of the video data:
• determine a transform mode for the block using a prediction mode for the block, wherein:
• the transform mode comprises a first transform mode responsive to the prediction mode being an inter-prediction mode,
• the transform mode comprises a second transform mode responsive to the prediction mode being an intra-prediction mode of a plurality of available intra-prediction modes,
• the first transform mode comprises one of a plurality of available first transform types, each of the available first transform types comprising respective combinations of transforms selected from the group consisting of:
• first fixed transforms, each comprising a respective fixed transformation matrix, and
• first mode-dependent, learned transforms, each comprising a respective transformation matrix that was generated iteratively using blocks predicted using the inter-prediction mode,
• the second transform mode comprises one of a plurality of available second transform types, each of the available second transform types comprising respective combinations of transforms selected from the group consisting of:
• second fixed transforms, each comprising a respective fixed transformation matrix, wherein the second fixed transforms comprise a proper subset of the first fixed transforms, and
• a second mode-dependent, learned transform comprising a transformation matrix that is generated iteratively using blocks predicted using the intra-prediction mode;
• and
• encode the block into the video bitstream using the prediction mode and the transform mode.