Adaptive determination of multiple transform candidates for video coding

A device for decoding video data, the device comprising:

• a memory configured to store video data;
• and
• one or more processors implemented in circuitry and configured to:
• determine a set of multiple transform selection (MTS) candidates for a current block of video data,
• wherein to determine the set, the one or more processors is configured to select the set, based on a number of non-zero coefficients of a transform block, between a first set consisting of a single MTS candidate, a second set of MTS candidates having at least four MTS candidates, and a third set of MTS candidates having more MTS candidates than the second set, the MTS candidates of the second and third set being MTS candidates other than a DCT2 candidate;
• determine one of the MTS candidates of the determined set of MTS candidates for the current block;
• inverse transform the transform block for the current block using the determined one of the MTS candidates to produce a residual block;
• and
• decode the current block using the residual block.

A device for decoding video data, the device comprising:

• means for determining a set of multiple transform selection (MTS) candidates for a current block of video data,
• wherein the means for determining the set comprises means for selecting the set, based on a number of non-zero coefficients of a transform block, between a first set consisting of a single MTS candidate, a second set of MTS candidates having at least four MTS candidates, and
• a third set of MTS candidates having more MTS candidates than the second set, the MTS candidates of the second and third set being MTS candidates other than a DCT2 candidate;
• means for determining one of the MTS candidates of the determined set number of MTS candidates for the current block;
• means for inverse transforming the transform block for the current block using the determined one of the MTS candidates to produce a residual block;
• and
• means for decoding the current block using the residual block.