Transform and last significant coefficient position signaling for low-frequency non-separable transform in video coding

A device for decoding video data, the device comprising:

• a memory to store the video data;
• and
• one or more processors implemented in circuitry, the one or more processors configured to:
• determine, based on a block size of a current block and a low-frequency non-separable transform (LFNST) syntax element, a zero-out pattern of normatively defined zero-coefficients, wherein the LFNST syntax element is signaled at a transform unit (TU) level, wherein a normative restriction limits a last significant coefficient position of the current block to a position in the current block allowed to be non-zero by the determined zero-out pattern;
• based on the normative restriction limiting the last significant coefficient position of the current block to the position in the current block allowed to be non-zero by the determined zero-out pattern, infer that the last significant coefficient position of the current block is in a region of the current block that is not zeroed out by the determined zero-out pattern;
• determine transform coefficients of the current block, wherein the transform coefficients of the current block include transform coefficients in an LFNST region of the current block and transform coefficients outside the LFNST region of the current block, and the one or more processors are configured such that, as part of determining the transform coefficients of the current block, the one or more processors:
• apply an inverse LFNST to determine values of one or more transform coefficients in the LFNST region of the current block;
• and
• determine that transform coefficients of the current block in a region of the current block defined by the zero-out pattern are equal to 0;
• apply an inverse transform to the transform coefficients of the current block to determine residual data for the current block;
• and
• reconstruct the current block based on the residual data for the current block.