Image coding method based on conversion, and device for same

An image encoding method performed by an image encoding apparatus, the method comprising:

• deriving prediction samples for a current block;
• deriving residual samples for the current block based on the prediction samples;
• deriving transform coefficients for the current block based on a primary transform for the residual samples;
• constructing image information to derive flag information indicating whether a significant coefficient exists in a second region other than a top-left first region of the current block;
• and
• encoding residual information derived based on the transform coefficients for the current block and a multiple transform selection (MTS) index based on the flag information indicating that no significant coefficient exists in the second region,
• wherein the constructing of the image information comprises:
• parsing a flag related to whether the significant coefficient is present in a scan subblock by scanning the scan subblock in a preset scan subblock unit according to an reverse diagonal scan order;
• determining whether the scan subblock belongs to the second region based on a value of the flag being 1;
• and
• determining whether a color index of the current block indicates a luma component based on the scan subblock belonging to the second region, and
• wherein the flag information specifies that the significant coefficient is present in the second region based on the color index indicating the luma component.

A non-transitory computer-readable digital storage medium that stores a bitstream generated by a method, the method comprising:

• deriving prediction samples for a current block;
• deriving residual samples for the current block based on the prediction samples;
• deriving transform coefficients for the current block based on a primary transform for the residual samples;
• constructing image information to derive flag information indicating whether a significant coefficient exists in a second region other than a top-left first region of the current block;
• and
• encoding residual information derived based on the transform coefficients for the current block and a multiple transform selection (MTS) index based on the flag information indicating that no significant coefficient exists in the second region to generate the bitstream,
• wherein the constructing of the image information comprises:
• parsing a flag related to whether the significant coefficient is present in a scan subblock by scanning the scan subblock in a preset scan subblock unit according to an reverse diagonal scan order;
• determining whether the scan subblock belongs to the second region based on a value of the flag being 1;
• and
• determining whether a color index of the current block indicates a luma component based on the scan subblock belonging to the second region, and
• wherein the flag information specifies that the significant coefficient is present in the second region based on the color index indicating the luma component.