Transform-based image coding method and device therefor

An image encoding method performed by an image encoding apparatus, 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;
• applying a low frequency non-separable transform (LFNST) to derive modified transform coefficients from the transform coefficients;
• generating residual information based on the modified transform coefficients;
• and
• encoding image information including the residual information,
• wherein applying the LFNST to derive the modified transform coefficients comprises:
• deriving a variable based on a first transform skip flag for a chroma Cb component in the current block and a second transform skip flag for a chroma Cr component in the current block, the variable indicating whether a significant coefficient exists only in a top-left sample position within the current block, the first transform skip flag for the chroma Cb component and the second transform skip flag for the chroma Cr component being encoded into a bitstream, respectively, based on a tree type of the current block being a dual tree chroma;
• and
• encoding an LFNST index based on the variable indicating that the significant coefficient exists at a position other than the top-left sample position,
• wherein based on the LFNST index being equal to 0, the LFNST is not applied to transform coefficients for the chroma Cb component and transform coefficients for the chroma Cr component,
• wherein based on the LFNST index being not equal to 0 and the first transform skip flag for the chroma Cb component being equal to 0, the LFNST is applied to the transform coefficients for the chroma Cb component,
• wherein based on the LFNST index being not equal to 0 and the first transform skip flag for the chroma Cb component being equal to 1, the LFNST is not applied to the transform coefficients for the chroma Cb component,
• wherein based on the LFNST index being not equal to 0 and the second transform skip flag for the chroma Cr component being equal to 0, the LFNST is applied to the transform coefficients for the chroma Cr component, and
• wherein based on the LFNST index being not equal to 0 and the second transform skip flag for the chroma Cr component being equal to 1, the LFNST is not applied to the transform coefficients for the chroma Cr component.

A non-transitory computer-readable digital storage medium storing a bitstream generated by an image encoding method, the image encoding 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;
• applying a low frequency non-separable transform (LFNST) to derive modified transform coefficients from the transform coefficients;
• generating residual information based on the modified transform coefficients;
• and
• encoding image information including the residual information to generate the bitstream,
• wherein applying the LFNST to derive the modified transform coefficients comprises:
• deriving a variable based on a first transform skip flag for a chroma Cb component in the current block and a second transform skip flag for a chroma Cr component in the current block, the variable indicating whether a significant coefficient exists only in a top-left sample position within the current block, the first transform skip flag for the chroma Cb component and the second transform skip flag for the chroma Cr component being encoded into the bitstream, respectively, based on a tree type of the current block being a dual tree chroma;
• and
• encoding an LFNST index based on the variable indicating that the significant coefficient exists at a position other than the top-left sample position, wherein based on the LFNST index being equal to 0, the LFNST is not applied to transform coefficients for the chroma Cb component and transform coefficients for the chroma Cr component,
• wherein based on the LFNST index being not equal to 0 and the first transform skip flag for the chroma Cb component being equal to 0, the LFNST is applied to the transform coefficients for the chroma Cb component,
• wherein based on the LFNST index being not equal to 0 and the first transform skip flag for the chroma Cb component being equal to 1, the LFNST is not applied to the transform coefficients for the chroma Cb component,
• wherein based on the LFNST index being not equal to 0 and the second transform skip flag for the chroma Cr component being equal to 0, the LFNST is applied to the transform coefficients for the chroma Cr component, and
• wherein based on the LFNST index being not equal to 0 and the second transform skip flag for the chroma Cr component being equal to 1, the LFNST is not applied to the transform coefficients for the chroma Cr component.