Modified transform-based image coding method and device for same

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

• deriving prediction samples for a target block;
• deriving residual samples for the target block based on the prediction samples;
• deriving transform coefficients for the target block based on a primary transform for the residual samples;
• deriving modified transform coefficients from the transform coefficients based on a predetermined transform kernel matrix for a non-separable transform;
• and
• encoding quantized residual information and a Low Frequency Non-Separable Transform (LFNST) index indicating the transform kernel matrix,
• wherein the target block is a coding block, and a transform block is partitioned from the coding block,
• wherein the non-separable transform is not performed when the size of the coding block is greater than the size of a predetermined maximum transform block,
• wherein the target block includes a luma coding block and a chroma coding block, and
• wherein based on the size of the luma coding block being equal to or smaller than the size of the maximum transform block and a color format being 4:2:0, the non-separable transform is applied to the chroma coding block when the chroma coding block is less than or equal to ½ of the size of the maximum transform block.

A non-transitory computer-readable digital storage medium that stores instructions that when executed by one or more processors cause the one or more processors to perform operations comprising:

• deriving prediction samples for a target block;
• deriving residual samples for the target block based on the prediction samples;
• deriving transform coefficients for the target block based on a primary transform for the residual samples;
• deriving modified transform coefficients from the transform coefficients based on a predetermined transform kernel matrix for a non-separable transform;
• and
• encoding quantized residual information and a Low Frequency Non-Separable Transform (LFNST) index indicating the transform kernel matrix to generate a bitstream,
• wherein the target block is a coding block, and a transform block is partitioned from the coding block,
• wherein the non-separable transform is not performed when the size of the coding block is greater than the size of a predetermined maximum transform applied block,
• wherein the target block includes a luma coding block and a chroma coding block, and
• wherein based on the size of the luma coding block being equal to or smaller than the size of the maximum transform block and a color format being 4:2:0, the non-separable transform is applied to the chroma coding block when the chroma coding block is less than or equal to ½ of the size of the maximum transform block.

A transmission method of data for an image, the method comprising:

• obtaining a bitstream for the image;
• wherein the bitstream is generated based on:
• deriving prediction samples for a target block,
• deriving residual samples for the target block based on the prediction samples, deriving transform coefficients for the target block based on a primary transform for the residual samples,
• deriving modified transform coefficients from the transform coefficients based on a predetermined transform kernel matrix for a non-separable transform,
• encoding quantized residual information and a Low Frequency Non-Separable Transform (LFNST) index indicating the transform kernel matrix to generate the bitstream, and
• transmitting the data comprising the bitstream,
• wherein the target block is a coding block, and a transform block is partitioned from the coding block,
• wherein the non-separable transform is not performed when the size of the coding block is greater than the size of a predetermined maximum transform applied block,
• wherein the target block includes a luma coding block and a chroma coding block, and
• wherein based on the size of the luma coding block being equal to or smaller than the size of the maximum transform block and a color format being 4:2:0, the non-separable transform is applied to the chroma coding block when the chroma coding block is less than or equal to ½ of the size of the maximum transform block.