Image coding method on basis of transform, and apparatus therefor
An image decoding method according to the present document comprises a step for deriving a modified transform coefficient, wherein the step for deriving the modified transform coefficient comprises a step for determining whether or not to parse an LFNST index on the basis of whether or not the width and height of a current block satisfy a condition about whether the LFNST can be applied, and whether or not the condition about whether the LFNST can be applied is satisfied is determined on the basis of a tree type and a color format of the current block and whether or not an ISP is applied to the current block.
1. A decoding apparatus for image decoding, the decoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
receive a bitstream including residual information;
derive transform coefficients for a current block based on the residual information;
derive modified transform coefficients by applying a low-frequency non-separable transform (LFNST) to the transform coefficients; and
derive residual samples for the current block based on an inverse primary transform for the modified transform coefficients,
wherein the at least one processor further configured to
determine whether or not to parse an LFNST index based on whether or not a width and a height of the current block satisfy a condition that the LFNST is applied,
wherein whether or not the condition that the LFNST is applied is satisfied is determined based on a tree-type of the current block, a color format of the current block, and whether or not an intra sub partitions (ISP) is applied to the current block, and
wherein in case the ISP is applied, based on the width and the height of the current block corresponding to a subpartition block being equal to 4 or more, the LFNST index is parsed.
2. An encoding apparatus for image encoding, the encoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
derive prediction samples for a current block;
derive residual samples for the current block based on the prediction samples;
derive transform coefficients for the current block based on a primary transform for the residual samples;
derive modified transform coefficients from the transform coefficients by applying a low-frequency non-separable transform (LFNST); and
encode residual information on the modified transform coefficients, and an LFNST index related to an LFNST matrix that is applied to the LFNST,
wherein the LFNST index is encoded based on whether or not a width and a height of the current block satisfy a condition that the LFNST is applied,
wherein whether or not the condition that the LFNST is applied is satisfied is determined based on a tree-type of the current block, a color format of the current block, and whether or not an intra sub partitions (ISP) is applied to the current block, and
wherein in case the ISP is applied, based on the width and the height of the current block corresponding to a subpartition block being equal to 4 or more, the LFNST index is encoded.
3. An apparatus for transmitting data for an image, the apparatus comprising:
at least one processor configured to obtain a bitstream for the image, wherein the bitstream is generated based on 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, deriving modified transform coefficients from the transform coefficients by applying a low-frequency non-separable transform (LFNST), and encoding residual information on the modified transform coefficients, and an LFNST index related to an LFNST matrix that is applied to the LFNST; and
a transmitter configured to transmit the data comprising the bitstream,
wherein the LFNST index is encoded based on whether or not a width and a height of the current block satisfy a condition that the LFNST is applied,
wherein whether or not the condition that the LFNST is applied is satisfied is determined based on a tree-type of the current block, a color format of the current block, and whether or not an intra sub partitions (ISP) is applied to the current block, and
wherein in case the ISP is applied, based on the width and the height of the current block corresponding to a subpartition block being equal to 4 or more, the LFNST index is encoded.