Image coding method based on transform, and device therefor
An image decoding method according to the present document comprises a step of deriving a residual sample for a current block on the basis of a transform coefficient, wherein the step of deriving the transform coefficient can comprise the steps of: determining whether a sub block transform for performing a transform by dividing a coding unit is applied to the current block; determining whether a multiple transform selection (MTS), for which a plurality of transform kernels are used, is applied to the current block; and setting the size of a transform block to which the transform is to be applied, on the basis of whether the sub block transform is applied to the current block and whether the multiple transform selection is applied to the current block.
1. An image decoding method performed by a decoding apparatus, comprising:
obtaining residual information from a bitstream;
deriving transform coefficients for a current block based on the residual information;
deriving residual samples for the current block based on the transform coefficients; and
generating a reconstructed picture based on the residual samples,
wherein deriving the transform coefficients comprises:
determining whether a multiple transform selection (MTS) using a plurality of transform kernels is enabled for the current block based on first flag information related to whether the MTS is enabled;
determining whether a subblock transform is used for a coding block including the current block based on second flag information related to whether the subblock transform is used for the coding block; and
setting a width and a height of a region where a non-zero transform coefficient can exist within the current block based on whether the MTS is enabled and whether the subblock transform is used for the coding block,
wherein based on the first flag information indicating the MTS is enabled and the second flag information indicating the subblock transform is used for the coding block, the width and the height of the region in the current block are set equal to 16, and
wherein based on the first flag information indicating the MTS is not enabled or the second flag information indicating the subblock transform is not used for the coding block, the width of the region in the current block is set equal to a minimum value of a width of the current block and 32, and the height of the region in the current block is set equal to a minimum value of a height of the current block and 32.
2. The image decoding method of claim 1 , wherein the second flag information is signaled at a coding unit level, and the coding unit includes the coding block.
3. The image decoding method of claim 1 , wherein the first flag information is signaled at a sequence parameter set level.
4. The image decoding method of claim 1 , wherein the residual information comprises last significant coefficient prefix information, and
wherein a maximum value of the last significant coefficient prefix information is derived based on the width and the height of the region.
5. An image encoding method performed by an image encoding apparatus, comprising:
deriving residual samples for a current block;
deriving transform coefficients based on the residual samples; and
encoding residual information comprising information on the transform coefficients,
wherein deriving the transform coefficients comprises:
determining whether a multiple transform selection (MTS) using a plurality of transform kernels is enabled for the current block;
determining whether a subblock transform is used for a coding block including the current block; and
setting a width and a height of a region where a non-zero transform coefficient can exist within the current block based on whether the MTS is enabled and whether the subblock transform is used for the coding block,
wherein based on first flag information indicating the MTS is enabled and second flag information indicating the subblock transform is used for the coding block, the width and the height of the region in the current block are set equal to 16, and
wherein based on the first flag information indicating the MTS is not enabled or the second flag information indicating the subblock transform is not used for the coding block, the width of the region in the current block is set equal to a minimum value of a width of the current block and 32, and the height of the region in the current block is set equal to a minimum value of a height of the current block and 32.
6. The image encoding method of claim 5 , wherein the residual information comprises last significant coefficient prefix information, and
wherein a maximum value of the last significant coefficient prefix information is derived based on the width and the height of the region.
7. A transmission method of data for an image, comprising:
obtaining a bitstream for the image, wherein the bitstream is generated based on deriving residual samples for a current block, deriving transform coefficients based on the
residual samples, and encoding residual information comprising information on the transform coefficients; and
transmitting the data comprising the bitstream,
wherein deriving the transform coefficients comprises;
determining whether a multiple transform selection (MTS) using a plurality of transform kernels is enabled for the current block;
determining whether a subblock transform is used for a coding block including the current block; and
setting a width and a height of a region where a non-zero transform coefficient can exist within the current block based on whether the MTS is enabled and whether the subblock transform is used for the coding block,
wherein based on first flag information indicating the MTS is enabled and second flag information indicating the subblock transform is used for the coding block, the width and the height of the region in the current block are set equal to 16, and
wherein based on the first flag information indicating the MTS is not enabled or the second flag information indicating the subblock transform is not used for the current block, the width of the region in the current block is set equal to a minimum value of a width of the current block and 32, and the height of the region in the current block is set equal to a minimum value of a height of the current block and 32.