Video coding method based on secondary transform, and device therefor
A video decoding method according to the present document comprises the steps of: deriving transform coefficients by performing de-quantization on the basis of quantized transform coefficients for a target block; deriving modified transform coefficients on the basis of an inverse reduced secondary transform (RST) of the transform coefficients; and generating a reconstructed picture on the basis of residual samples for the target block on the basis of an inverse primary transform of the modified transform coefficients, wherein an inverse RST using a transform kernel matrix is performed on transform coefficients of an upper-left 4×4 region of an 8×8 region of the target block, and modified transform coefficients of the upper-left 4×4 region, an upper-right 4×4 region, and a lower-left 4×4 region of the 8×8 region are derived through the inverse RST.
1 . A decoding apparatus for an image decoding, the decoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
derive transform coefficients through dequantization based on quantized transform coefficients for a target block;
derive modified transform coefficients based on an inverse non-separable transform using a transform kernel matrix for the transform coefficients; and
derive residual samples for the target block based on an inverse primary transform for the modified transform coefficients,
wherein the at least one processor is further configured to:
derive flag information related to whether or not a transform index is obtained, wherein the transform index is related to whether the inverse non-separable transform is applied to the target block and which one of transform kernel matrices is applied to the target block;
derive the transform index based on the flag information; and
perform a matrix operation of specific transform coefficients in the target block and the transform kernel matrix related to the transform index,
wherein the flag information is signaled in a sequence parameter set, and the transform index is present in a coding unit syntax,
wherein based on the inverse non-separable transform being applied to 8×8 area of the target block, 48 modified transform coefficients are derived by performing the matrix operation with 16 specific transform coefficients in the target block,
wherein the 16 specific transform coefficients for the matrix operation are ordered in a diagonal scan order from a top-left 4×4 area of the 8×8 area of the target block,
wherein based on an intra prediction mode for the target block being less than or equal to mode 34, the 48 modified transform coefficients are arranged in a horizontal scan order in an area excluding a bottom-right 4×4 area in the 8×8 area of the target block, and
wherein based on the intra prediction mode for the target block being more than the mode 34, the 48 modified transform coefficients are arranged in a vertical scan order in the area excluding the bottom-right 4×4 area in the 8×8 area of the target block.
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 based on an intra prediction mode applied to a target block;
derive residual samples for the target block based on the prediction samples;
derive transform coefficients for the target block based on a primary transform for the residual samples;
derive modified transform coefficients based on a non-separable transform using a transform kernel matrix for the transform coefficients;
derive quantized transform coefficients by performing quantization based on the modified transform coefficients; and
generate flag information related to whether or not a transform index is present and the transform index, wherein the transform index is related to whether the non-separable transform is applied to the target block and which one of transform kernel matrices is applied to the target block,
wherein the flag information is signaled in a sequence parameter set, and the transform index is present in a coding unit syntax,
wherein based on the non-separable transform being applied to 8×8 area of the target block, 16 modified transform coefficients are derived by performing a matrix operation with 48 transform coefficients in the target block,
wherein based on the intra prediction mode for the target block being less than or equal to mode 34, the 48 transform coefficients for the matrix operation are ordered in a horizontal scan order from an area excluding a bottom-right 4×4 area in the 8×8 area of the target block,
wherein based on the intra prediction mode for the target block being more than the mode 34, the 48 transform coefficients are ordered in a vertical scan order in the area excluding the bottom-right 4×4 area from the 8×8 area of the target block, and
wherein the 16 modified transform coefficients are arranged in a diagonal scan order in a top-left 4×4 area of the 8×8 area of the target block.
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 based on an intra prediction mode applied to 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 based on a non-separable transform using a transform kernel matrix for the transform coefficients, deriving quantized transform coefficients by performing quantization based on the modified transform coefficients, and generating flag information related to whether or not a transform index is present, and the transform index, wherein the transform index is related to whether the non-separable transform is applied to the target block and which one of transform kernel matrices is applied to the target block; and
a transmitter configured to transmit the data comprising the bitstream,
wherein the flag information is signaled in a sequence parameter set, and the transform index is present in a coding unit syntax,
wherein based on the non-separable transform being applied to 8×8 area of the target block, 16 modified transform coefficients are derived by performing a matrix operation with 48 transform coefficients in the target block,
wherein based on the intra prediction mode for the target block being less than or equal to mode 34, the 48 transform coefficients for the matrix operation are ordered in a horizontal scan order from an area excluding a bottom-right 4×4 area in the 8×8 area of the target block,
wherein based on the intra prediction mode for the target block being more than the mode 34, the 48 transform coefficients are ordered in a vertical scan order in the area excluding the bottom-right 4×4 area from the 8×8 area of the target block, and
wherein the 16 modified transform coefficients are arranged in a diagonal scan order in a top-left 4×4 area of the 8×8 area of the target block.