Transform-based image coding method and device
An image decoding method according to the present disclosure includes deriving transform coefficients through dequantization based on the quantized transform coefficients for the target block; deriving modified transform coefficients based on an inverse reduced secondary transform (RST) for the transform coefficients; deriving residual samples for the target block based on an inverse primary transform for the modified transform coefficients; and generating a reconstructed samples based on the residual samples, and prediction samples derived based on an intra prediction mode for the target block, wherein the inverse RST is performed based on a transform kernel matrix selected from a transform set including a plurality of transform kernel matrices, the transform set is determined based on a mapping relationship according to the intra prediction mode applied to the target block, and a plurality of intra prediction modes including the intra prediction mode of the target block are mapped to one transform set.
1. An image decoding method performed by a decoding apparatus, comprising:
deriving quantized transform coefficients for a target block from a bitstream;
deriving dequantized transform coefficients for the target block through dequantization based on the quantized transform coefficients for the target block;
deriving modified transform coefficients based on an inverse reduced secondary transform (RST) for the dequantized transform coefficients, wherein the inverse RST derives the modified transform coefficients through a matrix operation of the dequantized transform coefficients and a transform kernel matrix, a number of the modified transform coefficients is greater than a number of the dequantized transform coefficients, the modified transform coefficients are arranged in a top-left area of the target block, and the top-left area is a top-left 4×4 area of the target block or a top-left 8×8 area of the target block;
deriving residual samples for the target block based on an inverse primary transform for the modified transform coefficients; and
generating a reconstructed picture based on the residual samples for the target block and prediction samples derived based on an intra prediction mode for the target block,
wherein the inverse RST is performed based on the transform kernel matrix selected from a transform set including a plurality of transform kernel matrices,
wherein the transform set is determined based on a mapping relationship according to the intra prediction mode applied to the target block, and
wherein five or more directional intra prediction modes are mapped to one transform set.
2. The image decoding method of claim 1 , wherein:
a first intra prediction mode for a first target block to which the inverse RST is applied and a second intra prediction mode for a second target block to which the inverse RST is applied are different from each other, and
transform sets mapped to the first intra prediction mode and the second intra prediction mode are same.
3. The image decoding method of claim 1 , wherein the modified transform coefficients are mapped into the target block based on a diagonal scan order.
4. The image decoding method of claim 1 , further comprising:
obtaining information on whether the inverse RST is applied, wherein the information is included in a sequence parameter set (SPS) in the bitstream.
5. An image encoding method performed by an image encoding apparatus, comprising:
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 reduced secondary transform (RST) for the transform coefficients; and
deriving quantized transform coefficients by performing quantization based on the modified transform coefficients,
wherein the RST derives the modified transform coefficients through a matrix operation of the transform coefficients and a transform kernel matrix,
a number of the modified transform coefficients is less than a number of the transform coefficients, and
the transform coefficients are arranged in a top-left area of the target block,
wherein the top-left area is a top-left 4×4 area of the target block or a top-left 8×8 area of the target block,
wherein the RST is performed based on the transform kernel matrix selected from a transform set including a plurality of transform kernel matrices,
wherein the transform set is determined based on a mapping relationship according to the intra prediction mode applied to the target block, and
wherein five or more directional intra prediction modes are mapped to one transform set.
6. The image encoding method of claim 5 , wherein:
a first intra prediction mode for a first target block to which the RST is applied and a second intra prediction mode for a second target block to which the RST is applied are different from each other, and
transform sets mapped to the first intra prediction mode and the second intra prediction mode are same.
7. The image encoding method of claim 5 , further comprising:
encoding information on whether the RST is applied, wherein the information is included in a sequence parameter set (SPS) in a bitstream.
8. A transmitting method of data for an image, comprising:
obtaining 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 reduced secondary transform (RST) for the transform coefficients deriving quantized transform coefficients by performing quantization based on the modified transform coefficients, and encoding image information related to the quantized transform coefficients; and
transmitting the data comprising the bitstream,
wherein the RST derives the modified transform coefficients through a matrix operation of the transform coefficients and a transform kernel matrix,
a number of the modified transform coefficients is less than a number of the transform coefficients, and
the transform coefficients are arranged in a top-left area of the target block,
wherein the top-left area is a top-left 4×4 area of the target block or a top-left 8×8 area of the target block,
wherein the RST is performed based on the transform kernel matrix selected from a transform set including a plurality of transform kernel matrices,
wherein the transform set is determined based on a mapping relationship according to the intra prediction mode applied to the target block, and
wherein five or more directional intra prediction modes are mapped to one transform set.