Image decoding method for chroma component and device therefor
An image decoding method performed by a decoding device, according to the present document, comprises the steps of: deriving a chroma quantization parameter table on the basis of quantization parameter data; deriving a quantization parameter for combined chroma coding on the basis of the chroma quantization parameter table; deriving prediction samples of a current chroma block on the basis of prediction information; deriving transform coefficients of the current chroma block on the basis of residual information; deriving residual samples by dequantizing the transform coefficients on the basis of the quantization parameter; and generating a reconstructed picture on the basis of the prediction samples and the residual samples.
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:
obtain image information including prediction information and residual information for a current chroma block, and a flag representing whether quantization parameter data for combined chroma coding is present, based on a chroma type;
obtain the quantization parameter data for the combined chroma coding based on the flag;
obtain quantization parameter data for a chroma cb component and quantization parameter data for a chroma cr component;
derive a chroma quantization parameter mapping table for the combined chroma coding based on the quantization parameter data for the combined chroma coding;
derive a chroma quantization parameter mapping table for the chroma cb component and a chroma quantization parameter mapping table for the chroma cr component based on the quantization parameter data for the chroma cb component and the quantization parameter data for the chroma cr component;
derive a quantization parameter for the combined chroma coding based on the chroma quantization parameter mapping table for the combined chroma coding;
derive a quantization parameter for the chroma cb component and a quantization parameter for the chroma cr component based on the chroma quantization parameter mapping table for the chroma cb component and the chroma quantization parameter mapping table for the chroma cr component;
derive prediction samples of the current chroma block based on the prediction information;
derive transform coefficients of the current chroma block based on the residual information;
derive residual samples by dequantizing the transform coefficients based on a quantization parameter related to the current chroma block among the quantization parameter for the combined chroma coding, the quantization parameter for the chroma cb component and the quantization parameter for the chroma cr component; and
generate a reconstructed picture based on the prediction samples and the residual samples,
wherein the quantization parameter data for the combined chroma coding includes a syntax element for a starting quantization parameter of the chroma quantization parameter mapping table for the combined chroma coding and a syntax element for a number of quantization parameters in the chroma quantization parameter mapping table for the combined chroma coding,
wherein the quantization parameter for the combined chroma coding is derived based on a quantization parameter corresponding to a quantization parameter of a luma component in the chroma quantization parameter mapping table for the combined chroma coding.
2. An encoding apparatus for an 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 of a current chroma block based on inter prediction or intra prediction;
derive residual samples of the current chroma block based on the prediction samples;
generate quantization parameter data for combined chroma coding of the residual samples based on a chroma type of the current chroma block;
generate a flag representing whether the quantization parameter data for the combined chroma coding is present;
generate quantization parameter data for a chroma cb component and quantization parameter data for a chroma cr component; and
encode image information including prediction information and residual information for the current chroma block, the quantization parameter data for the combined chroma coding, the quantization parameter data for the chroma cb component, the quantization parameter data for the chroma cr component and the flag,
wherein the quantization parameter data for the combined chroma coding includes a syntax element for a starting quantization parameter of a chroma quantization parameter mapping table for the combined chroma coding and a syntax element for a number of quantization parameters in the chroma quantization parameter mapping table for the combined chroma coding,
wherein a quantization parameter for the combined chroma coding is derived based on a quantization parameter corresponding to a quantization parameter of a luma component in the chroma quantization parameter mapping table for the combined chroma coding.
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 of a current chroma block based on inter prediction or intra prediction, deriving residual samples of the current chroma block based on the prediction samples, generating quantization parameter data for combined chroma coding of the residual samples based on a chroma type of the current chroma block, generating a flag representing whether the quantization parameter data for the combined chroma coding is present, generating quantization parameter data for a chroma cb component and quantization parameter data for a chroma cr component, encoding image information including prediction information and residual information for the current chroma block, the quantization parameter data for the combined chroma coding, the quantization parameter data for the chroma cb component, the quantization parameter data for the chroma cr component and the flag and generating the bitstream including the image information; and
a transmitter configured to transmit the data comprising the bitstream,
wherein the quantization parameter data for the combined chroma coding includes a syntax element for a starting quantization parameter of a chroma quantization parameter mapping table for the combined chroma coding and a syntax element for a number of quantization parameters in the chroma quantization parameter mapping table for the combined chroma coding,
wherein a quantization parameter for the combined chroma coding is derived based on a quantization parameter corresponding to a quantization parameter of a luma component in the chroma quantization parameter mapping table for the combined chroma coding.