A picture decoding method, comprising obtaining image information including information related to a quantization parameter, QP, from a bitstream; deriving a luma QP based on the information related to the QP; deriving chroma QPs based on the information related to the QP, wherein the chroma QPs include a first chroma QP for a Cb component and a second chroma QP for a Cr component; generating a reconstructed picture including reconstructed samples based on the luma QP, the first chroma QP for the Cb component and the second chroma QP for the Cr component, wherein the deriving luma QP comprises: deriving an average luma value of a current block based on neighboring samples; deriving a quantization parameter offset, QP offset, for deriving the luma QP based on the average luma value and the information related to the QP; and deriving the luma QP based on the QP offset, characterized in that deriving the chroma QPs comprises: deriving the first chroma QP for the Cb component based on a first chroma QP mapping table for the Cb component; and deriving the second chroma QP for the Cr component based on a second chroma QP mapping table for the Cr component, wherein first chroma QP mapping table for the Cb component is a separate mapping table from the second chroma QP mapping table for the Cr component. obtaining image information including information related to a quantization parameter, QP, from a bitstream; deriving a luma QP based on the information related to the QP; deriving chroma QPs based on the information related to the QP, wherein the chroma QPs include a first chroma QP for a Cb component and a second chroma QP for a Cr component; generating a reconstructed picture including reconstructed samples based on the luma QP, the first chroma QP for the Cb component and the second chroma QP for the Cr component, wherein the deriving luma QP comprises: deriving an average luma value of a current block based on neighboring samples; deriving a quantization parameter offset, QP offset, for deriving the luma QP based on the average luma value and the information related to the QP; and deriving the luma QP based on the QP offset, characterized in that deriving the chroma QPs comprises: deriving the first chroma QP for the Cb component based on a first chroma QP mapping table for the Cb component; and deriving the second chroma QP for the Cr component based on a second chroma QP mapping table for the Cr component, wherein first chroma QP mapping table for the Cb component is a separate mapping table from the second chroma QP mapping table for the Cr component. deriving an average luma value of a current block based on neighboring samples; deriving a quantization parameter offset, QP offset, for deriving the luma QP based on the average luma value and the information related to the QP; and deriving the luma QP based on the QP offset, characterized in that deriving the chroma QPs comprises: deriving the first chroma QP for the Cb component based on a first chroma QP mapping table for the Cb component; and deriving the second chroma QP for the Cr component based on a second chroma QP mapping table for the Cr component, wherein first chroma QP mapping table for the Cb component is a separate mapping table from the second chroma QP mapping table for the Cr component. deriving the first chroma QP for the Cb component based on a first chroma QP mapping table for the Cb component; and deriving the second chroma QP for the Cr component based on a second chroma QP mapping table for the Cr component, wherein first chroma QP mapping table for the Cb component is a separate mapping table from the second chroma QP mapping table for the Cr component.