Coding and deriving quantization parameters

A decoder, wherein:

• said decoder is configured to parse quantization parameter, QP, control information valid for a block of samples;
• said decoder is configured to derive a QP to use for at least one color component of said block of samples according to a first QP derivation process independent on said QP control information responsive to residual data of said block of samples being not color transformed;
• and
• said decoder is configured to derive said QP to use for said at least one color component of said block of samples according to a second QP derivation process dependent on said QP control information responsive to said residual data of said block of samples being color transformed.

An encoder, wherein:

• said encoder is configured to determine non-color-transformed quantization parameter, QP, offsets valid if residual data of a block of samples is not color transformed;
• said encoder is configured to determine color transformed QP offsets valid if residual data of a block of samples is color transformed;
• and
• said encoder is configured to generate QP control information included in an encoded bitstream based on said non-color-transformed QP offsets and said color transformed QP offsets.

A computer program product comprising a non-transitory computer readable medium storing a computer program comprising instructions, which when executed by at least one processor, cause said at least one processor to:

• parse quantization parameter, QP, control information valid for a block of samples;
• derive a QP to use for at least one color component of said block of samples according to a first QP derivation process independent on said QP control information if residual data of said block of samples is not color transformed;
• and
• derive said QP to use for said at least one color component of said block of samples according to a second QP derivation process dependent on said QP control information if said residual data of said block of samples is color transformed.

A computer program product comprising a non-transitory computer readable medium storing a computer program comprising instructions, which when executed by at least one processor, cause said at least one processor to

• determine non-color-transformed quantization parameter, QP, offsets valid if residual data of a block of samples is not color transformed;
• determine color transformed QP offsets valid if residual data of a block of samples is color transformed;
• and
• generate QP control information included in an encoded bitstream based on said non-color-transformed QP offsets and said color transformed QP offsets.