QP control information valid for a block of samples is parsed. A QP to use for the block of samples is derived according to a first QP derivation process independent on the QP control information if residual data of the block of samples is not color transformed and according to a second QP derivation process dependent on the QP control information if the residual data of the block of samples is color transformed. The embodiments thereby decouple QP parameters for blocks of samples that are coded with color transform and those that are coded without color transform. This decoupling achieves better flexibility during encoding and results in better visual quality.