A quantization parameter, QP, deriving method, said method comprising: parsing a quantization parameter QpY, and parsing control information valid for a block of samples comprising a luma component Y and two chroma components Cb, Cr, by: wherein parsing control information comprises: parsing (S20, S25), QP offset syntax element (pps_cb_qp_offset, pps_transformed_cb_qp_offset) from a picture parameter set valid for a picture comprising said block of samples; parsing a slice transformed offset present flag from said picture parameter set; and parsing (S22, S27), if said slice transformed offset present flag is equal to one, a QP offset syntax element (slice_cb_qp_offset, slice_transformed_cb_qp_offset) from a slice header of a slice of said picture, wherein said slice comprises said block of samples; deriving a quantization parameter QP for said chroma component Cb of said block of samples according to a first QP derivation process as a function of (Qp Y + pps_cb_qp_offset + slice_cb_qp_offset) if a flag indicating whether said residual data of said block of samples is color transformed is equal to zero; and deriving said quantization parameter QP for said chroma component Cb of said block of samples according to said second QP derivation process as a function of (Qp Y + pps_transformed_cb_qp_offset + slice_transformed_cb_qp_offset) if said flag is equal to one; wherein Qp Y represents a value of a luma QP for said luma component Y; pps_cb_qp_offset represents an offset to said luma QP, is present in said picture parameter set valid for said picture comprising said block of samples and is valid for non-color-transformed blocks of samples; slice_cb_qp_offset represents an offset to said luma QP, is present in said slice header of said slice of said picture and is valid for non-color-transformed blocks of samples; pps_transformed_cb_qp_offset represents an offset to said luma QP, is present in said picture parameter set and is valid for color transformed blocks of samples; and slice_transformed_cb_qp_offset represents an offset to said luma QP, is present in said slice header and is valid for color transformed blocks of samples. wherein parsing control information comprises: parsing (S20, S25), QP offset syntax element (pps_cb_qp_offset, pps_transformed_cb_qp_offset) from a picture parameter set valid for a picture comprising said block of samples; parsing a slice transformed offset present flag from said picture parameter set; and parsing (S22, S27), if said slice transformed offset present flag is equal to one, a QP offset syntax element (slice_cb_qp_offset, slice_transformed_cb_qp_offset) from a slice header of a slice of said picture, wherein said slice comprises said block of samples; deriving a quantization parameter QP for said chroma component Cb of said block of samples according to a first QP derivation process as a function of (Qp Y + pps_cb_qp_offset + slice_cb_qp_offset) if a flag indicating whether said residual data of said block of samples is color transformed is equal to zero; and deriving said quantization parameter QP for said chroma component Cb of said block of samples according to said second QP derivation process as a function of (Qp Y + pps_transformed_cb_qp_offset + slice_transformed_cb_qp_offset) if said flag is equal to one;