Data encoding and decoding

A data encoding method comprises generating encoding parameters include at least one quantisation parameter which varies a quantization relationship between a primary (e.g luma) and one or more secondary components (e.g. chroma) for an image region. The quantisation parameters are generated by: a) generating residual data for the primary and secondary components, the residual data showing a difference between an image data sample and a predicted version of that image data sample, S1400; b) determining a complexity measures of the primary component and the one or more secondary components in dependence upon the generated residual data, wherein the complexity measures are indicative of the energy in the image region, S1402; and c) generating the at least one quantisation in dependence upon the complexity measure of the primary component and the complexity measure of the one or more secondary components which have been determined, S1404.