A method of decoding a plurality of coding units from a bitstream to produce an image frame, the coding units being the result of decompositions of coding tree units, the plurality of coding units forming one or more contiguous portions of the bitstream. The method comprises determining a subdivision level for each portion of the bitstream, each subdivision level being applicable to the coding units of the respective contiguous portion of the bitstream; decoding a quantisation parameter delta for each of a number of areas, each area based on decomposition of coding tree units into coding units of each contiguous portion of the bitstream and the corresponding determined subdivision level; determining a quantisation parameter for each area according to the decoded delta quantisation parameter for the area and the quantisation parameter of an earlier coding unit of the image frame; decoding using the determined quantisation parameter of each area.