An encoder encodes input data to generate corresponding encoded output data. The encoder includes an analysis unit for analysing a portion of the input data and directing the portion to appropriate encoding unit(s), wherein the encoding unit(s) are operable to encode the portion thereat to generate the encoded output data. At least one of the encoding units is operable to compute an average value of data values present in each portion received thereat, to subdivide the data values into at least two sets, to compute average values of the data values in each set, and for each set to allocate the average value for that set to all data values in that set, whilst retaining a spatial mask of the portion, and wherein the spatial mask and information representative of the average values computed for the at least two data sets is included in the encoded output data.