A set of encoding options to use when encoding an array of data elements is selected based on a bit count value and a distortion value for that set of encoding options. The distortion value is determined from a set of error values that represents the difference between a set of frequency domain coefficients and a set of de-quantised coefficients. The set of frequency domain coefficients are generated by applying only a subset of row transformations or only a subset of column transformations. A set of quantised coefficients are generated by quantising only a subset of the set of frequency domain coefficients. This arrangement provides a more efficient way of selecting encoding options, but without a detrimental reduction in the efficacy of the selection process.