1. A method of performing spatial decorrelation on a block of data values at a processing module as part of a data compression process, wherein the processing module comprises a plurality of processing pipelines each comprising a first stage and a second stage, the method comprising, iteratively: processing a line of data values at the first stages of the processing pipelines, and outputting first coefficients resulting from the processing to the second stages of the processing pipelines; and implementing, at the second stages of the processing pipelines, spatial decorrelation on one or more first coefficients, and outputting spatially decorrelated data values; wherein, in each of a plurality of iterations, a respective line of data values is received at the first stages of the processing pipelines, wherein the lines are received over the plurality of iterations in a particular order such that if the first coefficients for a particular line will be used for the spatial decorrelation for another line then the data values of the particular line are received at the first stages in an earlier iteration to that in which the data values of said another line are received at the first stages.