A data array to be stored is first divided into a plurality of blocks. Each block is further sub-divided into a set of sub-blocks.Data representing sub-blocks of the data array is stored, together with a header data block for each block that the data array has been divided into.For each block, it is determined whether all the data positions for the block have the same data value associated with them, and, if so, an indication that all of the data positions within the block have the same data value associated with them, and an indication of the same data value that is associated with each of the data positions in the block, is stored in the header data block for that block of the data array.