Apparatus comprises data compression circuitry to process a set of data values, the data compression circuitry comprising: detector circuitry to detect, for each of n complementary groups of m data values of the set of data values, a first subset of the groups for which the data values in the group have a predetermined pattern of data values, where m and n are integers and m×n is the number of data values in the set of data values; generator circuitry to generate a compressed data packet comprising at least: a representation of a second subset of the groups, the second subset being each of then complementary groups other than groups in the first subset; and an indication of a group position, with respect to the set of data values, of each group in the second subset of groups. Complementary decompression apparatus is also described.