A method of decompressing a selected sub-block of image element values from a compressed block of image element values, the compressed block of image element values is divided into a plurality of sub-blocks, the compressed block of image element values comprises information, for each sub-block of the plurality of sub-blocks, identifying an encoding format for that sub-block, the information identifying the encoding format of a sub-block identifies (i) one pattern of a plurality of patterns of image element values or (ii) another sub-block that matches that sub-block, the method comprising: identifying, from the information identifying the encoding format of the selected sub-block, a pattern of the plurality of patterns associated with the selected sub-block by: if the information identifying the encoding format of the selected sub-block identifies one pattern, identifying that pattern as the pattern associated with the selected sub-block (1904, 1906), and if the information identifying the encoding format of the selected sub-block identifies another sub-block that matches the selected sub-block, identifying a pattern associated with the matching sub-block as the pattern associated with the selected sub-block (1908, 1910, 1912); identifying, from the compressed block of image element values, one or more image element values associated with the selected sub-block; and generating the selected sub-block from the pattern and the one or more image element values associated with the selected sub-block; characterised in that the compressed block of image element values comprises, for each sub-block whose corresponding information identifying the encoding format of the sub-block identifies one pattern, an image element value unit that comprises the one or more image element values of that sub-block that form the identified pattern, and the one or more image element values associated with the selected sub-block are the image element values in a relevant image element value unit, the relevant image element value unit being the image element value unit associated with that sub-block or the image element value unit associated with the matching sub-block. identifying, from the information identifying the encoding format of the selected sub-block, a pattern of the plurality of patterns associated with the selected sub-block by: if the information identifying the encoding format of the selected sub-block identifies one pattern, identifying that pattern as the pattern associated with the selected sub-block (1904, 1906), and if the information identifying the encoding format of the selected sub-block identifies another sub-block that matches the selected sub-block, identifying a pattern associated with the matching sub-block as the pattern associated with the selected sub-block (1908, 1910, 1912); if the information identifying the encoding format of the selected sub-block identifies one pattern, identifying that pattern as the pattern associated with the selected sub-block (1904, 1906), and if the information identifying the encoding format of the selected sub-block identifies another sub-block that matches the selected sub-block, identifying a pattern associated with the matching sub-block as the pattern associated with the selected sub-block (1908, 1910, 1912); identifying, from the compressed block of image element values, one or more image element values associated with the selected sub-block; and generating the selected sub-block from the pattern and the one or more image element values associated with the selected sub-block; characterised in that the compressed block of image element values comprises, for each sub-block whose corresponding information identifying the encoding format of the sub-block identifies one pattern, an image element value unit that comprises the one or more image element values of that sub-block that form the identified pattern, and the one or more image element values associated with the selected sub-block are the image element values in a relevant image element value unit, the relevant image element value unit being the image element value unit associated with that sub-block or the image element value unit associated with the matching sub-block.