1. A method of encoding an array of texture data elements to be used in a graphics processing system, the method comprising: encoding the array of texture data elements as a plurality of encoded texture data blocks, each encoded texture data block representing a sub-set of the texture data elements of the array of texture data elements to be encoded and containing data to allow decoded data values for the sub-set of the texture data elements that the block represents to be determined; and wherein: at least one of the encoded texture data blocks represents a non-rectangular sub-set of texture data elements from the array of texture data elements to be encoded.