IMAGE ACTIVITY DATA COMPRESSING AND DECOMPRESSION METHOD AND APPARATUS

Image data compression apparatus for generating groups of encoded image data representing respective sub-areas of an image, in which the degree of data compression applied to each sub-area is specified at least in part by an attribute value associated with that sub-area, comprises selection logic for selecting the attribute value to be associated with each sub-area of an image; an attribute encoder for encoding the attribute value for each sub-area of the image according to a selected one of a plurality of predetermined sets of attribute codes, different sets of attribute codes being encoded using different respective quantities of data; a comparator for comparing attribute values for a group of the data blocks to detect whether the attribute values for the group of sub-areas could be encoded using a replacement one of said sets of attribute codes requiring a smaller quantity of data; and replacing logic responsive to a detection that the attribute values for the group of sub-areas could be encoded using a replacement set of attribute codes requiring a smaller quantity of data, for replacing the attribute codes from the selected set of attribute codes with corresponding codes from the replacement set of codes.