Method and apparatus for compressing and decompressing data

The apparatus as claimed in 1, wherein if the modulation indicators for a group are not all the same then the set of one or more modulation values indicated by the modulation value choice bits includes more than one modulation value, and wherein the modulation indicator for a respective element in the group indicates which of the set of candidate modulation values is to be used for that element in the group.

An apparatus for compressing electronic image data comprising two sets of elements which are arranged in an alternating pattern, the apparatus being configured to:

• generate at least two sets of reduced size image data from the electronic image data, each element of each set of reduced size image data being representative of a plurality of elements of the electronic image data, wherein the sets of reduced size image data can be combined using modulation values to provide a representation of the electronic image data;
• assign, for each of the elements of a first of the two sets of elements of the electronic image data, a modulation value selected from a plurality of modulation values;
• assign a respective flag corresponding to each of the elements of a second of the two sets of elements of the electronic image data, wherein the flag assigned for an element of the second set indicates whether a modulation value for that element is to be derived by interpolating modulation values assigned to adjacent horizontal elements or adjacent vertical elements.

The apparatus as claimed in 17, wherein if the modulation indicators for a group are not all the same then the set of one or more modulation values indicated by the modulation value choice bits includes more than one modulation value, and wherein the modulation indicator for a respective element in the group indicates which of the set of candidate modulation values is to be used for that element in the group.