can be compressed raw i e resulting from the discarding of an adjacent line or require the repetition of a previous line each line of data in the store 5 is preceded by a header this header includes a start of header marker followed by control bits which indicate the form of the data i e compressed or raw and whether or not that data is to be repeated for the next line in one modification of this method each block of pixels may comprise more than two lines with the microprocessor 2 attempting to compress the volume of data to that needed for a single line it will be appreciated that the ram 6 can be small and correspond at the minimum to two lines of raw pixel data in an alternative method fig4 the ram 6 is able to store more than two lines of data for example ten lines and the microprocessor 2 will operate on the data in a different manner in this case a required compression ratio is set by the microprocessor 2 for example 2 1 the microprocessor 2 accesses step 24 a certain number of lines of data from the memory 4 for example 10 lines constituting a block and performs step 25 a compression on those lines in an attempt to reduce them to the required number of lines to achieve this the data is read out from the memory 4 compressed and then stored step 26 in the ram 6 in sequence the microprocessor 2 checks.