A computer program and method for generating from a 1-bit image data file a multiple-bit image data file, which, when displayed on a computer display screen, resembles an image that would appear on an output medium as a result of an output device printing the 1-bit file, the program being operable to carry out, and the method comprising, the steps of: receiving a 1-bit image data file and first and second values that characterize actual regions of the output medium that would be marked by the output device when printing single and consecutive "on" pixels of the 1-bit file; calculating from the "on" pixels and the first and second values, proportions of corresponding notional rectangular regions of the output medium that would be marked by the output device when printing the "on" pixels; and setting values of the pixels of a multiple-bit image data file according to the calculated proportions.
