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 ou...