The basic concepts of image histogram modification are described, including in particular the technique of histogram equalisation or flattening, which is the basis for a large variety of transformations. A fast and versatile digital system is described which is capable of acquiring image histogram data, and under microprocessor control, of performing many transformations interactively. Images are input in the form of standard T.V. rate video signals and processed with < 150ns throughput delay. Transformation calculations are performed in well under a second.