Abstract: Apparatus for image processing which consists of a display (1) and controller (2), a CPU (3) connected to accept user instructions from a mouse (4), an image processor (5) arranged to process image data and a memory (6) for storing image (and other data). A user can specify a sequence of image processing operations on the display (1) using the mouse (4), and interactively modify the sequence depending on the displayed image. The apparatus operates three independent routines ("virtual machines"); a user interface routine accepting instructions and updating the screen display; a sequence evaluation routine constructing a table (20) representing the sequence graph, and evaluating the table (20) to generate corresponding image processing instructions for the image processor (5); and the image processing routine executing specified image processing operations and storing the result in the memory (6) to be accessed for display. Evaluation only occurs after an output is required.