1. A system for rapidly generating an image of a pattern in successive lines, each line being formed of a plurality of pixels, in which each pixel is represented by a pixel value, said system comprising: means for storing the pixel values of a previously generated line; means for producing one or more replacement pixel values; output means for providing successively generated lines of the image; control means for selectively delivering output of said means for storing and said means for producing to said output means in accordance with commands defining the boundaries of the pattern to generate the successive lines of the image; and means for updating said means for storing with the replacement pixel values from said means for producing.