Generating pictures using random forbidding context

We use random context picture grammars to generate pictures through success ive refinement. At any stage a picture consists of a shape divided into sma ller shapes, each containing a variable or terminal. A variable may be rewr itten according to a production of the underlying grammar. This entails eit her dividing the shape containing it into smaller shapes, or substituting a variable or terminal for it. For visualization purposes every terminal is associated with a color, and its shape is filled with that color. We show p ictures generated with random context picture grammars, then concentrate on grammars which use forbidding context only and present a shrinking lemma f or the corresponding languages.