The specification of visual language syntax

Visual programming languages use pictures as programs. One approach to building a visual programming environment is to parameterlze a generic environment with a language specification. We describe a mechanism for specifying visual languages that can be used as the basis of a language-independent visual programming environment. Our mechanism is a new type of grammar, called a picture layout grammar. We show how this type of grammar can describe the two-dimensional syntax of a visual language and give an example of its use. A picture layout grammar permits the syntactic structure of visual program to be recovered by parsing. The parsing ability provides the basis of our visual programming environment.