Pictures Depicting Pictures : On the Specification of Visual Languages by Visual Grammars
Growing interest in visual languages has triggered new extended research into the specification and parsing of multi-dimensional structures. Though some approaches to general descriptive formalisms for visual structures have been made in the last years, there is still no visual specification formalism for visual languages. The paper discusses the need for a visual formalism and introduces such a technique by augmenting logic programming with picture terms. Picture terms can be considered as partially specified pictures. We define how to match picture terms and how to integrate matching with the execution of logic programs. Based upon this extension, picture clause grammars (PCGs) are introduced. Due to their declarative nature PCGs are formal visual specifications of visual languages and executable parsers at the same time. Like Definite Clause Grammars, PCGs can be used for parsing and syntax directed translation. The executablity of PCGs is demonstrated by defining their translation to logic programs employing picture terms. From the different types of PCGs that are needed to describe a picture a Chomsky-like hierarchy of regular, context-free, and non-context-free pictures can be derived. We will give examples for picture languages and the grammars that are used for parsing and translating them.
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