Growing interest in visual languages has triggered new extended research into the specification and parsing of multi-dimensional structures. The author discusses the need for a visual specification formalism and introduces such a technique by augmenting logic programming with picture terms which can be considered as partially specified pictures. He defines 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. PCGs are formal visual specifications of visual languages and can be used for parsing and syntax directed translation of visual languages like DCGs are used in the case of textual languages. The executability of PCGs is demonstrated by defining their translation to logic programs employing picture terms.
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