Ambiguity in visual language theory and its role in diagram parsing

To take advantage of the ever-increasing volume of diagrams in electronic form, it is crucial that we have methods for parsing diagrams. Once a structured, content-based description is built for a diagram, it can be indexed for search, retrieval, and use. Whenever broad coverage grammars are built to parse a wide range of objects, whether natural language or diagrams, the grammars will overgenerate, giving multiple parses. This is the ambiguity problem. This paper discusses the types of ambiguities that can arise in diagram parsing, as well as techniques to avoid or resolve them. One class of ambiguity is attachment, e.g., the determination of what graphic object is labeled by a text item. Two classes of ambiguities are unique to diagrams: segmentation and occlusion. Examples of segmentation ambiguities include the use of a portion of a single line as an entity itself. Occlusion ambiguities can be difficult to analyze if occlusion is deliberately used to create a novel object from its components. The paper uses our context-based constraint grammars to describe the origin and resolution of ambiguities. It assumes that diagrams are available as vector graphics, not bitmaps.