A Diagram Editor for Line Drawings with Inscriptions

Abstract

A major obstacle to the use of diagrams in system design is the time required to draw and edit such diagrams. The primary purpose of the Diagram Editor is to reduce this time. Initial layout is facilitated by a technique that permits rapid visual evaluation of shape, size and positioning alternatives. Editing is supported in the same sense that a word processor supports the preparation of text. Thus a minor change which might normally require hours of mindless work redrawing an entire diagram can be accomplished in minutes.

The editor works with diagrams that consist of line drawings and text. The line drawings are composed of a relatively small number of basic shapes with connection paths between these shapes. The text is a set of inscriptions associated with the shapes and connectors. The editor produces both hardcopy of a diagram and a data structure which represents its formal and visual content. This can then be used in subsequent sessions by the editor and other programs. Ease of construction and user aesthetics are of paramount importance. User-definable descriptions of diagram elements, default algorithms and user interaction are employed to achieve these objectives.

A simple diagram is represented as a single ‘field’ of diagram elements. Such elements are either ‘node type’ or ‘connector type’ where nodes (shapes) have a position relative to the field center and connectors have a position relative to the two nodes they connect. Associated with both nodes and connectors are ‘regions’ which may contain inscriptions — e.g. text, formulae, or special symbols.

A complicated diagram is represented as a multilevel field structure where the structuring is accomplished by the reversible operations of simplification and elaboration. Petri Nets and related theoretical work have influenced the editor approach. This paper describes key concepts in the editor design.