Icola — Incremental Constraint-Based Graphics for Visualization

Visualization is valuable in monitoring and debugging programs. The goal of the Wand research project at the University of Saskatchewan is to provide both a framework and tools for rapid development of visualization aids for logic programming languages. The ICOLA (Incremental Constraint-based Object Layout Algorithm) system is the newest graphics facility within Wand. ICOLA positions graphical objects according to object declarations and constraints specifying relative positional relationships among the objects. Three important features of ICOLA are that it is capable of creating reasonable pictures from highly under-constrained specifications, it uses an incremental constraint solution algorithm and hence generates those pictures efficiently, and it supports incremental (i.e. progressive) insertions and deletions of objects and constraints. The ability of the incremental algorithm to support such deletions is particularly noteworthy. This paper describes: PDI, the language supported by ICOLA; the incremental constraint solution algorithm itself; a successful implementation in Prolog and C; and results of a performance evaluation of the implementation.