Indigo: A Local Propagation Algorithm for Inequality Constraints

Inequality constraints are useful for specifying various aspects of user interfaces, such as constraints that one window is to the left of another, or that an object is contained within a rectangle. However, current local propagation constraint solvers can't handle inequality constraints. We present Indigo, an efficient local propagation algorithm for satisfying acyclic constraint hierarchies, including inequality constraints. KEYWORDS: Constraints, inequality constraints, local propagation INTRODUCTION Constraints are useful for a number of aspects of user interface construction, including layout, model-view consistency, and consistency of multiple views. In selecting the class of constraints to be supported in a UI toolkit, there are various tradeoffs between simplicity and power. Some of these tradeoffs include: ffl one-way or multi-way constraints ffl functional constraints only, or more general kinds of relations ffl required constraints only, or constraint hierarchies A c..