The General Consistent Labeling (or Constraint Satisfaction) Problem

https://deepblue.lib.umich.edu/bitstream/2027.42/154162/1/39015099114897.pd

SCENELAB: Scene Labelling by a Society of Agents; A Distributed Constraint Propagation System

This paper describes SCENELAB, a computer system for labelling line drawings of scenes in simple polyhedral worlds. The key idea behind SCENELAB is to bring together the concept of contraint-based filtering algorithms and the paradigm of societies of cooperating agents. The problem of finding labellings for pictures drawn from blocks world scenes has been taken as a sample application. Clearly, this makes SCENELAB no vision system, but we claim that a system designed along these lines could be part of a real vision system. Following e.g. Alan Mackworth, we argue that constraint exploitation on resp. between various representational levels is a key technique of ‘seeing things'. Furthermore, constraints and constraint propagation neatly fit into the framework of societies of agents, realized by asynchronuously concurrent processing units and message passing mechanisms. SCENELAB, as it is actually running, can be used to specify and solve arbitrary Labelling problems that can be seen as instances of a particular class of simple constraint problems based on finite, pseudo-transitive binary constraints. However, it is felt that the overall approach generalizes to arbitrary constraint problems. Emphasis is given to a mathematical model of the problem and its solution, to be able to specify the reasoning techniques of SCENELAB, and to identify the class of problems it can handle. I tried to shed some Light onto the methodological background of SCENELAB, which seems necessary to judge the achievements and disachievements of the present work. After some introductory chapters on the key concepts involved in SCENELAB, (scene) labelling problems, constraint propagation, and societies of agents, an overview on both the structure and behavior of SCENELAB is given in part B of the paper. In part C, then, an algebraic model is introduced, which serves as a base for discussing several approaches to labelling problems, namely Waltz's original Landmark algorithm, a synchronized parallel solution suggested by Azriel Rosenfeld, and clearly, the present approach. A proof of the correctness of SCENELABs algorithms is included. This proof takes into account the specifities of systems of asynchronously communicating agents where no global state is observable