Abstract
Biederman & Cooper (1991) showed that observers were better at classifying degraded line drawings of objects when shown only contour junctions than when shown only middle segments. We here provide an account of these results based on a low-level algorithm for object completion. The human visual system infers geometry not only for visible but also for occluded contours and surfaces. This provides a central theme to figural completion which lays out a computational framework to estimate paths that connect a set of boundary fragments. In our model, we use the Fokker-Planck Equation to extract a set of points with assigned orientations. These points represent the sources and sinks for a stochastic completion field (SCF) algorithm (Williams and Jacobs, 1995). The SCF algorithm produces a distribution of possible completion fields, where each field is a probability density function (PDF) that enables us to score each possible completion path. We tested the algorithm on the Snodgrass and Vanderwart (1980) dataset of 260 manually traced line drawings of objects. The traced objects were separated into two half images; one half with contour segments containing junctions, and the other with segments between junctions. We attempted to complete the half-drawings using the SCF algorithm. The completions replicated the original, intact drawings more faithfully for the half-drawings with junctions than those with middle segments. Our computational results show that contour completion is easier in objects with junctions than with middle segments, which aligns with Biederman & Cooper's behavioural result. Ultimately, the SCF is a method that is potentially useful for predicting which types of incomplete line drawings can be more easily completed by the human visual system. Moreover, SCF may form the computational basis for an image-computable implementation of the Good Continuation Gestalt grouping rule.