Abstract.
Figure/ground assignment is a key step in perceptual or- ganization which assigns contours to one of the two abutting regions, providing information about occlusion and allowing high-level process- ing to focus on non-accidental shapes of figural regions. In this paper, we develop a computational model for figure/ground assignment in com- plex natural scenes. We utilize a large dataset of images annotated with human-marked segmentations and figure/ground labels for training and quantitative evaluation. We operationalize the concept of familiar configuration by construct- ing prototypical local shapes, i.e. shapemes, from image data. Shapemes automatically encode mid-level visual cues to figure/ground assignment such as convexity and parallelism. Based on the shapeme representation, we train a logistic classifier to locally predict figure/ground labels. We also consider a global model using a conditional random field (CRF) to enforce global figure/ground consistency at T-junctions. We use loopy belief propagation to perform approximate inference on this model and learn maximum likelihood parameters from ground-truth labels. We find that the local shapeme model achieves an accuracy of 64% in predicting the correct figural assignment. This compares favorably to previous studies using classical figure/ground cues [1]. We evaluate the global model using either a set of contours extracted from a low-level edge detector or the set of contours given by human segmentations. The global CRF model significantly improves the performance over the lo- cal model, most notably when using human-marked boundaries (78%). These promising experimental results show that this is a feasible ap- proach to bottom-up figure/ground assignment in natural images.