title: Convexity Shape Prior for Binary Segmentation.
abstract: Convexity is a known important cue in human vision. We propose shape convexity as a new high-order regularization constraint for binary image segmentation. In the context of discrete optimization, object convexity is represented as a sum of three-clique potentials penalizing any 1- 0- 1 configuration on all straight lines. We show that these non-submodular potentials can be efficiently optimized using an iterative trust region approach. At each iteration the energy is linearly approximated and globally optimized within a small trust region around the current solution. While the quadratic number of all three-cliques is prohibitively high, we design a dynamic programming technique for evaluating and approximating these cliques in linear time. We also derive a second order approximation model that is more accurate but computationally intensive. We discuss limitations of our local optimization and propose gradual non-submodularization scheme that alleviates some limitations. Our experiments demonstrate general usefulness of the proposed convexity shape prior on synthetic and real image segmentation examples. Unlike standard second-order length regularization, our convexity prior does not have shrinking bias, and is robust to changes in scale and parameter selection.
authors.forename: Lena
authors.initials: L
authors.lastname: Gorelick
authors.forename: Olga
authors.initials: O
authors.lastname: Veksler
authors.forename: Yuri
authors.initials: Y
authors.lastname: Boykov
authors.forename: Claudia
authors.initials: C
authors.lastname: Nieuwenhuis