Reduce, Reuse & Recycle: Efficiently Solving Multi-Label MRFs
Karteek Alahari, Pushmeet Kohli and Philip Torr
In this paper, we present novel techniques that improve the computational and memory efficiency of algorithms for solving multi-label energy functions arising from discrete MRFs or CRFs. These methods are motivated by the observations that the performance of minimization algorithms depends on: (a) the initialization used for the primal and dual variables; and (b) the number of primal variables involved in the energy function. Our first method (dynamic $\alpha$-expansion) works by recycling' results from previous problem instances. The second method simplifies the energy function by reducing' the number of unknown variables, and can also be used to generate a good initialization for the dynamic $\alpha$-expansion algorithm by `reusing' dual variables. We test the performance of our methods on energy functions encountered in the problems of stereo matching, and colour and object based segmentation. Experimental results show that our methods achieve a substantial improvement in the performance of $\alpha$-expansion, as well as other popular algorithms such as sequential tree-reweighted message passing, and max-product belief propagation. In most cases we achieve a 10-15 times speed-up in the computation time. Our modified $\alpha$-expansion algorithm provides similar performance to Fast-PD. However, it is much simpler and can be made orders of magnitude faster by using the initialization schemes proposed in the paper.