Semantic Image Labelling as a Label Puzzle Game

In this work we introduce a novel solution to the semantic image labelling problem, i.e. the task of assigning semantic object class labels to individual pixels in a test image. Conventional methods are typically relying on random fields for modelling interactions between neighboring pixels and obtaining smooth labelling results using unary and pairwise cost functions. Instead, we consider the labelling problem as a puzzle game, where the final labelling is obtained by assembling discriminatively learned candidate sets of label puzzle pieces, each representing a topological and semantically plausible label configuration. The puzzle game is set up by means of a modified random forest classifier, designed to learn the local, topological label-structure and hence the local context associated to the training data. To solve the puzzle game we propose an iterative optimization technique that maximizes an agreement function by alternatingly seeking for the best label puzzle piece per pixel and the resulting semantic labelling per image. We provide both, theoretical properties of our puzzle solver algorithm as well as experimental results on the challenging MSRC and CamVid databases. In a direct comparison with a conditional random field we obtain superior results, indicating the practicability of our proposed method.