Label propagation in complex video sequences using semi-supervised learning

We propose a novel directed graphical model for label propagation in lengthy and complex video sequences. Given hand-labelled start and end frames of a video sequence, a variational EM based inference strategy propagates either one of several class labels or assigns an unknown class (void) label to each pixel in the video. These labels are used to train a multi-class classifier. The pixel labels estimated by this classifier are injected back into the Bayesian network for another iteration of label inference. The novel aspect of this iterative scheme, as compared to a recent approach [1], is its ability to handle occlusions. This is attributed to a hybrid of generative propagation and discriminative classification in a pseudo time-symmetric video model. The end result is a conservative labelling of the video; large parts of the static scene are labelled into known classes, and a void label is assigned to moving objects and remaining parts of the static scene. These labels can be used as ground truth data to learn the static parts of a scene from videos of it or more generally for semantic video segmentation. We demonstrate the efficacy of the proposed approach using extensive qualitative and quantitative tests over six challenging sequences. We bring out the advantages and drawbacks of our approach, both to encourage its repeatability and motivate future research directions.