  Amodal completion is a visual task that humans perform easily but which is
difficult for computer vision algorithms. The aim is to segment those object
boundaries which are occluded and hence invisible. This task is particularly
challenging for deep neural networks because data is difficult to obtain and
annotate. Therefore, we formulate amodal segmentation as an out-of-task and
out-of-distribution generalization problem. Specifically, we replace the fully
connected classifier in neural networks with a Bayesian generative model of the
neural network features. The model is trained from non-occluded images using
bounding box annotations and class labels only, but is applied to generalize
out-of-task to object segmentation and to generalize out-of-distribution to
segment occluded objects. We demonstrate how such Bayesian models can naturally
generalize beyond the training task labels when they learn a prior that models
the object's background context and shape. Moreover, by leveraging an outlier
process, Bayesian models can further generalize out-of-distribution to segment
partially occluded objects and to predict their amodal object boundaries. Our
algorithm outperforms alternative methods that use the same supervision by a
large margin, and even outperforms methods where annotated amodal segmentations
are used during training, when the amount of occlusion is large. Code is
publicly available at https://github.com/YihongSun/Bayesian-Amodal.
