1. An apparatus, comprising: a face detection engine configured to determine whether a face is present in one or more gray images of respective image frames generated by a depth camera; a hand tracking engine coupled to the face detection engine, and configured to track a hand in one or more depth images generated by the depth camera, on determination by the face detection engine that a face is present in the one or more gray images; and a feature extraction and gesture inference engine coupled to the hand tracking engine, and configured to extract features based on results of the tracking by the hand tracking engine, and infer a hand gesture based at least in part on the extract features; wherein either the face detection engine or the hand tracking engine is further configured to determine a measure of a distance between the face and the camera, using the one or more depth images.