1. Apparatus adapted for being worn by a person, which apparatus comprises: a camera, a projector for projecting images on at least one surface, which surface is not part of said apparatus and is not dedicated for display of said images, and a digital computer for: analyzing visual data captured by said camera to recognize objects and track their motion, and outputting instructions to change said projected images, in such a way that said changes depend at least in part on the results of said analysis.