  The observation of objects located in inaccessible regions is a recurring
challenge in a wide variety of important applications. Recent work has shown
that indirect diffuse light reflections can be used to reconstruct objects and
two-dimensional (2D) patterns around a corner. However, these prior methods
always require some specialized setup involving either ultrafast detectors or
narrowband light sources. Here we show that occluded objects can be tracked in
real time using a standard 2D camera and a laser pointer. Unlike previous
methods based on the backprojection approach, we formulate the problem in an
analysis-by-synthesis sense. By repeatedly simulating light transport through
the scene, we determine the set of object parameters that most closely fits the
measured intensity distribution. We experimentally demonstrate that this
approach is capable of following the translation of unknown objects, and
translation and orientation of a known object, in real time.
