The prototype uses two cameras: one that captures the driver's view and a second that sees the scene behind a view-blocking wall. A computer takes the feed from the second camera and layers it on top of the images from the first so that the wall appears to be transparent. This makes it simple to glance "through" a wall to see what's going on behind it. But the techniques needed to combine them were challenging to develop, says Yaser Sheikh of Carnegie Mellon University in Pittsburg, Pennsylvania.