How is a projector handled in a Ray Tracing environment?
Just treat it as a special case of a point light + shadow ray. Whenever a ray hits the screen (or any surface onto which you'd like to project the slide), fire another "projector" ray back towards the projector. If it is not obscured, then use the X &amp; Y angles (relative to the coordinate system of the projector) to look up a texture map of the slide.