Photon maps in bidirectional Monte Carlo ray tracing of complex objects

This paper describes a bidirectional Monte Carlo ray tracing method simu lating global illumination in models containing complex objects that do not have to be tessellated The two pass method combines a rst pass light ray tracing ray casting with a second pass optimized Monte Carlo ray tracing In the rst pass the light emitted from the light sources hit objects in the scene and may be re ected or transmitted a kind of backward path tracing This step handles all kinds of re ections and not only the specular to di use re ections This turns out to be a valuable optimization At every object interaction energy is stored on the surface of the object For simple objects an illumination map is used For complex objects e g procedurally based objects like fractals energy is stored in a photon map This new concept makes it possible to treat caustics upon such objects without having to parameterize the surface of the objects The second pass Monte Carlo ray tracing from the eye visualizes the scene based upon the result from the rst pass We use the irradiance gradient method to model di use re ections seen directly from the eye All secondary re ections are taken from the photon maps or the illumination maps Only the caustic part of the ray casting step is visualized directly Photon Maps in Bidirectional Monte Carlo Ray Tracing of Complex Objects Henrik Wann Jensen Niels J rgen Christensen Department of Graphical Communication Technical University of Denmark Building DK Lyngby Denmark