Going back a couple of years, raytracing, particularly raytraced reflections, was nothing more than a fake simulation with the refmap.gif file for 3ds(max) users whereas the term "Refractions" were arousing just a simple question: “Is it possible?”. But today we have state-of-the art rendering engines which are available both as standalone and bundled rendering solutions for major 3d software applications aiding the 3d artists to create hyper realistic imagery.

"Global Illumination" or "GI" is one of the techniques available within the contemporary rendering engines which accurately simulates the real world lighting conditions given the proper cpu power and enough time for computations. In today’s production environments usually time is of concern if not the cpu power thus makes GI an “expensive” method for visualisation. Optimizing quality rendering time with alternative methods has become an essential need for the production process.

If you are dealing with 3d rendering, you probably have heard of the “Occlusion” method. In the Siggraph 2002 presentation “Production-Ready Global Illumination”,  ILM Technical Director Hayden Landis tells about this alternate method they developed for simulating GI effectively with speed.

Apparently this relatively new technique has been used by many huge production houses for their projects but it has just began to be available as a common toolset for the regular 3d artist like myself :)

With the introduction and integration of mental ray, 3dsmax has gained a strength in its rendering department, particulary the photo-realistic section. 3dsmax 7 now has the ability to render ambient and reflective occlusion images with the help of a mental ray 3.3 shader.

The Ambient Occlusion method is basically rendering out an occlusion pass(which is a grayscale or a tinted image based on environment map) and then overlaying this image over a flat lit diffuse pass(achieved by ambient lighting, I’ll describe it later) to represent the shadowing caused by the environment. That’s why the method is called “Ambient Occlusion”, to describe the occlusion(blocking) of the ambient light amount to simulate GI shadows. It is just like rendering your scene with a GI skylight, with no specular highlights or cast shadows and faster than GI.