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Description
In my new job at VRAC in Iowa State University I am working toward providing John Deere with new ways to display their models in real time and more visually pleasing than their current CAD programs allow.
We are using OpenSG as a framework which is widely used at VRAC. All models are rendered in Fedora Core 4 linux with my GeForce 6600gt in real time. The trick displayed here is to precalculate lighting using estimated ambient occlusion. The theory is that points on objects with more access to the outside world are generally brighter, so in a well lit room, the tip of your nose is brighter than the inside of your ear.
In this implementation I build a KD-tree of all triangles in the scene, then cast random rays for each vertex in the hemisphere in the direction of the vertex normal from a point just barely offset from the vertex. The percentage of rays that do not hit any triangles is the "accessibility" of the vertex and directly translates to the brightness used here. For production uses, this value will be blended with the diffuse lighting and perhaps other forms of lighting.
The top two images are just my test torus and cylinder and you can see that the algorithm has troubles where the shadowing on the end of the sphere looks off because there is no vertex in the center. The availability is calculated in a second or so. The next two images show a pair of free 3DS models available on the net. They are in the realm of 10k triangles and at higher quality estimations can take more than an hour. The production models will have millions of triangles and several levels of detail so clearly further optomization is needed. However, this will be part of a process that can be automated over night so it is not necessary that the calculation be instantaneous.
We are looking into implementing in GLSL an algorithm from Nvidia in GPU Gems which does an approximation in real time in their CG language. We need this CPU implementation because even though it takes time to precalculate, the actual rendering is just as fast as if no lighting calculations have been done at all even on lower end graphics cards.
The biggest point of optomization is going to be the KD-tree node splitting algorithm so that the ray intersect tests go faster.
