POVRay

POV is a 3-dimensional raytracing engine. It takes information you supply and simulates the way light interacts with the objects you've defined to create stunning 3d pictures and animation. In addition to the process known as "raytracing," newer versions of POV can also use a variant of the process known as "radiosity" to add greater realisim to scenes, particularly those which use diffuse light such as the fluorescent lighting you might find in an office building.
POVRay is free. The sourcecode for POVRay is also availible.

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