It's all semantics. Usually the effect is named according to how it was generated.
Computer games use quick hacks and tricks to create such an effect because speed is more important than the visual quality. Games usually use polygons with color at vertices tricks, lightmaps, or other projected texture tricks.
The effect as computed in mental ray is called "volumic lighting". This is because a ray is cast within a specified enclosed space called a "volume". The volume can be a cube, sphere, implicit shape or whatever you can think of and derive mathmatically. Softimage|3D makes a distinction between a volume and the atmosphere, but under the hood, atmostpheric lighting is just volumic lighting with a big volume that encases the entire scene.
Anyway, volumic lights are calculated by tracing a light ray along it's path and evaluating it's color and intensity at incremental steps along the way. This technique is called "raymarching" and can be quite slow (See March_Fractal and March_Smoke shaders). However, most shaders that use this technique usually expose a "step" parameter to adjust the distance between evaluations of the light ray. Larger steps means more distance between evaluations and faster computing, but also lesser quality as the end result will appear jaggy. Small steps produce better results, but at the expense of speed.
Light-Fog as termed by the folks at A|W have different meaning. Some of the light shaders are computed in post (optiFX for example). This type of lighting is faster to compute, but less accurate as it's a 2D effect. This can be verified by creating a mirror like object to reflect objects casting shadows within the scene. You'll see where shadows are cast in the scene, but not in the reflections on the mirror. A|W also has volume based lights in the raytracer, but I'm not sure what technique they use to compute the effect. Offhand I'd say it's a combination of shadow maps and raytracing.
Matt
Matt Lind
Animator / Technical Director
Softimage certified instructor:
Softimage|3D
Softimage|XSI
speye_21@hotmail.com
