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So, Andrew Price mentioned, in a random tutorial(can't find it atm), that Cycles will render all geometry/particles, even if they are not visible to the camera; This suggests that Cycles doesn't use a frustum check, that is, check whether or not(yay redundancies!) an object is in view.

I would assume using it would improve performance, since all games do it, so: Is it true that it doesn't use it? Why doesn't it use it?

I can actually see a potential problem, a reason not to use it; That would be Global Illumination, where objects out of view would create bounce lighting. Is this why?

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No, Neither Cycles nor Blender-Internal use a camera frustum check when rendering.

While it could work in some cases, but would obviously fail with reflections. Also the chances of flickering in animations where the camera moves is high, objects outside the camera may cast shadows or emit light.

This feature could be useful for preview renders (an additional simplify option).

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It doesn't. Cycles is a backwards ray tracer, which means a that, in each sample, for each pixel, it sends out a ray into the scene, which then bounces off any object it finds in it's way, depending on it's materials, or it goes on into infinity - the background. When a ray reaches the background, each surface it hit get a proper color, depending on it's material and the bounces the ray did. So if a ray was to bounce out of the view and get clamped, then most things would fail, including shadows and reflections.

The way it works on games is entirely different, because they use a different rendering technique, and a lot of Baking.

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