Blender Cycles Slow Rendering Interior Scene

Question

I have create a very basic blender scene with Suzanne and a modified cube with no UVs and only basic materials. The scene contains about 3 emissive lights.

Using the cycles rendering engine, when rendering the exterior scene (3 lights, Suzanne and cube), blender takes 55 seconds.

However when rendering inside of the cube which contains only 1 emissive light, it takes 3 minutes and is relatively slower. Why is it so?

Even the empty inside of Suzanne which contains nothing takes longer time. Why so? Am I doing something wrong or is it natural?

Note: All the scenes are rendered with exactly the same settings and 6 samples

Answer 1

In outdoor scenes most rays don't bounce much and just fly off into the sky.

In indoor scenes almost all the rays will end up reflecting around up to the maximum number of bounces. Not only does this often mean more noise, but also slower rendering (more ray intersection calculations etc.)

You can speed this up a little by reducing the number of bounces. You can also try and reduce noise by using MIS and non-mesh lamps where appropriate.

Related:

• Spherical emitter does not illuminate very well?
• When should MIS be used and when should it be disabled?
• Managing/Reducing noise in a night scene

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Note that in 2.75 cycles will have Light Portals, which can be used to guide rays through openings. This is useful for indoor scenes lit by an outside source.

To set up a light portal, add an area lamp which covers an opening (e.g. a window), then enable Portal in the lamp settings:

The direction matters, the lamp should be pointing into the interior.

Portals will actually make sampling slower, but converge the scene with far fewer samples. So much so that if you reduce the sample count, you can get less noise with fewer samples and a shorter render time than if you didn't use portals: