Global illumination is the simulation of all light interreflection effects in a scene (except caustics). This includes effects such as color bleeding: if a red table is next to a white wall, the white wall gets a slightly pink tint. This effect is not possible with ordinary ray tracing algorithms. But if the pink tint is lacking in an image, the image looks fake, even though it might be hard to point out precisely why. Global illumination effects are subtle but add realism to a scene.
Simulation of global illumination has at least two distinct uses:
The computation of global illumination requires photon tracing, just like computation of caustics. In fact, the same photon material shaders can be used. Since caustics are treated separately in mental ray, the global illumination simulation does not include caustics. So if all light interreflections should be simulated, both global illumination and caustics must be enabled.
The photons stored during global illumination simulation are stored in a separate photon map, the global illumination photon map. When the material shader calls mi_compute_irradiance, the irradiance from both the caustics photon map and the global illumination photon map are computed.
To turn global illumination on, specify globillum on in the options or give command-line option -globillum on.
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