Fine-Tuning Global Illumination

To change the number of photons used to compute the local intensity of global illumination, specify a globillum accuracy (and optionally a maximum radius) in the options. For example,

globillum accuracy 300 2.0

The default number is 500; larger numbers reduce the noise and make the global illumination smoother but increase render time. The default radius depends on the scene extent.

In order to reduce the noise in the image, but keep the global illumination details, the number of photons used for the lookup should be increased by the same factor as the number of photons emitted from the lights.

For some scenes this may lead to large photon maps which exceed the available memory. In order to reduce the photon map size, the string option

"globillum merge" distance

can be used. If this option is set, the photons within the specified distance are merged. For scenes with uneven photon distribution, this can decrease the size of the photon map dramatically.

Alternatively, large number of photons can be handled with per-object photon map files^3.5. The photons would be saved into several files, which could be loaded and unloaded on demand. Per-object photon maps are not recommended for usage in a combination with final gathering as the spatial coherence of photon maps lookups would be very poor, causing frequent reloading.

The distribution of photons may be inspected with the photon density and photon intensity diagnostic modes. If the former one shows small regions with very high photon density, the setting the "globillum merge"^3.5 option may be useful.