By default, all objects can participate in global illumination computations. This is necessary for simulation of real global illumination. However, sometimes one might just be interested in color bleeding from one object to another, and the rest of the scene does not need to participate in the global illumination simulation.
To simulate global illumination more efficiently in such cases, objects can be flagged such that the photons are only emitted towards certain objects and stored only on selected objects. Objects are then divided into globillum-casting ( globillum 1 flag) and globillum-receiving ( globillum 2 flag), or both (globillum 3 flag), or neither ( globillum 0 flag). For example, color bleeding from a red diffuse table onto a diffuse white wall requires a globillum-casting table and a globillum-receiving wall. Objects can also be flagged globillum off, which means that globillum photons will not hit them at all (the objects will be "invisible" to globillum photons), or flagged globillum on which is the same as globillum 3. The globillum mode is an object attribute. Photons are emitted only in the direction of globillum-casting objects, and only stored on globillum-receiving objects.
To use this optimization requires that the default object globillum flag (specified in the options) is set to something different than 3 (which is the default value, enabling all objects to cast and receive global illumination).
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