Importons are "virtual particles". In some aspects they are similar to photons: they bounce in the scene. There are significant differences though: unlike photons, importons do not distribute energy. Instead, they contain a color describing with which factor an illumination at a certain location would contribute to the final image. That way, they provide an information for the rendering core on how the computational efforts should be distributed in order to get an image of best quality with limited computational resources.
Unlike photons, importons are emitted from the camera and bounce towards lights, i.e. in the direction opposite to the photons. However, the duality of lights makes it possible to apply photon shaders to importons. For simplicity reason, mental ray does not introduce a new type of shaders but uses photon shaders for importons as well.
Importons are used as a supplementary mechanism which improves the rendering quality and performance. It is used as the primary mechanism to drive the quality of irradiance particles. Another usage in current mental ray is to improve the quality of the globillum photon map. It allows to dramatically reduce the size (and thus speed up lookup) of photon maps with a help of the merging within a variable merging distance which is determined with the help of importons. Such variable merging distance is superior to the existing constant merging distance.
If enabled, importons are shot before photon maps are created. They are used to during the globillum photon map creation. As the are not relevant for globillum photon map nearest neighbor lookups, importons are discarded once the photon map is created.
The importons map is controlled by scene options.
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