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Importons3.6

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. The primary usage in mental ray 3.6 is to improve the quality of the globillum photon map. It allows 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 constant merging distance introduced in mental ray 3.5.

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 importon map is controlled by the following string options:
"importon" bool

If enabled, the importons are emitted and importon map is created. The default value is off.

"importon density" float

Specifies the approximate number of importons shot from the camera per pixel. The minimal value for this option in the current implementation is 0.02, i.e approximately 1 importon per 50 pixels. The default and recommended value is 1. Lower values will speed up importon emission, but could lead to a worse photon map and final image quality.

"importon merge" float

Specifies the world-space distance used to merge importons closer than that distance. The default value is 0.0, which means that merging is disabled.

"importon trace depth" int
Controls the diffusion of importons in the scene. If set to zero, importons will not scatter on the diffuse bounces. In some cases it may be required to use more than a single diffuse bounce. This is the case if the combination with finalgathering is used, or when the "importon traverse" option is switched off.
"importon traverse" bool

Enables a special behavior of importons shoot from the camera. Such importons would not be blocked by even completely opaque geometry. Instead, they would be stored for all intersections with geometry on the ray from the camera to infinity. This leads to a significantly higher number of importons stored in the scene. However, it removes the discontinuity in the distribution of the importons originated from the visibility to the camera function. The default value is off

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