When rendering interior scenes, one frequently encounters an efficiency problem where the "portals" through which light can enter the scene from outside (e.g. an open window) are relatively small, making it quite difficult to sample.
Exit portals can greatly improve rendering efficiency in these situations by marking important areas through which light can enter the scene, which Indigo will directly sample to produce valid light carrying paths.
Although it is a material type, exit portals don't have any particular appearance of their own, they simply provide an "open window" through which the background material is seen, and through which it can illuminate the scene.
For more information and example images, please see the SketchUp exit portal tutorial.
Requirements for exit portal usage:
- If any exit portals are present in the scene, then all openings must be covered by exit portals.
- The face normals must face into the interior of the scene.
- The exit portal material should only be applied to one side of a mesh (e.g. a cube), otherwise it will lose its effectiveness.