PortalA technique that assists the render kernel with exterior light sources that illuminate interiors. In interior renderings with windows, it is difficult for the path tracer to find light from the outside environment and optimally render the scene. Portals are planes that are added to the scene with the Portal material applied to them. materials optimize rendering light sources. They accomplish this by helping the render kernel find important light sources in the scene. For example, for interior scenes illuminated by an outside light source that comes in through windows, it can be difficult for the path tracer to optimize the light as it enters the interior environment. To help the path tracer find these light sources, you can place a Polygon plane outside the window and then apply a Portal material to the plane, which creates a Portal plane. This setup improves the light quality and increases the render efficiency.
Other considerations when using Portal materials include:
Figure 1 shows the results of rendering with and without a Portal material. The scene shows a glass sphere rendered in a room lit by light coming through a window. The scene is rendered using 128 samples. The first image does not have a Portal plane over the opening. The indirect light quality is significantly better in the second scene where Portals are present.
Figure 1: The first image is rendered without a Portal material, the second is rendered with a Portal material