Portal Materials are used to optimize the rendering of light sources. They accomplish this by helping the render kernel find important light sources in the scene. For example, in the case of 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, a polygon plane can be placed outside the window and then a Portal Material can be applied to the plane - thus creating a “portal plane”. This set-up will improve the quality of the light and increase the efficiency of the render.
A Portal material can be created from the Hypershade window (figure 1) or from the OctaneRender shelf (figure 2), just like the other standard materials available in OctaneRender. The Portal material does not have any particular options available to control its functionality.
Figure 1: Create a Portal Material in the Hypershade.
Figure 2: Create a Portal Material using the Octane shelf buttons.
To set up a scene using Portal Materials it is important to make sure that every window or opening in the environment is covered by a portal plane. It will not work if only one window has a portal over it when all other windows do not have a portal over them. And the normal direction of the portal plane should be facing inwards towards the interior or the scene will not render properly. Portal planes should not be blocked by other geometry such as a glass surface. Objects with the Portal material applied will not be visible in the rendering as geometry.
It is best to try to use the least amount of geometry for portals, eg only a few simple rectangular planes are best, dense geometry used for portal planes can slow down rendering. It is possible to use a single piece of portal geometry to cover several openings such as multiple windows on a single wall however if the geometry is too large that can reduce rendering efficiency. Its important to strike a balance between coverage of openings and the size of the geometry that uses the portal material.
The Portal Material option should be used with the Pathtracing and PMC kernels, it will not work when rendering with the Direct Light kernel.
The two images in figure 3 show the results of rendering without and with 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 500 samples. Notice the first image, that does not have a portal plane placed over the opening, is noisier than the second image which does use a portal plane.
Figure 3: Two images rendered without and with a portal plane. Notice that the second image has less noise thanks to the use of a portal plane.