How to use the Portal Material in Corona Renderer for Cinema 4D?
In Corona Renderer, the Portal Material is not an actual light source, and it will not change the final appearance of your scene by adding or taking away any light. Instead, it tells Corona how to sample light more efficiently, and so help get rid of noise much faster than randomly sampling the whole scene.
How to create a portal?
Create a plane with no thickness, then apply the Portal material to it (this material has no parameters).
How place a portal in your scene?
Using the Portal Material is easy - they should be the right size to perfectly cover the window hole, and should be placed outside the window. The detailed guidelines regarding portals are the same for all 3D software, and are covered in How do light portals work in Corona Renderer?
Why do I need it?
The benefits of using the Portal Material will not only improve your render time but also the overall quality and assist in reducing unwanted noise.
Cropped image: with Portal on the left and without on the right.
This scene was rendered for approximately 15 passes with and without the use of the Portal Material, denoising has been turned off.
The difference in noise levels is rather noticeable.
NOTE: This free Paris Apartment scene from Slashcube is set up with portals, and can be used as an example of how to set them up (and compare rendering times and noise with and without them).