Light portal

A light portal is an invisible helper surface placed over windows, doors or skylights in interior scenes. It does not emit any light of its own; it tells the render engine that light from the sky comes in through this opening. The engine can then aim its samples at the openings where light actually arrives instead of firing them into walls.

Why light portals are needed

With methods such as path tracing, sky light in an interior is only found by the lucky rays that make it out through a window. Most rays sent in random directions from inside the room hit a wall; few reach the glazing. The result is a grainy image despite long render times. A portal lets the engine sample the window directly, giving a much cleaner image in the same time, or the same quality in less time.

Practical points

A portal should cover the whole opening and face the right way. Adding a separate portal to dozens of small windows can sometimes slow a scene down, so the largest openings come first. Their benefit drops in scenes with very large openings, direct sunlight pouring in or views that are mostly outdoors. Some current engines sample lights cleverly enough to reduce the need for portals, but understanding the idea is still key to understanding interior noise. For more on interior work, see our interior rendering services guide.