Simon’s Graphics Blog

# Now You're Lighting With Portals

path tracing maths

I hate dome lights. You always waste a ton of rays that are occluded by geometry, and the situation gets even worse when lighting indoor scenes with exterior dome lights!

So why not help your renderer out and place portals that, when hit, teleport to the dome light. Then instead of sampling the whole skydome, we just sample the portals, and avoid sending rays where we know they will be occluded.

As an example, here’s the Sponza scene using an exterior (uniform) dome light, rendered using unidirectional path tracing with multiple importance sampling:

Lots of rays never manage to find the open roof, so we get plenty of noise. Now let’s replace the dome light with a portal that covers the open roof, then allow that to be sampled instead:

Noise is greatly reduced, for exactly the same number of rays.

The sampling algorithm is simple enough to implement in your GPU path tracer of choice: sample the portal and use the usual conversion between pdf wrt area (the portal) and pdf wrt solid angle (the dome):

$$P_\sigma = \frac{P_A \|\mathbf{v}\|^2}{\cos(\theta)} = \frac{P_A \|\mathbf{v}\|^3}{\mathbf{v}.\mathbf{n}}$$

Where $\mathbf{v}$ is the vector between target point and the portal point, and $\mathbf{n}$ is the portal normal.