Distributed Ray-Tracing

Perhaps someone can help me with this. I was doing some experimentation with lenses and PBTR v4. This was going great, I was able to model the projection of an object through a lens onto a surface quite well. However, now I want to simulate doublets: lenses which consist of two parts so with two materials. I don't know how to model this in PBTR. It seems that it is not possible to have a shape (lens) touch more than one other material.

> PBRT's MediumInterface system can only represent a single "inside" medium and a single "outside" medium per shape. If a shape physically touches multiple different media (for example, a glass sphere sitting at the interface between water and air), PBRT cannot directly represent this configuration.

I think this is kind of odd for a renderer which is otherwise quite capable. Can anyone explain why this is the case, and how I can work around this limitation?

There is a way to model this type of situation for watertight dielectrics with interface tracking: you assign each material a priority value, and a transition between materials occurs when entering that material only if it has a higher priority than your current material. Yining Karl Li has a great article about it:

https://blog.yiningkarlli.com/2019/05/nested-dielectrics.htm...

that inspired me to add the feature to my renderer (rayrender.net).

The downside to priority tracking (and possibly why PBRT does not include it) is it introduces a lots of overhead to ray traversal due to each ray needing to track a priority list. Modern raytracers use packets of rays for GPU/SIMD operations, and thus minimizing the ray size is extremely important to maximize throughput and minimize cache misses.