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?

> Conventional ray-tracing is estimating illumination using a single sample across the entire domain, which constitutes a particularly crude approximation.

Straw man.

> Shadows have a hard edge, as only infinitesimally small point light sources of zero volume can be simulated

Uh, no. Raytracing can definitely have emitting surfaces and volumes.

> Reflection / Refraction can only simulate a limited set of light paths, for perfect mirror surfaces, or perfectly homogeneous transparent media.

You sure about that?

> More complex effects like depth of field are not supported.

https://www.povray.org/documentation/view/3.60/248/

Also, the title should get a "2019" tag.