This is something I’ve been wanting to get to the developers to for ages. I’m primarily making this thread in hopes that one might see it, and also to link to it in case I find anyone in the #blendercoders IRC channel. Thoughts from anyone are appreciated however!
I wish to ask for what’s hopefully a simple yet effective addition to Cycles, which would make currently inaccessible effects possible to do natively in a realistic fashion: Implementing a volume refraction shader. Either by allowing the existing Refraction BSDF to work on the Volume material output, or by defining a new Volume Refraction shader if that is a better approach.
Suggested functionality: When a ray passes through a volume affected by this shader, its trajectory is bent, just like Glass BSDF or Refraction BSDF does for surfaces. Optionally, an IOR value may let users define the probability and / or angle at which rays have their course modified, whereas a Roughness value adjusts how blurry the view gets through such a volume… the two ideally operate in an independent way so that refraction and blur can work without one another.
Purpose 1: The most important practical use of this shader is proper heat haze, emulating the air deformations we see above hot ovens / jet engines / roads during summer days. As a workaround, this is already possible to do with render nodes, by rendering your volume as a black & white mask on a different render pass then plugging it into the Vector input of a Displace node through which the main render is passed. However this is just a workaround that so happens to work: It leads to quality loss due to smudging and stretching the render result, causes empty spaces if the transformation of the mask warps near an edge, is much less realistic compared to the ray actually traveling a different trajectory, and might ultimately be slower depending on what calculations are implied.
Purpose 2: This would allow a greater level of detail for objects such as crystals, diamonds, gems, ice crystals, and other types of rough glass. You can already design realistic crystals in Cycles, by customizing the IOR and Roughness of a Glass BSDF or Refraction BSDF, causing different spots to refract or blur in different amounts. However you can’t differentiate such spots volumetrically inside the crystal itself, and can’t create rough blurry strands or inner glass pieces that have a different refraction angle from others. This can of course be worked around by adding smaller meshes inside the main crystal and giving them different materials, but this technique will cause sharp edges and is less correct and flexible compared to volumes in many cases.
