Being faced with a design choice, I have to decide between supporting anisotropic surfaces or not in my ray tracer. Is there any common use case of this feature other than velvet (so that I may be motivated to go with the former decision)?
- Most manufactured items that are made by grinder, milling, shaper or lathe that have not been polished. Due to tiny grooves caused by the cutting head.
- Intentionally brushed surfaces. Mainly to make it harder to see scratching. Used for example on metal panels of elevators, escalators etc.
- Hair, animal or human. Hair has a microstructure much like a surface that has been turned with a lathe. But also a surface covered by small hair has a anisotropic behaviour even without this as you mention in your velvet example.
- Many Fabrics are anisotropic on account of them being made out of strands etc.
- Wood grain causes wooden structures to be anisotropic. Although the effect is subtle as they are usually not terribly reflective and can probably be modeled by macroscale bumpmapping in closeup shots only.
- 3D printed items etc.
As you can see a designer of industrial goods might be super interested in seeing anisotropic materisls in their scenes.
A really common case in the real world are industrially fabricated metal surfaces which often contain patterns (due to their creation process) with strongly anisotropic surface distributions, especially if they haven't been polished.
Some of these patterns may only occur at the macro scale from common viewpoints (i.e. irrelevant for your BRDF), but on a lot of them this will happen at the microfacet scale.
In a common virtual scene where this pan covers a few dozen square pixels, these will be way too small to be modeled as detail in something like a normal map or geometric detail. Some of the patterns are already hard to see in the image, but undoubtedly contribute to the overall look of the material.