Refraction IOR still affects fresnel when Lock Fresnel IOR is disabled

[1funk]

Modo 11.0v1 138707
V-Ray 3.52.01 27395

When "lock fresnel IOR" is disabled, the "fresnel IOR" channel should control fresnel completely. I just found that the "refraction IOR" channel still has an affect.

The left has Fresnel IOR = 1.5, lock Fresnel IOR = OFF, refraction IOR = 1.5 (this looks correct)
The right has Fresnel IOR = 1.5, lock Fresnel IOR = OFF, refraction IOR = 1.0 (this looks incorrect - notice the fresnel reflections near the edge are missing)

So even with "lock Fresnel IOR" disabled, you have to set both "fresnel IOR" and "refraction IOR" to 1.5 to get the correct result

[vladimir.nedev]

The behavior is the same in V-Ray for 3ds Max from what I tested.
I am not sure whether the behavior is intentional or a bug.
Vlado will know.

Greetings,
Vladimir Nedev

[Druski]

Has there been any resolution on this issue? We have just come across this very same issue in Maya and would love to understand if it's a bug, or expected behavior. If it's expected, could you please provide a brief explanation as there is currently nothing in the docs, or forums.

[vlado]

The explanation is related to total internal reflections, usually from the back sides of an object but also when an object with lower IOR is immersed inside an object with a higher IOR (like air bubbles in water). If the Fresnel IOR and the refraction IOR are completely unrelated, the reflections produced by the Fresnel formula do not match those that follow from Snell's law for total internal reflections and artifacts can happen (in the form of black spots or missing reflections/refractions). When the two IORs are different, V-Ray attempts to use a formula that ensures consistency between reflections and refractions, but it does mean that the refraction IOR still has some effect on reflections.

We've investigated a few different ways around this issue but they had various problems. Hopefully we'll manage to find a working solution.

Best regards,
Vlado