PBR vs Fresnel in V-Ray

[marty030]

Hello readers,
I´ve been reading a lot about the PBR workflow lately following Disney´s technical papers, Marmoset, etc..

Transfering the handy colorcharts for a quick pick over to V-Ray, what is the correct setting for Fresnel reflections?
I´m a bit confused how to implement the different RGB-values for F0.

For instance if I want just to build a normal material with 4% reflectivity:

I know I could set it up like:

a) - Reflection-Slot with RGB 255/255/255
- "Fresnel Reflections" ON with 1.5

The value of the reflection slot, does it represent F90° in this case and with the IOR I get control over F0°
(and the reflection curve in retrospect)? (This looks always too reflective to me, no matter of glossiness.)


What if I want to make use of the RGB charts for control of F0 instead of the IOR-values:

b) - Reflection-Slot with RGB 56/56/56
- "Fresnel Reflections" ON with 1.5

Would that be correct? (Looks not right to me.)

Or:

c) do I have to use a FallOff-Map in the Reflection-slot instead with FallOff-Type "FallOff", "Override OFF" and 56/56/56 in the Front-slot and 255/255/255 in the Side-slot?


And in addition, if I want a specific reflection curve according to the Reflective-Index website:

Do I use the FallOFff-Type "Perpendicular/Parallel" or "FallOff" to setup the Output curve?


Kind regards
martin

[JulianT_CD]

a) The Reflection Color slot is a multiplier, that gets multiplied on top of the reflection curve. It equally affects F0 and F90. So Putting in a 50% gray in the reflection color would halve the F0, the F90 and everything in between.

You can't use the reflection color slot to emulate the Specular F0 colors from PBR directly. For dielectrics you would solely use the IOR to get something equivalent to the F0 PBR specular colors. F0 for dielectrics is mostly in the range between 2% and 8% reflectivity. That roughly translates to an IOR between 1.4 - 1.8 IIRC.

b) I don't know how Disney calculates the Fresnel curve for metal. If all they do is color the F0 and make it much stronger, then calculate the rest of the Fresnel curve like for dielectrics, then yes, you could use a Falloff curve in the reflection slot with the Specular chart color in the facing color slot.

You are much closer to the real reflectivity of materials by using the curves from ReflectiveIndex.info though. You would set the type to "Perpendicular/Parallel" and then adjust the R, G and B output curves separately according to the curves you see on ReflectiveIndex.info for that specific material. Don't forget to uncheck "Fresnel" in the material, so that only your manually created reflection curve is driving the reflectivity.
One issue with this workflow is however, that the distribution of values along the X-axis of the output curve is not linear. So when you set a curve point half way along the X-axis of the reflection curve, you are not exactely adjusting the 45° reflectivity. You will be off by some degrees. Have a look here: http://viscorbel.com/vray-materials-part-3-metals/
I think vlado or someone else wrote and OSL textures which is linear. You could also use the free Complex OSL texture from Siger Studios which let's you type in the n and k values from the complex IOR directly.

[marty030]

@JulianT_CD
Thanks for answering.

Ok, so basically I´d be better off using individual FallOff curves if I want to adjust F90 seperately. Fair enough!

[JulianT_CD]

@JulianT_CD
Thanks for answering.

Ok, so basically I´d be better off using individual FallOff curves if I want to adjust F90 seperately. Fair enough!

Yes, correct. With the reflection curves you have full control over the reflectivity of the material at all angles. Just keep the slight non-linearity in mind. In practice this is not a huge issue though.