metalness vs IOR

janshao

Hi.

I am sorry if this topic appears again but i am still confused about the metalness workflow and was wondering if someone could help me out and shed some light on it

Lets take the IOR first. First of all i am confused by all those different IOR lists excisting online.
If I take "Iron" for example, I can find totally different values online.
For example on cgsociety its 1.51 and on pixelandpoly i get 2.950
I even found values underneath 1, which confuses me even more.

https://forums.cgsociety.org/t/a-complete-ior-list/1070401
https://pixelandpoly.com/ior.html

If i take 1.51 for example then i get something close to plastic, 2.95 looks already more like Iron to me.
Chrome I believe has a similar IOR, close to 2.95
When setting up my chromeball as a reference to the IBL I was using an IOR close to 2.95.

When using the metallness workflow i get a completely different result depending on my base color (albedo) map.
When setting the base color albedo to 1 which would be pure white, I get a much brighter and stronger reflection then with the IOR 2.95. 
I think i have to set it to 10 or 15 instead to get a similar result. But on the other hand, i thought there can`t be anything more reflective then chrome, which means in theory i shouldn`t go much higher then 3. 
So what is correct and what is wrong? How would i set up a correct chromeball with the metalness workflow?
Should I never go that high with my albedo map when using metalness?
On the other hand, when i check under presets arnold gives me a chrome preset with metalness 1 and albedo completely white, which means to get the same effect the old fashion way i have to set the IOR way above the actual chrome IOR of 2.95.


As a last question. Is there metal which is less reflective arround the edge? I can remember in good old mental ray times i could darken the edge(fresnel) for metal.

musashidan

From the Arnold docs:

Using a very high IOR value can look quite similar to Metalness. It looks the same if you set the Base Color to the Specular Color and the Specular Color to black. The difference is that you get an extra reflection at the edges, with the Specular Color controlling the edge tint. The metal fresnel works the same as in the new complex IOR shader, with the artistic parameters.

You should normally use IOR for materials like plastic, glass, or skin (dielectric fresnel) and Metalness for metals (conductive fresnel with Complex IOR). The other reason is that Metalness is easier to texture since it's in the 0..1 range, and using textures from applications like Substance painter works best when using Metalness rather than IOR.

In old school offline rendering(like in MentalRay) you would crank up the IOR past 20, for metals, but that's not necessary anymore. The GGX microfacet shading models of recent times accounts for the IOR. You would generally leave it at the default of 1.52, and only change it for materials like skin, wax, water, etc.

janshao

Thank you very much for your help

But i still don`t understand why cranking up the IOR to 20 for metal when in the IOR list its actually somewhere arround 3?
I initialy thought the IOR is more correct then the metalness but metallness makes it easier to paint maps for.
So when setting the IOR to 3 which is roughly chrome, then i would at least expect a similarity when setting metallness to 1. But to get the same result i have to set the IOR to 20. Thats what i don`t understand right now

musashidan

Cranking it up to 20 was old-school, pre-GGX microfacet shading model. You shouldn't have to do that anymore. No material is IOR 1. That would be for air or gases.

Metalness automatically takes care of the IOR(fresnel) Before metalness, cranking the IOR to 20 was a hack. All it is doing is widening the angle of the fresnel. Use metalness and ignore the IOR setting(leave it at default)