fresnel reflection and refraction connection

[playmesumch00ns]

Fresnel reflection depends on the Index Of Refraction (IOR) of the material. For dielectrics this is a single number, e.g. ~1.3 for water, ~1.5 for glass etc. This controls the blend between reflection and transmission for the material, so when you get to really high IOR values (>10) the material becomes fully opaque.

[mister3d]

Fresnel reflection depends on the Index Of Refraction (IOR) of the material. For dielectrics this is a single number, e.g. ~1.3 for water, ~1.5 for glass etc. This controls the blend between reflection and transmission for the material, so when you get to really high IOR values (>10) the material becomes fully opaque.

Thank you for reply. When you say that for dielectrics it's a single number you mean IOR and IOR in reflection slot? Because these are two different refrations, so for dielectrics we set these values the same. And what about the other materials, these values differ?

[playmesumch00ns]

What software are you using? It's likely that the shader allows you to control the IOR for reflection and refraction separately, even though the values should be the same (regardless of material).

For metals, it's more complex (literally) and the fresnel function is described by 2 numbers: the index of refraction (usually represented by the greek letter eta, which looks like 'n') and an absorption coefficient.

[mister3d]

What software are you using? It's likely that the shader allows you to control the IOR for reflection and refraction separately, even though the values should be the same (regardless of material).

For metals, it's more complex (literally) and the fresnel function is described by 2 numbers: the index of refraction (usually represented by the greek letter eta, which looks like 'n') and an absorption coefficient.

I'm using 3dsmax, so there sre 2 numbers - the falloff slot for fresnel IOR in reflection slot, and one more for IORefraction. So I always should keep IOReflection and refraction the same? This is interesting, because it must be an explanation for this in real life. When I change IOR in reflection slot, if I make it higher, it reflects more(the fresnel effect becomes less and less pronounced).
About the 2 values you've mentioned, I have only IOR number and 2 slots that control forward/backward reflectivity of fresnel reflection.

About the number of IOR Christopher Nichols says the following:
"Let me explain IOR and reflections. Usually, harder materials, like metals etc... have a higher IOR. Plastics and woods and plaster, and other softer materials tend to have lower IOR. Usually I would keep that range between 1.4 and 2.0 but can got as far as 1.2 to 2.5."
So for metals it's like 2.0, for plastics it's like 1.4, right? Maybe there are more precise numbers somewhere.

[ienrdna]

Here some IORs

http://www.ps.missouri.edu/rickspage/refract/refraction.html
http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html

[playmesumch00ns]

For metals it works completely differently than for plastics, water etc (i.e. things that don't conduct electricity).

The IOR in the refraction slot is probably the value used to calculate the refraction vector, and the IOR in the reflection slot sounds like it's being used to calculate the falloff in reflection intensity. So yeah, the two values want to be the same (technically speaking)

[mister3d]

Thank you for reploes, alot of new information to think aobut.
What is strange that though these IOR values are measured, I stumbled across that IOR looks not correct with 1.33 water, so I changed to 1.2 and i looked more correct(visually, if to compare with original in real life).

[cpnichols]

The IOR is actually exactly related to both Refraction and Reflection in terms of both bending through the material as well as the energy of light and how that energy stops being absorbed by the material and when it starts to reflect it. Hence all materials (not just transparent ones) have an IOR value which deal with all levels of specular/reflective/refractive light. Therefore, if you want to be scientific about it, the values should be equal in both reflection and refraction.

[mister3d]

The IOR is actually exactly related to both Refraction and Reflection in terms of both bending through the material as well as the energy of light and how that energy stops being absorbed by the material and when it starts to reflect it. Hence all materials (not just transparent ones) have an IOR value which deal with all levels of specular/reflective/refractive light. Therefore, if you want to be scientific about it, the values should be equal in both reflection and refraction.

Thank you, Christopher, for your scientific explanation! It's good to understand it on scientific level. Hoping to see more videos from you on Gnomonology soon.
(I wish that this would be a bottleneck in my scenes' realism, but it's not at all for now :) )

[mister3d]

I found a strange discrepancy in IOR values in different tables

http://www.m3corp.com/a/tutorials/refraction.htm
and here
http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html

They are entirely different, look for metals.

Im mental ray there's a possibility to link reflection to IOR
http://img523.imageshack.us/img523/3839/iorws8.jpg (http://imageshack.us)
http://img523.imageshack.us/img523/3839/iorws8.fc2a087888.jpg (http://g.imageshack.us/g.php?h=523&i=iorws8.jpg)

EDIT:sorry, looks like this option is best for dielectrics. But how can I use IOR for metals at all then?

playmesumch00ns, how can the absorption coefficient of fresnel reflection of metals be used in renderers such as Mental ray?

[playmesumch00ns]

Not sure. A quick google didn't turn up anything interesting. For PRMan I just wrote a little app that would load nk data files (nabbed the ones from indigo), calculate the fresnel function for 0-PI/2 angle of incidence and spit out an image I can look up in the shader to get the reflectivity value. You could do something similar for mental ray I should imagine. Sorry but I can't share the textures.

[mister3d]

Not sure. A quick google didn't turn up anything interesting. For PRMan I just wrote a little app that would load nk data files (nabbed the ones from indigo), calculate the fresnel function for 0-PI/2 angle of incidence and spit out an image I can look up in the shader to get the reflectivity value. You could do something similar for mental ray I should imagine. Sorry but I can't share the textures.

Thank you for the reply, I just wanted to know if there is an acces for it in standard controls which I missed maybe. Anyway I'm not a shader specialist, so I will stick with ordinary controls. As far as I understand it's a flaw of absence of a dedicated shader for metals.

[playmesumch00ns]

Yup. You can get away with just ignoring it though to be honest. It's a really subtle effect

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