CIORs (Complex Index Of Refraction)

#21

Straycat, you can test it easily asking yourself this question: If you are taking a photo of something reflected on a mirror…, ¿You focus the mirror or the subject?. If you focus the subject, the mirror blurs, if you focus the mirror, the subject blurs. :slight_smile:

#22

Yeah seems completely logical when you think about it. I guess I just never gave it much thought.

#23

Yes, I know. Sorry. I was replying to the highjackers comments earlier. Forgot to quote. :slight_smile:
Interesting topic btw. I’ve been looking for an answer to that myself. For now, I think the closest we can come to CIOR is using fresnel reflections with a very high IOR, Like 20-40. Unfortunately maya doesn’t display the reflectivity curve like for example 3ds max, so it’s a bit of a blind man’s trial and error situation. :confused:

#24

I find your highjacking to be almost as illuminating as mine even if not as relevant. Please continue.

For now, I think the closest we can come to CIOR is using fresnel reflections with a very high IOR, Like 20-40. Unfortunately maya doesn’t display the reflectivity curve like for example 3ds max, so it’s a bit of a blind man’s trial and error situation. :confused:

Thank you for repeating my prior point, but that doesn’t work as well as one would hope. Thus, my initial posts of my crappy sink. I was using a mia_mat_x with Fresnel BRDF and IORs of “like 20-40”. I was the blind man trial-and-erroring, and never found a valid solution, just like the original poster. Having every element in common with the original post, it seemed a valid topic for me to comment on. However it is good to know you agree with my methods entirely.

I’ll post up the scene if anyone’s interested and we can play with it? I’m very open to a more empirical scene too however, but I’m not on a NDA for that project so it’s fair game if anyone wants to take a shot at it as a test-bed.

#25

Share that scene InfernalDarkness, I will be checking it in my spare time.

You saw the image I posted earlier (the bike, yeah)?, What rate do you asign to the metals?.

#26

Hrm… yes, well… You write such long posts… sometimes I’m too lazy to read the whole thing.
sorry about that. :stuck_out_tongue:

#27

Share that scene InfernalDarkness, I will be checking it in my spare time.

You saw the image I posted earlier (the bike, yeah)?, What rate do you asign to the metals?.

Sure thing, I’ll clean it up a bit and compress the textures or something so it’ll squeeze across the web, tomorrow when I’m back at work.

I’m not sure what you mean by “rate”, however? Metals generally aren’t so difficult, but in this particular scene and its accompanying master bathroom scene, I was unable to get good, predictable results for some reason. Trying the IOR technique has worked for me in the past, especially on brushed nickel and stainless steel metals.

Usually the only metal that thwarts me is “oil-rubbed bronze”, which pretty much requires texture map-painting to achieve the rubbed-edges effect.

Hrm… yes, well… You write such long posts… sometimes I’m too lazy to read the whole thing.
sorry about that. :stuck_out_tongue:

You’ll love or hate this song! Hope you get a laugh out of it anyway, my friend:


(clean version, by the way)

#28

well this topic is very interesting ! but as i really don’t understand the CIOR concept can someone help me to get the idea.

i will speak as the fool, but when i do metal shader i only use diffuse + specularity/reflection/Anisotropic ppties + nice texture. How does refraction come into the story ? do you guys use refraction for your metal shaders ? i don’t see how metal can have a refraction index ?

could you please post some photo of a metal that need CIOR or refraction and that can’t be done in mental ray , cause i’ve never seen that , and i feel like i’m missing something crucial.

Thanks for your help !

cheers !

#29

From Zap’s the article on the topic:

Well… actually… no. Metals are indeed not refractive, and are indeed not dielectrics (meaning, electrical insulators). They are Conductors, and for some baroque reason these are also considered to have an “Index of Refraction”.

Now, don’t ask me how on earth someone came up with the idea of refractive metals or how this is actually calculated… I didn’t write the laws of Physics (I just abuse them) so just trust me it’s there… and these values are high. Not your average “1.3” ish like for water, but values like “25” or “50”.

Making Better Metal with mia_material

His example render:

The theory behind using higher IORs for metal is to simulate or mimic microsurface scattering of highlights, basically. It can add a lot more zazz to your metals with a lot less work.

Edit:

In addition, when one searched “metal index of refraction” and finds some relevant sites, one can find the IORs of each metal type calculated for oneself.

Chromium = 3.68
Silver = .329

That site will also calculate your BRDF curves for you, towards the bottom, if Fresnel isn’t helpful for realism.

#30

thanks for the links !

Well i’m not sure i get all the story but for what i understand. he said

  • metal are not refractive but
  • to get an optimized BRDF curves its better to use fresnel reflections and High indice of refraction to draw the BRDF curves
    - But the goal of all this is to control the look of the curves that blend Diffusion/Reflection

In maya you can put all the indice you want you have no preview on your BRDF curves like said previously.

So to do stuff like this i prefer to use

  • a facing ratio node
  • i remap my facing ratio with a remap value node
  • now with the remap value you have full control on the look of your BRDF curves, you draw by hand the exact look of your BRDF curves
  • you then plug the out value in your reflectivity attribute
  • and you plug the inverse of this value in the diffusion weight attribute

then you get diffusion/reflectivity compensation and you have the full control on How your BRDF curves looks like.

its looks more intuitive to me than using obscur IOR to draw my curves !

cheers !

#31

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