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Thread: Fresnel IOR

  1. #1
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    Default Fresnel IOR

    I've been reading up on this explanation of vray materials and I'm getting confused with the whole Fresnel IOR or Index of Refraction. In the guide it says:

    As a general guideline, here are the Reflect IOR values for some common object types:
    water 1.33
    plastic 1.45
    glass 1.5-1.8
    diamond 2.4
    compound materials like wood, stone, concrete etc 3-6
    metals 20-100


    I know that in vray the Fresnel IOR and IOR are linked. I believe that the Fresnel IOR is actually locked by default. My thing is which one to use.?



    Later in the guide it says:


    Index of Refraction has been calculated for many materials, so you donít need to guess. You can find various IOR tables on the internet. Here is one of them:
    Steel 2.50
    Chrome Green 2.4
    Chrome Red 2.42
    Chrome Yellow 2.31
    Chromium 2.97



    I thought that since the two were linked that you would use the same value for both, but as stated in the Fresnel IOR section metals are 20-100. If you look in the Index of refraction list, steel is 2.5.


    By the way this is the guide:

    http://viscorbel.com/vray-materials-theory/

  2. #2
    Veteran Member RyderSK's Avatar
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    Default Re: Fresnel IOR

    I see how this can be confusing and while I always recomend Viscorbel's great tutorials, he's including this info without really understanding it making it look even more confusing.

    But I can assure you, you can just ignore all this. Creating materials in Vray is EYE-BALLING, nothing more, nothing less. The IOR driven Fresnel effect used in Vray (and bunch of other renderers, with exception of Maxwell, which feature the whole (n,K) part of IOR equation, but for God knows what reason..) is simply simplification of the full equation and doesn't work for all materials, like above mentioned Metals for example.

    So with this in mind, you can just keep 1.6 for 95perc. of materials, 1.6-3 with lot of eye-balling for bunch of plastics/wood/special materials and NO fresnel for Metals (then you can play with fallof curve if you're confident but...not needed in 90perc. cases) for reflective properties and 1.333-1.6 for various forms of Water/Glass/other liquids in refraction.


    I think this guy (he also has a bunch of nice tutorials) describes it more coherently:

    http://www.workshop.mintviz.com/tuto...terials-guide/


    Anyway, I think that by now, Bertrand Benoit has finished 2 articles about "Materialism" and there is NO better info on how to approach photorealistic material creation (in Vray, but applicable elsewhere). He goes through IOR, value and texture driven parameters,etc.. everything.

    So once you finish Viscorbel's 3 mat parts, hit up BB :- )

    http://bertrand-benoit.com/blog/category/cg-techniques/
    benjaminbogaert and elipan like this.
    talcikdemovicova.com

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    Default Re: Fresnel IOR

    Quote Originally Posted by RyderSK View Post
    I see how this can be confusing and while I always recomend Viscorbel's great tutorials, he's including this info without really understanding it making it look even more confusing.

    But I can assure you, you can just ignore all this. Creating materials in Vray is EYE-BALLING, nothing more, nothing less. The IOR driven Fresnel effect used in Vray (and bunch of other renderers, with exception of Maxwell, which feature the whole (n,K) part of IOR equation, but for God knows what reason..) is simply simplification of the full equation and doesn't work for all materials, like above mentioned Metals for example.

    So with this in mind, you can just keep 1.6 for 95perc. of materials, 1.6-3 with lot of eye-balling for bunch of plastics/wood/special materials and NO fresnel for Metals (then you can play with fallof curve if you're confident but...not needed in 90perc. cases) for reflective properties and 1.333-1.6 for various forms of Water/Glass/other liquids in refraction.


    I think this guy (he also has a bunch of nice tutorials) describes it more coherently:

    http://www.workshop.mintviz.com/tuto...terials-guide/


    Anyway, I think that by now, Bertrand Benoit has finished 2 articles about "Materialism" and there is NO better info on how to approach photorealistic material creation (in Vray, but applicable elsewhere). He goes through IOR, value and texture driven parameters,etc.. everything.

    So once you finish Viscorbel's 3 mat parts, hit up BB :- )

    http://bertrand-benoit.com/blog/category/cg-techniques/
    So what your saying and from the article, I should not use Fresnal IOR and IOR for metal materials at all? Also for other materials, I should leave the Fresnel IOR locked and just change the Index of Refraction to reflect something listed under the Index of Refraction list such as:

    Acetone 1.36
    Actinolite 1.618
    Agalmatoite 1.550
    Agate 1.544
    Agate, Moss 1.540

  4. #4
    Veteran Member RyderSK's Avatar
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    Default Re: Fresnel IOR

    Vray uses the IOR driven fresnel effect for both reflection and refraction, but each only aplies to its property so either reflection or refraction. If you keep reflection IOR locked (i.e default 1.6) you're fine, there is no need to change it to 1.544,1.618, etc...those values don't correspond, you can freely ignore those charts, there is no place for physical correctness in materials to start with in Vray or any other biased engine apart from energy conservation law. The simplified material system is just eye-balling stuff with very basic physic properties set (fresnel).

    LOCKED=Default 1.6.
    Aren't you confusing the IOR at refraction ? You only set it for materials that refract light, i.e liquids/ glasses/etc. whatever you set here will not affect reflective properties at all.

    For the fresnel and Metals, yes, you should not use it. I see people use various values between 12-20 for steel and etc. but...there is little difference between 12-eternity... If you want to really create kickass metal, you will drive both glossiness/roughness and reflectivity with custom fallof curve and ideally multilayered material.
    talcikdemovicova.com

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    Veteran Member nicnic's Avatar
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    Default Re: Fresnel IOR

    hmm i was wondering what the IOR of moss was the other day
    most useful!
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    Default Re: Fresnel IOR

    Hi Jojo,
    I totally agree with all Juray wrotes...
    if I may I would add some tought about :-)

    Actually if you create a plastic material, you can similarly unlock "fresnel IOR" and increase it or simply increase "IOR" in the refractive area, it's exactly the same! - Few users do this test:



    As you can see in EX2 and EX3 you get the same result!
    Nothing special if you use Fresnel IOR or simply IOR.

    Controlling Fresnel IOR is really useful
    in case you create a refractive object, a diamond, for instance. Increasing the IOR its reflections will increase automatically since in nature their are linked. So in this case it really makes sense control Fresnel IOR: to let IOR affects differently reflections and refractions (v-ray allow it.. not nature!)

    Of course anyone can follow the way he prefers, final output is the only important thing. Personally I prefer to make these considerations becasue I love to work with things making sense, but it's just my taste

    Hope you'll like my contribute
    Ciro


    PS
    a part this theoretical considerations if your goal is creat great metals, Juray has the perfect suggestion:
    "you will drive both glossiness/roughness and reflectivity with custom fallof curve and ideally multilayered material."
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    Default Re: Fresnel IOR

    Thank you very much for your excellent explanation. How do you know when to change the Fresnel IOR from 1.6? Do you have some guiding values for architectural materials such as rough concrete, polished concrete, brick, wooden facade, non-lacquered wooden flooring and plants?

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    Senior Member TomasEsperanza's Avatar
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    Default Re: Fresnel IOR

    Quote Originally Posted by emilschmidt View Post
    Thank you very much for your excellent explanation. How do you know when to change the Fresnel IOR from 1.6? Do you have some guiding values for architectural materials such as rough concrete, polished concrete, brick, wooden facade, non-lacquered wooden flooring and plants?
    There are many lists online. Lots of them don't really offer the every day materials that one would hope to see, but instead they consist of chemicals, and minerals, etc. However, with a bit of determination, you can often deduce from various sources, what would be a relevant IOR number to use for your material.

    A key componant that effects the prodominant IOR of paint may be 2.71, thus trying this figure for a paint material and seeing if it improves the look sometimes takes a bit of the guesswork out of it.
    Last edited by TomasEsperanza; February 1st, 2015 at 05:48 AM. Reason: spelling

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    Veteran Member RyderSK's Avatar
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    Default Re: Fresnel IOR

    Just keep it 1.6 for everything except metals. Most non-conductor (but really, for CGI purpose, just non-metals), have extremely little front angle reflectance, that varries between 0.02 to 0.05 linear. That's basically 1.333 for water, 1.52 for most woods, plastics,etc.. and 1.6-1.8 for certain super shiny plastics and metaloids. But the difference is so small that you shouldn't bother.

    http://refractiveindex.info/ has already new section for 3D Artists, but keep in mind, these are complex IOR formula, meaning refractive index (n) and extinction coefficient (K). For simplified fresnel formula like in Vray, it was decided the K should be ignored (K=1), since it only applies for metal materials predominantly.
    For this exact reason, it's not possible to create fully physically correct metals using IOR value. But since metals on other hand, vary little in frontal reflectance (70 to 100 perc.), you can just use the color they look to your eye.
    The more complex approach is to replicate their reflectance curve as fallof in specular reflection slot of material.

    But really, there is no magic. Don't go and follow the tables that say Zinc is 1.8875 (and K something like 6.54.....blablah), thinking you can use it in rendering. You can't and shouldn't.

    Simplify your life, and ignore bro-science. It's all actually easier than it seems.

    So, TL-DR:

    Wood, Plastic, Paint, etc... everything !! = Default IOR + Diffuse Texture.

    Metals= Diffuse ZERO (Black!), IOR= Off (OR 12-80 by eye, 12 being low-reflective metals, 80+ being mirrors) and specular color for given metals (gold for gold...medium grey for silver,etc... there are tables in http://www.allegorithmic.com/pbr-guide )

    talcikdemovicova.com

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