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#1 2015-04-05 12:36:23

doubleailes
Guerilla dev, the guy to hug

Specular

Hi guys,

I just read the "vol.2 - Practical guidelines for creating PBR textures" form Allegorithmic (nice job).
They explain all the dielectric got a F0 between 2% and 5%
How i can set my F0 with the right value?


Philippe Llerena

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#2 2015-04-05 18:57:13

Ben
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Re: Specular

Hi,

Not really sure what F0 is.
I assume that F0 is Fresnel reflection term at normal incidence, and the paper refers to Schlick approximation for Fresnel formula.
Basically, F0 maps to the IOR, so the only thing you have to choose is the correct IOR of your material.

Ben


Benjamin 'Ben' Legros
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#3 2015-04-06 11:56:16

doubleailes
Guerilla dev, the guy to hug

Re: Specular

Yep this is F0.

http://i.imgur.com/7EgLI4Q.jpg

And they are giving this chart:
http://i.imgur.com/rR05wJE.jpg
Completed by this one
http://i.imgur.com/WzA25GG.jpg

Of course entering the sRGB in specular color doesn't giving me the right image.

Last edited by doubleailes (2015-04-06 13:17:11)


Philippe Llerena

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#4 2015-04-06 20:10:32

Gam
Member

Re: Specular

doubleailes :

Of course entering the sRGB in specular color doesn't giving me the right image.

for a dielectric the specular color should be set to white to reflect the incoming light
Only conductive shader (gold, steel, aluminium, copper etc) should have an albedo and K coeficient (kappa) for the ior.

Last edited by Gam (2015-04-06 20:12:42)

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#5 2015-04-07 15:32:52

Ben
Guerilla dev, the guy to hug

Re: Specular

Hi doubleailes,

Keep Spec colors to white, there's no reason why a dielectric would have a colored spec (unless coated, but the Spec2 layer accounts for this.)
Only Metal can be colored.
And eventually, dealing only with IOR will give the correct F0. If you try to setup some color in spec, you'll get great loss of energy. The lower the IOR (towards 1), the lower the F0, the higher the IOR, the higher the F0.

Some tabulated values:

Code:

IOR     F0
1.0     0.0
1.1     0.00227
1.2     0.00826
1.3     0.017
1.4     0.0278
1.5     0.04
1.6     0.053
1.7     0.067
1.8     0.082
1.9     0.0963
2.0     0.11

Also, don't use sRGB in the color picker, colors in the picker should be linear. Or pick using the color picker, as it automatically linearize the screen color.

Cheers,
Ben


Benjamin 'Ben' Legros
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http://www.guerillarender.com

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#6 2015-04-08 11:25:10

doubleailes
Guerilla dev, the guy to hug

Re: Specular

Thanks a lot Ben,

Your table was exactly what i need.

F0 = (1-IOR)^2 / (1+IOR)^2
Is this the formula?

Last edited by doubleailes (2015-04-08 11:26:35)


Philippe Llerena

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#7 2015-04-08 12:31:14

hulud
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Re: Specular

Yes, (1-IOR)^2 / (1+IOR)^2 is the Schlick's approximation for \theta == 0.

http://en.wikipedia.org/wiki/Schlick%27s_approximation


Cyril 'Hulud' Corvazier
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http://www.mercs-eng.com

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#8 2015-04-08 18:37:01

doubleailes
Guerilla dev, the guy to hug

Re: Specular

Got what i need for my tool.
Thanks.

See u.


Philippe Llerena

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#9 2015-04-09 00:48:40

Gam
Member

Re: Specular

I was using this one to convert ior to schlick on Arnold

( ( x - 1,0 ) / ( x + 1,0 ) ) ^ 2

Cheers

Last edited by Gam (2015-04-09 00:48:58)

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