Fresnel channel

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noldo
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Fresnel channel

Post by noldo » Thu Jan 10, 2013 8:35 pm

I wanted to ask a question. It 'correct to use the fresnel channel as a channel of specularity? In other words, it is appropriate to use a specularity texture in the fresnel channel to control the amount of specularity? Thank you.

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Zom-B
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Re: Fresnel channel

Post by Zom-B » Thu Jan 10, 2013 9:02 pm

Actually there is no fresnel channel in Indigo, only a fresnel scale for phong materials, and that isn't controllable by a map afaik!

Maybe you should explain your needs a little detailed, and also tell what Exporter you work with ;)
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noldo
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Re: Fresnel channel

Post by noldo » Thu Jan 10, 2013 9:38 pm

Ok, I'm sorry but my English is not very good.
With fresnel channel, i mean fresnel scale, as you specify.
I use skindigo exporter, but for the moment the plug-in dont'have the fresnel scale control, then i add the fresnel scale in indigo rt application.
It is possible to apply a map in the fresnel scale channel (why not?).
I noticed that a black value of map, cause no specularity (diffuse channel only), while a white value mean full fresnel value (1.0?), so I thought that the specularity was driveable with the fresnel scale value (with a map).
Thanks.

noldo
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Re: Fresnel channel

Post by noldo » Thu Jan 10, 2013 9:40 pm

In other words:
- exponent channel control the roughness;
- fresnel scale control the specularity (reflection);

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Zom-B
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Re: Fresnel channel

Post by Zom-B » Thu Jan 10, 2013 9:47 pm

The "roughness" of a reflective Phong material (and also Glossy Transparent!) IS possible my using a texture.
Its the exponent you define by that, so the controle where the material is rough, and where it is shiney.
B controls the spectrum of glossiness and brighter pixels on the texture set the glossiness to more reflective on the defined spectrum of B.

Fresnel Scale is afaik a 0 to 1 value that most exporters still haven't included yet :/
If unchanged the value is at 1 as default (afaik).
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noldo
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Re: Fresnel channel

Post by noldo » Thu Jan 10, 2013 9:54 pm

Zom-B wrote:The "roughness" of a reflective Phong material (and also Glossy Transparent!) IS possible my using a texture.
Its the exponent you define by that, so the controle where the material is rough, and where it is shiney.
B controls the spectrum of glossiness and brighter pixels on the texture set the glossiness to more reflective on the defined spectrum of B.
Exactly, but in this channel (exponent), a lower value mean more rough, while a higher value mean more shiny.

In the fresnel scale channel (with a bitmap applied, for example a black & white checkerboard), the black value mean NO reflection (someone like lambertian material), the white value mean normal reflection value (1.0) in according with the glossiness in the exponent map.

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Re: Fresnel channel

Post by PureSpider » Fri Jan 11, 2013 2:56 am

As I understand it, he pretty much wants an "IOR Map" for Phong materials, something I'd also love a lot!

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Re: Fresnel channel

Post by OnoSendai » Fri Jan 11, 2013 3:25 am

Zom-B wrote:only a fresnel scale for phong materials, and that isn't controllable by a map afaik!
It is in 3.6.x.

FakeShamus
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Re: Fresnel channel

Post by FakeShamus » Fri Jan 11, 2013 3:28 am

noldo wrote: In the fresnel scale channel (with a bitmap applied, for example a black & white checkerboard), the black value mean NO reflection (someone like lambertian material), the white value mean normal reflection value (1.0) in according with the glossiness in the exponent map.
this is possible, are you using the newest version, 3.6.4?
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fresnel map.jpg

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Zom-B
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Re: Fresnel channel

Post by Zom-B » Fri Jan 11, 2013 3:39 am

OnoSendai wrote:
Zom-B wrote:only a fresnel scale for phong materials, and that isn't controllable by a map afaik!
It is in 3.6.x.
Seems I totally missed that -.-'
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Re: Fresnel channel

Post by CTZn » Fri Jan 11, 2013 6:09 am

Zom-B wrote:
OnoSendai wrote:
Zom-B wrote:only a fresnel scale for phong materials, and that isn't controllable by a map afaik!
It is in 3.6.x.
Seems I totally missed that -.-'
Me too. Worse in fact, I noticed the feature . But I didn't integrate the fact, and was nearly going to explain why it wasn't implemented yet early today :mrgreen:
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galinette
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Re: Fresnel channel

Post by galinette » Fri Jan 11, 2013 7:04 am

OnoSendai wrote:
Zom-B wrote:only a fresnel scale for phong materials, and that isn't controllable by a map afaik!
It is in 3.6.x.
By the way, why not making it spectral, and by this way make the "specular color" parameter obsolete? As it removes the diffuse color, it is quite useless as it is.

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Re: Fresnel channel

Post by FakeShamus » Sat Jan 12, 2013 10:14 am

OnoSendai wrote:
Zom-B wrote:only a fresnel scale for phong materials, and that isn't controllable by a map afaik!
It is in 3.6.x.
is ISL working as expected in the Fresnel channel? I was trying something similar to what I've used previously within a Blend shader, using a normalWS check to return somewhere from 0.0 to 1.0 for the Fresnel Scale, but I get an error, saying that it "failed to find function eval(vec3)"?
I'm trying the exact same code that I used for the "Snowy Rock" blend - actually your code, Ono:

Code: Select all

def eval() real :
	if(dot(normalWS(), vec3(0.0, 0.0, 1.0)) > 0.3,
                1.0,
                0.0)
shouldn't the above "eval() real" work in Fresnel, just as in the Blend channel? also looks like this doesn't work as a shader for Exponent, but I would expect it to (correct me if I'm wrong, cause I only have the most amateur understanding of what's going on in these functions...)

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CTZn
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Re: Fresnel channel

Post by CTZn » Sat Jan 12, 2013 11:24 am

It is working as expected, yes. Indigo is certainly registering the blend and fresnel scale as different data types, wich explains the different respective signatures.

You can see on a new phong material that by default the fresnel scale shader uses the signature:

Code: Select all

def eval(vec3 pos) real :
[/size]

In general you can trust and just abide by the ISL debugger.

Now, why the blend parameter could not, even as a ratio, be 'pos dependent', this is above my head yet and again :lol:
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Re: Fresnel channel

Post by OnoSendai » Thu Jan 17, 2013 2:59 am

galinette wrote:
OnoSendai wrote:
Zom-B wrote:only a fresnel scale for phong materials, and that isn't controllable by a map afaik!
It is in 3.6.x.
By the way, why not making it spectral, and by this way make the "specular color" parameter obsolete? As it removes the diffuse color, it is quite useless as it is.

Etienne

This is an interesting idea.
It's a bit tricky to change fresnel_scale to be spectral, as it is tied up with energy conservation and sampling.
What I could do is to introduce a new (optional) fresnel_colour parameter, that would multiply the specular part of the BRDF. Having non-unity fresnel colour what model some kind of absorption happening during specular scatters.

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