## "color sigma_a"

Discussions about LuxRender's materials, and how to make the most of them.

Moderator: coordinators

### "color sigma_a"

Hi

Lets say i want to use absorption
[0.00000717 0.00000063 0.00000047]

Then i input in LuxBlend
[0.71703359 0.06316786 0.04673223]
with scale 0.00001

But in exported file i find
"color sigma_a" [0.00000333 0.00002762 0.00003063]

Is this correct?

Meelis

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Joined: Sat Oct 17, 2009 2:16 am

### Re: "color sigma_a"

Gamma correction?
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SATtva

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### Re: "color sigma_a"

SATtva wrote:Gamma correction?

I see, perhaps you are right.

Meelis

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Joined: Sat Oct 17, 2009 2:16 am

### Re: "color sigma_a"

If you're using luxblend25, then by default it does "color at depth".

Currently the only way to bypass this is to texture the sigma parameter, and use a constant color texture as input, with your values.

I think perhaps we should check if the "at depth" value is zero, if so use the input as is, and only apply the scale?
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Lord Crc

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Joined: Sat Nov 17, 2007 2:10 pm

### Re: "color sigma_a"

Thx Lord Crc

Yes luxblend 2.5

I test the constant color texture way.
Is it "fresnel color" (reflectance color) texture i have to use?

Color at depth is reverse value
My Color R255 G22 B17 (red)
Exported color R28 G230 B255 (blue-greenish)
Soo = renders correct (atlest lexblend preview is red), i figured asorption is also reversed, but it's not.

Meelis

Posts: 873
Joined: Sat Oct 17, 2009 2:16 am

### Re: "color sigma_a"

No, this is the color at depth formula:

Code: Select all
depthed = (-math.log(max([(float(i)),1e-30]))/(self.depth)) * self.absorption_scale * ((float(i))==1.0 and -1 or 1)

What you need is the "color" version of the constant texture...except that isn't exposed in luxblend25.

Really, the color at depth system in lb25 needs to be overhauled a bit. It should also be using the colordepth texture instead of doing the conversion itself, since that would allow "texture at depth". I was thinking maybe have a checkbox for "color at depth" which uses the colordepth texture if enabled, or simply exports the value if disabled, then either way runs things through a scale texture to handle the absorption scale.
-Jason

J the Ninja

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### Re: "color sigma_a"

Checkbox would be nice.

Meelis

Posts: 873
Joined: Sat Oct 17, 2009 2:16 am

### Re: "color sigma_a"

J the Ninja wrote:What you need is the "color" version of the constant texture...except that isn't exposed in luxblend25.

Well that's dumb I mean it shouldn't be useful most of the time, but why not have it there?

J the Ninja wrote:Really, the color at depth system in lb25 needs to be overhauled a bit. It should also be using the colordepth texture instead of doing the conversion itself, since that would allow "texture at depth". I was thinking maybe have a checkbox for "color at depth" which uses the colordepth texture if enabled, or simply exports the value if disabled, then either way runs things through a scale texture to handle the absorption scale.

Could we use a dropdown? Then we could have "color at depth" or "sigma".
If color at depth is shown we have the regular color + T button and the "at depth" field, while "sigma" shows r,g,b text inputs like asymmetry, and the scale input.
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Lord Crc

Posts: 4455
Joined: Sat Nov 17, 2007 2:10 pm