Iray Uber Shader [Documentation] Questions

Arnold C

Tobor said:

 If "color" suddenly involves both chrome and luminance -- which we know is coming -- it's anyone's guess how that will change the appearance of existing shaders.

Your main interest is skins; mine is exhaust flumes and laser beams. In the end, we'll probably need different treatments of volumetric shaders to satisfy all the needs out there.

I'd say it would open a wider range of possibilities. And confusion.  But let`s see if and when they implement that.

It isn't that I'm not interested in laser beams, but I take it step for step and prefer to have some good-looking first... to then get lasered.

As I said in the other post, in optics, concerning absorption and scattering, things are mostly treated alike. The main thing is to have the valid values at hand to simulate that stuff. And for exhaust flumes I'd also would like to have some particle effects available.

Tobor

Szark said:

ok given that SSS Amount only goes to 1.00 I am not sure what to use now. Yes I know I can increase the threshold so I might give it a shot. I did see more SSS effect when I sent the SMD to 0.75 at SSS Amoun of 0.50

Fortunately going above the limits of 1.0 doesn't kill it, and it does lend some interesting results.

The forumula in IrayUber for scattering coefficient is:

sub_surface_scattering_amount / scattering_distance_of_measurement

which corrlates to SSS Amount / Scattering Measurement Distance

I think in Arnold's examples above he's factoring in IOR (and maybe something else), but we really don't know how Iray incorporates these other factors in the engine. They is not defined in the MDL itself. I leave those numbers up to his research.

 

Arnold C

>>Edited due to new insights.<<

 

Arnold C

UPDATE, 2015-09-29:

Uh, sorry, I just got further explanations from JanJordan, there has been a little misunderstanding regarding the material.ior! Although expressed as color, the colors within the RGB channels of the Transmitted Color will be left alone when it comes to calculations for the absorption and scattering coefficients.

Due to her, there are no further adjustments made to the Transmitted Color color values, it's just that the logarithm for the respective color will be divided by the (negative) value for the distance. Simple as that, nothing else.

Transforming the equation, you're be able to derive the appropriate value for each of the color channels for a given absorption coefficient:

Absorption Coefficient * (-1 * Transmitted Measurement Distance) = log(Transmitted Color); the exponent to the base e of the logarithm will result in the color for the respective RGB channel. 

Example for a caucasian skin type: absorption coefficent for the Red RGB color channel of 0.45, divided by a Transmitted Measurement Distance of 1.00 (cm) multiplied with -1, will result in a logarithm of -0.45, and the exponent for that will be: 0.637628. That's the setting for the "valve" for the color channel, 63,7628 % of red light will be transmitted through a material of 1 cm thickness, the difference to 100 % will be absorbed. This procedure will have to be repeated for the green and blue color channels alike. For starters you could try an absorption coefficent of 1.25 for green and 1.85 for blue. That's within range for a caucasian skin type measured at 550 and 450 nm.

Since scattering is restricted to greyscale, we won't be able to calculate appropriate values for that by now. I considered to go by a scattering coefficient for a red wavelength (16.7 at 700 nm); this will result in less scattering for green and blue wavelegths, but avoids to have too strong scattering at the red one. That would be an SSS Amount of 1.670 and a Scattering Measurement Distance of 0.10 (cm).