OTOY Forums

There were two new features added to the specular materials recently:

Dispersion

The IOR (index of refraction) depends on the wavelength of the light entering,
in many materials it is slightly larger for blue light than for red light.
This for example allows to split a beam of white light in a rainbow with a glass prism.

To model the dispersion we use the two-term form of Cauchy's equation.

• index is used as the A parameter. This input increases or decreases the IOR for
  all wavelengths.
• dispersion_coefficient_B is used as the B parameter, and controls the amount
  of dispersion. Increasing this parameter increases the dispersion effect, and also slightly
  increases the average IOR over all wavelengths. A value of 0.0 means no dispersion, the
  index input parameter then just gives the IOR for all wavelengths (like in previous versions
  of Octane).

The following image compares a prism with index set to 1.3549, and the B parameter
to 0.0 (left) and 0.02 (right). The beam in the right image is split into colors and slightly
shifted down.


Absorption

NOTE: The absorption parameters have been removed from the specular material as of
version 1.0 beta 2.55. You may use the absorption and scale parameters of the new medium
nodes to achieve the same effect.

Absorption means that the material slightly absorbs light while passing through.
The color resulting from this absorption is dependent on the distance light travels through
the material. With increased distance it will get darker, and if the absorption is colored
it will get more saturated.

This is very different from the transmission input, which only controls absorption at
the surface of the material, which is independent of the size of the object.

To specify the absorption two input parameters are used:

• absorption_color gives the spectrum remaining from white light after traveling
  through the material for a certain distance. Only constant colors are supported
  correctly.
• depth_of_absorption specifies the aforementioned distance (in meters). The smaller
  this distance, the stronger the absorption will be.

I hope this helps understanding the new parameters,
Roeland

Hey roe land, is it possible if I can grab ahold of the obj file you demonstrated for dispersion.

Yes, this is the obj and mtl file.

--
Roeland

cheers roeland, doing some major testing.

Question:

Is there any greater angle of the triangle (facing the light) which shows the out coming caustics at its best?

I don't know the optimal angles for the triangle. One thing that will definitely improve the brightness of the caustic is tilting upwards the plane behind the prism, so the beam hits it at a less acute angle.

--
Roeland

Thanks for this. I have a question - as Octane is a "physically based renderer," I must ask - are there any specular materials in the REAL world that do NOT absorb light? i.e., is there any reason to use transmission instead of absorption, aside from the fact that it might require less tweaking for simple models or models with consistent thickness?

Hi Roeland

is it possible to bring this thread back to life with reference to the newest Octane version with all its parameters, especially the Absorption Medium? Can you reproduce this example scene for Octane 2.x?
I am trying to get a similar result but it looks quiet modest.

Thank you very much in advance!

I am also interested in seeing if this can recreated within the current C4D build.