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I am looking for help figuring out what extactly the "Dispersion coefficient" of a specular material node does.

The mouse over help text reads:

B parameter of the Cauchy Dispersion Model

I tried to google for an answer but then got lost in a lot of theory of different related models without finding that "B parameter".

1) Can anyone provide a quick overview of the physical theory behind the "Cauchy Dispersion Model" and the "B parameter"?

2) What is the intended use for "Dispersion coefficient" with OctaneRender?

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removed test series and screenshots that were just confusing to keep this post shorter

Side Note:

General request:
Could it be possible to add additional more detailed versions of mouse over help text to the OctaneRender parameters when holding CTRL?

Example:
In Zbrush you get a quick one sentence summary for a parameter for a normal mouse over action.
But when holding CTRL during mouse over a longer text is displayed in an info box the provides an often much needed longer explanation.

Hi linvanchene,
your disoersion values are way too high
In real life, look at the water in a trasparent plastic bottle to understand the effect.
from the OctaneRender documentation:

• Dispersion Coefficient (dispersion_coefficient)
  The dispersion in OctaneRender™ is based on Cauchy’s equation which has two terms: A which is the index of refraction and B which is the dispersion coefficient. Increasing the value increases the amount of coloration and dispersion in the object and in caustics.

Here is a quick link to wikipedia, use the C values instead of the B values in the table at the end of the page for realistic results
ciao beppe

bepeg4d wrote:Hi linvanchene,
your disoersion values are way too high
In real life, look at the water in a trasparent plastic bottle to understand the effect.
from the OctaneRender documentation:

• Dispersion Coefficient (dispersion_coefficient)
  The dispersion in OctaneRender™ is based on Cauchy’s equation which has two terms: A which is the index of refraction and B which is the dispersion coefficient. Increasing the value increases the amount of coloration and dispersion in the object and in caustics.

Here is a quick link to wikipedia, use the C values instead of the B values in the table at the end of the page for realistic results
ciao beppe

Salut Beppe

Thank you a lot for pointing me in the right direction.

In addition to the links you posted reading up on dispersion in general helped me as well.

https://en.wikipedia.org/wiki/Dispersion_(optics)

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After some basic understanding it seems a test scene wih a pyramid, a ground plane and a small light emitter is more suited to observe the effect.
A difference between 0.030 and .00345 shows the effect in color of those smaller value changes.

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If someone wants to experiment further with the same scene I attached the ORBX in the .zip file:
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Look values up or use a menu with example parameter settings for often used material types?

I guess I never really bothered about actually understanding what the Dispersion coefficient does because in the DAZ Studio OctaneRender plugin there are presets to choose for different type of materials.

So instead of looking up index of refraction or Dispersion coefficient values I was just able to select values of a similar material directly from a menu when editing the parameters.
Learning and understanding the values we use may be important in the long run.

Nevertheless new users might be overwhelmed with a lot of information already.

It could be helpful to have some menu selections for Index of Refraction and the Dispersion Coefficient in OctaneRender standalone as well so users can quickly select some standard values for commonly used materials like glass etc.

^best forum reply ever goes to^

If you're at all familiar with the film/video term 'chromatic abberation', I am pretty certain dispersion coefficient is a measurement of the "dispersion" of the color specturum that causes less precision(cheaper) lenses to have chromatic abberation. If you're not familiar with this term, forget i said anything.

garytyler wrote:^best forum reply ever goes to^

If you're at all familiar with the film/video term 'chromatic abberation', I am pretty certain dispersion coefficient is a measurement of the "dispersion" of the color specturum that causes less precision(cheaper) lenses to have chromatic abberation. If you're not familiar with this term, forget i said anything.

I can follow this statement in so far that chromatic aberration can be considered as a result of dispersion.

But the specific dispersion coefficient in OctaneRender is operating on an object level while the chromatic aberration would be visible on a camera or scene level.

compare:

https://en.wikipedia.org/wiki/Chromatic_aberration

https://en.wikipedia.org/wiki/Dispersion_(optics)

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The way I understand it now at least in OctaneRender the "Dispersion coefficient" is very loosely put the setting that adjust how coloured the caustics of a specific object or volume are.
The dispersion coefficient is set on an object / surface / volume level.

The Chromatic aberration is very loosely put a purple fringe around many different objects in the scene caused by a lower quality camera lense.

To actually simulate "Chromatic aberration" on a scene level we would need to have that setting somewhere in the camera options or maybe in the post processing settings.

An example of a very extreme and not realistic chromatic aberration effect created in photoshop:
But there may certainly be more pressing things to worry about than simulating bad lense effects.

Hi linvanchene,
just as an example, here is a couple of chromatic aberration effect obtained with specular dispersion:
viewtopic.php?f=6&t=38504&p=212671&hili ... on#p212671
viewtopic.php?f=5&t=45517&hilit=fisheye
happy GPU rendering
ciao beppe

bepeg4d wrote:Hi linvanchene,
just as an example, here is a couple of chromatic aberration effect obtained with specular dispersion:
viewtopic.php?f=6&t=38504&p=212671&hili ... on#p212671
viewtopic.php?f=5&t=45517&hilit=fisheye
happy GPU rendering
ciao beppe

Thank you for the links to those very creative threads.

I did not expect that placing the camera in a box could actually yield such interesting looking dispersion and fish eye effects on a scene level.

Especially thank you bepeg4d for taking the time to keep experimenting with the fish eye effect and sharing the result.

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While tracking uses of the specular box this post by Olitech also helped me to get some additional ideas how to use it

viewtopic.php?f=6&t=38312&p=172854&hili ... ox#p172854

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Goldisart wrote:https://render.otoy.com/forum/viewtopic.php?f=39&t=50195

In case anyone else was puzzled by following that link.

Its actually a list of materials and their dispersion coefficient values in Russian.

Thank you for sharing.

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Google Auto Translated Version:

The dispersion in the range of G-

Rutile 0.28
Anatase 0.213 and 0.259 *
The plot of 0.19
Sphalerite 0.156
Cassiterite 0,071
Dzhevalit 0,063
Demantoid 0,057
Melanie 0,057
Cerussite 0,051
Titanite 0,051
Benitoite 0.039 and 0.046 *
Diamond 0.044
Zircon 0,039
Benitoite 0.046 and 0.039 *
Galliant 0,038
Smithson 0.014 and 0.031 *
Epidote 0.03
Tanzanite 0.03
Grossular 0,027
Hessonite 0,027
Spessartite 0,027
Willem 0,027
Scheele 0,026
Spinel 0,026
Almandine 0,024
Rhodolite 0,024
Staurolite 0,023
Dioptase 0,022
Pyrope 0,022
Kyanite 0.02
Peridot 0.02
Taafeit 0,019
Vesuvianite 0,019
Kornerupine 0,018
Rubin 0,018
Sapphire 0,018
Sinhalese 0.018
Calcite 0.008 and 0.017 *
Cordierite 0,017
Danburite 0,017
Giddenit 0,017
Kunze 0,017
Scapolite 0,017
Tourmaline 0,017
Andalusite 0,016
Apatite 0,016
Datolite 0,016
Euclase 0,016
Alexandrite 0,015
Chrysoberyl 0,015
Hamberg 0,015
Phenacite 0,015
Sillimanite 0,015
Aquamarine 0,014
beryl 0.014
brazilianite 0,014
petals 0,014
Emerald 0,014
Smithson 0.031 and 0.014 *
Topaz 0.014
Amethyst 0,013
Amethyst 0,013 quarts
aventurine 0,013
Rhinestone 0,013
citrine 0,013
prasiolite 0,013
Smoky quartz 0,013
Rose Quartz 0.013
Tiger Eye 0.013
0.012 amazonite
Moonstone 0.012
Orthoclase 0.012
Beryllonite 0.01
cancrinite 0.01
leucite 0.01
obsidian 0.01
Quartz glass 0.01
Calcite 0.017 and 0.08 *
Fluorite 0,007

Thanks Goldisart for sharing and big thanks to linvanchene for translation
ciao beppe