Indigo Renderer Forum

Posted: **Mon Jul 31, 2006 4:28 am**

Hey Guys...

**UPADTED - rendered 14h**

I started a Diamond Caustic dispersion Test, by using this material:

<material>
  <name>diamant</name>
    <specular>
      <normal_reflectance>0.05</normal_reflectance>
      <transparent>true</transparent>
      <ior>1.7280</ior>
      <cauchy_b_coeff>0.1342</cauchy_b_coeff>
      <rgb_absorptivity>0 0 0</rgb_absorptivity>
    </specular>
</material>

Does anybody got an Idea how to optimize the material to get a Result as close as possible to the Real World?
I found some NK-Data for minerals, but at this Point Indigo doesn't support transparent NK Materials

http://minerals.gps.caltech.edu/FILES/Visible/index.htm

Any Suggestions?

Posted: **Mon Jul 31, 2006 5:11 am**

Hmm, ... not easy to get it 100% right. First I notice that you are using
the old syntax for specular ... the <normal_reflectance> element is not
needed anymore.

Note: Your IOR is to low for a diamond and your cauchy_b is way to
high. I haven't tested this material, just a fast research ...

Code: Select all

<material>
	<name>diamond</name>
	<specular>
		<transparent>true</transparent>
		<ior>2.417</ior>
		<cauchy_b_coeff>0.044</cauchy_b_coeff>
		<rgb_absorptivity>0.00 0.00 0.00</rgb_absorptivity>
	</specular>
</material>

Unfortunately I have not yet found information about the absorbtion
of a diamond ... so you have to play a bit ...

Hope this will get you started! ;o)

take care
psor

Posted: **Mon Jul 31, 2006 1:27 pm**

psor wrote:Note: Your IOR is to low for a diamond and your cauchy_b is way to
high. I haven't tested this material, just a fast research ...

Hey psor, thank you very much for the Info!!!!

I Also found the following interesting informations in this Monster Document:

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         Index of    Critical
  Colour Refraction  Angle     Source Line    Wavelength

  Red      2.407     24° 33'   Solar  B-line   687   nm
  Yellow   2.417     24° 26'   Sodium D-line   589.3 nm
  Violet   2.452     24°  4'   Solar  G-line   431   nm

Is it possible to tweak the Color based IOR the right way by changing the rgb_absorptivity to:

Code: Select all

<rgb_absorptivity>to -0.01 0 0.035</rgb_absorptivity>

...or do I undestand this comand a wrong way???

I try a new Render with a better Diamond Model, and tweaked IOR & cauchy_b_coeff!
Somehow I get always one or two "force accepting" ech cycle, I just checked the normals, all fine (C4D)...

Again, thank you for the help

**EDIT**

psor wrote:the <normal_reflectance> element is not
needed anymore.

I found this command in the dispersion_test.xml Material for the prism...

Posted: **Mon Jul 31, 2006 1:40 pm**

Heya Zom-B,

force accepting isn't anything "bad" it is just an information.
If you decrease max_num_consec_rejections a lot you'll get
a lot of force accepting output in the display ...

As far as I do recall Nick said Indigo displays "force accepting"
if a ray is trying to "bounce" more then you allowed with
max_num_consec_rejections and the ray gets killed. ;o)

For the <normal_reflectance> you've found, I guess Nick
missed to delete it ... it get's ignored anyway - I guess.

Code: Select all

         Index of    Critical
  Colour Refraction  Angle     Source Line    Wavelength

  Red      2.407     24° 33'   Solar  B-line   687   nm
  Yellow   2.417     24° 26'   Sodium D-line   589.3 nm
  Violet   2.452     24°  4'   Solar  G-line   431   nm

As far as I know the IOR that we have to use gets it's value from
the average of those three values ... so it has nothing to do with
the absorbtion of light ... I think ... I do not have enough knowledge
in this matter to explain it exactly. ;o)

Nick? Any info on how to obtain the values for absorbance? Hmm, ...
maybe full range .nk file support would help.

take care
psor

Posted: **Mon Jul 31, 2006 9:06 pm**

Thak you for the explanation psor!

here is an updated Render of an even better Diamond Model, with better lightning, and the updated material.

** UPDATE rendered for 8.40h **

Posted: **Mon Jul 31, 2006 9:22 pm**

Hi,

I think that 0.044 is not Cauchy B for diamond (it is another measure for dispersion used in the gem industry). I computed a value of 0.013948 for Cauchy B and 2.3744 for Cauchy A, based on data in Diam.nk.

Cheers,
ryjo

Posted: **Mon Jul 31, 2006 9:51 pm**

ryjo wrote:Hi,

I think that 0.044 is not Cauchy B for diamond (it is another measure for dispersion used in the gem industry).
I computed a value of 0.013948 for Cauchy B and 2.3744 for Cauchy A, based on data in Diam.nk.

Cheers,
ryjo

Wow.... sombody who can understand the the wired numbers in the NK-Files

But now I'am interested.... there is a <cauchy_a_coeff> commmand too?!??

Posted: **Mon Jul 31, 2006 11:00 pm**

Zom-B wrote: Wow.... sombody who can understand the the wired numbers in the NK-Files

But now I'am interested.... there is a <cauchy_a_coeff> commmand too?!??

I assume that if cauchy_b_coeff is non-zero then ior should be given as Cauchy A.

Perhaps Ono can comment?

Posted: **Mon Jul 31, 2006 11:20 pm**

Heya ryjo,

thanx mate for your contribution. Could you explain in a
simple way how to calculate the cauchy_b ... ?

take care
psor

Posted: **Tue Aug 01, 2006 12:24 am**

psor wrote:Heya ryjo,

thanx mate for your contribution. Could you explain in a
simple way how to calculate the cauchy_b ... ?

take care
psor

Hi,

In the Cauchy equation ior is computed as IOR(L) = A + B/L^2. By taking the difference between two wavelengths you get

Code: Select all

IOR( L1 ) - IOR( L2 ) = B / L1^2 - B / L2^2

From this I get Cauchy B as

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B = ( L1 * L2 )^2 / ( L2^2 - L1^2 ) * ( IOR1 - IOR2 )

Putting in some numbers:

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L1 = 431 nm = 0.431 um ; IOR1 = 2.452
L2 = 687 nm = 0.687 um ; IOR2 = 2.407

B = ( 0.431 * 0.687 )^2 / ( 0.687^2 - 0.431^2 ) * ( 2.452 - 2.407 )
B = 0.01378

Cauchy A can then be computed as 2.377.

These numbers came from psor's post.

Hope this helps!

Cheers
ryjo

Posted: **Tue Aug 01, 2006 12:32 am**

Thanx ryjo, much appreciated!

take care
psor

Posted: **Tue Aug 01, 2006 2:37 am**

Thanks ryjo...

I just started a new Render with your settings, I have to admit that I havn't understood anything
of your math, shure others are smarter at this point than me...

Code: Select all

<material>
   <name>diamond</name>
   <specular>
      <transparent>true</transparent>
      <ior>2.3744</ior>
      <cauchy_b_coeff>0.013948</cauchy_b_coeff>
      <rgb_absorptivity>0 0 0</rgb_absorptivity>
   </specular>
</material>

I just slightly modified the Model, adjusted the light, changed the floor Material
and added some DoF, so the Diamond doesn't look this gigantic

This Render will be the last for this little diamond experiment, I keep the Scene,
and if Nick someday integrates the support for Transparent NK-Data,
I will rerender and compare (I know NK files are much more complex, its just for "fun")
I would love to make this Scene & Model available as a Testscene if transparent
NK-Date will be supported some day...

Posted: **Tue Aug 01, 2006 10:20 am**

OMG! this is so useful! thanks alot dudes! (im currently working on a CGI dark side of ye moon cover)

Posted: **Tue Aug 01, 2006 7:26 pm**

Ok guys...

This will be the last version of the rendering, stilll not finised, but DOF
needs some time to become "noiseless":
When its done, I will post the Render at the "finished artwork" Forum
together with the XML & Modell files, so everybody kann use it

** Rendered 15h **

Code: Select all

<material>
   <name>diamond</name>
   <specular>
      <transparent>true</transparent>
      <ior>2.3744</ior>
      <cauchy_b_coeff>0.013948</cauchy_b_coeff>
      <rgb_absorptivity>0 0 0</rgb_absorptivity>
   </specular>
</material>

I like the diamond Material, with less dispersion it looks "better" I think,
but I'm not realy satesfied with the DOF: not well tweaked, but I dont want to do this again & again &... ^^

bye

Posted: **Tue Aug 01, 2006 11:33 pm**

Finalised this small Project... thanks for your Help psor & ryjo!!!

Indigo Renderer Forum

Diamond, big as a fist ;)

Diamond, big as a fist ;)

update...

v3