clipka <ano### [at] anonymousorg> wrote:
> Am 17.07.2012 14:54, schrieb Chaanakya:
> > I've been trying to figure this out for a while.  Let's say one specifies an ior
> > of 1.5 (in the interior {} block).  Can't POV-Ray automatically calculate the
> > reflectance and transmittance?  It knows the angle of the incident light and the
> > index of refraction of the two media.  Even with the fresnel keyword, however,
> > POV-Ray still requires me to manually calculate and plug in the reflectance and
> > transmittance.  Am I missing something?
>
> (1) Historical reasons; early versions didn't have the fresnel keyword,
> and instead required you to tweak various parameters until the result
> looked reasonably ok. The parameters are still there.
>
> (2) The fresnel formulae for reflectance and transmittance are correct
> for ideal, perfectly smooth surfaces, but real surfaces aren't that way;
> a surface with plenty of microscopic scratches, for instance, will
> exhibit a mix of specular reflection (from light rays that hit undamaged
> areas) and lamtertian diffuse reflection (from rays that hit a scratch).
> Lambertian law would also imply that in order to reduce the specular
> reflections on LCD displays you'd have to blur them, but researchers
> recently developed a surface microstructure (at below-wavelength scale)
> that will actually /suppress/ reflections.
>
> (3) The fresnel formulae are correct for single interfaces, but real
> surfaces frequently involve some coating, making it a stack of multiple
> interfaces; it is much easier (in terms of geometric modelling and/or
> computational effort) to handle such surfaces with a tweaked
> single-interface model, rather than simulate the interactions between
> the material layers.
>
> (4) Manually calculating reflectance and transmittance for an ideal
> surface shouldn't be necessary if you use both fresnel and conserve_energy.
>
>
> (BTW, is there any reason you ask this in the unix-specific newsgroup?)

1) Okay. I understand

2) For my application, I'm assuming that the surface is perfectly smooth, so as
long as POV-Ray uses the fresnel equations, that's fine by me.

3) I am again assuming that there is no coating on those interfaces and am
layering slabs in order to force computation of interactions between the layers,
so this is also fine.

4) I have found that if I only specify the IOR, conserve_energy, and fresnel
(with no pigment { transmit N.N } block), the object looks black.  This is the
one thing I don't understand.

Also, no, there is no specific reason - I just have gotten pretty good responses
from the folks over here, so I decided to post here :)

Sincerely,

Chaanakya

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