Am 03.07.2015 um 10:34 schrieb scott:
>>>> If you are using fresnel, is it valid (from a PBR point of view) to
>>>> even
>>>> scale the results using a maximum reflection other than 1?
>>>
>>> Absolutely - if, for instance, your surface is littered with
>>> non-specular material at a microscopic level. Fine rust on iron steel,
>>> for instance, or a thin coating of dust.
>
> But are you sure simply scaling down the Fresnel reflection term in
> those cases gives a physically correct result for those materials?

If it's soot, then yes (provided you also tune down the diffuse term); 
any other stuff mixed in, and things get more complicated, hence my 
recommendation:

>>> In those cases it might be better, however, to use an average of two
>>> materials.


>> What about plastic? It reflects, but it's not highly reflective. I don't
>> think I would go higher than 0.5 maximum reflection on plastic.
>
> Agreed completely, but does using the Fresnel equations to calculate
> reflection (which gives an absolute value) and scaling the result to
> simulate some other effect (surface roughness, sub-surface scattering
> etc) give a physically correct result? Or is it just a "fudge factor" to
> give something that looks correct?

For surface roughness, "roughness" is the POV-Ray parameter to tweak; 
this also tunes down the brightness of individual pixels in highlights 
(provided you use the "albedo" keyword) and reflections (provided you 
also use the "roughness" keyword in the reflection block), by spreading 
the highlights over a larger area (so much that you might not even 
notice the specular reflection anymore), but just tuning down the 
maximum brightness obviously doesn't get you the same effect, as it 
leaves reflections crisp and sharp.

Sub-surface scattering has nothing to do with specular reflections - to 
the contrary: It is the result of all the light that is /not/ reflected 
in a specular manner (which is to say, at the object's surface). As a 
matter of fact, sub-surface scattering (in a very simple case) is what 
the traditional "diffuse" mechanism models.

Post a reply to this message