On 3-7-2015 10:34, scott wrote:
>>>> 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?
>
>>> In those cases it might be better, however, to use an average of two
>>> materials.
>
> Yes that's my concern, that if you are using the reflection scaling to
> "fake" a different effect (roughness, absorbtion or scattering etc) then
> it *might* not be physically correct to just scale it.
>
> In other words, if you created a geometrically microscopic rough surface
> (or added physical dust, sub-surface scattering or whatever) and
> rendered it with max reflection 1, would it be possible to get the same
> result by rendering a perfectly smooth, clean surface with max
> reflection set to some lower value?
>
>> 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?
>

I don't know, but in all these cases different from the /simple/ 
reflective surfaces, we need some good working examples demonstrating 
the properties. I confess that this is too difficult for me - and 
probably most people - to do and expect them from our own gurus :-) I 
have also difficulties visualising an averaged texture or a layered 
texture staying within the properties boundaries.

-- 
Thomas

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