Am 06.04.2016 um 13:24 schrieb Thomas de Groot:
> On 6-4-2016 12:28, And wrote:
>> Or the carton.
>>
> 
> Powdery/chalk, carton, yes that makes sense indeed for oren_nayar.
> 
> Got back to the original clipka message:
> 
> [quote] The Lommel-Seeliger model has its roots in astronomy, where it
> is used to model "lunar type" (i.e. rocky or dusty) surfaces, while the
> Lambertian model is used for "icy" surfaces, and weighted averages of
> the two are used to model anything in between. [/quote]
> 
> What would you say for the following:
> 
> - Lambertian for icy or shiny surfaces;
> - oren_nayar for fine, powdery-like surfaces;
> - lommel_seeliger for coarser, dusty-like surfaces.

Actually, having read up a bit on the models' origins and technical
background by now, I'd say the following:

- There is probably no significant practical difference between powdery,
dusty or coarse surfaces.

- Lommel-Seeliger is a niche model, designed to be computationally
"lightweight" while reasonably approximating dark (*) coarse surfaces;
from all I know, its use is almost exclusively restricted to astronomy.

(* Many people don't realize it, but the moon is about as bright as a
lump of anthrazit coal.)

- Pure Lambertian is only reasonably realistic for /some/ (**) bright
coarse surfaces.

- With an additional fresnel-based term to compensate for light lost to
specular reflection (finish-level "fresnel on" in recent POV-Ray
versions), the Lambertian model is also reasonably realistic for
[non-metallic] uncoated shiny surfaces, and /some/ (***) coated shiny
surfaces.

- For most coarse surfaces, including /some/ (**) bright ones,
Oren-Nayar [with noteworthy roughness] is better suited.

- For /some/ (***) coated shiny surfaces, Oren-Nayar with an additional
fresnel-based compensating term (as mentioned above) is better suited.

(**) For bright coarse surfaces, the choice betwen the Lambertian and
Oren-Nayar model depends on the underlying material's optical
properties: If, at the dimensions of the surface's roughness, the
material exhibits some degree of translucency, the Lambertian model is
sufficient (e.g. snow, not-yet-licked ice cream, wood, Spectralon,
possibly Styrofoam), though it may call for SSLT if shown up close. If
on the other hand the material is opaque at those dimensions, Oren-Nayar
is better suited (e.g. Shapeways' unpolished "strong & flexible"
plastic, unglazed ceramics).

(Actually this is also true for dark coarse surfaces, but they are
almost exclusively opaque at the dimensions in question.)

(***) For coated shiny surfaces, the choice between the Lambertian and
Oren-Nayar model depends on which one would be appropriate if the
surface was uncoated. In both cases, use of finish-level "fresnel on" is
recommended.


Also, for the sake of completeness:

- POV-Ray's "brilliance"-based variation of the Lambertian model is an
abomination that has no physical justification whatsoever; my current
guess is that it originated as a botched attempt to implement the
so-called Minnaert model.

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