Am 29.02.2016 um 02:33 schrieb Bald Eagle:
> Strange thoughts pop into my head, and was wondering about the phenomenon where
> when you get a piece of paper wet or greasy, you can have it look dark or light
> depending upon its orientation.

That's because when wet or greasy, the paper's diffuse reflectivity is
reduced, but the specular reflectivity is increased. Specular
reflectivity is highly angle-dependent -- even if you make sure that you
look at the surface from the direction "opposite" to the incoming light,
specular reflection is strongest at shallow angles. (See "Fresnel
equations".)

> I've also noticed that on engraved reflective surfaces - you can get an
> inversion, so to speak.

This is also related to the angle-dependent nature of specular reflections.

> 3.  Has anyone to date modeled this with POV-Ray, perhaps with an animation?

There are a few things at work here:

- Translucency: Wet paper is translucent (dry paper is, too, but to a
much lesser degree). You can model this either by using a solid shape
(e.g. a thin box) with media or SSLT, or by using a non-solid shape
(e.g. a pair of triangles) with POV-Ray 3.7's backside illumination
feature (specify a second parameter to "diffuse" to specify how much a
light behind the surface should illuminate the front side).

- Angle-dependent specular reflection: If the object is supposed to be
made of metal, use "metallic" in both the reflection block and in the
finish block itself. If the object is supposed to be made of anything
else, specify an interior and ior, use "fresnel" in the reflection
block, and if you use POV-Ray 3.7.1 also use "fresnel" in the finish
block itself.

- Angle-dependent diffuse reflection: Yes, this exists, and it behaves
just the opposite of the specular reflection. Think of it as specular
reflection "stealing" light from the diffuse reflection. To model this,
you need to use either an angle-of-incidence ("aoi") patterned texture,
or POV-Ray 3.7.1 and "fresnel" in the finish block itself.

- Etchings: I guess surface normal perturbations should do the trick.

- Rough specular reflections: You may want the specular reflections to
be somewhat blurry, especially in case of paper. You can achieve this by
using very small-scale random surface normals (e.g. bumps) and some
means of oversampling (e.g. using many averaged textures with different
surface normals, a slight bit of focal blur, or high-quality anti-aliasing).

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