Am 23.09.2018 um 15:20 schrieb ttyUSB0:
> I have a material that creates a shadow of varying density, depending on the
> angle to the direction of the light rays.
> Is it possible to specify a variable transmission of light through the surface,
> depending on the angle of light rays incidence?

That's actually normal behaviour for pretty much all transparent
real-life materials; usually, the difference in shadow is due to
specular reflection at the surfaces that "steals" part of the light; in
real life, specular reflection is stronger at shallow angles, as
expressed by Fresnel's law.


As Jim already hinted at, POV-Ray can take into account Fresnel's law
for /some/ effects, if you specify an index of refraction.

However, it doesn't normally do so for shadows; to make POV-Ray do so,
you have to:

- Specify the object's refractive index (via `ior` in an `interior` block).

- Set up the material to respect fresnel's law for reflection (using
`reflection { fresnel on }` in a finish block).

- Set up the material to account for reflected light in computing
transmission (using `conserve_energy` in a finish block).

- Use photon mapping to coax POV-Ray into actually computing advanced
transmission effects for shadows.


An alternative is to use the `aoi` (angle of incidence) pattern for the
pigment, varying the `trans` component of the colour according to the angle.

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