Am 25.07.2016 um 00:36 schrieb peyrol:
> In the FAQ is the question "Why doesn't my mirror reflect light? Why won't my
> lens focus light?" and the answer has to do with backwards ray tracing versus
> forwards ray tracing. Here's a quote:
...
> My OS is Linux Mint 18, which finally includes POVray in its repositories. Here
> is a quote from the blurb I read in my Software Manager:
...
> Prisms. That would be refraction, yes? Which the FAQ says does not work. Is this
> blurb correct in what it says? How can prisms make a spectrum if mirrors don't
> reflect light from light sources?

There are two different things involved here:

(1) The scene as /viewed/ through a prism or in a mirror: Backwards
raytracing, as used by POV-Ray, does model this properly. As for prisms
and lenses, POV-Ray also models dispersion. This is what the Mint repo
is talking about.

(2) The /illumination/ of the scene by light refracted through a prism
or reflected in a mirror: Backwards raytracing does /not/ model this;
you need forward raytracing for this. This is what the FAQ is referring
to. POV-Ray does provide a mechanism called Photon Mapping, which embeds
a forward raytracing algorithm in an otherwise backward raytracing
engine, and therefore can model such effects; however, this mechanism is
not active by default, and needs some extra statements in the scene to
enable it.

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