"Florian Siegmund" <flo### [at] gmxat> wrote:
> Is it possible to design a scene containing a "blacklight" source? What I
> mean is: I want an object (or single parts of an object) only to be visible
> when being lit by special light sources. Other parts of the scene should
> not be affected by these LS. I know that one can achieve this by using
> light groups, but the main problem I have to solve is, how do I create an
> object that becomes invisible (translucent, not black!) when the LS is
> switched off or when other objects cast shadows on it?

Ouch.

This one's not presently doable as is - the closest solutions would be to
set either the fade_color appropriately or to put in a media, and neither
of these would work quite how you want it.

Your next best bet is to fake it by way of running the lighting check
yourself (cf. the demo on how to write a raytracer in the SDL, contained in
the help file) and use the intersection points to produce a mesh that
contains only what you want, namely the areas lit by these light sources.
This would get rather messy eventually, especially if you want things to
look right; however, you do have the "advantage" of being able to speed up
the render by lowering the resolution of the lighting checks as needed.
(The worst part of this is that it'd be a forward step, as with the photon
map, rather than the standard reverse method.  Ah well.)

So your options are either to fake it by coloring the interior of the
object, or to spend (potentially) a lot of extra time writing a small
forward-raytracing engine so this works out properly.

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