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?

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"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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Wasn't it Florian Siegmund who 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?

POV blacklights don't behave anything like that.

Lights with negative colours illuminate objects with negative pigments,
but the negative lights do affect object with conventional pigments
(they darken them) and an undarklit negative-pigment object is black,
not transparent.

-- 
Mike Williams
Gentleman of Leisure

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Florian Siegmund 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?

I have made a small test scene that might be what you are looking for, 
see the posts I will make i a few minutes to p.b.i and p.t.s-f.
I've got the idea for the additive pigment used in the scene from a post 
  by someone on these newsgroups. Credits should go to whoever that was 
(I can't remember and can't find the post right now).

-- 
Daniel Nilsson

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> 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.)

A much simpler and faster approach would be to render the image with only
the specific object in white, everything else in black, and the special
lights set to white as well. You'd get an image where white represents the
visible object, and black the background.
Then do another render of the scene, this time with the appropriate colors
and all lights switched on.

You the black/white image as a function, multiply it with the colored image
(as a function) and place that close in front of the camera when rendering
the scene without the object. Voila! Composite image!

If you're using focal blur though, that might be a problem, so then I'd
suggest using trace() to find the 3d-Locations of the pixels, and color them
using the colored image.

All together not very suitable for animations, but nontheless, that could
still be automated rendering the animation three times, using the frame
number as reference which image-maps to load.

Regards,
Tim
-- 
aka "Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

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Daniel Nilsson <pov### [at] daniel-nilssoncom> wrote:
> I have made a small test scene that might be what you are looking for,
> see the posts I will make i a few minutes to p.b.i and p.t.s-f.
> I've got the idea for the additive pigment used in the scene from a post
>   by someone on these newsgroups. Credits should go to whoever that was
> (I can't remember and can't find the post right now).
>
> --
> Daniel Nilsson

Thx Daniel, this concept is exactly what I was looking for. If you are
interested in, I posted an include file for creating very realistic
rainbows to the POV-Ray Binary Files (scene files) a few minutes ago.

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Florian Siegmund wrote:
> Thx Daniel, this concept is exactly what I was looking for. If you are
> interested in, I posted an include file for creating very realistic
> rainbows to the POV-Ray Binary Files (scene files) a few minutes ago.

If you make a scene with ultraviolet light please consider posting it to 
p.b.i so I can have a look at it.
I'm not in need of a rainbow right now but I've put on my "to look at" 
list. I rarly have the time to make a complete scene, but I like playing 
around with new concepts now and then.

-- 
Daniel Nilsson

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