Stefan Viljoen <spa### [at] removethispolardcom> wrote:
> Now look at image 2 - the moment I turn on photon mapping, I get this black
> shadowed area where the caustics caused by the glass plate are supposed to
> appear - only it is a pitch black shadow, and STAYS that way whenever
> photons are turned on.

  It seems to me that POV-Ray calculates the refraction of light (for
the photons) too accurately and realistically, and what you want is just
some fake effect which looks good, regardless of physical accuracy.

  Photon mapping is not a "magic trick" which will produce sparkling
caustics-looking effects regardless of the object and other properties
of the scene. The older fake caustics (used with the 'caustics' keyword)
is more like that, but it's of course quite limited.

  Photon mapping calculates *accurately* how light coming from a point
light reflects/refracts from objects and hits other surfaces. This means
that the shape of the refracting object is quite relevant if you want to
get sparkling beautiful caustics. Caustics are produced when refraction
concentrates light from a larger area into a smaller area, thus
intensifying its brightness in that area. Naturally the object must
have a certain shape for this to happen. For example a planar sheet of
glass will not concentrate light and thus there will be no sparkling
caustics behind it.

> How can I eliminate this black area and get caustics to show up in there?

  The area is black because light is not reaching it. What other color
would you expect it to be? The object is diverging the light so that it
won't reach all the shadowed parts.

  Of course in real life light will reach those areas from elsewhere,
reflected from other surfaces around, and this can be simulated with
radiosity, but that's a whole different question altogether.

-- 
                                                          - Warp

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