> First, here's bad news:
>
> We have a bug in photons { pass_through }.
>
> The current implementation mechanism is (and apparently has always been)
> causing an "amplification" effect on photons. See
> http://bugs.povray.org/task/93 for an example scene.
>
> Then, there's good news:
>
> I located the error.
>
> There is a piece of code in PhotonTrace::ComputeLightedTexture that
> causes pass_through objects to *both* let the photon pass unaffected
> *and* have a clone of it pass trough it with all effects - transmit &
> filter attenuation, reflection & refraction - the whole smash.
>
> Duh!
>
> Question is, how to fix this thing? Do we just want to suppress
> reflection & refraction but do transmit & filter attenuation? That would
> be my favorite, though according to the documentation only media-based
> attenuation should occur.

When I add pass_through, I expect the object to have no effect on the 
photons, but I like your suggestion of allowing filter and transmit to 
affect the result.

With your suggestion, if there is something looking like a stained 
glass, or similar, in the way of the incoming light, you'll get all the 
effect of it, without having to compute photons for areas that only get 
affected by that filtering object.

If you go that way, the documentations will have to be changed to match 
that behaviour.



Alain

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