"Rich" <SrP### [at] ricoswebcom> wrote...
> "Tom Melly" <tom### [at] tomandlucouk> wrote in news:3c629a46@news.povray.org:
>
> > Umm, I think water is the only thing you want "collect off" for (as the
> > docs say, most things that are photon targets don't show photons, they
> > just refract or reflect 'em).
>
> Hmm, wouldn't that defeat the purpose of using photons in this scene?  I'd
> think putting collect off on the water would prevent it from producing
> caustics, etc.

Actually, you DO want to use "collect off" for the water.  This means that,
although photons won't show up ON the water, they will still be produced
THROUGH the water.

The reason we don't collect photons on clear surfaces is twofold:
1) It is a waste of time. Because clear surfaces have such a small diffuse
component, it doesn't make a difference if we show them or not.
2) On thin clear surfaces, we save photons on both surfaces of the object.
However, when we go to collect the photons, we will get a little "bleed"
from one surface when we're trying to gather from the other.  This is also a
problem when we have a clear object sitting directly on top of an opaque
object.  We _really_ want to turn collect OFF for the clear object, because
otherwise, when we try to collect photons on the opaque object, we will end
up gathering twice as many photons, since we'll be gathering the photons
from both the clear layer and the opaque layer.

-Nathan

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