Turning on dispersion seemed to produce a really huge speed reduction - 
even with it set to only 2 samples. I was expecting it to be 
approximately 2 times slower... but it seems vastly slower than that. 
(It goes from thousands of pixels per second down to 40 or so.)

Can anyone explain to me why that is? What have I missed here?

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Andrew the Orchid <voi### [at] devnull> wrote:
> Turning on dispersion seemed to produce a really huge speed reduction -
> even with it set to only 2 samples. I was expecting it to be
> approximately 2 times slower... but it seems vastly slower than that.
> (It goes from thousands of pixels per second down to 40 or so.)
>
> Can anyone explain to me why that is? What have I missed here?

I'm only guessing, but I think that the dispersion is allowing rays to get
to places in your scene that they weren't reaching without dispersion and
they're getting involved in more reflections. Also, dispersion does weird,
unreal-looking things with less than 5 samples.

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Andrew the Orchid <voi### [at] devnull> wrote:
> Turning on dispersion seemed to produce a really huge speed reduction -
> even with it set to only 2 samples. I was expecting it to be
> approximately 2 times slower... but it seems vastly slower than that.
> (It goes from thousands of pixels per second down to 40 or so.)
>
> Can anyone explain to me why that is? What have I missed here?

Really you have missed nothing - dispersion is one of the most CPU intensive
effects that POV can do, especially if you are using photon mapping, media
or radiosity at the same time.  Using them all together on my 2.10 GHz
Athlon 3000+ routinely results in renders that take days or even weeks.  A
rate of less than 10 PPS is common.  A high dispersion_samples value (50 or
more) slows things down but it's worth the time for your final render. If
you really want to see the full-blown effect of dispersion, using photons
is the only way to go since they will project caustics outside the object
anywhere, even onto atmospheric or object media, not just in the shadow.
(tip: when using photons with media you must add a photon block with target
to your media container.) If it is a CSG object with difference you are
using with photons and dispersion(like a cut gemstone), consider bounding
the object by hand - it can really speed things up.  Also, careful use of
spotlights can make a difference since the light only goes where it's
needed.  Check out the consts.inc file docs for a list of materials, their
iors, and dispersion amounts.  Good luck

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