I have a pretty typical glass-sphere-on-grey-floor scene,
and I'm using dispersion and photons in it.  
I'm using Nathan Kopp's color map macro on three spotlight sources.
My questions:

The shadow of the sphere starts out black at the edge, stays black for a
medium
distance, and then the colors and bright spot from the dispersion begins.
What do I change so that the black shadow is brighter or starts to fade
into the bright spot more thoroughly (i.e. closer to the shadow's edge?) . 
I guess what I'm looking for is an increase in caustics around the base,
but without messing up the prism effect too much.

Second, I've played with the disp_nelems for both the light source macro
and the target sphere but I still see color banding.  Which do I need to
raise the nelems on to eliminate it (I've gone up to 20 so far) and how
high should it be to look nice at, say, 640x480?

Lastly, the ground surface is flat, but I get all sorts of tiny, regular
streaks in the
dispersion highlight.  For some reason, this looks right to me, but I want
to know
where it's coming from - the roughness on the sphere (.005) or the photons.

I know I should trial-and-error this stuff myself, but my computer is
pretty slow.

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And did I mention the scene file is in the scene files group? oops.

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CreeD wrote:
> 
> I guess what I'm looking for is an increase in caustics around the base,
> but without messing up the prism effect too much.

Generally speaking, the higher the IOR of your sphere, the tighter the caustics. 
Dispersion is also effected by this, so you might have to tweak the dispersion
amount as you change the IOR.

> 
> Which do I need to raise the nelems on to eliminate it (I've gone up to 20 so > far)
and how high should it be to look nice at, say, 640x480?

I've gone up to 50. Your mileage may vary.

> 
> Lastly, the ground surface is flat, but I get all sorts of tiny, regular
> streaks in the
> dispersion highlight.

I'm guessing these streaks are just aliasing artifacts of the photon map. You
might try increasing the number of photons or gather radius. But if it doesn't
bother you, let it be.

-- 
Margus Ramst

Personal e-mail: mar### [at] peakeduee
TAG (Team Assistance Group) e-mail: mar### [at] tagpovrayorg

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CreeD <mes### [at] nqinet> wrote:
: The shadow of the sphere starts out black at the edge, stays black for a
: medium
: distance, and then the colors and bright spot from the dispersion begins.
: What do I change so that the black shadow is brighter or starts to fade
: into the bright spot more thoroughly

  That depends on whether you want a physically correct result or an
artistically good-looking image?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Warp <war### [at] tagpovrayorg> wrote in article <398a96d5@news.povray.org>...
> CreeD <mes### [at] nqinet> wrote:
> : The shadow of the sphere starts out black at the edge, stays black for
a
> : medium
> : distance, and then the colors and bright spot from the dispersion
begins.
> : What do I change so that the black shadow is brighter or starts to fade
> : into the bright spot more thoroughly
> 
>   That depends on whether you want a physically correct result or an
> artistically good-looking image?

Good looking would be fine.  Physically, I would imagine that the shadow
wouldn't look so dark anyway, but I've never sat in a darkened room with a
spotlight and a crystal ball, and until I do, I'm just full of conjecture.

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In article <01bfffdd$d34a9940$b913a1d0@mk>, "CreeD" <meshe@nqi.net> 
wrote:

> Good looking would be fine.  Physically, I would imagine that the 
> shadow wouldn't look so dark anyway, but I've never sat in a darkened 
> room with a spotlight and a crystal ball, and until I do, I'm just 
> full of conjecture.

This is realistic for the situation of one point light. When light is 
refracted, *all* of it is refracted, none of it gets past without having 
it's path bent. This means the edges of the shadow, where the light is 
being refracted away, will be black.
You usually don't see this in real life, because diffusely reflected 
light illuminates the shadow(both directly and indirectly, through the 
refracting object), and you usually have more than one light source. You 
should get more realistic effects by using more lights and radiosity.

-- 
Christopher James Huff - Personal e-mail: chr### [at] maccom
TAG(Technical Assistance Group) e-mail: chr### [at] tagpovrayorg
Personal Web page: http://homepage.mac.com/chrishuff/
TAG Web page: http://tag.povray.org/

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CreeD wrote:

> but I've never sat in a darkened room with a spotlight and a crystal ball,
> and until I do...

...you are not an advanced user. :)

-- 
Ken Tyler - 1400+ POV-Ray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/

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CreeD <mes### [at] nqinet> wrote:
: Physically, I would imagine that the shadow
: wouldn't look so dark anyway, but I've never sat in a darkened room with a
: spotlight and a crystal ball, and until I do, I'm just full of conjecture.

  If you want a physically correct result you'll have to use radiosity.
Some ambient light simulates it faster (although not as good). The shadow
should be physically correct.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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