Check
 a) max_trace_level... more surfaces to go through (this should be OK,
      though)
 b) autostop angle.  If autostop is zero and the first few photons go through
      the hole in the middle, it will stop right there.  setting autostop
      to a higher angle forces the patch to keep going in the spiral pattern
      until at least the autostop angle is reached (try 45 degrees).
      You'll know if autostop is the problem if the "Building photon
      maps..." step takes almost no time.

Are there no photons (black shadow) or just no highlight (normal looking
shadow)?

-Nathan

Nieminen Mika wrote:
> 
>   If I make a cylinder like this:
> 
> cylinder { -y*2,y*2,1 material { Glass } photons { ... } }
> 
> and put it on a plane, I get a nice caustic highlight on the plane, but if I
> make the cylinder hollow this way:
> 
> difference
> { cylinder { -y*2,y*2,1 }
>   cylinder { -y*1.8,y*2.01,.8 }
>   material { Glass }
>   photons { ... }
> }
> 
> I don't get any caustic highlight anymore.
> 
>   This may:
>   a) just work fine. I don't get a caustic higlight on the plane because I
> shouldn't, because this kind of object doesn't refract the light in that way.
>   b) be a bug in the program.
>   c) be because I haven't understood how photons work.
> 
>   Which one of those answers is correct and why?
> 
> --
> main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
> ):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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