Please forgive the noob nature of this post, but I'd really like to
understand what is going on with HDRI-lit renderings.  In playing around
toward that end I took the standard MLPOV hdri.pov scene and put a disc in
like this:

disc {
  <0, 0, 0> y, 8.0, 0
  pigment { color rgb 1 }
  finish { ambient 0 diffuse 1 }
}

the resulting rendering is attached below as HDRI_disc.jpg  I wanted to have
more defined shadows, and so asked Alan Holding about the settings he used
in his very fine skeleton.jpg image ( located here:
http://news.povray.org/povray.binaries.images/29668/ )

I lifted his globals and stuck them into the HDRI_disc scene; here they are:

global_settings {
  radiosity {
    pretrace_start 0.16
    pretrace_end   0.01
    count 300
    nearest_count 3
    error_bound 0.1
    recursion_limit 1
    low_error_factor 1
    gray_threshold 0.0
    minimum_reuse 0.015
    brightness 1  // was 1.2 in Alan's scene

    adc_bailout 0.01/2
    normal on
  }
}

and the result is attached below as HDRI_disc_lichens.jpg  Strange dark
spots...photons?

Finally, by changing the "recursion limit" from 1 to 3 bright spots that
look an awful lot like refractive caustics appear.  This one is attached as
HDRI_lichens_caustics.jpg.

The Questions:
1) Based on the orignal HDRI_disc.pov, what I settings do I need to change
just to get smoother, more believable shadows?  Setting low_error_factor
lower smooths the shadows, but doesn't touch the spots...
2) What *are* those dark spots?  How do I make them either go away or
resolve into something more attractive?
3) Are those really caustics?  How can I eliminate them?  How can I refine
them?
4) The contrast in the upper right of the middle sphere is exceptionally low
in the two later images--any idea why?

Thanks for your patience--RTFM responses completely welcome, especially if
they come with section numbers!

-Chad

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Attachments:
Download 'HDRI_disc_lichens_caustics.jpg' (36 KB) Download 'HDRI_disc_lichens.jpg' (32 KB) Download 'HDRI_disc.jpg' (24 KB)

Preview of image 'HDRI_disc_lichens_caustics.jpg'

Preview of image 'HDRI_disc_lichens.jpg'

Preview of image 'HDRI_disc.jpg'