Reading this link

http://news.povray.org/povray.binaries.images/thread/%3Cweb.45dbe0082f42fd2caa2e049f0%40news.povray.org%3E/

And remember similar problems I have in proper experience

I think, only for a few points we need rendering entire image at slow speed
rate (ie applying higher max_trace_level or some other stuffs)

I suggest a small mod for POV, wend rendering a image with multi
frame_number, give the possibility of keep the pixels of the prior
frame_number, and rendering only the pixels with "zero render" (black dots)
in the new frame, this it makes possible to make images with multipass
render.

Activate this feature with a "+multipass" switch or so combined with
animations swiches, but the target is a static frame, not animation, we
reuse the animations routines for multipassing

example:

#if (frame_number=0) // first fast pass
  global_settings {
    max_trace_levels 5 // fast render with many black areas
  }
#end
#if (frame_number=1) // second pass
  global_settings {
    max_trace_levels 50 // render with some black dots
  }
#end
#if (frame_number=2) // final render
  global_settings {
    max_trace_levels 500 // slow render without black dots
  }
#end

....


The idea is save render time calculating a high percentage of the image at
"low trace rate", then recalculate the rays at more trace level for pixels
undefined in prior pass.

Sorry for my english

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