Abe wrote:
> Mike Kost <con### [at] povraytashcorpnet> wrote:
>> Sebastian H. wrote:
>>
>> > Mike Kost wrote:
>> >> http://povray.tashcorp.net
>> >
>> > Your sky sphere tutorial is really helpful.
>> > I'll try it out on the weekend. Good work.
>> >
>> > Sebastian
>>
>> Thanks - I'm glad it's helpful.
>>
>> I've started using a series of Python scripts to automate the whole
>> process, so what I'm doing now has advanced beyond my tutorial. Something
>> new to write-up when it settles down, I guess.
>>
>> Mike
> 
> How about letting POV do the work? I was able to do a reasonable facsimile
> of the sampling process in POV sdl.
> 
> -- SNIP --
> -- /SNIP --
> 
> Abe

A very elegant solution. This will be fun to try out.

In the past, I have had poor experiences with auto-generated color_maps. All
the results I've gotten are prone to having banding problems. Some of it is
due to unlucky pixel distributions, but it becomes difficult to get good
results over a wide range of images without lots of trial and error (and by
then, I can do it faster by hand sampling). Over-sample too much, and it
picks up high frequency noise that creates banding. Under-sample and the
colors look cheesy (and again has visual bands). 

I have not tried averaging along the X axis yet, so I'm curious to try your
Povray code out, but I think the problem goes a little deeper: real skies
and linear interpolation don't mix. Sunsets do not have discontinuous first
derivatives, but linearly interpolated color maps do. I believe that the
visual problems in rendered skies come from large changes in slope when
passing a control point in the color map. Averaging and using weighted
color map entries distribution work to cover up the problem. The real
solution, in my (by now not so humble) opinion, is to extend color maps to
allow cubic or b-spline interpolation. A higher order interpolation will
match the original sunset image with just 5 - 10 point color map entries. 

Perhaps it's time to solicit the Povray coders for an enhancement...

All the best,

Mike
--
http://povray.tashcorp.net/

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Mike, thanks for tutorials.
A axample:

sky_sphere {

  pigment {
    gradient y
    color_map {
        [  0/269 color rgb <120/255, 79/255, 51/255>]
        [  1/269 color rgb <141/255, 83/255, 46/255>]
        [  2/269 color rgb <177/255, 86/255, 41/255>]
        [  3/269 color rgb <235/255,128/255, 72/255>]
        [  5/269 color rgb <255/255,159/255, 72/255>]
        [  8/269 color rgb <255/255,203/255, 94/255>] // New
        [ 10/269 color rgb <255/255,218/255,112/255>]
        [ 13/269 color rgb <255/255,233/255,148/255>] // New
        [ 15/269 color rgb <251/255,241/255,172/255>] // New
        [ 20/269 color rgb <255/255,246/255,203/255>]
        [ 30/269 color rgb <255/255,240/255,219/255>]
        [ 40/269 color rgb <236/255,223/255,214/255>]
        [ 50/269 color rgb <205/255,204/255,212/255>]
        [ 55/269 color rgb <185/255,190/255,209/255>] // New
        [ 60/269 color rgb <166/255,176/255,201/255>]
        [ 65/269 color rgb <149/255,163/255,190/255>] // New
        [ 70/269 color rgb <129/255,149/255,182/255>]
        [ 80/269 color rgb <103/255,127/255,171/255>]
        [ 90/269 color rgb < 79/255,110/255,154/255>]
        [100/269 color rgb < 66/255, 97/255,143/255>]
        [110/269 color rgb < 52/255, 84/255,131/255>]
        [120/269 color rgb < 47/255, 75/255,122/255>]
        [140/269 color rgb < 37/255, 60/255,102/255>]
        [160/269 color rgb < 32/255, 51/255, 84/255>]
        [180/269 color rgb < 27/255, 42/255, 71/255>]
        [200/269 color rgb < 25/255, 36/255, 58/255>]
        [220/269 color rgb < 22/255, 31/255, 48/255>]
        [240/269 color rgb < 18/255, 27/255, 42/255>]
        [260/269 color rgb < 15/255, 21/255, 33/255>]
        [269/269 color rgb < 15/255, 21/255, 33/255>]

    }
    scale 0.5
  }


    pigment {
      bozo
      turbulence 0.65
      octaves 6
      omega 0.7
      lambda 2
      color_map {
          [0.0 0.1 color rgb <0.85, 0.85, 0.85>
                   color rgb <0.75, 0.75, 0.75>]
          [0.1 0.5 color rgb <0.75, 0.75, 0.75>
                   color rgbt <1, 1, 1, 1>]
          [0.5 1.0 color rgbt <1, 1, 1, 1>
                   color rgbt <1, 1, 1, 1>]
      }
      scale <0.2, 0.5, 0.2>


    scale 0.6
    rotate -90*x
    rotate -90*z
    }

}




plane {
  y, 0
  texture {
    pigment {
      checker
      color rgb <0.0, 0.1, 0.1>
      color rgb <0.0, 0.0, 0.0>
      scale 2
    }
  }

}

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