There's a problem in the sky_sphere tutorial which I think should be
corrected. I'm talking about the "Adding some clouds" section.

  Ok, the example is good in the sense that it demonstrates how you can
use multiple pigments in a sky sphere. However, the generated clouds don't
look good at all, and I have seen several times how it teaches a bad habit
to people (ie. to put clouds in the sky sphere).

  The reason why the clouds look bad is that the sky sphere is like a
sphere which is centered at the camera, while the real sky is like a sphere
which is centered more than 6000 km below the camera.

  The easiest way of getting realistic-looking clouds is, of course, using
a plane. The sky_sphere can be used for coloring the sky, like this:

camera { location <0,-1,-10> look_at 0 }
light_source { <1000,2000,-200>, 1 }
plane { y,-2 pigment { green 1 } }
sphere { 0,1.2 pigment { rgb 0 } finish { reflection 1 } }

sky_sphere
{ pigment
  { gradient y color_map
    { [0 rgb <.8,.9,1>]
      [.2 rgb <.2,.5,1>]
      [1 blue 1]
    }
  }
}

plane
{ -y,-100
  pigment
  { bozo color_map
    { [0 rgb .5]
      [.1 rgb 1 filter .2]
      [.5 rgbf 1]
    }
    turbulence .8
    scale 100
  }
  finish { ambient 1 }
}


  I wonder if the tutorial should be changed a bit so that it doesn't teach
the bad habit of putting the clouds in the sky_sphere.

-- 
plane{-x+y,-1pigment{bozo color_map{[0rgb x][1rgb x+y]}turbulence 1}}
sphere{0,2pigment{rgbt 1}interior{media{emission 1density{spherical
density_map{[0rgb 0][.5rgb<1,.5>][1rgb 1]}turbulence.9}}}scale
<1,1,3>hollow}text{ttf"timrom""Warp".1,0translate<-1,-.1,2>}//  - Warp -

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