Wasn't it John VanSickle who wrote:
>#declare Pedantic=true;
>
>It is really not necessary to use a special shape to model an entire
>planet.  Earth's tallest mountain (Mt. Everest) is 8848 meters high; the
>radius of the Earth is about 6366 kilometers.  On an 800x600 image
>depicting the entire planet, situated such that Everest was on the very
>edge of the disk, the height of the mountain would be about
>300*8848/6000000 = roughly .41 pixels.  Anti-aliasing will turn this
>into a bit of fuzz at the edge of the disk.
>
>To get the shadows of mountains, use a normal pattern in the texture.

However, if you attempt to render the Moon with only a normal pattern
you'll get a result that shows a clearly unrealistic terminator line.

The tip of a mountain on the equator that's half a pixel high will still
be in sunlight when it's 4 degrees (acos(800/800.5)), past the
terminator. That's a clearly visible 18 pixels. Normals don't cope with
this effect, and plunges the mountain into darkness exactly at the
terminator.

E.g. in this example, which admittedly has mountains a little higher to
exaggerate the effect, exactly half of the moon is illuminated, and
you'll see a very artificial looking straight shadow line. 

camera { location  <0, 0, -2.5> look_at <0, 0, 0>}

light_source {<-1000,0,-0> colour rgb 1}

sphere {<0,0,0> 1 pigment {rgb 0.9}
  normal {granite
    scale 0.5
    slope_map{
      [0 <1,1>]
      [1 <0,0>]
    }
  } 
}

>#declare Pedantic=false;

-- 
Mike Williams + #
Gentleman of Leisure

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