The documentation reads, "The sky sphere can contain several pigment 
layers..." But if I use more than one transparent pigment on a 
sky_sphere, the transparent areas show black instead of the first 
pigment layer.

In all three images, a blue gradient is used as the first pigment layer. 
  In cases 1 and 2, one partially transparent pigment is layered over 
the blue gradient, and the result is as I expect.  In the third case, 2 
partially transparent pigments are layered over the blue gradient, and 
the blue does not show at all.

The render stats indicate that only one trace level is used.

As a sanity check, I applied the same pigments to a sphere at the lower 
left.  In all cases, the sphere renders as I expect.

Am I missing something, or is this a bug?

----------[BEGIN CODE]----------
// +KFF3
#version 3.6;

#include "colors.inc"
#include "skies.inc"

global_settings { assumed_gamma 1 max_trace_level 5 }

camera
{ ultra_wide_angle
   location <0, 0.2, 0>
   look_at <0, 5.2, 5>
   angle 135
}

plane { y, 0 pigment { rgb 1 } }

#declare P1 = pigment
{ gradient y color_map
   { [0 rgb <0.5, 0.75, 1.0>]
     [1 rgb <0.1, 0.15, 0.2>]
   }
}

#declare P2 = pigment
{ wood color_map
   { [0.0 rgb <1, 0.8, 0.6> transmit 0]
     [0.7 rgb <1, 0.8, 0.6> transmit 1]
   }
   ramp_wave
}

sky_sphere
{ pigment { P1 }
   #switch (frame_number)
     #case (1)
       pigment { P2 }
       #break
     #case (2)
       pigment { P_Cloud1 scale 0.1 }
       #break
     #case (3)
       pigment { P2 }
       pigment { P_Cloud1 scale 0.1 }
       #break
   #end
}

#default { finish { ambient 1 diffuse 0 } }

sphere
{ 0, 1
   texture { pigment { P1 } }
   #switch (frame_number)
     #case (1)
       texture { pigment { P2 } }
       #break
     #case (2)
       texture { pigment { P_Cloud1 scale 0.1 } }
       #break
     #case (3)
       texture { pigment { P2 } }
       texture { pigment { P_Cloud1 scale 0.1 } }
       #break
   #end
   translate 2.5 * z
   rotate -35 * y
}
-----------[END CODE]-----------

The results are the same in POV-Ray versions 3.6.1, 3.7, and 
3.7.1-alpha.8150025.

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Attachments:
Download 'sky_layers1.jpg' (9 KB) Download 'sky_layers2.jpg' (20 KB) Download 'sky_layers3.jpg' (25 KB)

Preview of image 'sky_layers1.jpg'

Preview of image 'sky_layers2.jpg'

Preview of image 'sky_layers3.jpg'