This is day 2 of my use of POV, so I won't be startled if there's just 
something I'm missing. I'm trying to render a very simple scene - 2 objects, 
3 lights - and it works exactly as I'd expect until I turn on +UA. This is 
on Vista, 2 Intel cores, way more than enough memory.

The scene is a glass sphere floating just above a slab of marble. It's 
viewed from directly above, giving the false impression of a half-sphere 
sitting on the slab. There's a light inside the sphere, and a couple outside 
it as well. The light inside refracts the way I'd hoped, giving the 
impression of an glowing sphere.

But I want the background transparent, with the eventual goal of cutting 
away at the slab's corners and having a non-rectangular object to use in 
openGL code. So I specify
+A +Q11 Output_File_Type=N  +UA

What I get is a jet black circle where the sphere was. When I display the 
.png file in other tools, I get my slab of marble with a circular hole cut 
through it. I've tried Output_File_Type=S  +UA in the vague hope of some 
sort of nonstandard .bmp file working out, and that's not better.

If I'm doing this wrong, what's the right way? Suggestions very welcome, 
thanks.


// +A +Q11 Output_File_Type=N with  +UA added,  makes the sphere act 
unexpectedly. -UA is fine.
#include "colors.inc"
#include "textures.inc"
#include "shapes.inc"
//#include "metals.inc"
#include "glass.inc"
#include "stones.inc"

global_settings {max_trace_level 12}

//look down
camera {
   location <0,0,3>
   look_at <0,0,0>
}

//stolen from samples
#declare Dist=80.0;
light_source {< 50, 25, 50> color White
     fade_distance Dist fade_power 2
   area_light <-40, 0, -40>, <40, 0, 40>, 3, 3
   adaptive 1
   jitter
}
light_source {< 50, 10,  4> color White
     fade_distance Dist fade_power 2
   area_light <-40, 0, -40>, <40, 0, 40>, 3, 3
   adaptive 1
   jitter
}

/*
sky_sphere {
    pigment {
        gradient y
        color_map {
            [0, 1  color White color White]
        }
    }
}
*/

//darkish marble
#declare T_StoneA =
texture{T_Grnt15 scale <.5, .5, .5> rotate <0, 0, -30>
        finish{ambient 0.3 diffuse 0.6  crand 0.003}
}
texture{
pigment {
 granite
  turbulence 0.05
  color_map
   {[0.0, 0.3 color White color DimGray]
    [0.3, 0.4 color DimGray color DimGray]
    [0.4, 0.6 color DimGray color Black]
    [0.6, 1.0 color DimGray color DimGrey]
   }
 }
 finish {
  crand 0.02 diffuse 0.5 phong .7}
}

//glass ball with dispersion, over slab of rock
union {
        box {<-0.5, -0.5 ,0>, <0.5, 0.5, 0.25> texture {T_StoneA} }
        sphere {<0,0,0.501>, 0.25 texture {T_Glass1} 
interior{I_Glass_Dispersion1} }
}

//light the sphere from within
light_source {< 0, 0,  0.333> color <255/255, 255/255,248/255> }

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