I abandoned transparency, since I actually don’t want to see through anything. I am using uncolored to get the fade. It works great with a picture that is made with fill and draw, but it doesn’t work with shading. The example below has a flat disk and a round ball. The disk looks fine, but the shaded ball is not surviving the application of inverted / uncolored.


\startMPinclusions
  picture disk, ball;
  disk := image(
    fill fullcircle scaled 1cm withcolor .4white;
    draw fullcircle scaled 1cm withpen pencircle scaled 0.8pt ;);
  ball := image(
    fill fullcircle scaled 1cm
      withshademethod "circular"
      withshadecenter (.25,.25)
      withshadecolors (.8white, black);
    draw fullcircle scaled 1cm withpen pencircle scaled 0.8pt ;);
\stopMPinclusions

\startMPpage
   draw inverted (disk uncolored .1white) ;
   draw inverted (disk uncolored .2white) shifted (0.25cm,0) ;
   draw inverted (disk uncolored .5white) shifted (1cm,0) ;
   draw disk shifted (2.25cm,0) ;

   draw inverted (ball uncolored .1white) shifted (0, -2cm) ;
   draw inverted (ball uncolored .2white) shifted (0.25cm, -2cm) ;
   draw inverted (ball uncolored .5white) shifted (1cm, -2cm) ;
   draw ball shifted (2.25cm, -2cm) ;
\stopMPpage