PRO Test

  ; Load display colors.

  LoadCT, 0
  TVLCT, 255, 255, 255, !D.Table_Size-2 ; White
  TVLCT,   0, 255,   0, !D.Table_Size-3 ; Green
  TVLCT, 255,   0,   0, !D.Table_Size-4 ; Red

  ; Create the plot in the Z-buffer to insure an 8-bit image
  ; for output to the PNG file.

  thisDev = !D.Name
  Set_Plot, 'Z', /Copy
  Erase
  Plot, findgen(11), Color=!D.Table_Size-4, $
     Background = !D.Table_Size-2, /NoData
  OPlot, findgen(11), Color=!D.Table_Size-3
  snapshot = TVRD()
  Set_Plot, thisDev

  ; Create the transparent vector. Set all values to 255 (opaque).

  t = BytArr(256) + 255B

  ; Set the background value (!D.Table_Size-2) to 0

  t[!D.Table_Size-2] = 0

  ; Create the PNG file.

  WRITE_PNG,'transparent_test.png', snapshot, r, g, b, TRANSPARENT = t

END