[Image-SIG] More PNG battles

[Ray Pasco]

OK, now I'm getting strung out.  I'm creating and saving a RGBA PNG,
but when it's converted to a P mode not only is the image damaged, but 
the encoder
*insists* on adding a value of 10 to the given palette index used to set 
transparency:
Does anyone know what is going on ?
(XP using 1.1.5b1 so I can use the get/setpalette methods)
--------------------------------------------
import Image

# (mode, size, color=r,g,b,a), no transparency
imgRgba = Image.new ('RGBA', (100, 100), (0, 85, 185,  255))         # 
dark blue

imgRgba.paste ( (185,  85,   0,  255), ( 10,  10,    90,  90) )      # 
dark red

# paste an "alpha-tized" color in the center of the image
# (r, g, b, a),  (L, Top, R, Bot)
imgRgba.paste ( (  0,   0,   0,    0), ( 20,  20,    80,  80) )      # 
black center

imgRgba.save ('TRANSPARENT.RGBA.png')

# Now convert to a palettized format and save to a new file
imgPlt = imgRgba.convert ('P')

# I happen to know that the palette index for the transparency color is 
index 0.
# ???  Why must 10 be subtracted from the transparency color index value
#      for the encoder to get it right ???
imgPlt.save ('TRANS.PLT.png', transparency=0-10)