[Image-SIG] feature request: 8-bit alpha transparency PNG;s

[Tibor Arpas]

Hi,
I'm using little attached program to convert my RGB and RGBA PNG's to png8.
But with RGBA pngs it's limited because it uses only GIF-like 1-bit
transparency. Seems that the alpha transparency in 8-bit (and 4-bit) PNG's
is little known or supported. But the main browsers seem to support it
including IE 6.0. One opensource utility which supports writing these is:
http://www.libpng.org/pub/png/apps/pngquant.html .

It would be greate if PIL would also do that.

Have a nice day,
Tibor



import Image
import random
import optparse
import sys

def quantize_and_invert(alpha):
    if alpha <= 64: # You may change the threshold value into something
other.
        return 255
    return 0

im = Image.open(sys.argv[1])

if im.mode=='RGBA':
    print "Warning, converting alpha transparency to 1-bit transparency"
    al = im.split()[3]
    al = Image.eval(al, quantize_and_invert)
    colors=255
else:
    al = None
    colors=256
if im.mode!='RGB':
    im = im.convert('RGB')
im=im.convert('P', palette=Image.ADAPTIVE, colors=colors)
if al: im.paste(255,None,al)
im.save('out.png',optimize=1, transparency=255)
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