[Image-SIG] Saving GIFs with added transparency

[Stephen Purcell]

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I've been trying to do the following with the latest PIL beta release:

	1. Load a non-transparent GIF image
	2. Mark one of its colours as transparent
	3. Save as a new GIF

I tried doing this by setting the 'transparency' entry of the image's
'info' dictionary.  On saving, this information was ignored.  Maybe this
was the wrong way to do it, but if it's not, I have a patch to offer for
'GifImagePlugin.py'.

It involves the addition of the following lines before the 'return' of
the 'getheader' function:

    if im.info.has_key('transparency'):
        transparentIndex = im.info['transparency']
        s.append('!' + chr(0xf9) + chr(4) + chr(1) + chr(0) + chr(0) +
                 chr(transparentIndex) + chr(0))

Perhaps this is useful to someone else out there.

Regards,


-Steve P.

BTW, I based my patch/hack on:

	http://www.acme.com/resources/classes/Acme/JPM/Encoders/GifEncoder.java
--
Steve Purcell
Software Design Engineer
WebSentric Software AG
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