[Image-SIG] Support for semi-transparent in PNG palette

[Denilson Figueiredo de Sá]

Hello, I think I found a bug/limitation in PIL.

If I have a PNG with palette, and with semi-transparent colors at that  
palette, then PIL will ignore the semi-transparent information. In other  
words, it will always load the palette as RGB, instead of RGBA.

Let's reproduce the bug, step-by-step:

1. Create an RGBA image using Gimp. Or, even easier, download this one I  
created:
http://denilsonsa.selfip.org/~denilson/pil/rgba.png

2. Convert that image to RGBA palette. What I've done was:

cp pil_rgba.png pil_palette.png
optipng -o7 pil_palette.png

You can download optipng at http://optipng.sourceforge.net/
Or you can download the converted image here:
http://denilsonsa.selfip.org/~denilson/pil/palette.png

3. Try opening it using PIL, convert back to RGBA, and save as PNG:

 from PIL import Image
f = Image.open("pil_palette.png")
print f.palette.mode  # This will print RGB, but should be RGBA
f = f.convert("RGBA")
f.save("pil_saved.png")

Or you can download it from here:
http://denilsonsa.selfip.org/~denilson/pil/saved.png


== What did I expect? ==

I expected that PIL would correctly load RGBA palette, and thus correctly  
preserve the semi-transparent information. I expected that the  
pil_saved.png file still looked the same as pil_rgba.png.


== What actually happens? ==

PIL throws away the Alpha channel, and the final pil_rgba.png image has  
all semi-transparent pixels converted to opaque.


== Extra information ==

The "feh" image viewer correctly supports RGBA palettes. Also, I think it  
is a better replacement for xv, for use in image.show()
http://www.linuxbrit.co.uk/feh

Both Firefox and Opera browsers correctly support RGBA palettes.

Surprisingly, Gimp does not support RGBA palettes, and will convert  
semi-transparent colors to full transparency.


== Similar messages ==

While searching, I found these messages about the same subject:
25 Jan 2009 -  
http://www.mail-archive.com/image-sig@python.org/msg02275.html
15 Nov 2008 -  
http://www.mail-archive.com/image-sig@python.org/msg02182.html
27 Aug 2008 -  
http://www.mail-archive.com/image-sig@python.org/msg02065.html
26 Jan 2005 -  
http://www.mail-archive.com/image-sig@python.org/msg00069.html



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Thanks.

-- 
Denilson Figueiredo de Sá
Rio de Janeiro - Brasil