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

[Wayne Watson]

I a bit tardy as explained by my Spam msg just posted.  This is a very 
difficult list to post to for several reasons just mentioned in a msg I 
posted a few minutes ago.

Interesting. Thanks. I did get what I was looking for (just posted a few 
mins ago), but this has some good twists.

Denilson Figueiredo de Sá wrote:
> 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
>
>
>
> I'm not subscribed to this mail list, so please add me to CC when 
> replying to this message.
> Thanks.
>

-- 

           Wayne Watson (Watson Adventures, Prop., Nevada City, CA)

             (121.01 Deg. W, 39.26 Deg. N) GMT-8 hr std. time)****

          "In arithemtic as in politics, the importance of one is
           determined by the number of zeros behind it." -- Anon