Error in alpha channel when decoding GIF.
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=3536
Reported by Anders Johansson firstname.lastname@example.org
For some transparent gif images the decoder (or encoder?) seam to make an error in the alpha channel.
- Take the attached image (org.gif) 9x4 pixels with 4 grey pixels and the rest of it transparent.
2. Decode and write the alpha channel as alpha.gif. Stdio.write_file("alpha.gif",Image.GIF.encode(Image._decode(Stdio.read_file("org.gif"))->alpha));
- The produced image has an extra white pixel that should not be there.
If the original image is slightly modified, like adding a grey pixel some where, the error suddenly does not show up. It is not limited to this image (org.gif) but has been observed in other images as well.