image/gif: Decode(All) replacing transparent background of frame with black

[jasdel]

Optimized GIFs that use a color palette combined with transparency over a colored
background when decoded will have the background color ignored. causing the formerly
transparent pixels to be black once decoded.

What does 'go version' print?
Verified against 1.2.1, 1.2.2, 1.3beta1

What steps reproduce the problem?
Decoding optimized gif with transparency over colored background. 

If possible, include a link to a program on play.golang.org.
Original Image: http://i.imgur.com/nGsYOhn.gif

1. gif.DecodeAll, then gif.EncodeAll out new file.  The frames will have black pixels
replaced where transparency should of been.
- http://play.golang.org/p/z8VwdM7jfS
- Resulting image: http://i.imgur.com/59gSWgw.gif

2. gif.Decode, then jpeg.Encode out new file. The single frame that is encoded will have
black pixels where the transparency should of been.
- http://play.golang.org/p/WPvkNmpiw4
- Resulting image: http://i.imgur.com/ZSxA5nO.jpeg

What happened?
Transparency replaced with black pixels

What should have happened instead?
transparency over colored background should be maintained.

Please provide any additional information below.

Attachments:

1. original.gif (1298657 bytes)
2. AfterDecodeAll-EncodeAll.gif (1316042 bytes)
3. AftergifDecode-jpegEncode.jpeg (14923 bytes)

[jasdel]

Comment 1:

Look like gifs have to be downloaded from the issue board before they animate.

[nigeltao]

Comment 2:

Owner changed to @nigeltao.

Status changed to Accepted.

[rui314]

Comment 3:

Is this image corrupt? ImageMagick's display command also displays broken images when I
gave the original file to it. The command even crashed when I tried to display the fifth
frame with "display 'nGsYOhn.gif[5]'".

[jasdel]

Comment 4:

Not to say nGsYOhn.gif isn't corrupt but I see similar black pixelization occurs with
http://thechive.files.wordpress.com/2014/05/tumblr_n2gyjqaxz81s8ve6co1_500.gif.
Also since you mentioned image corruption, ubuntu's default Image Viewer(3.10.2)
actually refuses to display, and ubuntu(14.04) showing thumbnails, for any of the gifs
after they have been read then written by image/gif. I don't know if this is directly
related to the black instead of transparent coloring, or not. Both Firefox and chrome
will display this image/gif written images.

Attachments:

1. tumblr_n2gyjqaxz81s8ve6co1_500.gif (976200 bytes)

[rsc]

Comment 5:

Labels changed: added release-go1.4.

[gopherbot]

Comment 6 by james.jdunne:

I forked the standard image/gif package and fixed these bugs myself but unfortunately it
required breaking the GIF struct contract (changing *image.Paletted to *image.RGBA).
This means that the writer.go code no longer compiles, so in my fork I just removed it
since I didn't need it. Perhaps this change wasn't strictly necessary but it did resolve
a few bugs I was banging my head against the wall with.
Please see my work here: https://github.com/JamesDunne/go-util/tree/master/imaging/gif
If anything, it could serve as a baseline for correcting the standard image/gif
implementation. I have tested it extensively on my own library of animated GIFs so I
know it's good.

[gopherbot]

Comment 7 by james.jdunne:

I've reverted my changes in my repository back to using `image.Paletted` instead of
`image.RGBA`. Decoding and encoding should work now, with minor changes to the `GIF`
struct contract.

[rsc]

Comment 8:

Nigel, will you get to this for 1.4? If not postpone to 1.5 please.

[gopherbot]

Comment 9:

CL https://golang.org/cl/138600043 mentions this issue.

[nigeltao]

Comment 10:

This issue was closed by revision 0be3176.

Status changed to Fixed.