image/gif: decoding of some GIFs fails with error `gif: invalid pixel value` yet they open alright.

[odeke-em]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go version devel +ea306ae Fri Apr 1 04:13:44 2016 +0000 darwin/amd64
2. What operating system and processor architecture are you using (go env)?
   GOARCH="amd64"
   GOBIN="/Users/emmanuelodeke/go/bin"
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="darwin"
   GOOS="darwin"
   GOPATH="/Users/emmanuelodeke/go"
   GORACE=""
   GOROOT="/Users/emmanuelodeke/go/src/go.googlesource.com/go"
   GOTOOLDIR="/Users/emmanuelodeke/go/src/go.googlesource.com/go/pkg/tool/darwin_amd64"
   CC="clang"
   GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -gno-record-gcc-switches -fno-common"
   CXX="clang++"
   CGO_ENABLED="1"
3. What did you do?
   If possible, provide a recipe for reproducing the error.
   A complete runnable program is good http://play.golang.org/p/_8PzZ6RffI.
   A link on play.golang.org is best.
   Tried to decode a gif https://brobible.files.wordpress.com/2016/03/joker1.gif but it failed with error gif: invalid pixel value.
   Complete runnable program here http://play.golang.org/p/_8PzZ6RffI
4. What did you expect to see?
   Successful GIF decoding
5. What did you see instead?
   error gif: invalid pixel value

[nigeltao]

It is indeed an invalid GIF image: the frame's palette only has 128 entries (i.e. valid values are from 0 to 127 inclusive), but the 'transparent index' is given as 128, which is outside the range [0, 127]. The GIF spec at https://www.w3.org/Graphics/GIF/spec-gif89a.txt says in section 22, Table Based Image Data, "Each index must be within the range of the size of the active color table".

Imagemagick similarly complains:

$ convert joker1.gif x.png
convert.im6: invalid colormap index `joker1.gif' @ error/colormap-private.h/ConstrainColormapIndex/34.

Nonetheless, it treats it as a warning, not an error. Furthemore, joker1.gif is viewable (with no visible error message) in both Firefox and Google Chrome, and by Eye-of-Gnome (which uses gdk-pixbuf). I suppose that it's a case of reality trumping the standard, and we should 'fix' this for Go.

[gopherbot]

CL https://golang.org/cl/22546 mentions this issue.

[odeke-em]

Thank you @nigeltao, this helps me quite a lot as I was encountering a lot of such gifs on the web and needed to import them