image/gif: encoder does not honor image bounds.

[gopherbot]

by dan.pupius:

1. Open a image using gif.DecodeAll
2. Replace frame(s) with a `SubImage`, e.g. to make a square crop
3. Save image using gif.EncodeAll

What is the expected output?
Saved image should correctly show cropped region

What do you see instead?
Resultant image is incorrectly offset, see lucha.out.gif vs lucha.out.jpg

Which operating system are you using?
OSX

Which version are you using?  (run 'go version')
$ go version
go version devel +560ca6cc94b5 Sun Oct 20 18:29:15 2013 -0700 darwin/amd64

Please provide any additional information below.
I think the sImageDescriptor block in gif/writer.go needs to have the X/Y values
adjusted by the Min bounds of the first frame.

Attachments:

1. gifs.go (966 bytes)
2. lucha.gif (7598 bytes)
3. lucha.out.jpg (17750 bytes)
4. lucha.out.gif (6843 bytes)

[robpike]

Comment 1:

Labels changed: added priority-later, packagebug, removed priority-triage.

Owner changed to @nigeltao.

Status changed to Accepted.

[gopherbot]

Comment 2 by dan.pupius:

Here's a workaround, should anyone else hit this issue.  You have to create a new zero
based image for each frame and instead of using SubImage copy the crop region
35c36,41
<        im.Image[index] = frame.SubImage(dstBounds).(*image.Paletted)
---
>        // Creates a new image bounded at (0,0) and instead of creating a SubImage,
>        // copy the cropped region into a new image.
>        b := image.Rect(0, 0, dstBounds.Dx(), dstBounds.Dy())
>        pm := image.NewPaletted(b, frame.Palette)
>        draw.Draw(pm, b, frame, dstBounds.Min, draw.Src)
>        im.Image[index] = pm

[nigeltao]

Comment 3:

I'll look at this after the 1.2 release. Feel free to ping this bug (after then) if I've
forgotten.

[rsc]

Comment 4:

Labels changed: added go1.3maybe.

[rsc]

Comment 5:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 6:

Labels changed: added repo-main.