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List:       graphicsmagick-bugs
Subject:    Re: [GM-bugs] Problem resizing animated gif with transparency
From:       Bob Friesenhahn <bfriesen () simple ! dallas ! tx ! us>
Date:       2010-02-16 17:06:14
Message-ID: alpine.GSO.2.01.1002161049350.25017 () freddy ! simplesystems ! org
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On Tue, 16 Feb 2010, Dan Tsohar wrote:
> 
> If I take the following picture:
> http://dl.dropbox.com/u/574561/tiger.gif
> 
> And resize it with the command:
> "gm convert tiger.gif -coalesce -resize 80x80+0+0% out.gif"
> 
> I get the following result:
> http://dl.dropbox.com/u/574561/out.gif
> 
> (Transparency is gone and turned black)
> 
> I have also seen that transparency sometimes turns to grey.

The problem with simple 'transparency' when the base image does not 
start with a color is that the resulting color depends on whatever 
background color or image the animation is played on.  Undefined 
pixels still need to have a color even if you can't see it.  The 
default background color is black.  If you expect a background color 
of white, then use

gm convert tiger.gif -background white -coalesce -resize 80x80+0+0% out.gif

It does seem like a bug that -coalesce does not preserve binary 
transparency and I notice that recent ImageMagick does better in this 
regard.  If you can submit a formal bug report via the GraphicsMagick 
SourceForge bug tracker, then I will work to fix this.

Bob
--
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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