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The most notable change in GIMP 2.5 happened under the hood. The color tools in GIMP have been ported to GEGL. This does not yet have much impact on the user experience but it is a first and important step forward. With full GEGL integration GIMP will finally get support for higher color depths, more colorspaces and eventually non-destructive editing.
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Developers, artists and user interface designers from all over the world worked together to make GIMP more powerful and easier to use than ever. The changes from version 2.2 are too numerous to list here, please check the Release Notes to get an overview. If you want to compile GIMP 2.4 yourself, you can grab the tarball from ftp.gimp.org or one of its mirrors. Binary packages for the various supported platforms are becoming available as we speak. Our Downloads section should help you to locate the package for your system.