XML is very well suited to describe the structure of a multi-layered, multi-framed image/animation and it can be used perfectly to embed meta information as well as vector layers, paths and the like. XML namespaces make it easy to add application-specific extensions later.
We discussed to bundle one XML file with a set of files that store the image data. These files would preferably be known image formats or perhaps even strict subsets of known image formats.
I would suggest that these files are then put together in an ar archive. Such an uncompressed archive has the advantage that the XML metadata can easily be extracted.
You just described a JAR file, which is not unlike the OpenOffice file format ;).
A JAR is a special type of ZIP archive, which contains one or more "data files" along with an XML "manifest" about the contents. I've worked on a number of projects (both commercial and "open") that have used such a format - it works quite nicely and is compatible with existing tools and technologies. Always better than reinventing the wheel!!
I think the approach of a JAR-like file for the future GIMP (and possibly CinePaint) file format is an excellent choice and allows for many avenues of expansion.
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