Our metadata library exiv2 only registers the most commonly used xmp
namespaces. Other namespaces need to be explicitly registered. We did not
read or try to store these namespaces, which caused a lot of warnings about
"No namespace info available for XMP prefix '...' and then we could not
process that metadata or save/export those tags.
We had to wait for gexiv2 support for registering/reading namespaces, but
that was added in gexiv2 version 0.12.2 (and the "try" version in 0.14.0).
When reading xmp metadata we process all namespaces and add them to our
metadata xml when we haven't seen them before in the same image.
A GHashTable is used to keep track of the prefixes we have seen before.
The new namespace xml tag is skipped in older GIMP versions, but will be
used now to add the namespaces when exporting images with xmp metadata.
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