Metadata conversion enhancements
The new XMP specification has more details on metadata conversions. The implementation in Exiv2 is mostly correct, but some parts are missing or need some improvement.
Exiv2::syncExifWithXmp is insufficient, the algorithms on page 64 are more complex. It needs access to all kinds of metadata - XMP, Exif, IPTC and also to the IPTC digest which is stored in Photoshop Image Resources (I suppose that this will have to wait for the unified metadata). Maybe it also makes sense to hardcode the conversion to readMetadata and writeMetadata as it is described on page 55 of the specification, because otherwise there may be a data loss if an image is edited first by Exiv2 and then by some Adobe software. Another reason is that the conversion rules for each format may be slightly different.
Reported by Vladimir Nadvornik