Hi,

The new XMP specification[1] 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.

I think that these issues should be added to the TODO list.

Vladimir

[1] http://www.adobe.com/devnet/xmp/pdfs/XMPSpecificationPart3.pdf