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Like i have previously said in this post, for the next major release of Exiv2, the Tiff metadata writting mode will be available.

Due to a lack of Exif metadata support in libtiff, we cannot use this library to backup all metadata from a converted RAW file. We cannot use this library to play with the metadata section only without touching the whole image data. Of course we use libtiff in editor to convert image to Tiff, but that's all.

Andreas Huggel, Exiv2 lead developper, has been working hard to re-design the whole architecture of the core library to be more flexible and maintainable. Now, with the current implementation from svn (0.17.1), Exiv2 can write Exif/Iptc/Xmp metadata to TIFF files, without touching image data section... like with JPEG files in fact. Note that old Exiv2 release can only Read and extract these metadata from Tiff files.

Since one week ago, i have followed the Exiv2 developements and updated the digiKam Exiv2 interface (libkexiv2) to be ready to play with Tiff files on the fly. KDE3 and KDE4 code have been patched in this way. See below a demo of digiKam 0.10.0 playing with a Tiff file to set captions and tags from album interface:

Another good news is, that we will be able to handle metadata from DNG files. Like this RAW format is fully based on TIFF/EP ISO standard (it's an fully documented TIFF extension from Adobe in fact), it is easy to store digiKam data in your Digital NeGative photos. For other RAW formats based on Tiff, like NEF for example, we need more time to look if we can play like this without to break image files.