[Digikam-devel] better metadata support : added exiv2 depency ?

[Gilles Caulier]

Hi all,

To support better metadata, especially Exif and IPTC, we need to use a new 
lib. Exiv2 seem to be the better choose actually, to manipulate EXIF and IPTC 
metadata from image file.

http://home.arcor.de/ahuggel/exiv2/

 Actually, Exiv2 support :

- Exif data from JPEG files with a large suppport of special marker notes from 
constructors.
- IPTC data from JPEG files
- Exif data from CRW RW file (Canon tm)

In Digikam::DImg, metadata are just stored like bytes array (DImg is a 
metadata container, no a metadata parser). It's easy to move this metadata  
from an original image to a target image. Actually, JPEG<->PNG Exif/IPTC 
metadata preservation is complete. If you convert a JPEG file including Exif 
data to a PNG file using Image Editor (and vis-versa), you can see the Exif 
metadata including to PNG file with Exif tab from main interface. Look here :

http://digikam3rdparty.free.fr/Screenshots/digikam0.9.0-alpha4.png

Exif/IPTC are store to PNG using standard ImageMagick Raw profile tag...

But we need more metadata preservation :

- RAW Exif/IPTC extraction 
==> http://www.rags-int-inc.com/PhotoTechStuff/RawStandards/RawSummary.html
- TIFF/EP Exif/IPTC manipulation.
- DNG Exif/IPTC manipulation (DNG==TIFF/EP like file. We can plan this file 
format for the future).

Exiv2 will support others image file format in the future, especially more RAW 
files and TIFF/EP files.

In all case, Extracting RAW Exif metadata is possible using dcraw::parse.c 
method. This way is used in neftags2jpeg program dedicaced to extract Exif 
from NEF file :

http://pages.quicksilver.net.nz/pepe/d70/Nikon_D70_on_Linux.html#exif

neftags2jpeg program using Exiv2 to store like a byte array Exif informations 
(:=)))...

In digiKam, this way can be used to make a RAW file parser to extract Exif 
data until that Exiv2 support better RAW file format. To get a complete list 
of RAW file tags, we can get any inspiration from the great exifprobe 
program :

http://www.virtual-cafe.com/~dhh/tools.d/exifprobe.d/exifprobe.html

If we using Exiv2 for 0.9.0, we will using 2 lib about Exif : Exiv2 and 
libkexif (using libexif). Sure in the future, all code depending of libkexif 
must be ported to Exiv2, but this can do later 0.9.0... 

Note : Exiv2 Exif bytes array data is compatible with libkexif. We can extract 
exif metadata using Exiv2 and displaying that using libkexif without 
problem...

Any suggestions welcome.

Gilles Caulier