[Digikam-users] jpeg compression

Gilles Caulier caulier.gilles at gmail.com
Fri Jun 29 12:43:51 CEST 2007


2007/6/29, Liquiddoom <liquidgarnet at gmail.com>:
>
> Actually, I believe it is the other way around. PNG destroys EXIF
> metadata,


Completly wrong !!! PNG support Exif, makernote, Iptc, Xmp. In digiKam it's
fully supported as well

Just try a simple test using ImageMagick with a JPEG file which have Exif
and IPTC for ex :

# convert foo.jpg foo.png
# convert foo.png foo2.jpg

open foo.jpg, foo.png and foo2.jpg in showfoto and look like Exif and iptc
still here !

This method to embeded metadata in PNG come from ImageMagick. I have
personally backported this code in digiKam core !

I'm use PNG everywhere and my pictures are always taken in RAW. I convert
all RAW file to PNG when i work. I never use TIFF because ...


while TIFF keeps it intact.


False... under Linux !  Of course, TIFF norm ask than tiff file format must
preserve metadata, but this is not the reality.

All open source software (UFRAW, ImageMagick, Gimp, etc) use Libtiff to
handle tiff files. With this library, XMP and IPTC can be preserved, but
never Exif and Makernote, duing a big limitation in libtiff implementation.

In fact try this second ex. :

# convert foo.jpg foo.tiff
# convert foo.tiff foo4.jpg

... and look like Exif and makernote have disappear ! It's  a shame for a
photograph !!! This is why i _NEVER_ use TIFF file format.


As for which to use, it really depends. I tend to use TIFF because it can
> support other color spaces and greater bit depth (as I shoot RAW).


PNG support 16 bits color depth and ICC color profiles. You can work on
Adobe RGB and Wide Gamut RGB to convert you RAW pictures as well

This situation around TIFF will change when  Exiv2 will support Tiff file
acces in writting mode. Like this we will delegate to Exiv2 all metadata
management for TIFF file, and Exif/Makernote will  be preserved.

But at this moment, PNG is the best solution to work with digiKam under
Linux.

Gilles Caulier
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