[Digikam-users] jpeg compression

Gilles Caulier caulier.gilles at gmail.com
Sat Jun 30 17:47:01 BST 2007

2007/6/30, Thorsten Schnebeck <thorsten.schnebeck at gmx.net>:
> Am Freitag 29 Juni 2007 schrieb Hugo Palma:
> > This discussion has really helped me understand some more things about
> file
> > formats.
> >
> > So if i'm only taking into the account picture quality and flexibility
> and
> > don't care about metadata tiff is better than png for editing right ?
> >
> No :-)
> TIFF is a very flexible but also complicate container for image data. This
> flexibility is the reason that many raw formats are based on TIFF internal
> directory structures but this leads also to the problem that a library
> like
> libtiff can not cope with every special vendor solution.
> PNG is simple and has everything you need for storing an image.
> To keep it simple in PNG you can e.g. not handle CYMK (print) or real
> number
> based (HDR) image data nor handle multiple images in one file.
> But being simple PNG often has better compression ratio than standard
> To make it short: Only looking at lossless storing images to a file TIFF
> and
> PNG are equal. But digiKam has better PNG support compared to TIFF as the
> used PNG libraries are simply better than the free and open tiff libs.
> So, for long time storage today PNG is the way to go as it preserves the
> metadata if the image.
> This may change in the future when TIFF-based DNG-format gets better
> support. ;-)

Ah the famous DNG format...

To have studied it indeep, i can said than :

DNG == TIFF + new tag.
DNG is now an ISO standard (i have seen a message about it into libtiff ML)
DNG is limited to store image data in 2 way : the first is a pseudo lossless
JPEG compression supporting 16 bits color depth (in fact JPEG algorith with
compression level set to 100, but supporting 16 bits/color/pixels). The
compression ratio is good, but it still JPEG stuff... This way is used by
all camera which support DNG as well, and not the second way...
DNG support only a _real_ lossless image data storage, which is in fact the
linear 16 raw image data. DNG do not support the famous Adobe Deflate
compression algorithm provide by TIFF file format. It really stupid... This
one give the equivalent compression ratio results of PNG. If you try to use
DNG converter from Adobe, and you use the RAW linear storage of image data,
the DNG file will be huge !

For me DNG do not give any advantages against PNG...


>   Thorsten
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