[Digikam-users] keeping pictures in PNG instead of JPG format

Jean-François Rabasse jean-francois.rabasse at wanadoo.fr
Wed Feb 20 18:17:57 GMT 2013

Hi Marco,

On Wed, 20 Feb 2013, M. Fioretti wrote:

>> Digikam uses the libexiv2 library for metadata handling. So,
>> supported files types are/should be the types supported by exiv2.
>> ... the complete info here (what is possible for which file format):
>> http://dev.exiv2.org/wiki/exiv2/Supported_image_formats
> According to that page for exiv2, hence for digiKam, Jpg and PNG are
> perfectly equivalent when it comes to both read and write any of the
> three metadata formats. At the same time, the digiKam handbook
> recommends PNG before JPG because, of course, only the first is
> lossless.

There are two differents things.

The exiv2 page I mentionned relates to files formats that support
embedding metadata in the file itself. For some formats it's very
well implemented and trustable, for some other (specialy some RAW
formats) it's a bit more hazardous or experimental.
(From my point of view, concerning an original RAW file, experimental
means « Don't use ! » :-)

And this is different from discussing about which format to use for
images edition, JPEG, PNG, TIFF or other.
(Some users use as working format the native format of the image
editor. As for me, I'm a Gimp user and my intermediate working
format is Gimp .xcf)

> Combining these two informations, what I understand is that:
> - I should save all my pictures in PNG format, so I'll still have all
>  the metadata read/write/export capabilities as if they were JPG, but
>  I'll be able to edit metadata, geotag, remove red eyes, rotate or do
>  any other image processing I forgot to do before converting to PNG,
>  without degrading the pictures
> - and the ONLY thing I'd lose is that a backup DVD of those pictures
>  would not be readable by today's living-room DVD players, that only
>  read jpg. So if Aunt Mary ever asks for a copy of the pictures to
>  see on her TV, I'd have to make a second DVD with the jpg versions
> Does all this make sense? Did I miss something?

Perfectly right. Choose your storing/archiving images format on technical
considerations, PNG could be a good candidate.
And be prepared to produce copies of images, depending on the desired
usage. Desired usage will imply final format and also dimensions.

- If you want to put some images on web pages, with a 600 or 700 pixels
width, build JPEGs with that size. Uploading to a web hoster PNG images
with 4500 or more pixels width will just waste storage space and bandwith.
And the visitor of your pages will see a 600 pixel width image :-)

- If you want to give a CD to Aunt Mary, God bless her, generate JPEGs
with some width between 1200 and 1600 pixels, enough for a TV screen.

- If you want to order paper prints, say in 4x5" format, and your print
service works in 300 dpi mode, the width in 5" will be 1500 pixels.
Multiply by two if you want to respect Shannon's sampling theorem and
upload to your print service images with 3000 pixels max. You won't
see any the difference on your paper prints.

The conclusion : keep your archives at best quality, keep your original
files (I 100% agree with what Elle Stone said), and rebuild images on
demand for each usage.
Programs such as ImageMagick convert can do that automatically, e.g. :
  convert Original.png -resize '1280x1280>' -quality 95 AuntMary.jpg


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