[Digikam-users] slightly OT question: convert jpg->png

[Paul Hartman]

On Wed, Jan 13, 2010 at 10:39 AM, Daniel Bauer <linux at daniel-bauer.com> wrote:
> Hi,
>
> when I convert jpgs's to png's in digikam with compression 5 they get approx.
> half as big (in MBs) as if I convert them using
> "convert xy.jpg -quality 50 -strip -verbose xy.png"
>
> I thought "-quality 50" in imagemagick equals to "compression 5" in
> digiKam/gimp, but obviously I'm wrong. Can somebody tell me how I can achieve
> the same compression in imagemagick as in digiKam?

Assuming PNG is lossless and no material changes are being made to the
image (that the size difference is solely due to compression level) I
would recommend using a tool like optipng to compress your PNG files
to the max, and it also handles simple things like removing the
alpha-channel if you don't use it (an instant 25% size savings). It's
almost always able to make the files smaller than the software that
generated them. Similar programs include advancetools and pngcrush.

I've found that combining optipng, advpng, and advdef and running them
in different orders I can sometimes squeeze a little bit more out of
the file, too.