slightly OT question: convert jpg->png

[Daniel Bauer-2]

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?

thanks a lot...

Daniel

(When I have many pics to convert I prefer imagemagick, because I can start
several jobs and then go out having a coffee or two, while my PC does the hard
work... On my old machine with OpenSuse 11.2 digiKam crashes when I start a
batch job and then try to add a new batch job from another folder/album while
the first one is already running. I guess this is a problem of my machine and
not of digiKam, unless others tell me that they see the same)
--
Daniel Bauer photographer Basel Barcelona
professional photography: http://www.daniel-bauer.com
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[Paul Hartman-2]

On Wed, Jan 13, 2010 at 10:39 AM, Daniel Bauer <[hidden email]> 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.
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[Milan Knížek]

Daniel Bauer píše v St 13. 01. 2010 v 17:39 +0100:
> 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?

Did you try the various filter types? As per ImageMagick help:

For the MNG and PNG image formats, the quality value sets the zlib
compression level (quality / 10) and filter-type (quality % 10).
Compression levels range from 0 (fastest compression) to 100 (best but
slowest). For compression level 0, the Huffman-only strategy is used,
which is fastest but not necessarily the worst compression.

If filter-type is 4 or less, the specified filter-type is used for all
scanlines:

0: none
  1: sub
  2: up
  3: average
  4: Paeth

If filter-type is 5, adaptive filtering is used when quality is greater
than 50 and the image does not have a color map, otherwise no filtering
is used.

If filter-type is 6, adaptive filtering with
minimum-sum-of-absolute-values is used.


Regards,

Milan Knizek
knizek (dot) confy (at) volny (dot) cz
http://www.milan-knizek.net - About linux and photography (Czech
language only)

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