[Digikam-users] Image compression confusion

[Gilles Caulier]

2007/7/12, Paweł Marciniak <pave at o2.pl>:
>
> Hello!
>
> I'm a bit confused with various DigiKam's compression options. To make it
> clear: this is not complaining about anything, but rather a try to discuss
> some issues that, IMHO, may confuse users and make digiKam less
> accessible.
>
> First of all, I don't know if it's a bug or a feature, but the PNG
> compression setting (the one in Configure Digikam...->Save Images) doesn't
> seem to affect images converted to PNG at the time of downloading them
> from
> camera. An example photo I play with is 7.0 MiB in size after downloading,
> independently of that setting.
>
> But what's even more strange is that when I use the KIPI convert or
> recompress plugin on that photo, the resulting file size varies between
> 7.9
> MiB and 10.8 MiB depending on the plugin setting - but when I save it from
> the image editor the resulting size is between 7.0 MiB and 14.6 MiB
> depending on the slider in the save dialog. I understand that KIPI plugins
> are _not_ digiKam, but the same people stand behind both projects, so
> maybe
> it would be better to make it behave more uniformly?


sure. I have not maintened this plugin since a long time. digiKam  have been
improved faster. so a sync between must be done.

This require only an adjusment of slider limit. It's easy to do


After some investigation I found out that the general PNG setting is used
> only when saving (not "saving...") PNGs in the image editor. Don't you
> think it may be a little confusing for ordinary users? Perhaps a short
> description in the options dialog (saying that it is default setting
> overriden by various dialogs etc.) would be a nice thing?


Why not...


OK, I'm done with PNGs, now something concerning JPEGs, namely the lossless
> compression checkbox in compression level option in KIPI recompress or
> convert plugins. I understand that some operations on JPEGs (e.g.
> rotation)
> may be done in a lossless manner, but what's the meaning of lossless
> conversion or recompression of already lossy JPEG image to another lossy
> JPEG image with different quality option?


ah, the famous lossless option with jpeg. Well, it come from ImageMAgick
(which is used in background by kipi-plugins)

In fact, if i remember in IM source code, this option set compression levels
to mininum to reduce jepg compression artifact, but of course, this is not
really a loosless stuff...

Perhaps recent version of IM have improved this way. Look IM man page for
details.

Of course, LossLess is not really a good term to use...


As to JPEGs, there's another thing that bothers me. After reading (in some
> recent thread) nice Gilles' explanation of why it's good to store images
> as
> PNGs rather than TIFFs for example, I decided to follow this advice. But
> now I realized that digiKam is able to save files (although this option is
> not available in the Camera GUI) as lossless JPEG 2000 as well. My test
> photo saved that way has only 5 MiB instead of 7 MiB with PNG, what seems
> an interesting alternative to consider. Is there anything wrong with JPEG
> 2000 that discourages using it as the main format of storing lossless
> photos? I can see that all the metadata has been lost while converting to
> JPEG 2000, but I assume that this is simply not implemented just yet but
> will get better in the future, right?


JPEG2000 and metadata is a mess.
JPEG2000 is really slow to decompress (jasper algo is not optimized,
especially with 16 bits image)

This is why PNG is always better for me

I hope that someone is able to clarify things for me a little :) Thanks in
> advance!


Gilles
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