Image compression confusion

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?

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?

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?

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?

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

Paweł

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[Gilles Caulier-4]

2007/7/12, Paweł Marciniak <[hidden email]>:

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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