[Digikam-users] png huge

[Gilles Caulier]

2010/1/19 Martin Senftleben <DrMartinus at drmartinus.de>:
> Hi,
>
> with the many replies to my question I still believe I haven't been
> understood properly, even though I am very grateful for the replies
> which helped me to understand things better.
>
> I recall the situation or my question:
> from a jpg image file, a png image file is created. The jpg file can
> contain only what is stored in it, right?

It contain image data, image informations, metadata, icc color profile, etc...

>The compression method is
> lossy,

yes, definitively. Never use JPEG for archiving purpose. Use TIFF,
PNG, PGF or JPEG2000. JPEG is for web published, printing, etc...
never use JPEG in an editor.

>so whatever is stored in there isn't complete already. This is
> also - in my view - proven by the png that results when I transfer the
> jpg file to the png format: they look absolutely alike, the png image
> is in no way better than the "original" jpg image, it's only about 5
> times larger.

This is wrong. PNG compression is lossless. Editing the image and
saving again do not lost quality

> The source of the png file is not the original raw image, but the
> lossy compressed jpg file. Is it possible for the png to extract more
> from the jpg file than there is in it?

There is nothing more. don't forget: jpeg decompression is lossless,
recompression YES ! Look this video :

http://vimeo.com/3750507

Or is it actually possible to
> revert the compression made by the jpg format to get basically the raw
> image (I doubt that, because then the jpg format wouldn't have lost
> any of the original info)?

Definitively, no. Also, jpeg is 8 bits color depth, RAW is 12/14 bits
colors depth. You lost color quality too ! Never use JPEG for
archiving purpose.

PNG support 16 bits color depth. It's perfect to store demosaiced RAW
image data. (note JPEG2000, TIFF, and PGF support 16 bits color depth
too...)

Gilles Caulier
 If that's the case, I would understand, but
> so far I couldn't read that from your replies.
>
> Sorry if I appear a bit dumb on this...
>
> Thanks for all replies
>
> Martin
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