[Digikam-users] jpeg compression again ( sorry )
linux at daniel-bauer.com
Mon Aug 18 17:53:30 BST 2008
On Monday 18 August 2008 14:52:18, Francesco Scaglioni wrote:
> Experiments have shown that if I use a jpeg compression
> setting after minor edit ( eg curves only ) of about 90-95%
> then the resultant file size is about the same as the
File size is not the only measure for image quality. If you save a jpg you
will always loose info, because it's always freshly compressed from the last
version. Often you wouldn't realize immediately just by looking at it, only
after many re-saves or if you want to retrieve some details in shadows etc.
that "suddenly" are lost...
> Should I therefore be using that setting in order
> to minimise quality loss ?
No. You should use another (lossless) file format as long as you are working
on the image and use jpg only for the end product, if needed.
So, a "save" workflow would be something like:
a) copy files from camera into a directory "x.y.originals" and never touch
b) copy "x.y.originals" to a new directory "x.y.work". Or, if originals from
camera are jpg's, transfer them into lossless file format and save
in "x.y.work". Now work with these files or more copies of these files only.
c) save the finished files (that are in lossless file format) to jpg in a
third directory "x.y.jpgs". If you have to correct something, do it with the
file in "x.y.work" and again save it as jpg to "x.y.jpgs".
If you destroyed something in b) you can always get the original in a) again
and copy it to b). Within b) you can always work as much as you want without
loosing image information by saving the files (you can still loose by your
work, of course).
For copying and to transfer into other fileformats/sizes use imagemagick
(search google to find out how to do this for whole directories etc.) It's
easy and fast.
Then, backing up originals and final products is another topic again.
> Certainly a setting of 100%
> results in some excessively large files (which I would
> rather avoid if I can). What quality figure is generally
> recommended, or is that a stupid question ?
As a standard I use 88%.
But it depends on the image, what kind of details it contains etc. So, if you
use images for web purposes you can often use higher compression, but you
will have to try with every single image. For me this is too much work :-) so
all my web images are 88%, basta.
Daniel Bauer photographer Basel Barcelona
professional photography: http://www.daniel-bauer.com
erotic art photos: http://www.bauer-nudes.com
Madagascar special: http://www.fotograf-basel.ch/madagascar/
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