[Digikam-users] jpeg compression again ( sorry )

[Daniel Bauer]

On Monday 18 August 2008 14:52:18, Francesco Scaglioni wrote:
> Hi,
>
> 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
> original.  

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 
them again
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. 

regards

Daniel

-- 
Daniel Bauer photographer Basel Barcelona
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