jpeg compression again ( sorry )

[Francesco Scaglioni-2]

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.  Should I therefore be using that setting in order
to minimise quality loss ?  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 ?

TIA


Francesco

-----------------------------------------------------------------------------------------
The information in this e-mail and any attachments is
confidential and is intended for the attention and use of the
named addressee(s).  It  must not be disclosed to any other
person  without our authority.  If you are not the intended
recipient, or a person responsible for delivering it to
the intended recipient or are aware that this e-mail has been
sent to you in error, you are not authorised to and must not
disclose, copy, distribute, or retain this message or any part of it.

We sweep all outgoing messages for the presence of computer
viruses. However, we cannot accept any responsibility for any
loss or damage to your systems due to viruses or malicious
code not detected.

The statements and opinions expressed in this message are
those of the author and do not necessarily reflect those of the
organisations within the Cornwall & Isles of Scilly Health
Community.

This email may be disclosed under the Freedom of Information  
Act 2000 or the Environmental Information Regulations 2004.
-----------------------------------------------------------------------------------------
_______________________________________________
Digikam-users mailing list
[hidden email]
https://mail.kde.org/mailman/listinfo/digikam-users

[Daniel Bauer-2]

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
professional photography: http://www.daniel-bauer.com
erotic art photos: http://www.bauer-nudes.com
Madagascar special: http://www.fotograf-basel.ch/madagascar/
_______________________________________________
Digikam-users mailing list
[hidden email]
https://mail.kde.org/mailman/listinfo/digikam-users