[Gimp-user] image work

[JLuc <jluc no-log org>]

Hello

I'm using gimp and scribus to produce CMYK PDF
and i have issue with the photograph rendering of the printed books or magazines.

The original image files come from various cameras or smartphones
and lots of différente photographers, none of them being professionnal.
They all are RGB and many of them are not technicaly perfect
but at least i manage to get high-enough resolution pictures.
The issue is with colour rendering :
many of them are printed too dark,
and the dark parts are over-dark, flattened and with no details.

AFAICT my scribus settings for colour management are OK (ICC profile...).
I havent a calibrated monitor.
What is see on screen is very nice - or at least nice enough -
Only printing is the issue.
The industrial printer i send my PDFs to prints on offset paper
- which i was told adds 2 or 3% to the red channel.
When i print localy on my deskjet printer, i kindof reproduces the "too dark" effect,
which can help me "debug" and improve the rendering.

When scribus displays the "out of gamut" areas, i see that much of these pictures are out of gamut.
Bringing these picture more "in gamut" could help.
How can i achieve this ?

EG this public domain picture https://pxhere.com/en/photo/1575227
This one is not dark overall compared to others.
I can see the tomatoe are a bit dark but overall the picture looks bright and balanced enough,
or at least there is a clear color contrast between tomato and salad colors.
Dont you think so ?
When printed, both the salad and the tomato looks much darker
and there is no more contrast betwen tomato and salad.
I tried editing this photograph with gimp so as to improve print :
- selected tomato red area with progressive selection, boosted the red and calmed down the blue and green
- selected the salad green area with progressive selection and added some saturate effect
On screen the result is better, brighter.
But when printed, it is only very slightly (allmost not) improved.

Another example picture is here : https://pic.infini.fr/40kyb5S0/L6oygOf7.jpg
This is not so good a picture.
It's been edited a bit but that's not enough.
The dark part (door and water) are dark but we can see details in them on screen.
When printed, these dark parts look flat dark brown-black with no details at all.

How can i improve that ?

JLuc