gimp 2.10.32 rgb / cmyk

[rich2005]

Quote:...but on the mountain, except there's a cutting line appearing all around the rock.

Did you replace the background for the mountain ? It looks like edge pixels remaining from a colour or a fuzzy selection.

[SKILKINKS35]

No I didn't. All I did "lighten" the mountain leaving the background sky untouched.
The strange thing is that problem doesn't occur on the cross image.

[PixLab]

It's because that "cutting line" is already there. (from what I saw in your latest screenshot)

   

3 reasons which come to my mind. (there are many more)
➤ your workflow, the order you are processing/use of filters on this picture.
➤ your camera's filters/mode, firmware, lens quality
➤ excess of sharpening/contrast, and other processing

Also about what I saw on your latest screenshots(not having even a part of the original is not helping), you are trying to get back a light from what, 5-6 stops, more? It's black (yes I agree, there are informations in it), but you want to get that light back at which price? in my humble opinion, this will only deteriorate your picture unless you want to use that highly lightened photo as a layer mode or mask for something.

[SKILKINKS35]

Hello PixLab, Thank you for your message and possible reasons for that "cutting line".

The original photo is 2.02 Mo. It was taken in 2015 with a Sony DSC-HX9V small camera (I have bought myself a SONY RX100 iii since...)
The vertical and horizontal resolution is 72ppp

I have done another test in GIMP on the original photo this morning. Here's all I have done:

1) shadows and highlight correction: "Shadows" from 0.00 to 100.00
2) "Radius" from 100.00 to 0.10.
And that "cutting line" is still there.

Now to be fair, trying to 5-6 stops, I agree with you is asking for trouble.
The funny thing is that when I zoom on the image, that "cutting line" looks as if the camera had been using a HDR filter which I never use when filming.

So, I will take your 2nd option: "your camera's filters/mode, firmware, lens quality".

As I am a newcomer to GIMP, I just wanted to "see for myself" how far GIMP could go, being aware it won't do miracles!
Though I must confess I am already quite impressed with what it can do!!!

I have also found out why that "cutting line" didn't appear on the black cross image when I did exactly the same test.
More than likely because the camera used for that image was a FUJI-XM1. Much better lenses than on my SONY -HX9V camera.

In any case thank you.
While I am at it, one question:
Is it very difficult (for a newcomer in GIMP not very technically minded) to replace the sky in an image by another one?

[rich2005]

Quote:While I am at it, one question:
Is it very difficult (for a newcomer in GIMP not very technically minded) to replace the sky in an image by another one?

As with all graphics, it depends. A solid plain colour and you can colour pick or fuzzy select. Cut out the selection and replace with something else. That is what most beginners use, and often results in that border of pixels you found.
For solid colour backgrounds all explained here: 
https://www.gimp-forum.net/Thread-Backgr...r-graphics but it is a bit technical

For more complicated backgrounds, often the Gimp Foreground select tool will make a selection of a complex shape. There is also the path tool used to draw around a subject then converted to a selection.

For non-destructive editing, use a layer mask, and either paint out the background by hand, or use a selection or channel.

When it comes to complex shapes, you will find your own favourite methods, this black-and-white created from a reduced colour copy. Used as a mask to replace a blue sky with a cloudy sky.

   

[SKILKINKS35]

Thank you rich 2005 for your precious answers!
I will investigate those.
I'm off for a few weeks and hopefully I should haver some time to test them.