Managing levels in two different parts of the image?

[magnuslar]

Hi!

I cannot find out to do this so posting here: 

I have an image of a bright sky over a grass in the foreground. The sky is far brighter. Now I want to adjust levels, so that I get a lot of detail in the sky, that is, darkening it, without making the foreground black. So essentially treating the two parts of the image indepently in terms of levels, brightness, contrast. My idea is to do this with some kind of mask, or to separate in two different layers, but I cannot figure out how (I'm quite a newbie to GIMP, trying to learn). 

Anyone have advice or can point to a good tutorial somewhere? 

EDIT: GIMP 2.8.22, running on Ubuntu 18.04

A scaled down version of the image is here:

   

Magnus

[Blighty]

As always, there is more than one method.

1. Use Curves
See (1) below. Play around with curves to suit.

2. Use Levels
See (2) below. Play around with the sliders.

3. Make a selection. Only the area within the selection is altered.
3a. Make a selection on the sky. See (3) below. Use curves or levels on the sky. 
3b. Invert the selection. The lower area is now selected. Again, use curves or levels.

4. Use luminosity mask
Not illustrated. Requires a plug-in for easy use

[rich2005]

I would try the luminosity masks script. A lot about it here: https://patdavid.net/2013/11/getting-aro...masks.html

Since the GIMP Registry site is gone the script is attached.  One for Gimp 2.8 and one for Gimp 2.10 For Gimp 2.8, goes in your user scripts folder ~/.gimp-2.8/scripts

You can use like this.

Filters -> Generic -> Luminosity masks creates a selection of masks in the Channels dock. 

You can apply any of these as a selection but often a better way is use a layer mask. 
Duplicate the layer, apply layer mask to top layer. Choose a mask, black is transparent, white opaque 

   

With a suitable mask, the sky is separated on the top layer and is  adjusted

   

 and the remainder is adjusted on the bottom layer. (3)

   

[magnuslar]

Thanks!! Luminosity masks was exactly what I was looking for. Great help!!

Best,

Magnus