Say I wanted to bring out the colors in the shadowy bottom right hand part of the attached image, how would I go about it? I assume I need to restrict the processing to that part but I have no idea what technique I should use and how to limit the effect to part of the image.
For a smooth brightening (or darkening) of shadows (resp. highlights) I prefer the color curves tool (Shift+C) where you can define a non-linear curve for all channels, or a single RGB color channel.
In the example image I used a positive S-shaped curve for all values and a negative S-shaped curve to remove blue from the shadow parts of the image:
One year later:)
In fact on such image you want to use "Luminosity masks" that are progressive selections that cover the parts of the image within a given luminosity range. The good things about them is that the effect of the color tools apply in proportion to the lightness or the darkness of pixels so you never have visible transitions. There are scripts around that can generate the 5 canonical masks, but here is a quick demo with only two.
- Duplicate the image
- Open the Channels list, and copy any of the RGB channels by dragging it to the main list. Rename the Channel "Highlights"
- Duplicate the channel, and rename the copy "Shadows"
- Colors>Invert to invest that channel.
Once this is done, you can work on the Highlights parts of the image by going to Channels, right-clicking "Hilights" and doing
Channel to selection (and then reselecting the layer otherwise you'll be working on the selection channel). For instance, using this technique, I just dimmed a bit the highlights and lightened the shadows. No other adjustments necessary:
There is a more complete tutorial on the matter.
To restrict the processing to a part of the image you use a selection (Freehand selector, here). Once there is a selection most tools apply only to the selection. If you use that 'raw' selection there will be a very visible edge between the parts that were changed and those that were not, so to smooth the transition you usually feather the selection (this is an option ("Feather edges") in most selection tools, but it can also be done afterwards (menu Select>Feather)).
The simplest tool to use afterwards is Brightness/Contrast, but don't overdo it.
Your photo is already a heavily processed JPG. Lifting up the shadows will introduce visible blurriness and other errors, which have been less offensive as dark. 8 bit JPG simply hasn't enough data left. The result is quite coarse. There's noise, JPG compression errors and too few brightness levels in the dark areas.
The next version is a test what is available in the shadows. The image is opened in a RAW image processing program and got the same treatment as a real RAW image. There's fill light, local contrast boost and radical sky blue reduction in shadow areas. The shadow has fortunately sharp border for easy selection.
If you zoom in you can see that the lifted shadows are very coarse behind the lake, because they were noisy and blurry and the bit depth is too low for nice contrast increasing.
But the result can still be acceptable. We can try to get this in GIMP.
The old approach to fix your photo would have been to increase brightness and contrast in shadow areas with curves and then fight back the introduced excessive colorfulness. This is presented already by others. We try something else which wasn't possible before GIMP 2.10.
GIMP 2.10 has a polar version of CIELAB color system named LCH which is especially useful and makes a new answer worth writing. We make a good colorizable BW version of the image and colorize it with blending mode Color LCH.
Colorizable BW image has grey midtones where one expects color. Too full black and white are not colorizable. We can make one by desaturating and masked contrast increasing. GIMP hasn't adjustment layers like Photoshop, Krita, Affinity Photo etc... but we can work around with layer copies.
Start by making a couple of desaturated copies of the original image layer:
Insert a layer mask. Let it be the BW image as inverted
Disable temporarily the layer mask. Move the original colored image on top and let it have blending mode Color LCH.
The result is quite the same as the original, but you can make radical changes by editing the BW image and its layer mask.
At first find a good Color > Color curves > Value setting which lifts the dark areas acceptably. The bright areas get too flat, but we'll fix it later:
We do not want to lift already bright areas. Enable the layer mask and apply curves to the mask to get some area separation. Click the mask icon to get the mask under editing:
Unfortunately the mask has too much details. Details suffer badly. A real man would paint the whole mask manually to get the exactly right effect strength distribution, but we can cheat. Let's apply some blur to the mask and get boosted local contrast that way:
Even more local contrast is available by flattening the image to a single layer and applying Unsharp Mask:
The version from a RAW developer program clearly has some saturation boost which is missing here due the LCH colorizing. If that sweetening is wanted, increase the saturation:
I guess the whole tinkering is not worth the effort if you can invest the needed time to learn to cope with a RAW developer. Photoshop's Camera Raw is the easy one, but generally they are complex. Raw Therepee is free and it can be used as plugin from GIMP.
Total rank amateur, here.
Very tedious approach, but, it worked pretty well. I have a number of scanned old chemical photos. They have darkened across the top 1/3rd of the photo. I tried a number of tricky ways to do it and finally ended up doing this.
Incrementally work up the image.
1 - Select the top 1/2 of the image.
2 - Menu->Colors->Brightness-Contrast - turn up the brightness 2 (very little). 3 - Starting from the top select down to just above the previously selected portion (a little less that 1/2 of the top of the image). 4 - Repeat step 2. 5 - Repeat step 3. The top edge of the selection is always the top of the image. The bottom edge of the selection creeps up the image with each iteration.
6 - Then, repeat step 2. 7 - Continue to the top of the image.
This takes a few minutes, but, it effectively builds a gradient of increased lightening as you go up from the middle of the image to the top. The lightening effect is gradual enough to avoid obvious lines/edges. You'll likely want to modify the Brightness step (more or less) depending on how slowly or quickly you want the effect to move across the image.