This is for Photoshop, might work in Gimp and similar if there is a Curves tool that can be used as a adjustment layer.
First of all: I use this on B&W photos currently, yet to be tried on color photos.
It is useful on hard contrast images and allows to lessen contrast without artifacts. Might come in handy as a technique for other things too.
Behind the screnes; this is adjusitng the image with the inverted levels of itself.
Fill and opacity tells how much to apply. The form of the curve could be a straight line from upper left to lower right (inversion!), the example below has more effect applied – softly.
- Add a curves layer
- Create the points as shown in the red and blue circles below
32,180 is visble for blue, the red point is 76,224 – but these are just a start, meant for adjusting.
- Set “Fill” to 10%,
- Adjust amount of effect with Opacity – I had 66% set for the image I grabbed the screen from.
- Play with the settings and curve form to get the required effect