Almost any image-editor worthy of the name offers curves adjustments, but they can be tricky...Read More
Solarisation is an old darkroom technique for partially reversing a print during the development process. This produces a picture that’s part positive and part negative. The result is a picture that can add a surreal look to any subject from a portrait to a landscape. The lighter parts of the scene reproduce naturally, but the darker parts are reversed, so that bright skies, for example, become dark and foreboding.
You can reproduce this effect digitally in a couple of different ways. One way is to simply reverse the curve in the curves panels so that it peaks in the centre and drops back down to zero on the right (though a double-peak often works better).
The other way is to use a dedicated Solarisation filter like the one in Color Efex Pro, part of the free Google Nik Collection. This is quicker and easier and gives a wider choice of effects in both black and white and colour.