Everyone who’s done any processing of photos will know about the Auto button of their Colour Level tool. It’s something that probably appears in every decent photo manipulation application and it’s invaluable for correcting colour casts on photos. Unfortunately it doesn’t always work very well.
The GIMP offers 3 buttons next to the Auto function and with these three buttons it’s possible to pick out elements of the image under examination that are either black, white, or grey! Click the white button, then click an area of the photo that should be white. Do the same for grey and black if needs be.
I only (re?)discovered how useful these were this evening when I was trying to colour correct a photo taken indoors, but with the white-balance set to "cloudy". The picture appeared too rich, Auto levels made it green but by clicking the white button and then on a fridge in the background the photo was suddenly almost perfect! I’m not going to post that image but here are two I made earlier…
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