Severe contrast enhancements: on 45% rule — and cats

Started 10 months ago | Discussions thread
Tom Axford
Contributing MemberPosts: 927
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Changing contrast without distorting hue and saturation
In reply to ilza, 10 months ago

I sympathise with your frustration.

I spent a couple of years puzzling over how to apply a tone curve to an image without distorting the colours (both hue and saturation). After doing quite a lot of reading about colour models and experimenting, initially in Photoshop, later in GIMP (I have given up using Photoshop), I found a method of doing it, which is described here .

My method is mathematically correct, assuming I have correctly guessed what the various tools in GIMP do (unfortunately, I have not found any mathematical definition of the tools in GIMP, or in Photoshop for that matter, but the basic ones seem fairly obvious). The hue and saturation of every pixel is preserved, no matter how extreme the changes to the tone curve.

However, the method is a little tedious to apply (although you soon get used to doing it). It would be much preferable if GIMP implemented it directly in the tools that operate on the tone curve (Brightness-Contrast, Levels, Curves, etc.). In fact, it would be a minor change to the program code to implement it. I'm sure it will happen sometime in the future, let's hope sooner rather than later.

By the way, I'm not sure that this helps in finding the cat!

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