Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
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ilza OP Regular Member • Posts: 107
Re: Changing contrast without distorting hue and saturation

Tom Axford wrote:

I sympathise with your frustration.

I do not recollect mentioning any 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 .

What you describe in this reference is, essentially, “the 100% rule”: you keep the saturation of the initial image intact. This is the “variant 2” of my post (with the variant 3 being the “the 0% rule”).

The whole topic of my post was that your method gives unnatural colors; hence “the 45% rule”. For my eyes, 45 gives visually better results than “the 40% rule” or “the 50% rule” — which are both significantly better than “the 100% rule” or “the 0% rule”.

And, what you mentioned is trivial to achieve in GIMP (although until I’ve read the source code of “Mode”s, I have been using yet more convoluted way that yours! It is a pity that what GIMP does is completely undocumented). Just

  • Duplicate the layer;
  • Change the Mode of the top copy to “Level”;
  • Apply curve to the top layer.

My method is mathematically correct,

¿¿¿ What mathematical correctness has to do with this ??? Setting all R,G,B to 0 is “mathematically correct”. What is, IMO, the topic of discussion is the human perception of colors after severe contrast manipulations.

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).

GIMP has an advantage that the source code is there, so one can google for this. But the value hierarchy of GIMP developers was always a mystery to me — why spend a lot of time implementing nice features, then skip documenting what they do. So now users must work like script kiddies: experiment/report/do-as-I-do.

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.).

My thought exactly. Only if my understanding is correct, one would need an additional slider so that “45” of “the 45% rule” is not hardwired. (In my experiments, 45 was consistently the best; but such things are very often subjective and depend on a person.)

If someone experimented with this: did you find other numbers more suitable in some situations?

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.

I would not be so hopeful — I have some experience communicating with GIMP developers. (One gets an impression that they think they have much more clue than what it looks from outside.)

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

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