Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
OP ilza Regular Member • Posts: 107
Re: Changing contrast without distorting colors: 45% rule and another cat

Tom Axford wrote:

Forgive me, but I am very slow and am having considerable difficulty in fully understanding your argument.

I think that the crux of the problem is understanding what you mean by your "N% rule". In your image labelled 100%, I can see that it has 100% of the original saturation. But your image labelled 0%, has more saturation to my eyes, not less, and it certainly isn't 0% saturation as that would imply a b&w image. I am obviously misunderstanding what you mean by the N% rule. Can you explain in more detail, please?

As I said in the OP:

  • two strategies (variant 2 and variant 3) are marked as 100%-rule, and 0%-rule;
  • both strategies work with the original image, and the second image, which has the tonal curve applied separately to R,G,B channels;
  • both strategies keep the hue from the original image;
  • both take the brightness from the curve-applied image.

Now the variants differ in:

  • Variant 2: take the saturation from the original image (100% rule);
  • Variant 3: take the saturation from the curve-applied image (0% rule).

As you can see, the percentage reflects how much of the original saturation is preserved. And N%-rule just naively interpolates between two variants: it mixes N% of saturation of the original image with (100-N)% of saturation of the curve-applied image.

Is it more clear now?

However, I am not at all knowledgeable about this level of detail in theories of colour vision, and I rather doubt that there are many people on this forum who are experts in colour vision.

What I suspect is that the experts in color vision know close to nothing about color shift effects from the tone-mapping. (My judgement is based on one reference only — one I gave in the OP. It is one of the most influential papers on tone mapping, but what they propose to do with colors looks like a complete gobbledock to me.) I think that on this stage of research on this topic, what people who actually work with colors (and not just study them) say may have more weight than the opinions of academics.


Now: on differences between 40%, 45%, 50%. All of the images above are hideous; the middle ones are obviously less hideous than the 0% and 100% ones, but the general ugliness does not help in deciding which of 40%, 45%, 50% is better.  So maybe providing such an extreme case (γ=5) as an example was not my best judgement.

However, I tried this approach with “mild” curves (and more artistic images) many times.  45% was always inside the range of visually-the-best percentages.  This is why I say that for me it always work.  On the other hand, I have very limited experience with tonal curves; so I cannot realistically insist that my findings should better be followed by other people.

However, if “professional” retouchers voice their opinion here, this may eventually lead to more widely accepted (and probably better than what I propose) ways to treat saturation.  Then somebody might implement smarter tonal curves in their programs and/or academics may have bright unexpected insights on how all that may be looked-at/improved/reworked.

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