Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
Tom Axford Veteran Member • Posts: 5,210
Re: Changing contrast without distorting colors: 45% rule and another cat

ilza wrote:

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?

Yes, thank you. I think I understand it now. I will try out your method in GIMP on some of my own images and see what I think.

Unfortunately, going by what others have said, there is no equivalent method in Adobe software (PS, LR, etc.) so the great majority of readers of this forum will not be able to use it. That includes nearly all the professionals as I think they use PS almost exclusively (with one or two exceptions).

Anyway, although I'm strictly amateur, your method interests me and I will certainly give it a try over a period of time (not promising any quick response)!

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.

I hope you will not be offended by a little personal advice: it does not encourage readers to take your work seriously when you are so dismissive about the work of others.

I have had a look at the paper you referenced and there is one short paragraph about handling saturation. They state that the method they use is essentially the same as previous researchers have used. Clearly their paper is primarily about their HDR method for transforming luminance and they have just picked up whatever was to hand to handle the colour.

If I understand their maths correctly, what they do to the saturation is simply take a percentage of the original saturation (arrived at by trial and error). That looks pretty crude, but it seems to work ok in the examples they show. They do not discuss how that method is arrived at, simply because they are borrowing it from previous research - which may give a justification (but I haven't looked).

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