Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
ilza
Forum MemberPosts: 74
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Re: HSV works better than either RGB or LAB
In reply to Tom Axford, Jul 10, 2013

Tom Axford wrote:

For adjusting the tone curve, HSV works better than either RGB or LAB.

The V channel may be adjusted (even to extremes) without causing colour shifts and without the risk of channel clipping on conversion back to RGB.

My guts feelings are that HSL may have some advantages (comparing to HSV) in the situation when there is no clipping. And, of course, HSV is infinitely better in the cases when HSL clips. How to solve this?

If I would design a tonal-curve applicator, I would use a weighted average of two:

  • Apply the curve both to L of HSL and to V of HSV;
  • Calculate V of the converted HSL, take V of converted HSV (denote as V₁, V₂);
  • If V₁<0.5, keep it;
  • If V₁≥1, replace it by V₂;
  • For values of V₁ between 0.5 and 1, take the weighted average (with weights changing linearly between two boundary cases) V = tV₁ + (1-t)V₂;
  • t=2-2V₁

(This way, outside of zone IX, we use curve-on-L; in zone IX, we use something more complicated — but I expect it hardly matters what one does in zone IX.)

(There is no need to say that I would also “cook” saturation using the 45% rule.)

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