Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
Tom Axford Veteran Member • Posts: 4,529
LAB mode does not preserve the colours

gwlaw99 wrote:

Would converting to LAB mode and only applying contrast enhancements to the L channel work?

Unfortunately this doesn't work in general, because of the way that L (luminance, lightness or luminosity) is defined, which is something like:

L = 0.21R + 0.72G + 0.07B

So pure white (1.0,1.0,1.0) for (R,G,B) has a luminance of 0.21x1.0 + 0.72x1.0 + 0.07x1.0 = 1.0

But other colours will have luminance less than 1.0. For example, a bright blue (0.2,0.4,1.0) has a luminance of only 0.21x0.2 + 0.72x0.4 + 0.07x1.0 = 0.4

If you choose to apply a curve that tries to raise a luminance of 0.4 to a higher value, it can't be done to our bright blue colour because the blue component is already 1.0, which is the maximum that can be represented. So, the blue channel will be clipped, which changes the colour. In fact, this is likely to happen to any colours that are close to the maximum brightness than can be represented, if the tone curve is anywhere raised above its initial value.

Sorry about the maths, but I can't think of any other way to explain it.

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