Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
Deleted1929
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Re: LAB mode does not preserve the colours
In reply to Tom Axford, Jun 29, 2013

The L in LAB is not a linear mix of RGB.  It's considerably more complex.

The site I generally refer to for this sort of thing is Bruce Lindbloom's website, however Wikipedia has a page on it :

https://en.wikipedia.org/wiki/Lab_color_space

http://www.brucelindbloom.com/  ( look under "Math" ).

To get the L component you need first the Y component from the XYZ space.  That Y component is a linear mix of RGB.  However the L component is a non-linear function of Y.

LAB is designed to give an L component that is a reasonable representation of human perceived tonality.  So the L component actually makes a pretty good B&W version of an image.

However changing just L values ( leaving AB values unaltered ) and then converting back to RGB does change hue and saturation.

I find the HSV space most useful for preserving color, although no space is perfect at this in my experience.  In GIMP you can mix a layer using Value mode ( the V component referred to in HSV ).  This is useful for avoiding the worst of color shifts.

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StephenG

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