Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
knickerhawk Veteran Member • Posts: 4,978
Re: LAB mode does not preserve the colours

sjgcit wrote:

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 : ( 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.

What do you mean that it changes hue?  If I change L I see virtually no change in the H value of HSB in Photoshop as long as I don't go out of gamut.  The B and S do shift as expected.

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.

Since HSV and LAB are "cousins," why is it you think that changing V is going to be any more neutral with regard to color shifts than changing L?  Aren't they basically the same?

It seems like kind of an odd argument that adjusting L in LAB "does not preserve the colors".  By definition it does.  The problem comes when one has to return to the limitations imposed by the color space you're outputting to.  However, as long as you stay within the gamut of your output color space, L adjustments should be as color neutral as you could hope for.  Certainly as neutral as anything you're going to be able to do in an RGB space.

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