Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
Tom Axford Veteran Member • Posts: 5,499
Re: LAB mode does not preserve the colours

knickerhawk wrote:

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.

Unfortunately, there is a fly in the ointment.

Perhaps the best way to see the problem is through an example. Take the following image, which is a bright red in which the R channel is at the maximum (255):

Convert this image to LAB mode and then change the L component to maximum everywhere (i.e. make the L layer a pure white layer) and convert back to RGB. The result is (according to GIMP):

It has clearly changed colour. Why?

The problem is that the luminosity of the original colour is well below the maximum luminosity, but when you try to raise its luminosity, the red channel inevitably gets clipped (because the red channel is already at the maximum possible), causing the colour shift.

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