Color/Luminosity separation

Started 1 month ago | Discussions thread
Tom Axford Veteran Member • Posts: 8,435
Re: Color/Luminosity separation

Just to explain in more detail what I meant in my previous post:

Take a colour like the purple shown below. In RGB it is (128, 0, 255), but its luminosity (L* in LAB colour) is only 39.3% (pure white is 100%).

If we are working in the luminosity channel and double the luminosity to 78.6%, that seems perfectly reasonable. But for this purple colour, doubling the luminosity would change the RGB values to (256, 0, 510) which is way outside the gamut of RGB (each R, G and B value must be between 0 and 255).

I'm not sure exactly what the software does when converting out-of-gamut LAB values to RGB, but supposing it simply clips the channels to the maximum 255. Then the colour would be shown as (255, 0, 255) which is a very different colour.

RGB = (128, 0, 255)

RGB = (255, 0, 255)

I use HSV Value instead of Luminosity if I wish to change the lightness of the image in a way that is guaranteed to preserve the colours exactly. Unfortunately, HSV colour is not supported in Photoshop and most editors, so I use GIMP, where it is supported.

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