Re: [Gimp-developer] Luminosity in LAB does not agree with Wikipedia or Matlab

Richard Gitschlag wrote:

> If an image's raw RGB pixel values represent perceptual linearity (when viewed through your
> standard RGB monitor),

Well, no they don't. They're whatever the device space makes them. Yes CRT displays
native behaviour makes it roughly perceptual, which is why such display spaces
are good to encode in, but the precise relationship to anything else is device
dependent (hence ICC profiles).

> and L*a*b* values also model perceptual linearity, it should logically
> follow that converting a linear greyscale gradient from one to the other should yield a linear
> gradient on the Y* value 

I'm not sure what you mean by "Y*". Any conversion should yield the same
color values (assuming you don't hit gamut limits), so there should
be no change in appearance. If on the other hand you mean "re-interpret
the same numbers in a different colorspace", then yes, what looks
perceptually linear in a gamma encoded RGB space should look roughly
perceptually linear in L* space. Normal CRT gamma curves and the L*
curve are not a precise match though. If one or other are not
true of GIMP, then it has a problem....

Graeme Gill.

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