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Hm, it seems I had my thinking a bit mixed up when I was typing my last response, because when I typed "Y*" I actually meant to say "L*". (Color management terminology is something of a "New Math" for me, so I have to wing it.)
Other than that I agree: GIMP's current LAB decomposition does not work "correctly" because it does not account for gamma encoding in the soruce RGB values. It takes the input values as representations of linear intensity (not gamma-encoded perceptual intensity) and produces the corresponding LAB values for that -- mathematically correct in context, but it's missing an important step in the process.
Numbers may not lie, but neither do they tell the whole truth.
> Date: Tue, 20 Dec 2011 08:43:09 +1100
> From: graeme2@xxxxxxxxxxxxx
> To: gimp-developer-list@xxxxxxxxx
> Subject: Re: 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.
> gimp-developer-list mailing list
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