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

> Date: Tue, 20 Dec 2011 07:04:20 +1100
> From: graeme2 argyllcms com
> To: gimp-developer-list gnome org
> Subject: Re: [Gimp-developer] Luminosity in LAB does not agree with Wikipedia or Matlab
> How about just implementing the conversion correctly ? To convert from RGB to L*a*b*,
> one has to convert from RGB to XYZ, then XYZ to L*a*b*. A typical working RGB space (like
> sRGB, AdobeRGB etc.) has a gamma encoding. So RGB to XYZ removes the gamma
> encoding (since XYZ is linear light), and then L*a*b* uses a 1/3 power which models
> perceptual linearity. Of course to display an L*a*b* value you have to convert it
> to XYZ and then to the display's RGB space. [And yes there are many
> possible avenues for optimising the performance of such conversions.]
> Graeme Gill.
> _______________________________________________
> gimp-developer-list mailing list
> gimp-developer-list gnome org

If an image's raw RGB pixel values represent perceptual linearity (when viewed through your standard RGB monitor), 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 (like you get from performing an RGB -> YCC conversion), despite that the decomp must account for gamma encoding during the intermediate steps.

And this is not what you get when running a LAB decomp in GIMP.

So . . . yeah, I'm totally in agreement here.

-- Stratadrake
strata_ranger hotmail com

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