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> 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 > http://mail.gnome.org/mailman/listinfo/gimp-developer-list 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 -------------------- |