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

Notice that if you perform a Midtones correction of about 2.2 on Wikipedia's CIELAB L channel then the result roughly approximates GIMP's L channel decomposition.  (Conversely, apply a Midtones correction of 0.45 to GIMP's L channel and compare results).

GIMP documentation says "The LAB color model is used by Photoshop" but from what I have seen this is not true unless there's some small detail about GIMP's decomp that we are overlooking.

For example, one effect I like to do from time to time is a luminosity inversion.  In (an old version of) Adobe Photoshop I could simply switch the image from RGB to LAB color mode and invert the L channel.  But I cannot do the same thing in GIMP using its LAB color space decomp because the yielded L channel is a magnitude too bright -- but! if I add the aforementioned midtones correction to my steps I do get results that are comparable to Adobe Photoshop.

-- Stratadrake
strata_ranger hotmail com
Numbers may not lie, but neither do they tell the whole truth.

From: stecnico0506 hotmail com
To: gimp-developer-list gnome org
Date: Sun, 18 Dec 2011 11:36:38 -0300
Subject: [Gimp-developer] Luminosity in LAB does not agree with Wikipedia or Matlab

     In this article is displayed an image ( and its luminosity (*.jpg). However, if I apply Colors-Components-Decompose-LAB in Gimp, the image L does not agree. The same happens with Matlab.

     I'm not sure if this is an error in the program or a documentation problem. In the last case, for example, the LAB model of Gimp is not the CIELAB model, or the model is CIELAB but they use different white points.

                        Adrián del Pino N.

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