"Extracted L" from "Tools/GEGL Operation/Extract Component/Component LAB L" isn't really "L"
Submitted by Elle Stone
Link to original bug (#755376)
Description
The "extracted L component" from GIMP "Tools/GEGL Operation/Extract Component/Component LAB L" isn't really "L". It differs from true LAB L by the amount that the sRGB "almost perceptually uniform" TRC differs from a true perceptually uniform "LAB L" TRC (parameters below taken from http://ninedegreesbelow.com/photography/lcms-make-icc-profiles.html):
/* sRGB TRC */ cmsFloat64Number srgb_parameters[5] = { 2.4, 1.0 / 1.055, 0.055 / 1.055, 1.0 / 12.92, 0.04045 };
/* LAB "L" (perceptually uniform) TRC */ cmsFloat64Number labl_parameters[5] = { 3.0, 0.862076, 0.137924, 0.110703, 0.080002 };
It's easy to show that "Extract Component/Component LAB L" doesn't really produce LAB Lightness: Change the extracted "L component" layer's blend mode to LCH Lightness and set it over a copy of the original color layer from which the LAB L channel was extracted. If the extracted "L component" is really LAB Lightness, the image tonality won't change, but it does.
Version: git master