Opencv-Gimp Lightness

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For more complex image processing techniques, it could be worthwhile to dig into the source code of Gimp to replicate / translate the function into another programming langage or library.

Starting by the documentation and the Github search bar, you should find:

• gimphuesaturationtool.c
• gimpoperationhuesaturation.c
• gimpcolorspace.c

Also, if you are a Gimp fan, I recommend the G'MIC Gimp plugin that adds tons of image processing operations to Gimp (source code and C++ API).

though i think, you're somewhat "on the wrong boat here" (abusing a computer-vision lib for simple image manipulations), you'd basically do the same thing as in gimp:

(pseudo code, as i don't have any java around)

cvtColor(bgr, hsv, COLOR_BGR2HSV)
split() into seperate H,S,V channels, V is "lightness"
subtract(V, some_value, V)
merge() H,S,V back again to a single hsv image
cvtColor(hsv, bgr, COLOR_HSV2BGR)

btw, here are the javadocs