> Subject: Re: [Gimp-user] What is wrong with this picture?
> From: l...@holoweb.net
> To: strata_ran...@hotmail.com
> CC: gimp-user-list@gnome.org
> Date: Thu, 21 Feb 2013 14:35:07 -0500
> I don't know where you get your "results should be like this"... you are
> not "increasing the magenta channel by 30%", you are mapping all pixels
> whose colour value in RGB space is within 30 degrees of magenta to a
> completely different hue, to a different location in HSV space....
> The effect you show happens with all the colour "channels".
> I think you are wanting to increase saturation.
> Or am I missing something?
> Liam

You missed only one (very important) detail:  The tool's Overlap setting.

IF we left the overlap setting at zero, then we would get exactly what you 
described -- only those pixels within absolutely 30 degrees of Magenta would be 
affected, everything else remains the same.

BUT with an Overlap setting of 50%, this "blurs" the threshold between Magenta 
and Red (and likewise between Magenta and Blue).  All pixels with hues falling 
between a 15° ~ 45° deviation from Magenta (a 30° range, or 50% of the 60° 
between Magenta and its neighbors) will receive a variable percentage (100% ~ 
0%, respectively) of the Magenta channel's adjustment.

Test it yourself:  Paint a simple red-to-yellow gradient, then go to the 
Hue-Saturation tool and (with overlap = 0) drain all saturation out of Red.  
Next, slowly increase the Overlap slider and watch how the pixel-sharp 
threshold between the two channels becomes a smooth fade.  It's very useful 
behavior when you're using this tool on photographs or other images with smooth 
color transitions around the hue thresholds; it just happens to make one epic 
screwup in this particular usecase.

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