> Subject: Re: [Gimp-user] Layer addition - bug, feature, or user 
> misunderstanding?
> To: strata_ran...@hotmail.com; gimp-user-list@gnome.org
> From: ellest...@ninedegreesbelow.com
> Date: Tue, 13 Oct 2015 11:46:11 -0400
> So either GIMP intentionally uses a different algorithm for addition 
> blend mode. Or else there's a bug in the code, most likely the same bug 
> reported here: https://bugzilla.gnome.org/show_bug.cgi?id=744265. I'll 
> add a comment to that bug report.

On an aside, I was able to build the testcase in that GTK bug report and those 
results were ... interesting, to say the least:  created a black background 
layer, then two layers having gradients from black to white (one vertically and 
one horizontally).  Set both of their blend modes to Addition, both at 100% 
opacity.  Result - The lower right triangle of the image has been clamped at 
white (as expected).

Then I adjusted the opacity of the top layer down to 50%.  This caused the 
"clamp triangle" in the lower right to dim (but still remain a solid color) -- 
yeah, the clamping action is clearly wrong here since at < 100% opacity there 
should be fewer pixels running out of gamut.  Next I adjusted the middle 
layer's opacity down to 50% as well, which ... shrunk the aspect ratio of the 
clamp triangle?  At 50% opacity the triangle showed a clear 2:1 aspect ratio.  
Despite that with both layers at 50% nothing should even be out of gamut here.  
I also swapped the two layers with each other, and the clamp triangle is now 
flipped (1:2 aspect ratio instead of 2:1).  Which is also wrong because 
addition is a commutative operation and layer order should make no difference 

So, yeah, interesting but still wrong results, definitely a bug when using 
Addition blending (and possibly others) at less than 100% opacity.

Not likely to be something I'd encounter in the wild myself -- but at least 
good to know it's getting patched.

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

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