> Subject: Re: [Gimp-user] Layer addition - bug, feature, or user
> To: strata_ran...@hotmail.com; firstname.lastname@example.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.
Numbers may not lie, but neither do they tell the whole truth.
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