Re: [Gimp-user] Layer addition - bug, feature, or user misunderstanding?

[Richard <strata_ranger hotmail com>]

> Subject: Re: [Gimp-user] Layer addition - bug, feature, or user misunderstanding?
> To: strata_ranger hotmail com; gimp-user-list gnome org
> From: ellestone 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 here.

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
strata_ranger hotmail com
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Numbers may not lie, but neither do they tell the whole truth.