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

[Richard <strata_ranger hotmail com>]

My explanation ran a bit long, but the key thing is that a layer's opacity setting is generally independent 
of the layer's blending.  AFAIK* "Dissolve" is the only layer blending that actually uses the opacity setting 
in its blending; for everything else, the layer blending is performed first and the opacity setting just 
interpolates between that intermediate result and transparency.  (* - Let me know if I missed anything.)

I tried taking a look at your XCF directly, however I'm running stable GIMP (2.8) and GIMP 2.9 makes breaking 
changes to the XCF format, so that's out of my range.  And while I was able to reproduce the left half of 
your screenshot easily in GIMP 2.8, simply stuffing those layers into groups did not change the result any, 
leaving me unable to reproduce the right half of your screenshot (without further info).

Here's an idea you can try -- move your three layers around so they only partially overlap each other (like a 
Venn diagram).  Then you have the ability to see the final blended result plus each layer individually.

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


> 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
>
> On 10/13/2015 10:55 AM, Elle Stone wrote:
> > So does this seem like a bug in the Addition blend mode? Or is there
> > something I'm not still not understanding about how Addition layer blend
> > mode is supposed to work?
>
> Hmm, Krita produces the results I expected, that is, the three layers 
> add up to R=G=B=1.0f.
>
> 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.