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

[Elle Stone <ellestone ninedegreesbelow com>]

On 10/13/2015 11:45 PM, Richard wrote:
>  > Subject: Re: [Gimp-user] Layer addition - bug, feature, or user
> misunderstanding?

> 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.

It looks like the Addition blend mode issue has been been fixed in GIMP 
2.9. The error was subtle enough that it could easily go unnoticed in 
most use cases. The example images in the bug report are special because 
they added together to produce obviously wrong results, but now they add 
together correctly.

I probably never would have noticed anything odd about Addition blend 
mode, except I was working with a channel layer stack from a color image 
where I already knew what the result of adding the layers *should* look 
like. But the highlights were subtly wrong, which is why I asked on the 
list whether maybe Addition blend mode was supposed to do something 
different from what I thought it did. And I'm glad I asked - your 
mathematical explanation of how layer opacities work is excellent, very 
clear. That information should go in the GIMP documentation somewhere, 
if it isn't already there.

Best,
Elle