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