> Subject: Re: [Gimp-user] Layer addition - bug, feature, or user > misunderstanding? > To: [email protected]; [email protected] > From: [email protected] > 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 [email protected] -------------------- Numbers may not lie, but neither do they tell the whole truth. _______________________________________________ gimp-user-list mailing list List address: [email protected] List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list List archives: https://mail.gnome.org/archives/gimp-user-list
