If you lock the alpha of a layer ->choose smudge brush that has high smudge rate (disable color rate for better results) ->paint from outside alpha to inside alpha =>Black gets dragged in from nowhere and gets added to the alpha. This is IMO not wanted behaviour as you introduce a color that is not necessarily wanted for the painting. And since you can't influence it it will always be black,making you have to pay much attention when using alpha lock. The smudge brush ideally does not introduce a color that should not be there. if there is nothing to smudge...then don't smudge.
Ack, I can reproduce.
This issue is not exactly specific to "lock alpha". You can reproduce on a not-locked canvas too: -create new document -add new layer -hide background layer (so it's easier to see next steps..) -on the new layer, draw a white area using a fully opaque preset (like fill_circle). -select a smudge preset (like Smudge_block_tilt), and smudge the white area starting from outside to the inside. -see the black added.
Git commit 8adefa2cc486ba82a6919edb812e4c63b5ba8e7b by Dmitry Kazakov. Committed on 03/12/2013 at 13:29. Pushed by dkazakov into branch 'master'. Fix KoCompositeOpCopy2 to handle source alpha channel properly The value of the source aplha should be taken into account while blending two pixels. Otherwise black aerials appear on screen. This might not be too mathematically correct, but at least something. M +7 -0 krita/benchmarks/kis_composition_benchmark.cpp M +2 -0 krita/benchmarks/kis_composition_benchmark.h M +12 -6 libs/pigment/compositeops/KoCompositeOpCopy2.h http://commits.kde.org/calligra/8adefa2cc486ba82a6919edb812e4c63b5ba8e7b
Git commit f9d92a94dfb57fab07ed07fec7248f4914e99ba8 by Dmitry Kazakov. Committed on 03/12/2013 at 13:29. Pushed by dkazakov into branch 'calligra/2.8'. Fix KoCompositeOpCopy2 to handle source alpha channel properly The value of the source aplha should be taken into account while blending two pixels. Otherwise black aerials appear on screen. This might not be too mathematically correct, but at least something. M +7 -0 krita/benchmarks/kis_composition_benchmark.cpp M +2 -0 krita/benchmarks/kis_composition_benchmark.h M +12 -6 libs/pigment/compositeops/KoCompositeOpCopy2.h http://commits.kde.org/calligra/f9d92a94dfb57fab07ed07fec7248f4914e99ba8