Created attachment 119329 [details] Screenshot of the problem 1) Enable Vector Optimizations (VC) in the performance tab 2) Create a 32-bit float image 3) Draw with Basic-5 Size brush Observe random black tiles aligned by 32/16-byte level (8/4 pixels). See the attached screenshot. The bug appears on both Windows and Linux with Vc 1.3.3, with either AVX or SSE4 implementation used. It means that it is unrelated to bug 406209. It is somehow related to our implementation of AlphaDarkenCompositor128 and OverCompositor128. PS: Or a bug in Vc... PPS: Or a bug in GCC...
Created attachment 119337 [details] Rendering error on 32bit float The behaviour is a little different on macOS compiled sources. Vc 1.4.1 Clang Apple LLVM version 10.0.1 (clang-1001.0.46.3)
On Windows 10 and on Linux Mint with Qt 5.11 the artifacts are red, too. They seem to be multiplied with the stroke. To see the artifacts, instead of making the stroke, put one stamp on the canvas. *** This bug has been marked as a duplicate of bug 404133 ***