Colorize mask gives a jagged result (compared to GMIC colorize). In most cases the strokes hide the artifacts but in some cases they are very visible and ugly. See https://imgur.com/a/5DBrJk3 for a comparison.
Hi, cyaoeu! Thank you for your report! Your abservation is really interesting. I'm not sure if I can tell how to fix that... Btw, what it the usecase for these smooth borders? Do you want to fill without the strokes at all? Could you also provide with a test file with the mask you created?
Please attach images to the bug report, and also provide a test image so we can try to reproduce the issue.
Created attachment 113473 [details] Krita file for starting krita colorize/GMIC colorize .kra file.
Created attachment 113474 [details] Krita colorize result Left side has 2px strokes, right side has 1px strokes (bit extreme but shows the artifacts).
Created attachment 113475 [details] GMIC colorize result As before left side: 2px strokes, right side: 1px strokes. You can see there are less artifacts. The Krita colorize result has dots of red that poke through the stroke but the GMIC colorize result doesn't (at least way less) and looks better. The 1px stroke is almost eaten by the antialiasing but still looks okay because the aliasing has a constant pattern where the Krita result is very irregular and easily noticable that something is wrong.
updating status
I can confirm, the results of Colorize Mask have artifacts, while GMIC Colorize (interactive) gives smooth results. I tried various options of Colorize Mask to make it work better, but it didn't.
Created attachment 115369 [details] KritaColorizeMask Source of the picture: close-up of the second right circle/oval from the original lineart, filled up with Colorize Mask. There is also a line from lineart moved to the left to show the artifacts a little better.
Re-confirming this bug for 5.2.9