Update of bug #19988 (project mypaint):
Status: Ready For Test => In Progress
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Follow-up Comment #5:
Confirmed the new oddity with some fake tilt inputs (just feed in any faked
nonzero tilt in the event handler). To make it apparent use a brush whose
elliptical ratio follows its declination and whose elliptical dab axis depends
on its (right) ascension. Basically emulate a flat brush being used for broad
strokes :) Then doodle while rotating the canvas to draw a spiral. Even for
constant faked tilts, you'll get a weird little twirly break in the otherwise
constant stroke, looking like
------------|/-|/-----------
A B
The twirl weirdness happens whenever the ascension (after correction for
canvas tilt) passes through -180 and 180 degrees between one motion event at
time A above and the next. The brush engine, in mypaint-brush.c, does a linear
interpolation between the values over several subsequent dabs until it doesn't
need to any more, B, but that results in a 360 degree twirl for brushes like
these.
This is has always been happening, but it's _much_ more likely to be noticable
now we're doing a correction based on canvas rotation. To trigger it, the pen
doesn't need to be at approx ±180 and facing away from you any more. Just
passing through canvas "up" with your tilt direction and an extreme tilt will
trigger this now.
Probably the right thing to do is to fix it in the brush engine. I think
interpolating in the direction of the minimum distance around the circle
(clockwise or anticlockwise, whichever makes a smaller difference) and
clipping the values using modulo arithmetic would be a good start.
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