1) Set the Angle Expression to be Direction
2) Draw vertically with a slow or medium speed
The resulted brushstroke would look like this:
It can be seen that the brush Angle keeps changing between 0 degree and 90 degree for some reason. The Direction expression shouldn't work like this.
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I did another test to see how paint direction would trigger this problem:
It is clear that this problem only appears when drawing vertically.
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This test is to see how paint speed (with a digital tablet) would trigger this problem:
It can be seen that the slower the brush moves, the more often the problem appears. However, if use a mouse or touch pad instead, the probelm appears almost 100% regardless how fast the brush moves.
