Don’t use polygon borders above 20
Even if you use height units, polygon borders are measured in pixels. Artifacts appear locally when larger values are used. In the screenshots below, the circle on the right has a size of .3, a fill colour of None and a line width of lineWidth, the value shown above the circles. The circle on the left has a size of .3, a fill colour of white and a grey circle in front of it with a size of .3 - lineWidth / 1000.
Note that the overall size of the circle on the right gets bigger when the lineWidth increases. I could have emulated this on the left by also changing the white circle but felt that in real experiments, I would probably prefer to keep the external size constant.
These artifacts do not appear online.
However, the resolution is affected by the display scale. The above screenshots were on my laptop at 250%.
This is at 100%
and this is at 350%
Unfortunately there doesn’t seem to be a way for PsychoPy to detect or compensate for the participant’s display scale.