How to draw an arc of a circle?

Hi Nahuel
Thank you so much, it really helps.
Just one more step to go, since I need to use fullscreen(1920x1080) to show the stimuli, the pie-shape will be deformed like these…


(But after I changed the window size into a square like 800x800 it worked well … )

So how can I solve this problem?? Thank you again!!