I got a very strange optical illusion with the code below. Was trying to make a radial blur effect. Is interesting. 

'''
    Pythonista Forum - @Phuket2
'''
import ui

class MyClass(ui.View):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def draw(self):
        wh = 100
        r = ui.Rect(0, 0, wh, wh)
        r.center(self.bounds.center())
        s = ui.Path.oval(*r)
        rd = 1
        for i in range(int(r.width)):
            s = ui.Path.oval(*r)
            s.line_width = 1
            c = (0,  rd, 0, 1)
            ui.set_color( c)
            s.stroke()
            r = r.inset(.5, .5)
            rd -=.01


if __name__ == '__main__':
    w, h = 320, 480
    f = (0, 0, w, h)

    mc = MyClass(frame=f, bg_color='white')
    mc.present('sheet', animated=False)

If you change rd -= .01, to rd -= .02 then you get the below

Sort of looks like a radial blur. But with rd-=.01 you get a wacky illusion. Just for fun