Here's the code I use to create new image, draw a circle onto it and show it:
import numpy as np
import cv2
# create 50x50 image, filled with white
img = np.ones((50,50))
# draw a circle onto the image
cv2.circle(img, (25,25), 10, 0, 2, lineType=cv2.LINE_AA)
# show the image on the screen
cv2.imshow("i", img)
cv2.waitKey(0)
cv2.destroyAllWindows()
However, no antialiasing seems to be applied (I upscaled the image x10):
What am I doing wrong?