Question

What’s wrong with this – why won’t it draw the circle in the window I create?

    threeDWinName = "3D View"
    cv2.namedWindow(threeDWinName, cv2.CV_WINDOW_AUTOSIZE)
    img2 = cv2.imread('white.png', 0)
    cv2.imshow(threeDWinName,img2)
    cv2.circle(img2, (100,100),100,255,-1)
    cv2.imshow(threeDWinName,img2)

Answer 1

Your code works for me. I see the file you are reading is called white.png, and by loading it with the 0 you are loading it in grayscale, so the circle you are drawing is also white. That could be the problem 😉

img2 = cv2.imread('black.png', 0)
cv2.circle(img2, (100,100),100,255,-1)
cv2.imwrite('circle.png', img2)

or if you use img2 = cv2.imread('black.png'), then you get this: