ZeroDivisionError (Python)

Question

I am getting a Zero Division Error with some images (Even though a lot of them work just fine) :

Here's the code :

image = skimage.io.imread('test.png', False)
image_gray = skimage.io.imread('test.png', True)
blurred = cv2.GaussianBlur(img_as_ubyte(image_gray), (5, 5), 0)
thresh = threshold_li(blurred)
binary = blurred > thresh
binary_cv2 = img_as_ubyte(binary)

# find contours in the thresholded image
cnts = cv2.findContours(binary_cv2.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if imutils.is_cv2() else cnts[1]

# loop over the contours
for c in cnts:
    # compute the center of the contour
    M = cv2.moments(c)
    cX = int(M["m10"] / M["m00"])
    cY = int(M["m01"] / M["m00"])

    # draw the contour and center of the shape on the image
    cv2.drawContours(img_as_ubyte(image), [c], -1, (0, 255, 0), 2)
    cv2.circle(img_as_ubyte(image), (cX, cY), 7, (255, 255, 255), -1)
    cv2.putText(img_as_ubyte(image), "center", (cX - 20, cY - 20),
    cv2.FONT_HERSHEY_SIMPLEX, 0.5, (255, 255, 255), 2)

viewer = ImageViewer(image)
viewer.show()

---

Traceback (most recent call last):
  File "Center.py", line 26, in <module>
    cX = int(M["m10"] / M["m00"])
ZeroDivisionError: float division by zero

Thanks in advance!

Answer 1

Error is self-evident. You cannot divide a number by zero. If M["m00"] is zero, then you need to handle it appropriately. Check for 0 values in M["m00"].

if M["m00"] != 0:
    cX = int(M["m10"] / M["m00"])
    cY = int(M["m01"] / M["m00"])
else:
    # set values as what you need in the situation
    cX, cY = 0, 0

Answer 2

Probably you have bad contours

Note Since the contour moments are computed using Green formula, you may get seemingly odd results for contours with self-intersections, e.g. a zero area (m00) for butterfly-shaped contours.

Answer 3

Calculate the the center of mass, e.g.:

cx = 0
cy = 0
for p in contour:
    cx += p[0][0]
    cy += p[0][1]
cx = int(cx/len(contour))
cy = int(cy/len(contour))

or look into boundingRect() (3.4.3).

Related: Finding the center of a contour using opencv and visual c++