Set Dilation Kernel Anchor

Question

I am trying to understand how to control the kernel anchor in dilation's OpenCV. Here is my example code to explain my idea:

import cv2
import numpy as np
import matplotlib.pyplot as plt

img = np.zeros((7, 7))
img[3, 3] = 255
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (1, 2), anchor=(0, 0))
print(kernel)
img = cv2.dilate(img, kernel)
plt.imshow(img, cmap='gray')
plt.show()

and here is the corresponding output:

When I change the kernel anchor as to (0, 1),

kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (1, 2), anchor=(0, 1))

I expect that the dilation would be upwards, but I am getting the exact same result. Does anyone have an explanation for this?

Thanks in advance!

Answer 1

You need to set the anchor not in the function cv2.getStructuringElement, but in the function cv2.dilate.

img = cv2.dilate(img, kernel, anchor=(0, 1))

or

img = cv2.dilate(img, kernel, anchor=(0, 0))