open_by_reconstruction()

def open_by_reconstruction(src, iterations = 1, ksize = 3):
        # first erode the source image
        eroded = cv2.erode(src, np.ones((ksize,ksize), np.uint8), iterations=iterations)

        # Now we are going to iteratively regrow the eroded mask.
        # The key difference between just a simple opening is that we
        # mask the regrown everytime with the original src.
        # Thus, the dilated mask never extends beyond where it does in the original.
        this_iteration = eroded
        last_iteration = eroded
        while (True):
            this_iteration = cv2.dilate(last_iteration, np.ones((ksize,ksize), np.uint8), iterations = 1)
            this_iteration = this_iteration & src
            if np.array_equal(last_iteration, this_iteration):
                # convergence!
                break
            last_iteration = this_iteration.copy()

        return this_iteration