如何让skimage的活动轮廓被推到边角

import numpy as np
import matplotlib.pyplot as plt
from skimage import data, io, measure
from skimage.color import rgb2gray
from skimage.segmentation import active_contour

image = io.imread('image.jpg')
image_gray = rgb2gray(image)

s = np.linspace(0, 2*np.pi, 400)
x = 220 + 100*np.cos(s)
y = 100 + 100*np.sin(s)
init = np.array([x, y]).T

snake = active_contour(image_gray, init, alpha=0.015, beta=10, gamma=0.001)

fig, ax = plt.subplots(figsize=(7, 7))
ax.imshow(image_gray, cmap=plt.cm.gray)
ax.plot(init[:, 0], init[:, 1], '--r', lw=3)
ax.plot(snake[:, 0], snake[:, 1], '-b', lw=3)
ax.set_xticks([]), ax.set_yticks([])
plt.show()