import numpy as np
import matplotlib.pyplot as plt
from skimage import data
# get some image
image = data.coins()
image = image[:,0:303]
# create array of radii
x,y = np.meshgrid(np.arange(image.shape[1]),np.arange(image.shape[0]))
R = np.sqrt(x**2+y**2)
# calculate the mean
f = lambda r : image[(R >= r-.5) & (R < r+.5)].mean()
r = np.linspace(1,302,num=302)
mean = np.vectorize(f)(r)
# plot it
fig,ax=plt.subplots()
ax.plot(r,mean)
plt.show()