import pylab as p
import numpy as np
import mahotas
f = np.ones((256,256), bool)
f[200:,240:] = False
f[128:144,32:48] = False
# f is basically True with the exception of two islands: one in the lower-right
# corner, another, middle-left
dmap = mahotas.distance(f)
p.imshow(dmap)
p.show()