| [iavarfilter] [Up] [iahistogram] | Automatic Thresholding Techniques |
Compute the automatic thresholding level of a gray scale image based on the Otsu method.
>>> import Numeric
>>> f = iaread('cookies.pgm')
>>> iashow(f)
(174, 314) Min= 16 Max= 205 Mean=94.752 Std=57.61
>>> (t, eta) = iaotsu(f)
>>> print 'threshold at %f, goodness=%f\n' %(t, eta)
threshold at 90.000000, goodness=0.938940
>>> iashow(f > t)
(174, 314) Min= 0 Max= 1 Mean=0.527 Std=0.50
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| f | f > t |
def iaotsu(f):
from Numeric import product, shape, arange, cumsum, nonzero, sum
n = product(shape(f))
h = 1.*iahistogram(f) / n
x = arange(product(shape(h)))
w0 = cumsum(h)
w1 = 1 - w0
eps = 1e-10
m0 = cumsum(x * h) / (w0 + eps)
mt = m0[-1]
m1 = (mt - m0[0:-1]*w0[0:-1]) / w1[0:-1]
sB2 = w0[0:-1] * w1[0:-1] * ((m0[0:-1] - m1)**2)
v = max(sB2)
t = nonzero(sB2 == v)[0]
st2 = sum((x-mt)**2 * h)
eta = v / st2
return t, eta
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