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ImageStat.py
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2001-05-03
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#
# The Python Imaging Library.
# $Id$
#
# global statistics
#
# History:
# 96-04-05 fl Created
# 97-05-21 fl Added mask; added rms, var, stddev attributes
# 97-08-05 fl Added median
# 98-07-05 hk Fixed integer overflow error
#
# Notes:
# This class shows how to implement delayed evaluation of
# attributes. To get a certain value, simply access the
# corresponding attribute. The __getattr__ dispatcher takes
# care of the rest.
#
# Copyright (c) Secret Labs AB 1997.
# Copyright (c) Fredrik Lundh 1996-97.
#
# See the README file for information on usage and redistribution.
#
import Image
import operator, math
class Stat:
"Get image or feature statistics"
def __init__(self, image_or_list, mask = None):
try:
if mask:
self.h = image_or_list.histogram(mask)
else:
self.h = image_or_list.histogram()
except AttributeError:
self.h = image_or_list # assume it to be a histogram list
if type(self.h) != type([]):
raise TypeError, "first argument must be image or list"
self.bands = range(len(self.h) / 256)
def __getattr__(self, id):
"Calculate missing attribute"
if id[:4] == "_get":
raise AttributeError, id
# calculate missing attribute
v = getattr(self, "_get" + id)()
setattr(self, id, v)
return v
def _getextrema(self):
"Get min/max values for each band in the image"
def minmax(histogram):
n = 255
x = 0
for i in range(256):
if histogram[i]:
n = min(n, i)
x = max(x, i)
return n, x # returns (255, 0) if there's no data in the histogram
v = []
for i in range(0, len(self.h), 256):
v.append(minmax(self.h[i:]))
return v
def _getcount(self):
"Get total number of pixels in each layer"
v = []
for i in range(0, len(self.h), 256):
v.append(reduce(operator.add, self.h[i:i+256]))
return v
def _getsum(self):
"Get sum of all pixels in each layer"
v = []
for i in range(0, len(self.h), 256):
sum = 0.0
for j in range(256):
sum = sum + j * self.h[i+j]
v.append(sum)
return v
def _getsum2(self):
"Get squared sum of all pixels in each layer"
v = []
for i in range(0, len(self.h), 256):
sum2 = 0.0
for j in range(256):
sum2 = sum2 + (j ** 2) * float(self.h[i+j])
v.append(sum2)
return v
def _getmean(self):
"Get average pixel level for each layer"
v = []
for i in self.bands:
v.append(self.sum[i] / self.count[i])
return v
def _getmedian(self):
"Get median pixel level for each layer"
v = []
for i in self.bands:
s = 0
l = self.count[i]/2
b = i * 256
for j in range(256):
s = s + self.h[b+j]
if s > l:
break
v.append(j)
return v
def _getrms(self):
"Get RMS for each layer"
v = []
for i in self.bands:
v.append(math.sqrt(self.sum2[i] / self.count[i]))
return v
def _getvar(self):
"Get variance for each layer"
v = []
for i in self.bands:
n = self.count[i]
v.append((self.sum2[i]-(self.sum[i]**2.0)/n)/n)
return v
def _getstddev(self):
"Get standard deviation for each layer"
v = []
for i in self.bands:
v.append(math.sqrt(self.var[i]))
return v
Global = Stat # compatibility
if __name__ == "__main__":
im = Image.open("Images/lena.ppm")
st = Stat(im)
print "extrema", st.extrema
print "sum ", st.sum
print "mean ", st.mean
print "median ", st.median
print "rms ", st.rms
print "sum2 ", st.sum2
print "var ", st.var
print "stddev ", st.stddev