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#!/usr/bin/env python
# $URL: http://pypng.googlecode.com/svn/trunk/code/pipasgrey $
# $Rev: 187 $
# pipasgrey
# Convert image to grey (L, or LA), but only if that involves no colour
# change.
def asgrey(out, inp, quiet=False):
"""Convert image to greyscale, but only when no colour change. This
works by using the input G channel (green) as the output L channel
(luminance) and checking that every pixel is grey as we go. A non-grey
pixel will raise an error, but if `quiet` is true then the grey pixel
check is suppressed.
"""
from array import array
import png
r = png.Reader(file=inp)
_,_,pixels,info = r.asDirect()
if info['greyscale']:
w = png.Writer(**info)
return w.write(out, pixels)
planes = info['planes']
targetplanes = planes - 2
alpha = info['alpha']
width = info['size'][0]
typecode = 'BH'[info['bitdepth'] > 8]
# Values per target row
vpr = width * (targetplanes)
def iterasgrey():
for i,row in enumerate(pixels):
row = array(typecode, row)
targetrow = array(typecode, [0]*vpr)
# Copy G (and possibly A) channel.
green = row[0::planes]
if alpha:
targetrow[0::2] = green
targetrow[1::2] = row[3::4]
else:
targetrow = green
# Check R and B channel match.
if not quiet and (
green != row[0::planes] or green != row[2::planes]):
raise ValueError('Row %i contains non-grey pixel.' % i)
yield targetrow
info['greyscale'] = True
del info['planes']
w = png.Writer(**info)
w.write(out, iterasgrey())
def main(argv=None):
from getopt import getopt
import sys
if argv is None:
argv = sys.argv
argv = argv[1:]
opt,argv = getopt(argv, 'q')
quiet = False
for o,v in opt:
if o == '-q':
quiet = True
if len(argv) > 0:
f = open(argv[0], 'rb')
else:
f = sys.stdin
return asgrey(sys.stdout, f, quiet)
if __name__ == '__main__':
main()
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