An OpenCV script that you can use to transform multiple images into one single image containing the minimum pixel value for each pixel of the input images.

minimum-pixel-values.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
"""
Written on 2013-03-18 by Philipp Klaus <philipp.l.klaus →AT→ web.de>.
Check <https://gist.github.com/5188638> for newer versions.
 
An OpenCV script that you can use to transform multiple images into one
single image containing the minimum pixel value for each pixel of the
input images. The input images have to be of the same dimensions!
"""

import cv2
import numpy as np
import argparse
import sys

DEBUG = False

if __name__ == '__main__':
    parser = argparse.ArgumentParser(description='Take a couple of images and determine the minimum values for each pixel.')
    parser.add_argument('-o', '--outfile', metavar='output-image.png',
            help='The resulting image is being written to this file.', required=True)
    parser.add_argument('-g', '--gray', action='store_true',
            help='Use this option if your input images ar grayscale!')
    parser.add_argument('-v', '--verbose', action='store_true',
            help='Give more detailed output.')
    parser.add_argument('infiles', metavar='INFILE', nargs='+',
            help='The image files you want to be analyzed.')
    args = parser.parse_args()

    if len(args.infiles) == 1:
        sys.stderr.write("Using this tool to calculate the minimum with only one input image doesn't make sense.\n")
        sys.stderr.write("Continuing anyway.\n")

    outimg = None
    for infile in args.infiles:
        if args.verbose: print "Analyzing %s now." % infile
        inimg = cv2.imread(infile, 0 if args.gray else 1)
        if outimg == None:
            outimg = np.copy(inimg)
        if DEBUG:
            cv2.imshow('current input image', inimg)
            cv2.waitKey()
        outimg = np.minimum(outimg, inimg)
        # Here is an alternative to numpy.minimum() but it's much slower,
        # I left it here because it shows how to work with np.select():
        #condlist = [outimg<inimg, outimg>=inimg]
        #choicelist = [outimg, inimg]
        #outimg = np.select(condlist, choicelist)
        if DEBUG:
            cv2.imshow('current output image', outimg)
            cv2.waitKey()
    if args.verbose: print "Writing result to %s." % args.outfile
    cv2.imwrite(args.outfile, outimg)