Several people have complained that importing OpenCV is cheating and that claiming MintEye was broken in 23 lines is disingenuous. So, here’s a solution in 34 lines of Python:

import sys
import os
import matplotlib.pyplot as plt
import math
from PIL import Image

for dir in range(1,14):
    dir = str(dir)

    total_images = len(os.listdir(dir))+1

    points_sob = []
    for i in xrange(1,total_images):
        image = Image.open(dir+'/'+str(i)+'.jpg')
        im = image.load()

        #convert to grayscale (ITU-R 601-2 luma transform)
        grey = [[None for _ in range(image.size[1])] for _ in range(image.size[0])]
        for x in xrange(image.size[0]):
            for y in xrange(image.size[1]):
                grey[x][y] = im[x,y][0]*299/1000 + im[x,y][1]*587/1000 + im[x,y][2]*114/1000

        #sobel
        sum_of_sob = 0
        for x in xrange(1,image.size[0]-1):
            for y in xrange(1,image.size[1]-1):
                gx = -grey[x-1][y-1] + grey[x+1][y-1] - 2*grey[x-1][y] + 2*grey[x+1][y] - grey[x-1][y+1] + grey[x+1][y+1]
                gy = -grey[x-1][y-1] - 2*grey[x][y-1] - grey[x+1][y-1] + grey[x-1][y+1] + 2*grey[x][y+1] + grey[x+1][y+1]
                sum_of_sob = sum_of_sob + math.sqrt(gx*gx + gy*gy)

        print sum_of_sob
        points_sob.append(sum_of_sob)

    res = points_sob.index(min(points_sob)) + 1
    x = xrange(1,total_images)
    plt.plot(res,points_sob[res-1], marker='o', color='r', ls='')
    plt.plot(x, points_sob)

    #plt.savefig(dir+'.png')
    plt.show()

The point I’m trying to make is that Sobel is a very simple operator – that’s why it was created, as a crude approximation of the derivative of a 2D signal.