from PIL import Image
from math import *

im = Image.open("BarredRock.jpg")
width, height = im.size

pix = im.load()
print pix[500, 500]
for i in range(width):
    for j in range(height):
        r,g,b = pix[i,j]
        radius = sqrt(pow(width/2 - i, 2) + pow(height/2 - j, 2))
        spot_radius = 300
        if(radius < spot_radius):
            pix[i,j] = (r, g, b)
        else:
            brightness = 1 - ((radius-spot_radius) / spot_radius)
            if(brightness < 0): brightness = 0
            pix[i, j] = tuple([int(d * brightness) for d in [r, g, b]])

im.show()