def is_dark(x, y):
    """
    Determine if a given pixel coordinate is a 'dark' pixel or not.
    """
    # Odds are (0,0), (0,2), (1,1), (1,3)
    # Evens are (0,1), (0,3), (1,0), (1,2)
    return (x + y) % 2 == 0