Preprocess 210x160x3 uint8 frame into 6400 (80x80) 1D float vector.

def preprocess(img):
    """Preprocess 210x160x3 uint8 frame into 6400 (80x80) 1D float vector."""
    # Crop the image.
    img = img[35:195]
    # Downsample by factor of 2.
    img = img[::2, ::2, 0]
    # Erase background (background type 1).
    img[img == 144] = 0
    # Erase background (background type 2).
    img[img == 109] = 0
    # Set everything else (paddles, ball) to 1.
    img[img != 0] = 1
    return img.astype(np.float).ravel()