def sample_pixels(img, options):

    '''Pick a fixed percentage of pixels in the image, returned in random
order.'''

    pixels = img.reshape((-1, 3))
    num_pixels = pixels.shape[0]
    num_samples = int(num_pixels*options.sample_fraction)

    idx = np.arange(num_pixels)
    np.random.shuffle(idx)

    return pixels[idx[:num_samples]]
Pick a fixed percentage of pixels in the image, returned in random
order.