Script to generate a series of crops of a source image

dice_image.py

import Image
import random

def dice_image(input_image, width, height, num_pics):
    """
    Dice_image creates num_pics images from a source image. It passes
    a window of specified size over the image, crops and
    exports new images.

    Inputs:
    input_image - filename of the PNG image to dice up (in local directory)
    width - width of each resulting cropped image
    height - height of each resulting cropped image
    num_pics - number of thumbnails to create
    """
    input_img = Image.open(input_image)
    source_width, source_height = input_img.size
    start_x, start_y = 0, 0
    iteration, pic_num = 1, 1

    max_crops = (source_width / width) * (source_height / height)
    
    # Set of window IDs to crop, ignore the rest
    try:
        crops = set(random.sample(range(1, max_crops + 1), num_pics))
    except ValueError:
        crops = set(range(1, max_crops + 1))

    for start_x in range(0, source_width - width + 1, width):
        for start_y in range(0, source_height - height + 1, height):
            if iteration in crops:
                box = (start_x, start_y, start_x + width, start_y + height)
                output_img = input_img.crop(box)

                # Strip .png extension, append unique ID
                output_img.save(input_image[:-4] + "_" + str(pic_num) + ".png")
                pic_num += 1

            iteration += 1