1、crop 方法函数原型为：
Image.crop(box=None)Returns a rectangular region from this image. The box is a 4-tuple defining the left, upper, right, and lower pixel coordinate.
This is a lazy operation. Changes to the source image may or may not be reflected in the cropped image. To break the connection, call the load() method on the cropped copy.
Parameters: box – The crop rectangle, as a (left, upper, right, lower)-tuple.
Return type: Image
Returns: An Image object.
2、通过矩形的左上角的坐标和右下角的坐标，即可构造box，例如下面的代码：
box = (100, 100, 400, 400)
region = im.crop(box)
3、通过提取除ROI时，上面例子为 region，保存ROI到图像则使用类 Image 的 save 方法：
region.save(filename)
4、写一个Demo例子，使用人脸数据库GENKI部分的图像做实验，该数据的数字子集GENKI-SZSL提供人脸区域的坐标和大小。提取代码如下：
from PIL import Image
import os
src = '.'
imlist = open(src + '/GENKI-SZSL_Images.txt', 'r').readlines()
rs = [float(line.split()[1]) for line in open(src + '/GENKI-SZSL_labels.txt', 'r').readlines()]
cs = [float(line.split()[0]) for line in open(src + '/GENKI-SZSL_labels.txt', 'r').readlines()]
ss = [float(line.split()[2]) for line in open(src + '/GENKI-SZSL_labels.txt', 'r').readlines()]
for i in range(0, len(rs)):
path = src + '/images/' + imlist[i].strip()
filename = src + '/output/' + imlist[i].strip()
try:
im = Image.open(path)
except:
continue
r = rs[i]
c = cs[i]
s = ss[i]
xLeft = int(c - s/2)
yUpper = int(r - s/2)
xRight = int(c + s/2)
yLower = int(r + s/2)
region = im.crop((xLeft, yUpper, xRight, yLower))
region.save(filename)
