Image.getdata（）返回什么？

Question

import Image

i1 = Image.open("image1.jpg")

i2 = Image.open("image2.jpg")

pairs = zip(i1.getdata(), i2.getdata())

if len(i1.getbands()) == 1:

    # for gray-scale jpegs

    dif = sum(abs(p1-p2) for p1,p2 in pairs)

else:

    dif = sum(abs(c1-c2) for p1,p2 in pairs for c1,c2 in zip(p1,p2))

ncomponents = i1.size[0] * i1.size[1] * 3

print "Difference (percentage):", (dif / 255.0 * 100) / ncomponents    

上面的代码是要获取2张图片之间的百分比差异？ 如果每个像素值相减，除以255的必要性是什么？