python为图片加水印，求大神帮助啊，帮忙解释一下啊，有点捉急啊

import Image, ImageEnhance #引用所需的库
POSITION = ('LEFTTOP','RIGHTTOP','CENTER','LEFTBOTTOM','RIGHTBOTTOM',"title","scale") #Logo的位置
PADDING = 10
MARKIMAGE = r'C:\Users\Administrator\Desktop\期末复习\Python\02.png' # Logo图片的位置
def reduce_opacity(im, opacity): # 处理Logo的透明度
assert opacity >= 0 and opacity <= 1 #图片透明度
if im.mode != 'RGBA': #图片模式
im = im.convert('RGBA')
else:
im = im.copy()
alpha = im.split()[3]
alpha = ImageEnhance.Brightness(alpha).enhance(opacity)
im.putalpha(alpha)
return im
def watermark(imagefile, markfile, position=POSITION[4], opacity=0.1):
im = Image.open(imagefile)
mark = Image.open(markfile)
if opacity < 1:
mark = reduce_opacity(mark, opacity)
if im.mode != 'RGBA':
im = im.convert('RGBA')
# 创建一个透明层，根据位置 来画Logo.
layer = Image.new('RGBA', im.size, (0,0,0,0))
if position == 'title':
for y in range(0, im.size[1], mark.size[1]):
for x in range(0, im.size[0], mark.size[0]):
layer.paste(mark, (x, y))
elif position == 'scale':
# 保持原长宽比例
ratio = min(
float(im.size[0]) / mark.size[0], float(im.size[1]) / mark.size[1])
w = int(mark.size[0] * ratio)
h = int(mark.size[1] * ratio)
mark = mark.resize((w, h))
layer.paste(mark, ((im.size[0] - w) / 2, (im.size[1] - h) / 2))
elif position == POSITION[0]:
#左上角
position = (PADDING,PADDING)
layer.paste(mark, position)
elif position == POSITION[1]:
#右上角
position = (im.size[0] - mark.size[0]-PADDING, PADDING)
layer.paste(mark, position)
elif position == POSITION[2]:
#中间
position = ((im.size[0] - mark.size[0])/2,(im.size[1] - mark.size[1])/2)
layer.paste(mark, position)
elif position == POSITION[3]:
#左下角
position = (PADDING,im.size[1] - mark.size[1]-PADDING,)
layer.paste(mark, position)
else:
#右上角
position = (im.size[0] - mark.size[0]-PADDING, im.size[1] - mark.size[1]-PADDING,)
layer.paste(mark, position)
# 组合起来就可以啦
return Image.composite(layer, im, layer)
new_image_s_filename=r"C:\Users\Administrator\Desktop\期末复习\Python\02-1.png"
# 要加水印的图片
watermark(new_image_s_filename,MARKIMAGE,POSITION[2],opacity=0.2).save(new_image_s_filename,quality=100)
可以仔细解释一下不？？？谢谢大神啊