David Janes' Code Weblog

September 18, 2011

Adding alpha levels to images with PIL

code fragments,pil,python,tips · admin · 4:54 am ·

I recently was tasked by a client with replacing a number of images — PNGs with alpha levels, such that parts of the image were transparent — with new pictures. The issue was how to get the exact same alpha level holes in the new images.

Here’s how I did it. I renamed one of the original images “template.png” and assumed that all the source images will the exactly the same size as the template. I then wrote this Python script using the Python Imaging Library.

import sys
import os
import re
import Image

#
#   Get the template
#
itemplate = Image.open("template.png")
itemplate.getpixel(( 0, 0 ))
r, g, b, alpha = itemplate.split()

#
#   Get the JPGs - they must be exactly the same size
#
for in_file in os.listdir("."):
    if not in_file.endswith(".jpg"):
        continue

    isrc = Image.open(in_file)
    if isrc.size != itemplate.size:
        continue
    
    idst = isrc.convert("RGBA")
    idst.putalpha(alpha)

    idst.save(in_file[:-4] + ".png")

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