Using PIL to make all white pixels transparent?

Question

I'm trying to make all white pixels transparent using the Python Image Library. (I'm a C hacker trying to learn python so be gentle) I've got the conversion working (at least the pixel values look correct) but I can't figure out how to convert the list into a buffer to re-create the image. Here's the code

img = Image.open('img.png')
imga = img.convert("RGBA")
datas = imga.getdata()

newData = list()
for item in datas:
    if item[0] == 255 and item[1] == 255 and item[2] == 255:
        newData.append([255, 255, 255, 0])
    else:
        newData.append(item)

imgb = Image.frombuffer("RGBA", imga.size, newData, "raw", "RGBA", 0, 1)
imgb.save("img2.png", "PNG")

Answer 1

You need to make the following changes:

• append a tuple (255, 255, 255, 0) and not a list [255, 255, 255, 0]
• use img.putdata(newData)

This is the working code:

from PIL import Image

img = Image.open('img.png')
img = img.convert("RGBA")
datas = imga.getdata()
newData = list()

for item in datas:
  if item[0] == 255 and item[1] == 255 and item[2] == 255:
    newData.append((255, 255, 255, 0))
  else:
    newData.append(item)

img.putdata(newData)
img.save("img2.png", "PNG")

Answer 2

You can also use pixel access mode to modify the image in-place:

from PIL import Image

img = Image.open('img.png')
img = img.convert("RGBA")

pixdata = img.load()

for y in xrange(img.size[1]):
    for x in xrange(img.size[0]):
        if pixdata[x, y] == (255, 255, 255, 255):
            pixdata[x, y] = (255, 255, 255, 0)

img.save("img2.png", "PNG")

Answer 3

Here is a mailing list message that seems appropriate. The solution is much more complicated than those presented here, but possibly faster.

Answer 4

Or if you'd just like to download pre-computed white/black pixels, grab this .zip: http://transparentpixelpack.prestonlee.com