How to set alpha value of a pixel in Python

Question

I'm trying to edit an image in a way such that all white pixels are converted to transparent pixels (with 0 alpha value). Basically I want to get rid of the background.

I know of the image file's im.putpixel method, but from my experimentation this does not allow me to set alpha values. I tried the following:

for x in range(90):
    for y in range(80, 90):
        im.putpixel((x, y), (0, 0, 0, 0))

and just got blackness in the specified area, not transparency. Is there a way to alter the alpha value of a pixel? If so, how?

Answer 1

It sounds like you are using python image library (PIL).

If so, you can just do something like this:

from PIL import Image
image = Image.open("<path to image file>").convert('RGBA')
pixeldata = list(image.getdata())

for i,pixel in enumerate(pixeldata):
    if pixel[:3] == (255,255,255):
        pixeldata[i] = (255,255,255,0)

image.putdata(pixeldata)
image.save("output.png")

That should generate something like this:

Of course, that's probably not exactly what you wanted. One thing you can do is set pixels to transparent when the color is close to white instead of exactly white. You can define a function like

def almostEquals(a,b,thres=5):
    return all(abs(a[i]-b[i])<thres for i in range(len(a)))

And replace the color checking line with:

if almostEquals(pixel[:3], (255,255,255):

To get something like this

Still some haloing/border artifacts but better.

In your case, with a simple black and white image, you may just want to set the alpha based on how white your pixels are (I often do this in photoshop). You can use one of the 3 color channels or even the average. Something like:

for i,pixel in enumerate(pixeldata):
    avg = int(sum(pixel[:3])/3.0)
    pixeldata[i] = (pixel[0],pixel[1],pixel[2],255-avg)

Would give you a result like this:

Still a minor halo, since we are still only changing alpha values, but much better. To fix even that, you could do something like set all pixels to black as well instead of just modifying the alpha.

Anyways, a long answer for a simple question, but hopefully that helps.