howto: Transparent Image in Python

First I would recommend reading The pil handbook ,
it has a lot of info on the questions you're asking, and if you use pil alot you'll need to refer to it quite a bit.

To answer this specific question, you need to understand image alpha values.
Put simply an alpha value is an integer between 0 and 255, 0 being completely transparent, and 255 being completely opaque.

img.paste takes 3 args, (sourceimage ,position, mask).
The mask is a single-band image, or an image with an alpha band.

Wherever the mask image has a value of 0 the "underneath" pixel will show, wherever it has a value of 255 the "above" pixel will show,
and any value in-between will result in a mixture of both pixels.

if you want an image to hold its own alpha band it needs to be converted to
'RGBA' mode (if its not already) then you can just use an image as its own mask.
eg. image.paste(image2, pos, image2)

Now if all you want to do is remove one color (colorkeying),
I haven't found an "easy" way to do it but you can do something like this:
(not tested)

but I imagine this would be quite slow

[Edited by - Euronomus on January 24, 2007 3:12:11 PM]