Use a color to create an alpha channel for a PNG?

I have several images that claim to have a transparent background but are actually white. I'd like to use Python Image Library/PIL to set that white background color to actually be transparent.

Since PNG uses an alpha channel, I'd love to create the alpha channel by finding contiguous areas of white开发者_JAVA百科 from the edges of the image (so I don't get "holes" of transparency when the image contains white data).

Any tips on how to create the alpha channel this way?

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I'd guess you'd want to run across the image in a spiral from the outside, setting a pixel to transparent if it is white, and a pixel further towards the edge is also white transparent. Stop once you've done a whole circle without changing any pixels.

Shouldn't be too difficult to write such a loop.

Do some kind of flood fill, seeded from the white edge pixels.