How to make GdkPixbuf round-cornered

[Jianchun Zhou <jianchun zhou gmail com>]

Hi, all:

I want to load an image as a GdkPixbuf from a image file, then make it's
CORNERS totally transparency to create a round corner picture, and save it
back into an image file.

Now I just don't know how to make CORNERS transparency.

Anybody can help me?

Thank you in advance.

-- 
Best Regards