Problems with pixbufs

[Tuomas Peippo <tuomas peippo 2ndpoint fi>]

I'm trying to "blend" two pixbufs together using gdk_pixbuf_composite
but I'm having trouble understanding what it actually does.

I was hoping that it would blend two pixbufs according to alpha
channels e.g if I have a image with red square on white background
which has alpha value 0 ( which I hope means fully transparent ) and I
would like to merge it with image that has some random pixels on white
background so I guess that gdk_pixbuf_composite() doesn't blend it so
that only red square would transfer?

I would be very grateful if someone would explain true meaning of this  
function and/or how to achieve my wanted result.

Tuomas Peippo