the backbuffer does not store any alpha.
So it does not make any difference if you draw it with alpha or not for grabbing. it only makes a difference in the color mixing on the backbuffer.
if you need alpha you must operate on pixmaps directly.
There recently has been a thread on that as well that (not sure, so please don't nail me it on it) shows how to enable the alphabuffer beside the backbuffer to render with alpha into it and get information with alpha from it.
So it does not make any difference if you draw it with alpha or not for grabbing. it only makes a difference in the color mixing on the backbuffer.
if you need alpha you must operate on pixmaps directly.
There recently has been a thread on that as well that (not sure, so please don't nail me it on it) shows how to enable the alphabuffer beside the backbuffer to render with alpha into it and get information with alpha from it.