Capturing windows in an NSImage

FROM : Eric Summers
DATE : Wed Jul 19 23:31:11 2006

I'm trying to figure out how programs like Apple's Grab screen capture
application get an image of other application windows.  I thought of using
an AppleEvent to get the bounds of the windows then capturing the data using
a big transparent window, but that wouldn't work if the window is hidden.
Grab will get the image data even if the window is hidden.  Anyone know how
the Grab application works?