X11 programming is a b*tch. The little code I've written for dotgnu using libX11 must've damaged my brain more than second-hand smoke and caffeine overdoses put together. So, when someone asked for a quick program to look at the X11 window and report pixel modifications my immediate response was "Don't do X11". But saying that without offering a solution didn't sound too appealing, so I digged around a bit with other ways to hook into display code.
RFB: Remote Frame Buffer is the client-server protocol for VNC. So, to steal some code, I pulled down pyvnc2swf. But while reading that I had a slight revelation - inserting my own listeners into its event-loop looked nearly trivial. The library is very well written and has very little code in the protocol layer which assumes the original intention (i.e making screencasts). Here's how my code looks.
class VideoStream:
def paint_frame(self, (images, othertags, cursor_info)):
...
def next_frame(self):
...
class VideoInfo:
def set_defaults(self, w, h):
...
converter = RFBStreamConverter(VideoInfo(), VideoStream(), debug=1)
client = RFBNetworkClient("127.0.0.1", 5900, converter)
client.init().auth().start()
client.loop()
Listening to X11 updates from a real display is that simple. The updates are periodic and the fps can be set to something sane like 2 or 3. The image data is raw ARGB with region info, which makes it really simple to watch a particular window. The VNC server (like x11vnc) takes care of all the XDamage detection and polling the screen for incremental updates with that - none of that cruft needs to live in your code.
Take a look at rfbdump.py for the complete implementation - it is hardcoded to work with a localhost vnc server, but it should work across the network just fine.
--You can observe a lot just by watching.
-- Yogi Berra