In addition to what Carlos said about using damage evens and transparent overlays, you probably want some sort of notification of activity in windows not currently displayed on screen. This is similar to urgency wm hints, but triggered by window content change rather than explicit application requests. One approach is simply to set the urgency hint when change is detected.
Note that this is different from indicating what has changed between two points in time as in the image diff suggestion. One way to use this technology is to dim all windows except the focused one, and then change opacity of parts with activity. More in line with the research article would be to only dim windows on non-focused monitors. -Truls
