Detect "changing content events" on a osx screen

This osx script should wait until a partial change of the content of the osx screen happens. The change must be in a given rectangle on the screen - defined by two cursor positions (-> script parameter). The rectangle must be visible on the screen while waiting for the change. The script is called with a maxmium wait time (in seconds). After this "maxtime" the script cancels and returns an error code. If successful the "waittime" is returned (in seconds: [login to view URL] format). The script should be used within applescript with OSX 10.7 (or newer).