Answer of FAQ

Restricting the recording to a single window cannot be done directly while recording.
Therefore please proceed as follows to get the same end-result:

With any export filter you can select an arbitrary rectangular area when exporting to any target format.

This is done using the option --rectangle widthxheight+x+y

demorec-to-flv-with-player
 --rectangle 500x400+50+30
 ~/demorec/myrecording
 ~/videos/myrecording.flv

To speed this up, I suggest playing back the video with DemoRecorder and making a screenshot with gimp.
gimp displays the coordinates of the mouse cursor within a gimp-window. Based on these coordinates you can get some good values for the numbers.