Video grab using ffmpeg?

Does anyone know how to grab video (screen) on Terminal using ffmpeg (not X11).

I have written a unix library and I'd like to make a short movie (demo) of it.

Tried: I already own SNapz Pro2 but it hangs the system (I have an old Powerbook 15" PPC). I guess my system is too slow for version 2.

I have tried some sharewares too, am in no mood of buying new stuff if i can use ffmpeg. ScreenFlick is good but puts a watermark. The resultant file for a full screen 2 minute or so movie is only 1-2 MB which is great.

Screen Movie Recorder also made a good demo movie (mov format) but its a 4 GB file.

The ffmpeg howto gives commands for video grab on X11, btw.