How can I capture frames from X11 into a file?

I think there should b开发者_StackOverflow中文版e some sort of internal support on screen capture utilities with X11. So how can I capture frames from X11 frame-buffer into a file, e.g. AVI, JPEG, etc.?

Have a look at these apps:

• http://xvidcap.sourceforge.net/
• http://www.unixuser.org/~euske/vnc2swf/pyvnc2swf.html

If the apps don't work you can look at their source code to see how to do your own app.