Programatically video capture a window of a separate program on Windows, Linux or Mac

Is there a way in any language for Windows, Linux or Mac (I'll use whatever OS is easiest)开发者_Go百科 to programatically screen cap a different program's window?

Also programatically recording a specific box on the screen would be acceptable if necessary.

Yes, it's generally possible on all of those OSs; the details are OS-specific, of course, and it's not exactly trivial on any of them. There are commercial or freeware tools for all OSs that do it. On Linux, you could just use, for example, Xvidcap.