How to programmatically create videos?

I know there's mencoder (part of the mplayer project), and ffmpeg, which both can do this.

ffmpeg is a great (open source) program for building all kinds of video, and converting one type of video (a sequence of images in this case) into other types of video.

Usually it is utilized from the command line, but that is really just a wrapper around its internal libraries. It is expressly available to be used from within another program.

There are also python bindings that wrap the c api, though this particular project doesn't seem to be getting the best support (there are probably other projects out there doing the same thing).

There's also this link where someone has used ffmpeg to do something similar to what you're looking for.

GStreamer is a popular choice. It's a full multimedia framework much like DirectShow or QuickTime, has the advantage of having legally licensed codecs available, and has excellent Python bindings.