ffmpeg is just the overall project, probably most interesting to you is libavcodec which implements encoders and decoders. Here is an example: https://www.ffmpeg.org/doxygen/0.6/api-example_8c-source.html

And there is more to find on the internet.

Shitlord by trade and passion. Graphics programmer at Laminar Research.
I write blog posts at feresignum.com