FFmpeg

FFmpeg is a library for encoding and decoding multimedia. You can interact with FFmpeg using their command-line interface or using their C API. I find it useful for converting videos to gifs. You can also extract videos into a sequence of images or vice-versa.

CLI

Basic usage is as follows:

ffmpeg -i input_file [-s resolution] [-b bitrate] [-ss start_second] [-t time] [-r output_framerate] output.mp4

x264

x264 is a software h264 decoder and encoder.
[1]

Changing Pixel Format

Encode to h264 with YUV420p pixel format

ffmpeg -i input.mp4 -c:v libx264 -profile:v high -pix_fmt yuv420p output.mp4

Images to Video

Reference
Assuming 60 images per second and you want a 30 fps video.

ffmpeg -framerate 60 -i image-%03d.png -r 30 video.mp4

C API

A doxygen reference manual for their C api is available at [2].

Muxing to memory

You can specify a custom AVIOContext and attach it to your AVFormatContext->pb to mux directly to memory or to implement your own buffering.