Minimal example of an ffmpeg video filter which applies a pair of OpenGL shaders to each frame of its input, and emits the shaded frames.

~/FFmpeg$ ln -s ~/ffmpeg-opengl/vf_gearstitch.c libavfilter/
~/FFmpeg$ git apply ~/ffmpeg-opengl/FFmpeg.diff
~/FFmpeg$ ./configure  --enable-libx264 --enable-filter=genericshader \
          --enable-gpl --enable-opengl --extra-libs='-lGLEW -lglfw'
~/FFmpeg$ make

• You may want to pass --cc=clang on OS X.
• There may slight variation in how GLEW and glfw are named (with regard to--extra-libs, above), e.g. -lglew or -lglfw3 - check pkg-config.
• The above example builds a minimal FFmpeg binary with libx264, which you'll need to build/install if that's the route you go. There's nothing codec-specific about the filter itself.

./ffmpeg -i input.mp4 -vf genericshader -y output.mp4

ffmpeg-opengl is free and unencumbered public domain software. For more information, see https://unlicense.org/ or the accompanying UNLICENSE file.