Convert RGB video to GreyScale using FFMPEG API
https://github.com/leandromoreira/ffmpeg-libav-tutorial
$ cd ~/
$ git clone https://github.com/FFmpeg/FFmpeg.git
$ cd FFmpeg
$ ./configure --enable-shared --enable-libx264 --enable-libx265 --enable-gpl --enable-nonfree
$ make
$ make install
$ make
$ ./transcode input.mp4 output.mp4