### Useful ffmpeg commands ###
       
       
       Here I keep the few ffmpeg commands that I need from time to time to process videos that I record with my camera.
       
       === extract audio from a video ===
       ffmpeg -i 20180217_103328.mp4 -vn -acodec copy 20180217_103328-audio.aac
       
       === remove audio from a video ===
       ffmpeg -i 20180217_103328.mp4 -vcodec copy -an 20180217_103328-noaudio.mp4
       
       === mix audio+video ===
       ffmpeg -i 20180217_103328-noaudio.mp4 -i 20180217_103328-audio2.aac 20180217_103328c.mp4
       
       === modify start time and duration ===
       ffmpeg -i source.mp4 -ss 00:00:05 -t 00:00:10 -c copy cut_video.mp4
       
       === re-encode a video ===
       ffmpeg -i source.mp4 dest.mp4
       
       === convert mp4 to gif (rescale to 640x480 and lower framerate to 10hz) ===
       ffmpeg -i source.mp4 -s 640x480 -r 10 funnycat.gif
       
       === apply several video filters: rotate by 90 degrees and crop video ===
       ffmpeg -i source.mp4 -filter:v "transpose=3, crop=720:1000:0:0" newvideo.mp4
       
       === slow down or accelerate a video by x1.5 ===
       ffmpeg -i source -filter:v "setpts=1.5*PTS" slow.mp4
       ffmpeg -i source -filter:v "setpts=0.5*PTS" fast.mp4