ffmpeg is a powerful command line tool for processing video and audio files. ffmpeg can do just about anything you can imagine with media files. The trouble is in understanding how to invoke the program correctly. There are a few options that require some Linux and multimedia expertise to get right. This article covers a couple handy ffmpeg hacks that have made much of the audio/visual content on this website possible. The commands presented here are MP4 centric. That said, you can modify most of the commands to work with alternative formats (for example, WebM).
Screen Recording
You can use ffmpeg to create a desktop recording. The command below assumes you’re on a Linux machine with an X Server running.
ffmpeg -y -f x11grab -draw_mouse 0 -s $RESOLUTION -i $DISPLAY ${OUTPUT_FILE}.mp4
Lets dissect the options:
| Option | Description |
|---|---|
-y | Overwrite an existing output file without prompting the User for confirmation. |
-f x11grab | Use the x11grab device. This device allows one to capture a region of an X11 display. |
-draw_mouse 0 | Disables recording of the mouse. Set this to 1 or remove the option completely to include the mouse in the recording. |
-s $RESOLUTION | Sets the capture resolution. For example, -s 1920x1080. |
-i $DISPLAY | Sets the input X11 display. You likely want to put the value of $DISPLAY here to capture the default X display. |
${OUTPUT_FILE}.mp4 | The path to the output MP4 file. |
If you want to make a recording that includes audio, the command looks a little different:
ffmpeg -y -f x11grab -draw_mouse 0 -s $RESOLUTION -i $DISPLAY -f pulse -ac 2 -i default ${OUTPUT_FILE}.mp4
What does the newly added -f pulse -ac 2 -i default bit do? It tells ffmpeg to
record audio using the default PulseAudio device. If you instead use Alsa for
audio, replace the PulseAudio device with the equivalent Alsa device: -f alsa -ac 2 -i hw:0. Having trouble identifying your Alsa/PulseAudio device? See
“Capture/ALSA” and “Capture/PulseAudio” for help.
You can augment the capture command to change the framerate, recording area, and more. Checkout the original source of this info for more details on how to capture video/audio.
Concatenating Video Files
Suppose you wanted to concatenate a number of recordings. You can use ffmpeg’s concat demuxer to join all the files.
The first step is to create a text file with the list of recordings you want to concat in the order you want them concatenated in:
file record1.mp4
file record2.mp4
file record3.mp4
...
Suppose you had all your *.mp4 files in a directory. You can create ffmpeg’s
concat input file using printf:
printf "file $s\n" *.mp4 > mylist.txt
Run ffmpeg using the concat device with your file list as input:
ffmpeg -f concat -i mylist.txt -c copy output.mp4
Note, this command works for files with the same codec. If you want to join files with different codecs, checkout “Concatenation of files with different codecs”.
Adding Text Overlays to Videos
Need to add a small text box to your video? Look no further than the fun command below:
ffmpeg -y -i ${IN}.mp4 -vf "drawtext=:text='Hello World':fontfile=/path/to/font.ttf:fontcolor=white:fontsize=50:box=1:boxcolor=black@0.5:boxborderw=5:x=(w-text_w)/2:y=(h-text_h)/2:enable='between(t,5,10)'" -codec:a copy ${OUT}.mp4
The magic happens in the video filter flag’s "drawtext=..." argument. The
fields of the argument are mostly self explanatory though the enable and
x/y options could do with a little elaboration.
The enable option specifies a window of time (in seconds) when the text label
will be visible. In this example, enable='between(t,5,10)' means the text box
will be visible from the 5 second mark to the 10 second mark in the video.
The x/y options specify the location of the text box on screen. The w,
h, text_w, and text_h are variables provided by ffmpeg to make it easier
to compute a position. w/h are the screen width and height.
text_w/text_h are the text string’s width and height.
The drawtext filter takes many more options. Take a look at the official
docs for all the details.
Bonus: Video Playback
It’s likely that you want to playback your video after an edit. ffplay is a program that uses ffmpeg libraries and SDL to playback a media file. The following command will play an MP4 adding a small text box with a live timestamp at the bottom:
ffplay -vf "drawtext=text='%{pts\:hms}':box=1:x=(w-tw)/2:y=h-(2*lh)" ${MY_VIDEO}.mp4
This command isn’t limited to MP4s. You can pass any video format ffmpeg supports to ffplay.
Conclusion
This article doesn’t scratch the surface of what’s possible with ffmpeg. That said, these “simple” commands have helped make most of the video content on this site! Hopefully, these same commands save you some time in creating your own video content.