FFmpeg

FFmpeg is the assembler of audio and video.

As a result, Ciro Santilli who likes "lower level stuff", has had many many hours if image manipulation fun with this software, see e.g.:

the "Media" section of the best articles by Ciro Santillis.
Figure "Ciro knows how to convert videos to GIFs"

As older Ciro grows, the more he notices that FFmpeg can do basically any lower level audio video task. It is just an amazing piece of software, the immediate go-to for any low level operation.

FFmpeg was created by Fabrice Bellard, which Ciro deeply respects.

Resize a video: superuser.com/questions/624563/how-to-resize-a-video-to-make-it-smaller-with-ffmpeg:

ffmpeg -i input.avi -filter:v scale=720:-1 -c:a copy output.mkv

Unlike every other convention under the sun, the height in scale is the first number.

Table of contents

FFmpeg filter graph

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Filter graphs are a thing of great beauty. What an amazingly obscure domain-specific language, but which can produce striking results with very little!!!

A quick example from stackoverflow.com/questions/59551013/how-to-generate-stereo-sine-wave-using-ffmpeg-with-different-frequencies-for-eac/77730492#77730492 illustrates some of the fundamentals:

ffplay -autoexit -nodisp -f lavfi -i '
sine=frequency=500[a];
sine=frequency=1000[b];
[a][b]amerge, atrim=end=2
'

which creates a graph:

                              +--------+
[sine=frequency=500]--->[a]-->|        |
                              | amerge |-->[atrim]-->[output]
[sine=frequency=1000]-->[b]-->|        |
                              +--------+

and plays 500 Hz on the left channel and 1000 Hz on the right channel for 2 seconds.

So we see the following syntax patterns:

sine, amerge and atrim are filters
sine=frequency=500: the first = says "araguments follow"
- frequency=500 sets the frequency argument of the sine filter
- for multiple arguments the syntax is to separate arguments with colons e.g. sine=frequency=500:duration=2
;: separates statements
[a], [b]: sets the name of an edge
,: creates unnamed edge between filters that have one input and one output

A list of all filters can be obtained ith:

ffmpeg -filters

and parameters for a single filter can be obtained with:

ffmpeg --help filter=sine

TODO dump graph to ASCII art? trac.ffmpeg.org/wiki/FilteringGuide#Visualizingfilters mentions a -dumpgraph option, but haven't managed to use it yet.

Bibliography:

ffmpeg.org/ffmpeg-filters.html official documentation
trac.ffmpeg.org/wiki/FilteringGuide some handy tips from the FFMpeg Wiki

ffplay

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Awesome tool to view quick stuff quickly without generating files. Unfortunately it doesn't support all options that the ffmpeg CLI supports, e.g. ffplay multiple input files. One day, one day.

ffplay multiple input files

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TODO possible? superuser.com/questions/559768/ffplay-how-to-play-together-separate-video-and-audio-files

For synthesized streams like sine we can do it e.g.

ffplay -autoexit -nodisp -f lavfi -i '
sine=frequency=500[a];
sine=frequency=1000[b];
[a][b]amerge, atrim=end=2
'

but it does not seem to accept multiple -i for some reason. So is there a way to open a file from some filter? E.g.:

ffplay -i tmp.wav -i tmp.mkv -filter_complex "[0:a]atrim=end=2[a];[1:v]trim=end=2[v]" -map '[a]' -map '[v]'

fails with:

Argument 'tmp.mkv' provided as input filename, but 'tmp.wav' was already specified.

FFmpeg sound synthesis

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Simple sines and variants:

2 second 1000 Hz:

ffmpeg -f lavfi -i "sine=f=1000:d=2" out.wav

FFmpeg video synthesis

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Video with a solid color:

2 second white video:

ffplay -autoexit -f lavfi -i 'color=white:640x480:d=3,format=rgb24,trim=end=2'

Also add some audio:

ffmpeg -lavfi "color=white:640x480:d=3,format=rgb24,trim=end=2[v];sine=f=1000:d=2[a]" -map '[a]' -map '[v]' out.mkv

TODO how to ffplay the video + audio directly? -map does not seem to work unfortunately.

2 second white followed by 2 second black video:

ffplay -autoexit -f lavfi -i 'color=white:640x480:d=3,format=rgb24,trim=end=2[a];color=black:640x480:d=3,format=rgb24,trim=end=2[b];[a][b]concat=n=2:v=1:a=0'

bibliography:
- superuser.com/questions/1153930/how-to-generate-a-pure-white-background-video-with-ffmpeg

Display count in seconds on the video:

black text on white background. Start from 0 and count up to 2:

ffplay -autoexit -f lavfi -i "
color=white:480x480:d=3,
format=rgb24,
drawtext=
  fontcolor=black:
  fontsize=600:
  text='%{eif\:t\:d}':
  x=(w-text_w)/2:
  y=(h-text_h)/2
"