= FFmpeg filter graph
{c}
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 https://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
``
Related question: https://stackoverflow.com/questions/69251087/in-ffmpeg-command-line-how-to-show-all-filter-settings-and-their-parameters-bef
TODO dump graph to <ASCII art>? https://trac.ffmpeg.org/wiki/FilteringGuide\#Visualizingfilters mentions a `-dumpgraph` option, but haven't managed to use it yet.
Bibliography:
* https://ffmpeg.org/ffmpeg-filters.html official documentation
* https://trac.ffmpeg.org/wiki/FilteringGuide some handy tips from the FFMpeg Wiki