FFmpeg filter graph

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