"-f lavfi -i testsrc - declaration of an input. The input format is lavfi, the input file is testsrc. This is duplicated four times to imitate four different inputs."
You can feed a single input multiple times in a filtergraph, so you can make do with only one. In this case, it hardly matters, but the input packet will be demuxed and decoded only once, unlike in the examples.
You can feed a single input multiple times in a filtergraph, so you can make do with only one. In this case, it hardly matters, but the input packet will be demuxed and decoded only once, unlike in the examples.