How to access ffmpeg astats filter metadata through libmpv

[dweymouth]

I develop a desktop music player using libmpv and I would like to make use of the metadata from the astats filter to create a peak meter. I have tried prepending astats=metadata=1 to my af fliter list which is applied using mpv_set_property (I also have equalizer af filters and those are working properly). I then try to observe the peak property with mpv_observe_property(mpv, 0 /*userdata*/, "af-metadata/lavfi.astats.Overall.Peak_level", MPV_FORMAT_DOUBLE) but I am not getting any property change events. Observing a different property like "time_pos" is working.

I am not sure what I am doing wrong. Basically trying to do what was described in #2645 but unfortunately there's no sample code in that issue once the original reporter got it working.

[richardpl]

I do not think IIRC that lavfi metadata is converted to mpv metadata...

[dweymouth]

It seems like it should be available under af-metadata https://mpv.io/manual/stable/#command-interface-af-metadata/%3Cfilter-label%3E

[richardpl]

Is there way to list all metadata entries? Then you could see if its there.
Also may depend if you use -af lavfi=[astats] or just -af astats....

[kasper93]

mpv -af "astats=metadata=1"

mp.get_property_native("af-metadata/astats.00")

or with label

mpv -af "@myfilter:astats=metadata=1"

mp.get_property_native("af-metadata/myfilter")

This will return table with the keys from filter metadata like lavfi.astats.Overall.Peak_level.

You can do the same with C API with nodes.

Querying values directly like mp.get_property_native("af-metadata/astats.00/lavfi.astats.Overall.Peak_level") is currently broken, after a05b847.

[kasper93]

Querying values directly like mp.get_property_native("af-metadata/astats.00/lavfi.astats.Overall.Peak_level") is currently broken, after a05b847.

Will be fixed by #14527

[dweymouth]

Thanks @kasper93 this worked great! In case this helps anyone else here's how I implemented this. My project is written in Go but I implemented this particular functionality in C to avoid the mpv_node having to cross the FFI boundary.

My af property string has @astats:astats=metadata=1:reset=1:measure_overall=none to enable the astats filter to compute the data and assign the astats property label.

https://github.com/dweymouth/supersonic/blob/92d2fb0/backend/player/mpv/peaks.c