Read analysis-filter measurements¶

FFmpeg's analysis filters — cropdetect, blackdetect, silencedetect, ebur128, signalstats, astats, … — measure the media as it flows through the graph. afmpeg surfaces those measurements as structured data on the result (spec 0017 §Q): put the filter in your graph, run the job, and read ProcessResult.Analysis via afmpeg.ParseResult — no scraping stderr.

Detect the content crop¶

res, err := rt.RunJob(ctx, fs, afmpeg.Command{
    Inputs:        []afmpeg.Input{{Path: "in.mp4"}},
    FilterComplex: "[0:v]cropdetect[v]",
    Outputs:       []afmpeg.Output{{Path: "out.mp4", Map: []string{"[v]"}, VideoCodec: "libx264"}},
})
if err != nil { /* ... */ }

pr, err := afmpeg.ParseResult(res)
if err != nil { /* ... */ }

for _, m := range pr.Analysis {
    fmt.Printf("t=%.2fs %s=%s\n", m.Time, m.Key, m.Value)
    // t=0.08s cropdetect.w=160
    // t=0.08s cropdetect.h=112
    // t=0.08s cropdetect.x=80 …
}

Each Measurement is {Time float64, Key string, Value string} — the lavfi. prefix dropped, values as the filters' own strings (parse the numeric ones with strconv). The series is consecutive-deduplicated per key, so a stable measurement (a fixed crop) records once, while discrete events each record on their own.

Loudness, silence, black frames¶

The same pattern reads any analysis filter. A few need their metadata=1 option to populate the data (they only log otherwise):

// Integrated loudness (EBU R128). ebur128 needs metadata=1.
FilterComplex: "[0:a]ebur128=metadata=1[a]",
// → Analysis keys r128.I (integrated LUFS), r128.LRA, r128.TPK (true peak), …

// Silence spans — silence_start / silence_end events with their timestamps.
FilterComplex: "[0:a]silencedetect=n=-30dB:d=0.5[a]",

// Black segments.
FilterComplex: "[0:v]blackdetect=d=0.1[v]",

For a cumulative metric (e.g. ebur128's integrated r128.I), the last value for that key is the final measurement; for event filters, each *_start/*_end pair is a span.

Capability¶

The analysis filters live in the intermediate profile (and up) — see the ffmpeg-wasi filter reference. Works on both the WASM module and the native backend; Analysis is empty (nil) when the job ran no analysis filter.