Codecs¶

What the engine can decode (as inputs) and encode (as outputs[].video_codec / audio_codec), by profile. Every codec here is in-tree libavcodec — no external library, LGPL-clean (the GPL variant additionally links libx264 for H.264 encode).

Lean (all builds)¶

Intermediate (+ over lean)¶

The native codec batch (spec 0016) — all in-tree, LGPL-clean:

External encoders (spec 0018)¶

The LGPL/BSD encoder libraries cross-compiled into the build (build/deps.sh) and linked by libav — external code, but all LGPL-compatible, so both licence variants get them. Encode uses the lib* name (e.g. audio_codec: "libopus"); the produced stream probes back to the plain codec name (opus).

All are software, single-threaded — VP9 encode in particular is slow without threads, so pick VP8 unless VP9 is required. libvpx's encoder uses setjmp/longjmp, lowered to wasm exception-handling; the afmpeg runtime enables that feature to load the module.

Subtitles (spec 0019)¶

The intermediate profile also enables the subtitle codec batch, which rides the dedicated subtitle transcode lane (decode → re-encode), not the filter graph. Drive it with subtitle_codec on a subtitle output stream — see the job spec; for burning subtitles into video use the subtitles/ass/drawtext filters instead.

(movtext is the configure-time name; the encoder is mov_text at runtime — the MP4/MOV timed-text codec.)

Notes¶

• DTS is decode-only (dca) — the encoder is experimental in FFmpeg and left out (D-0016-B). TrueHD/AMR are not in this batch.
• eq is not a codec — see filters.
• HEVC/AV1 encode live in the full profile below (native only). AV1 decode is via libdav1d (spec 0023 D-0023-C) — in the intermediate profile (and full), both variants, both runtimes. FFmpeg's in-tree av1 decoder is hwaccel-only, so software AV1 decode needs the lib. dav1d ships a threaded-wasm build by default; ours is built single-threaded for WASM (it runs on wazero, which has no threads — decode is correct but slower than the threaded native driver). So intermediate decodes AV1 identically on the .wasm module and the native driver.
• Patent posture — AC-3, DTS, MPEG-2/4, VC-1 carry codec patents, the same class as the H.264/HEVC the engine already decodes; see the licensing explanation.
• The decode-only codecs (prores, dnxhd, mpeg2video, vc1, …) are enabled in the build but need a licence-clean media corpus to exercise end-to-end (spec 0016 §8) — the codecs with an encoder are verified by an encode→decode round-trip.

Full (native only, + over intermediate)¶

The heavy next-generation encoders (spec 0023). They are thread- and SIMD-hungry — impractical in WASM — so they ship only in the native driver's full profile (ffmpeg-wasi-driver-linux-amd64-full-<variant>, spec 0028), never in any .wasm.

HEVC decode needs no full profile — the native hevc decoder is in every build (lean and up). HW-accel encoders (NVENC/VAAPI/…) are the remaining full members, deferred until a GPU is available.