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Build from source

You don't need to build — every release ships both variants. But the whole pipeline is MIT and reproducible, so building it yourself is easy.

Prerequisites

  • Docker (the build runs in a wasi-sdk image; nothing is installed on your host).
  • ~2 GB of disk and a few minutes.

Build a variant

docker build -f build/Dockerfile \
  --build-arg VARIANT=lgpl \
  --target artifact -o dist .

docker build -f build/Dockerfile \
  --build-arg VARIANT=gpl \
  --target artifact -o dist .

The module lands at dist/ffmpeg-wasi-<variant>.wasm. To build a different upstream FFmpeg, add --build-arg FFMPEG_VERSION=n8.1.2 (any FFmpeg release tag).

With just:

just build lgpl

Run it

The repo bundles a tiny wazero harness that loads the module and runs it (it provides the env setjmp/longjmp imports and the WebAssembly feature set the build needs):

go run ./tools/run dist/ffmpeg-wasi-lgpl.wasm
# or: just run lgpl

You'll see the engine's capability report — the FFmpeg version and the available codecs/muxers/filters — confirming it links and runs:

ffmpeg-wasi engine
ffmpeg: n8.1.2
libavcodec 4070502  libavformat 4066406  libavfilter 724582
encoders:
  libx264    yes        # gpl variant only (no on lgpl)
  aac        yes
  ...
decoders:
  h264       yes
  ...

What the build does

Three small scripts under build/, orchestrated by build/Dockerfile:

  1. libav.sh — clones FFmpeg, configures it single-threaded for wasm32-wasi (libraries only), and makes the libav* archives.
  2. driver.sh — links the engine (src/driver.c) + the wasi compat shims against those archives into one .wasm command module.
  3. toolchain.sh — the shared wasi-sdk/clang cross-compile environment.

See The build for what makes it work (single-threaded config, setjmp/longjmp lowering, the POSIX/WASI compat shims).