fluvie_web_encoder

Render Fluvie videos to MP4 fully in the browser with ffmpeg.wasm. The frames never leave the page.

Why

Fluvie normally renders in two steps: capture the widget tree to raw frames, then encode them with FFmpeg. In a browser there is no FFmpeg binary — but there is ffmpeg.wasm, which is FFmpeg. So this package keeps Fluvie's capture step and runs the exact same encode argument plan through ffmpeg.wasm: H.264, GIF, and transparent WebM all work with no reimplementation and no native code.

It is opt-in. The ffmpeg.wasm payload is large, so only apps that depend on this package load it; apps that render through fluvie_server (a server) or only target mobile stay light.

How it works

WebVideoRenderer runs Fluvie's capture loop into an off-screen surface inside your app's own pipeline (a FluvieWebStage), writing the frames into an in-memory sandbox.
It feeds that sandbox to ffmpeg.wasm through Fluvie's WasmRuntime, runs the manifest's argument plan, and reads the MP4 back as bytes.

You write the same Video you would render anywhere.

Install

flutter pub add fluvie_web_encoder

The package loads ffmpeg.wasm lazily, on the first render, through a page-global FluvieFfmpeg bridge. The bridge wraps @ffmpeg/ffmpeg; see the setup guide for the script to add to web/index.html (or self-host the wasm for offline use). Use the single-threaded core to avoid needing cross-origin-isolation headers.

Quick start

Wrap your app once in a FluvieWebStage. It gives in-browser capture a surface inside your app's own pipeline (off-screen, never shown), which is what lets the render boundary mount and paint:

import 'package:fluvie_web_encoder/fluvie_web_encoder.dart';

void main() => runApp(const FluvieWebStage(child: MyApp()));

Then render anywhere in the app. The same Video you render on the desktop:

import 'package:fluvie/fluvie.dart';
import 'package:fluvie_web_encoder/fluvie_web_encoder.dart';

Future<Uint8List> renderInBrowser(Video video) async {
  final renderer = WebVideoRenderer();
  return renderer.render(
    composition: video,
    aspect: Aspect.square,
    duration: const Duration(seconds: 4),
    longEdge: 720,
  );
}

The returned bytes are an MP4: hand them to a download link or upload them.

What carries over, and what does not

Because both halves are Fluvie's own (capture) and real FFmpeg (encode), the full feature set works: every element, animation, transition, and Export mode (MP4, GIF, transparent WebM) renders exactly as on the desktop.

Performance: ffmpeg.wasm is roughly 10–50× slower than native FFmpeg. Web on-device suits previews and short clips; use fluvie_server for long renders.
Clips: a Clip decodes on-device through WebCodecs. WebVideoRenderer wires a decoder by default; it needs a browser with WebCodecs and the FluvieClipDecoder bridge on the page. Without them a clip fails with a clear error, so wire the bridge or render clip compositions on the server or mobile.
Audio: opt-in. Pass audio: true to mix and mux a Video's Audio tracks (the same amix plan the desktop uses — looping beds, fades, trims, multi-track). Bundle audio as an asset or fetch it from an allowlisted URL; the browser has no local-file source. See the audio guide.
Golden baseline: the encoded MP4 may differ from a native FFmpeg build, so give web its own golden baseline.

Documentation

See the Fluvie on-device web rendering guide.

License

MIT. See LICENSE.