webm-wasm

webm-wasm lets you create webm videos in JavaScript via WebAssembly. The library consumes raw RGBA32 buffers (4 bytes per pixel) and turns them into a webm video with the given framerate and quality. This makes it compatible out-of-the-box with [ImageData][imagedata] from a . With realtime mode you can also use webm-wasm for streaming webm videos.

Works in all major browsers (although Safari can’t play webm 🐼).

The wasm module was created by [emscripten’ing][emscripten] [libvpx], [libwebm] and [libyuv].

``
$ npm install --save webm-wasm
`

> Note: This is a proof-of-concept and not a production-grade library.

Usage

webm-wasm runs in a worker by default. It works on the web and in in Node, although you need Node 11+ with the --experimental-worker flag.

$3

`js
// 1. Load the webm-wasm.js file in a worker
const worker = new Worker("webm-worker.js");
// 2. Send the path to the .wasm file
worker.postMessage("./webm-wasm.wasm");
// 3. Wait for the worker to be ready
await nextMessage(worker);
// 4. Send the parameters for the constructor
worker.postMessage({
width: 512,
height: 512
// ... more constructor options below
});
// 5. Start sending frames!
while (hasNextFrame()) {
// ArrayBuffer containing RGBA24 data
const buffer = getFrame();
worker.postMessage(buffer, [buffer]);
}
// 6. Signal end-of-stream
worker.postMessage(null);
// 7. Get the webm file as an ArrayBuffer
const webm = await nextMessage(worker);
// 8. Cleanup
worker.terminate();
`

(You can find an implementation of nextMessage() in [src/worker/webm-worker.js][nextmessage])

$3

- width (default: 300): Width of the video
- height (default: 150): Height of the video
- timebaseNum (default: 1): Numerator of the fraction for the length of a frame
- timebaseDen (default: 30): Denominator of the fraction for the length of a frame
- bitrate (default: 200): Bitrate in kbps
- realtime (default: false): Prioritize encoding speed over compression ratio and quality. With realtime mode turned off the worker will send a single ArrayBuffer containing the entire webm video file once input stream has ended. With realtime mode turned on the worker will send an ArrayBuffer in regular intervals.

$3

Worker code can’t be loaded from another origin directly, even when the source is CORS-enabled. It is, however, still possible to load webm-wasm from a CDN like [unpkg.com] with a little workaround:

`js
const buffer = await fetch(
"https://unpkg.com/webm-wasm@/dist/webm-worker.js"
).then(r => r.arrayBuffer());
const worker = new Worker(
URL.createObjectURL(new Blob([buffer], { type: "text/javascript" }))
);
worker.postMessage("https://unpkg.com/webm-wasm@/dist/webm-wasm.wasm");
// Continue as normal
`

$3

If you just want to use the WebAssembly module directly, you can grab webm-wasm.wasm as well as the the [Emscripten] glue code webm-wasm.js.

The WebAssembly module exposes a C++ class via [embind]:

`c++
class WebmEncoder {
public:
// Same options as above. cb is a callback function that takes an ArrayBuffer.
WebmEncoder(int timebase_num, int timebase_den, unsigned int width, unsigned int height, unsigned int bitrate, bool realtime, val cb);
bool addRGBAFrame(std::string rgba);
bool finalize();
std::string lastError();
// ...
}
`

$3

[Transferable Streams] are behind the “Experimental Web Platform Features” flag in Chrome Canary. The alternative webm-transformstreamworker.js makes use of them to expose the webm encoder. Take a look at the demos to see the usage.

Demos

To run the web demos, start the websever using

`
$ npm run serve
`

To run the node demos, run them directly (requires Node 11+):

`
$ node --experimental-worker ./node-simple.js
`

Building

Because the build process is completely [Dockerized][docker], Docker is required for building webm-wasm.

`
$ npm install
$ npx napa
$ npm run build
``

---

Apache 2.0

[imagedata]: https://developer.mozilla.org/en-US/docs/Web/API/ImageData
[nextmessage]: https://github.com/GoogleChromeLabs/webm-wasm/blob/63b96a4f0e2821f34f972827f800259222ef9142/src/worker/webm-worker.js#L37-L46
[unpkg.com]: https://unpkg.com
[docker]: https://www.docker.com/
[transferable streams]: https://www.chromestatus.com/features/5298733486964736
[embind]: https://developers.google.com/web/updates/2018/08/embind
[emscripten]: https://kripken.github.io/emscripten-site/
[libvpx]: https://github.com/webmproject/libvpx
[libwebm]: https://github.com/webmproject/libwebm
[libyuv]: https://chromium.googlesource.com/libyuv/libyuv/