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Last pushed: 2 years ago
Short Description
Media Source Extension WebM streaming encoder
Full Description


The Media Source Extension (MSE) is a
W3C specification that allows JavaScript to send byte
streams to media codecs within web browsers that support HTML5 video. This allows
the implementation of client-side prefetching and buffering code for streaming
media entirely in JavaScript.

To formulate it more clearly, MSE is a way to generate a video stream programatically
from JavaScript within a browser. This is an essential feature when the data source
behind a video stream is not a simple flat file that the browser can pull from a web
server (e.g. multiple sources, AJAX queries, etc).


Unfortunately, the MSE is experimental at best, each major browser having various
issues with assembling and playing chunked streams of video data. These include
limited video/audio codec support, the need for specialized stream segmentation,
half baked implementations requiring various hacks to work around.

WebMCoder is a tiny tool to solve the painful problem of converting a video file
into a format suitable for streaming through the MSE. It should be able to convert
most input media streams into the open standard WebM format in a suitable way for
streaming via the media source extensions.

Installation and usage

WebMCoder is a Go tool using Docker
images. These must be installed prior to being able to use the encoder.

Installing the Go wrapper can be done via:

$ go get


In its crudest form, WebMCoder can be invoked to simply convert an input video
file into a MSE suitable output WebM file:

$ webmcoder input.ext output.webm

A few video and audio conversion options are also supported:

  • --achan: Number of audio channels to generate (0 = same as input)
  • --arate: Audio bitrate to encode the output to (0 = same as input)
  • --vrate: Video bitrate to encode the output to (0 = same as input)
  • --vres: Video resolution (WxH) to encode the output to (empty = same as input)

A full command to recode the Elephants Dream movie
for streaming via media source extensions with a bit of reprocessing (mono audio,
640 x 360 video resolution, 674 kbit/s video bitrate) would be:

Note, the Elephants Dream HD is a 815MB download!

$ curl -L -O
$ webmcoder --achan=1 --vres=640x360 --vrate=691200 ./ED_HD.avi ./elephants-dream.webm

... Approximately 10 mins later ...

$ ls -al
-rw-r-----  1 karalabe karalabe 854537054 Feb  7 18:00 ED_HD.avi
-rw-r--r--  1 karalabe karalabe 60962855 Feb  7 18:10 elephants-dream.webm


Just to provide a rough sketch on how to embed the above generated media stream
into a webpage, first an HTML5 video element is needed.

<video id="videosink" width="640" height="360" controls>
  <source src="" type="video/webm">Your browser does not support the video tag.

After which we need to retrieve our video stream chunks from some arbitrary data
source, and assemble them into a MediaSource element. Please note, the below
code is only pseudocode highlighting the stream parts, the rest is up to the user
to implement.

// Create a new media source to fill with the video stream
var player = document.getElementById("videosink");
var source = new MediaSource;

source.addEventListener('sourceopen', function() {
  // Media source ready, create a video buffer and an async queue to fill with data
  var queue  = [];

  var buffer = source.addSourceBuffer('video/webm; codecs="vorbis,vp8"');
  buffer.addEventListener('updateend', function() {
    if (queue.length > 0) {
  }, false);

  // Start downloading the video stream, inserting either into the queue or the buffer
  while (!complete) {
    var chunk = downloadNextChunk();

    // Queue up the next chunk or insert directly
    if (queue.length > 0 || buffer.updating) {
    } else {

// Initialize the video player with the chunked stream source
player.src = URL.createObjectURL(source);


The conversion tool is based on:

  • Docker: containerizing the tool dependencies
  • FFmpeg: converting the data streams to WebM
  • mse-tools: prepping WebM files for streaming


WebMCoder is licensed under the BSD 3-Clause License. For details please consult
the LICENSE file
contained within the repository root.

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