MPEG1 Video Decoder in JavaScript
JavaScript HTML
Latest commit 186666d Jan 16, 2017 @phoboslab committed on GitHub Merge pull request #94 from AnimationMentor/fix-seekToFrame
reset intraFrames in jsmpeg constructor


An MPEG1 Video Decoder in JavaScript

jsmpeg is a MPEG1 Decoder, written in JavaScript. It's "hand ported", i.e. not compiled with emscripten or similar. This will probably make it obsolete with the advent of asmjs.

Some demos and more info:



var player = new jsmpeg(file [, options])

The file argument accepts a URL to a .mpg file or a (yet unconnected) WebSocket instance for streaming playback.

The options argument to the jsmpeg() supports the following properties:

  • benchmark whether to log benchmark results to the browser's console
  • progressive whether to start playback as soon as the first chunks have been loaded. Uses HTTP range-requests. Default false.
  • progressiveThrottled whether to throttle downloading chunks until they're needed for playback. Requires progressive; default false.
  • canvas the HTML Canvas element to use; jsmpeg will create its own Canvas element if none is provided
  • autoplay whether playback should start automatically after loading
  • loop whether playback is looped
  • seekable whether a seek-index is build during load time; neccessary for seekToFrame and seekToTime methods
  • preserveDrawingBuffer whether the WebGL context is created with preserveDrawingBuffer - necessary for "screenshots" via canvas.toDataURL(); default false.
  • onload a function that's called once, after the .mpg file has been completely loaded
  • ondecodeframe a function that's called after every frame that's decoded and rendered to the canvas
  • onfinished a function that's called when playback ends


  • play() begin playback
  • pause() pause playback
  • stop() stop playback and revert play position to the beginning
  • seekToFrame(frame) seek to the specified frame (Number)
  • seekToTime(seconds) seek to the specified time (Number)
  • nextFrame() if playback is paused, decode and render the next frame; returns then HTML Canvas element

When live streaming, jsmpeg supports the following methods for recording the stream clientside

  • canRecord() returns true when streaming has started and recording can begin, false otherwise
  • startRecording(callback) attempts to start recording, calls the optional callback when recording started - usually when the next intraframe was received
  • stopRecording() stops recording and returns a Blob with the recorded .mpg data

Usage Examples

// Synopsis: var player = new jsmpeg(urlToFile, options);
// The 'options' argument and all of its properties is optional. If no canvas element 
// is given, jsmpeg will create its own, to be accessed at .canvas

// Example:
var canvas = document.getElementById('videoCanvas');
var player = new jsmpeg('file.mpeg', {canvas: canvas, autoplay: true, loop: true});


// If you pass 'seekable: true' in the options, you can seek to a specific frame
// or time in the video.

var player = new jsmpeg('file.mpeg', {canvas: canvas, seekable: true});

player.seekToFrame(1200); // Seek to intra frame before frame 1200
player.seekToTime(20); // Seek to intra frame before 20sec

// seekToFrame() and seekToTime() only seek to the closest, previous intra frame by
// default. If you want to seek to the exact frame or time, pass 'true' as second
// parameter.
// Depending on the input video, this can be potentially slow, as jsmpeg has
// to decode all frames between the previous intra frame and the seek target

player.seekToFrame(1200, true); // Seek to frame 1200 exactly

// Passing 'seekable: true' also populates the total frame count and duration
// of the video

console.log('Duration: '+player.duration+' seconds ('+player.frameCount+' frames)')

// An 'onload' callback can be specified in the 'options' argument
var mpegLoaded = function( player ) {
    console.log('Loaded', player);
var player = new jsmpeg('file.mpeg', {onload: mpegLoaded});

// If you don't use 'autoplay' and don't explicitly call .play(), you can get individual
// video frames (a canvas element) like so:
var frame = null;
while( (frame = player.nextFrame()) ) {
    someOtherCanvasContext.drawImage(frame, 0, 0);

Live Streaming

jsmpeg supports streaming live video through WebSockets. You can use ffmpeg and a nodejs server to serve the MPEG video. See this blog post for the details of setting up a server. Also have a look at the stream-server.js and stream-example.html.

To configure jsmpeg to connect to the stream server, simply pass a WebSocket connection instead of a filename to the constructor:

// Setup the WebSocket connection and start the player
var client = new WebSocket( 'ws://' );
var player = new jsmpeg(client, {canvas:canvas});

Stream Recording

To record an MPEG stream clientside in the browser jsmpeg provides the .startRecording(cb) and .stopRecording() methods. .stopRecording() returns a Blob object that can be used to create a download link.

    // Called when recording really starts; usually 
    // when the next intra frame is received

// ...

// Stop recording and create a download link
var blob = player.stopRecording();

var filename = 'jsmpeg-recording.mpg';
var a = document.getElementById('downloadLink');
a.innerHTML = filename; = fileName;
a.href = window.URL.createObjectURL(blob);


  • Playback can only start when the file is fully loaded (when not streaming through WebSockets). I'm waiting for chunked XHR with ArrayBuffers to arrive in browsers.
  • MPEG files with B-Frames look weird - frames are not reordered. This should be relatively easy to fix, but most encoders seem to not use B-Frames at all by default.
  • The width of the MPEG video has to be a multiple of 2.
  • Only raw MPEG video streams are supported. The decoder hates Stream Packet Headers in between macroblocks.

You can use FFmpeg to encode videos in a suited format. This will crop the size to a multiple of 2, omit B-Frames and force a raw video stream:

ffmpeg -i in.mp4 -f mpeg1video -vf "crop=iw-mod(iw\,2):ih-mod(ih\,2)" -b 0 out.mpg


jsmpeg is based on Java MPEG-1 Video Decoder and Player by Korandi Zoltan and inspired by MPEG Decoder in Java ME by Nokia.