Web/API/OfflineAudioContext/startRendering

The startRendering() method of the OfflineAudioContext Interface starts rendering the audio graph, taking into account the current connections and the current scheduled changes.

The complete event (of type OfflineAudioCompletionEvent) is raised when the rendering is finished, containing the resulting AudioBuffer in its renderedBuffer property.

Browsers currently support two versions of the startRendering() method — an older event-based version and a newer promise-based version. The former will eventually be removed, but currently both mechanisms are provided for legacy reasons.

Syntax

Event-based version:

offlineAudioCtx.startRendering();
offlineAudioCtx.oncomplete = function(e) {
  // e.renderedBuffer contains the output buffer
}

Promise-based version:

offlineAudioCtx.startRendering().then(function(buffer) {
  // buffer contains the output buffer
});

Parameters

None.

Returns

Void.

Example

In this simple example, we declare both an AudioContext and an OfflineAudioContext object. We use the AudioContext to load an audio track via XHR (AudioContext.decodeAudioData), then the OfflineAudioContext to render the audio into an AudioBufferSourceNode and play the track through. After the offline audio graph is set up, you need to render it to an AudioBuffer using OfflineAudioContext.startRendering.

When the startRendering() promise resolves, rendering has completed and the output AudioBuffer is returned out of the promise.

At this point we create another audio context, create an AudioBufferSourceNode inside it, and set its buffer to be equal to the promise AudioBuffer. This is then played as part of a simple standard audio graph.

Note: For a working example, see our [[../../../../../../../mdn.github.io/webaudio-examples/offline-audio-context-promise/index|offline-audio-context-promise]] Github repo (see the source code too.)

// define online and offline audio context

var audioCtx = new AudioContext();
var offlineCtx = new OfflineAudioContext(2,44100*40,44100);

source = offlineCtx.createBufferSource();

// use XHR to load an audio track, and
// decodeAudioData to decode it and OfflineAudioContext to render it

function getData() {
  request = new XMLHttpRequest();

  request.open('GET', 'viper.ogg', true);

  request.responseType = 'arraybuffer';

  request.onload = function() {
    var audioData = request.response;

    audioCtx.decodeAudioData(audioData, function(buffer) {
      myBuffer = buffer;
      source.buffer = myBuffer;
      source.connect(offlineCtx.destination);
      source.start();
      //source.loop = true;
      offlineCtx.startRendering().then(function(renderedBuffer) {
        console.log('Rendering completed successfully');
        var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
        var song = audioCtx.createBufferSource();
        song.buffer = renderedBuffer;

        song.connect(audioCtx.destination);

        play.onclick = function() {
          song.start();
        }
      }).catch(function(err) {
          console.log('Rendering failed: ' + err);
          // Note: The promise should reject when startRendering is called a second time on an OfflineAudioContext
      });
    });
  }

  request.send();
}

// Run getData to start the process off

getData();

Specifications

Specification	Status	Comment
Web Audio APIThe definition of 'startRendering()' in that specification.	Working Draft

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`startRendering`	Chrome Full support 14	Edge Full support 12	Firefox Full support 25	IE No support No	Opera Full support 15	Safari Full support 6	WebView Android Full support Yes	Chrome Android Full support 18	Firefox Android Full support 26	Opera Android Full support 14	Safari iOS ?	Samsung Internet Android Full support 1.0
Promise-based `startRendering()`	Chrome Full support 42	Edge Full support ≤18	Firefox Full support 37	IE No support No	Opera Full support 29	Safari No support No	WebView Android Full support 42	Chrome Android Full support 42	Firefox Android Full support 37	Opera Android Full support 29	Safari iOS No support No	Samsung Internet Android Full support 4.0

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown