Video processing with WebCodecs

Modern web technologies provide ample ways to work with video.
Media Stream API,
Media Recording API,
Media Source API,
and WebRTC API add up
to a rich tool set for recording, transferring, and playing video streams.
While solving certain high-level ta…

This content originally appeared on and was authored by Eugene Zemtsov

Modern web technologies provide ample ways to work with video. Media Stream API, Media Recording API, Media Source API, and WebRTC API add up to a rich tool set for recording, transferring, and playing video streams. While solving certain high-level tasks, these APIs don't let web programmers work with individual components of a video stream such as frames and unmuxed chunks of encoded video or audio. To get low-level access to these basic components, developers have been using WebAssembly to bring video and audio codecs into the browser. But given that modern browsers already ship with a variety of codecs (which are often accelerated by hardware), repackaging them as WebAssembly seems like a waste of human and computer resources.

WebCodecs API eliminates this inefficiency by giving programmers a way to use media components that are already present in the browser. Specifically:

  • Video and audio decoders
  • Video and audio encoders
  • Raw video frames
  • Image decoders

The WebCodecs API is useful for web applications that require full control over the way media content is processed, such as video editors, video conferencing, video streaming, etc.

Current status

Step Status
1. Create explainer Complete
2. Create initial draft of specification Complete
3. Gather feedback & iterate on design In Progress
4. Origin trial In Progress
5. Launch Not started

Video processing workflow

Frames are the centerpiece in video processing. Thus in WebCodecs most classes either consume or produce frames. Video encoders convert frames into encoded chunks. Video decoders do the opposite. Track readers turn video tracks into a sequence of frames. By design all these transformations happen asynchronously. WebCodecs API tries to keep the web responsive by keeping the heavy lifting of video processing off the main thread.

Currently in WebCodecs the only way to show a frame on the page is to convert it into an ImageBitmap and either draw the bitmap on a canvas or convert it into a WebGLTexture.

WebCodecs in action


It all starts with a VideoFrame. There are two ways to convert existing pictures into VideoFrame objects.

The first is to create a frame directly from an ImageBitmap. Just call the VideoFrame() constructor and give it a bitmap and a presentation timestamp.

let cnv = document.createElement('canvas');
// draw something on the canvas

let bitmap = await createImageBitmap(cnv);
let frame_from_bitmap = new VideoFrame(bitmap, { timestamp: 0 });
The path from ImageBitmap to the network or to storage.
The path from ImageBitmap to the network or to storage.

The second is to use VideoTrackReader to set a function that will be called each time a new frame appears in a MediaStreamTrack. This is useful when you need to capture a video stream from a camera or the screen.

let frames_from_stream = [];
let stream = await navigator.mediaDevices.getUserMedia({});
let vtr = new VideoTrackReader(stream.getVideoTracks()[0]);
vtr.start((frame) => {
The path from MediaStreamTrack to the network or to storage.
The path from MediaStreamTrack to the network or to storage.

No matter where they are coming from, frames can be encoded into EncodedVideoChunk objects with a VideoEncoder.

Before encoding, VideoEncoder needs to be given two JavaScript objects:

  • Init dictionary with two functions for handling encoded chunks and errors. These functions are developer-defined and can't be changed after they're passed to the VideoEncoder constructor.
  • Encoder configuration object, which contains parameters for the output video stream. You can change these parameters later by calling configure().
const init = {
output: handleChunk,
error: (e) => {

let config = {
codec: 'vp8',
width: 640,
height: 480,
bitrate: 8_000_000, // 8 Mbps
framerate: 30,

let encoder = new VideoEncoder(init);

After the encoder has been set up, it's ready to start accepting frames. When frames are coming from a media stream, the callback given to VideoTrackReader.start() will pump frames into the encoder, periodically inserting keyframes and checking that the encoder is not overwhelmed with incoming frames. Both configure() and encode() return immediately without waiting for the actual work to complete. It allows several frames to queue for encoding at the same time. But it makes error reporting somewhat cumbersome. Errors are reported either by immediately throwing exceptions or by calling the error() callback. Some errors are easy to detect immediately, others become evident only during encoding. If encoding completes successfully the output() callback is called with a new encoded chunk as an argument. Another important detail here is that encode() consumes the frame, if the frame is needed later (for example, to encode with another encoder) it needs to be duplicated by calling clone().

let frame_counter = 0;
let pending_outputs = 0;
let vtr = new VideoTrackReader(stream.getVideoTracks()[0]);

vtr.start((frame) => {
if (pending_outputs > 30) {
// Too many frames in flight, encoder is overwhelmed
// let's drop this frame.
const insert_keyframe = (frame_counter % 150) == 0;
encoder.encode(frame, { keyFrame: insert_keyframe });

Finally it's time to finish encoding code by writing a function that handles chunks of encoded video as they come out of the encoder. Usually this function would be sending data chunks over the network or muxing them into a media container for storage.

function handleChunk(chunk) {
let data = new Uint8Array(; // actual bytes of encoded data
let timestamp = chunk.timestamp; // media time in microseconds
let is_key = chunk.type == 'key'; // can also be 'delta'
method: 'POST',
headers: { 'Content-Type': 'application/octet-stream' },
body: data

If at some point you'd need to make sure that all pending encoding requests have been completed, you can call flush() and wait for its promise.

await encoder.flush();


Setting up a VideoDecoder is similar to what's been done for the VideoEncoder: two functions are passed when the decoder is created, and codec parameters are given to configure(). The set of codec parameters can vary from codec to codec, for example for H264 you currently need to specify a binary blob with AVCC extradata.

const init = {
output: handleFrame,
error: (e) => {

const config = {
codec: 'vp8',
codedWidth: 640,
codedHeight: 480

let decoder = new VideoDecoder(init);

Once the decoder is initialized, you can start feeding it with EncodedVideoChunk objects. Creating a chunk just takes a BufferSourceof data and a frame timestamp in microseconds. Any chunks emitted by the encoder are ready for the decoder as is, although it's hard to imagine a real-world use case for decoding newly encoded chunks (except for the demo below). All of the things said above about the asynchronous nature of encoder's methods are equally true for decoders.

let responses = await downloadVideoChunksFromServer(timestamp);
for (let i = 0; i < responses.length; i++) {
let chunk = new EncodedVideoChunk({
timestamp: responses[i].timestamp,
data: new Uint8Array ( responses[i].body )
await decoder.flush();
The path from the network or storage to an ImageBitmap.
The path from the network or storage to an ImageBitmap.

Now it's time to show how a freshly decoded frame can be shown on the page. It's better to make sure that the decoder output callback (handleFrame()) quickly returns. In the example below, it only adds a frame to the queue of frames ready for rendering. Rendering happens separately, and consists of three steps:

  1. Converting the VideoFrame into an ImageBitmap.
  2. Waiting for the right time to show the frame.
  3. Drawing the image on the canvas.

Once a frame is no longer needed, call destroy() to release underlying memory before the garbage collector gets to it, this will reduce the average amount of memory used by the web application.

let cnv = document.getElementById('canvas_to_render');
let ctx = cnv.getContext('2d', { alpha: false });
let ready_frames = [];
let underflow = true;
let time_base = 0;

function handleFrame(frame) {
if (underflow)
setTimeout(render_frame, 0);

function delay(time_ms) {
return new Promise((resolve) => {
setTimeout(resolve, time_ms);

function calculateTimeTillNextFrame(timestamp) {
if (time_base == 0)
time_base =;
let media_time = - time_base;
return Math.max(0, (timestamp / 1000) - media_time);

async function render_frame() {
if (ready_frames.length == 0) {
underflow = true;
let frame = ready_frames.shift();
underflow = false;

let bitmap = await frame.createImageBitmap();
// Based on the frame's timestamp calculate how much of real time waiting
// is needed before showing the next frame.
let time_till_next_frame = calculateTimeTillNextFrame(frame.timestamp);
await delay(time_till_next_frame);
ctx.drawImage(bitmap, 0, 0);

// Immediately schedule rendering of the next frame
setTimeout(render_frame, 0);


The demo below shows two canvases, the first one is animated at the refresh rate of your display, the second one shows a sequence of frames captured by VideoTrackReader at 30 FPS, encoded and decoded using WebCodecs API.

Feature detection

To check for WebCodecs support:

if ('VideoEncoder' in window) {
// WebCodecs API is supported.

Using the WebCodecs API

Enabling via a command line flag

To experiment with the WebCodecs API locally on all desktop platforms, without an origin trial token, start Chrome with a command line flag:


Enabling support during the origin trial phase

The WebCodecs API is available on all desktop platforms (Chrome OS, Linux, macOS, and Windows) as an origin trial in Chrome 86. The origin trial is expected to end just before Chrome 88 moves to stable in February 2021. The API can also be enabled using a flag.

Origin trials allow you to try new features and give feedback on their usability, practicality, and effectiveness to the web standards community. For more information, see the Origin Trials Guide for Web Developers. To sign up for this or another origin trial, visit the registration page.

Register for the origin trial

  1. Request a token for your origin.
  2. Add the token to your pages. There are two ways to do that:
    • Add an origin-trial <meta> tag to the head of each page. For example, this may look something like:
      <meta http-equiv="origin-trial" content="TOKEN_GOES_HERE">
    • If you can configure your server, you can also add the token using an Origin-Trial HTTP header. The resulting response header should look something like:
      Origin-Trial: TOKEN_GOES_HERE


The Chrome team wants to hear about your experiences with the Idle Detection API.

Tell us about the API design

Is there something about the API that doesn't work like you expected? Or are there missing methods or properties that you need to implement your idea? Have a question or comment on the security model? File a spec issue on the corresponding GitHub repo, or add your thoughts to an existing issue.

Report a problem with the implementation

Did you find a bug with Chrome's implementation? Or is the implementation different from the spec? File a bug at Be sure to include as much detail as you can, simple instructions for reproducing, and enter Blink>Media>WebCodecs in the Components box. Glitch works great for sharing quick and easy repros.

Show support for the API

Are you planning to use the WebCodecs API? Your public support helps the Chrome team to prioritize features and shows other browser vendors how critical it is to support them.

Send emails to or tweet to [@ChromiumDev][cr-dev-twitter] with the #webcodecs hashtag and let us know where and how you're using it.

Hero image by Denise Jans on Unsplash.

This content originally appeared on and was authored by Eugene Zemtsov

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