WebCodecs API
Note: This feature is available inDedicated Web Workers.
TheWebCodecs API gives web developers low-level access to the individual frames of a video stream and chunks of audio.It is useful for web applications that require full control over the way media is processed.For example, video or audio editors, and video conferencing.
Concepts and Usage
Many Web APIs use media codecs internally.For example, theWeb Audio API, and theWebRTC API.However these APIs do not allow developers to work with individual frames of a video stream and unmixed chunks of encoded audio or video.
Web developers have typically used WebAssembly in order to get around this limitation,and to work with media codecs in the browser.However, this requires additional bandwidth to download codecs that already exist in the browser,reducing performance and power efficiency, and adding additional development overhead.
The WebCodecs API provides access to codecs that are already in the browser.It gives access to raw video frames, chunks of audio data, image decoders, audio and video encoders and decoders.
Processing Model
The WebCodecs API uses an asynchronousprocessing model. Each instanceof an encoder or decoder maintains an internal, independent processing queue. When queueing a substantial amount of work, it's important tokeep this model in mind.
Methods namedconfigure(),encode(),decode(), andflush() operate asynchronously by appending control messagesto the end the queue, while methods namedreset() andclose() synchronously abort all pending work and purge theprocessing queue. Afterreset(), more work may be queued following a call toconfigure(), butclose() is a permanent operation.
Methods namedflush() can be used to wait for the completion of all work that was pending at the timeflush() was called. However, itshould generally only be called once all desired work is queued. It is not intended to force progress at regular intervals. Calling itunnecessarily will affect encoder quality and cause decoders to require the next input to be a key frame.
Demuxing
There is currently no API for demuxing media containers. Developers working with containerized media will need to implement their ownor use third party libraries. E.g.,MP4Box.js orjswebm can beused to demux audio and video data intoEncodedAudioChunk andEncodedVideoChunk objects respectively.
Interfaces
AudioDecoderDecodes
EncodedAudioChunkobjects.VideoDecoderDecodes
EncodedVideoChunkobjects.AudioEncoderEncodes
AudioDataobjects.VideoEncoderEncodes
VideoFrameobjects.EncodedAudioChunkRepresents codec-specific encoded audio bytes.
EncodedVideoChunkRepresents codec-specific encoded video bytes.
AudioDataRepresents unencoded audio data.
VideoFrameRepresents a frame of unencoded video data.
VideoColorSpaceRepresents the color space of a video frame.
ImageDecoderUnpacks and decodes image data, giving access to the sequence of frames in an animated image.
ImageTrackListRepresents the list of tracks available in the image.
ImageTrackRepresents an individual image track.
Examples
In the following example, frames are returned from aMediaStreamTrackProcessor, then encoded.See the full example and read more about it in the articleVideo processing with WebCodecs.
let frameCounter = 0;const track = stream.getVideoTracks()[0];const mediaProcessor = new MediaStreamTrackProcessor(track);const reader = mediaProcessor.readable.getReader();while (true) { const result = await reader.read(); if (result.done) break; let frame = result.value; if (encoder.encodeQueueSize > 2) { // Too many frames in flight, encoder is overwhelmed // let's drop this frame. frame.close(); } else { frameCounter++; const insertKeyframe = frameCounter % 150 === 0; encoder.encode(frame, { keyFrame: insertKeyframe }); frame.close(); }}