Media Stream Recording API

This folder contains the implementation of the W3C MediaStream Recording API. Image Capture was shipped in Chrome 49 (Mar 2016) -- support in other browsers can be found in the CanIUse entry.

Encoding video uses hardware accelerated capabilities where possible: check Encode Accelerator Implementation Status for the current situation.

API Mechanics

A MediaRecorder uses a MediaStream as its source of data. The stream may originate from a camera, microphone, <canvas>, <video> or <audio> tag, remote PeerConnection, web audio Node or content capture (such as the screen, a window or a tab).

Construction Options

The MediaRecorder() constructor accepts an optional MediaRecorderOptions dictionary giving hints as to how to carry out the encoding:

  • mimeType indicates which container and codec to use, e.g. video/webm;codecs="vp9" or video/x-matroska;codecs="avc1" (see the specific isTypeSupported() test).

    Chrome will select the best encoding format if mimeType is left unspecified; in particular, it will select a hardware accelerated encoder if available. (The actual encoding format can be found in ondataavailable Blobs type).

  • Users can vary the target encoding bitrate to accommodate different scenes and CPU loads via the different bitrate members.

Recording

Once a MediaRecorder is created, recording can begin with start().

This method accepts an optional timeslice parameter. Chrome will buffer this much of the encoded result (in milliseconds). If unspecified Chrome will buffer as much as possible. A value of 0 will cause as little buffering as possible.

Encoded chunks are received via the ondataavailable event, following the cadence specified by the timeslice. If timeslice is unspecified, the buffer can be flushed using requestData() or stop(). event.data contains the recorded Blob.

Implementation in Chromium

This API is structured around the MediaRecorder class, which owns a MediaRecorderHandler which in turn owns a number of VideoTrackRecorder/AudioTrackRecorders and a single WebmMuxer. VideoTrackRecorders are codec specific and encapsulate the necessary resources to get the job done. All this is illustrated in the diagram below.

MediaRecorder classes

Other topics

Can MediaRecorder record stereo?

Yes it can, but Chrome‘s implementation of audio streams doesn’t support this format, see crbug/706013 and crbug/596182.

The produced recording doesn't have duration or is not seekable

This is by design of the webm live format and is tracked in crbug/642012. The alternative is to use a Javascript library to reconstruct the Cues (see the discussion in the Spec), or feed the individual recorded chunks into a <video> via a SourceBuffer.

Testing

Media Recorder web tests are located in web_tests/fast/mediarecorder, and web_tests/external/mediacapture-record, unittests in content and media and browsertests.