This document defines a recording API for use with MediaStreams as defined in [[!GETUSERMEDIA]]

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This document is not complete. It is subject to major changes and, while early experimentations are encouraged, it is therefore not intended for implementation. The Media Capture Task Force expects this specification to evolve significantly based on:

This document was published by the Web Real-Time Communication Working Group as an Editor's Draft. If you wish to make comments regarding this document, please send them to public-media-capture@w3.org (subscribe, archives). All feedback is welcome.

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Overview

This API attempts to make basic recording very simple, while still allowing for more complex use cases. In the simplest case, the application instatiates the MediaRecorder object, calls record() and then calls stopRecord() or waits for the MediaStream to be ended. The contents of the recording will be made available in the platform's default encoding via the dataavailable event. Functions are available to query the platform's available set of encodings, and to select the desired ones if the author wishes. The application can also choose how much data it wants to receive at one time. By default a Blob containing the entire recording is returned when the recording finishes. However the application can choose to receive smaller buffers of data at regular intervals.

Media Recorder API

readonly attribute MediaStream stream
The MediaStream passed in to the constructor.
readonly attribute RecordingStateEnum state
The current state of the MediaRecorder object.
readonly attribute unsigned long imageWidth
The width of the video or photo image to capture. The initial value will be a platform-supplied default.
readonly attribute unsigned long imageHeight
The height of the video or photo image to capture. The initial value will be a platform-supplied default.
readonly attribute DOMString mimeType
The MIME type for recording. It specifies the container format as well as the audio and video capture formats. The initial value will be a platform-supplied default.
attribute EventHandler onstart
Called to handle the start event.
attribute EventHandler onstop
Called to handle the stop event.
attribute EventHandler ondataavailable
Called to handle the dataavailable event. Note that the Blob (see [[!FILE-API]]) of recorded data is contained in this event and can be accessed via the 'data' attribute.
attribute EventHandler onpause
Called to handle the pause event.
attribute EventHandler onresume
Called to handle the resume event.
attribute EventHandler onerror
Called to handle the recordingerror event.
attribute EventHandler onwarning
Called to handle the recordingwarning event.
void start()
When a MediaRecorder object’s start() method is invoked, then, if the state is not "inactive", the UA MUST raise an INVALID_STATE exception and return immediately. Otherwise, it MUST queue a task, using the DOM manipulation task source, that runs the following steps:
  1. Set state to 'recording' and wait until media becomes available from stream.
  2. Once data becomes available raise a start event and start gathering the data into a Blob (see [[!FILE-API]]).
  3. If the timeSlice argument has been provided, then once timeSlice milliseconds of data have been colleced, raise a dataavailable event containing the Blob of collected data, and start gathering a new Blob of data. Otherwise (if timeSlice has not been provided), continue gathering data into the original Blob.
  4. When the stream is ended set recording to 'false' and stop gathering data.
  5. Then raise a dataavailable event containing the Blob of data.
  6. Finally, raise a stoprecording event.

Note that stop(), requestData(), pause, and muteTrack also affect the recording behavior.

The UA must record the MediaStream in such a way that the original Tracks can be retrieved at playback time. If any Track within the MediaStream is muted at any time (i.e., if its readyState is set to muted), the UA must insert black frames or silence until the Track is unmuted. If the UA is unable to start recording or at any point is unable to contine recording, it must raise a recordingerror event, follwowed by a dataavailable event containing the Blob it has gathered, follwed by the stoprecording event.

optional long? timeslice
The number of milliseconds of data to return in a single Blob.
void stop()
When a MediaRecorder object’s stop method is invoked, then if state is "inactive", the user agent MUST raise an INVALID_STATE exception and return immediately. Otherwise it MUST queue a task, using the DOM manipulation task source, that runs the following steps:
  1. Set state to 'inactive' and stop gathering data.
  2. Raise a dataavailable event containing the Blob of data that has been gathered.
  3. Raise a done event
void pause()
When a MediaRecorder object’s pause()method is invoked, then if state is "inactive" the user agent must raise an INVALID_STATE exception and return immediately. Otherwise it must queue a task, using the DOM manipulation task source, that runs the following steps:
  1. Set state to "paused".
  2. Stop gathering data into its current Blob (but keep the Blob available so that recording can be resumed in the future).
  3. Raise a pause event
void resume()
When a MediaRecorder object’s resume() method is invoked, then if state is "inactive" the user agent must raise an INVALID_STATE exception and return immediately. Otherwise it must queue a task, using the DOM manipulation task source, that runs the following steps:
  1. Set state to "recording".
  2. Resume (or continue) gathering data into its current Blob.
  3. Raise a resume event.
void requestData()
When a MediaRecorderobject’s requestData() method is invoked, then if state is not "recording" the user agent must raise an INVALID_STATE exception and return immediately. Otherwise it must queue a task, using the DOM manipulation task source, that runs the following steps:
  1. Raise a dataavailable event containing the current Blob of saved data.
  2. Create a new Blob and gather subsequent data into it.
AvailableSettings getOptions()
When a MediaRecorder object’s getOptions() method is invoked, the user agent must return an AvailableSettings structure containing the set of recording options that it supports. These are the legal values that can be set via setOptions.
void setOptions(RecordingSettings optionValues)
When a MediaRecorder object’s setOptions() method is invoked, then if state is not "inactive", the user agent must raise an INVALID_STATE exception and return immediately. Otherwise it must run the following steps before returning:
  1. Let currentOptions be the current set of options that are specified to be used for recording by the MediaRecorder. (If setOptions() has not yet been called, these will be the UA's defaults.)
  2. For each option that is specified in the optionVlaues argument, if the UA does not support it, raise a UNSUPPORTED_OPTION exception, set currentOptions back to its value in step 2, and terminate these steps. Otherwise replace the corresponding member of currentOptions with the specified value.

The MediaRecorder() constructor takes one argument which must be of type MediaStream (see [[!GETUSERMEDIA]]). When the constructor is invoked, the UA must construct a new MediaRecorder object, set its mediaStream attribute to be the provided MediaStream, set its state attribute to 'inactive' and return the object.

RecordingStateEnum

inactive
Recording is not occuring. (Either it has not been started or it has been stopped.).
recording
Recording has been started and the UA is capturing data..
paused
Recording has been started, then paused, and not yet stopped or resumed.

Blob Event

readonly attribute Blob data
Returns a Blob object whose type attribute indicates the encoding of the blob data. An implementation must return a Blob in a format that is capable of being viewed in an HTML <img> tag. .

BlobEventInit

Blob data
A Blob object containing the data to deliver via this event.

Settings Dictionaries

CapabilityList MimeType
A list of the MIME types that can be selected as encodings for recording.
CapabilityRange imageWidth
The maximum and minimum width, in pixels, for the capture of video or photo images.
CapabilityRange imageHeight
The maximum and minimum height, in pixels, for the capture of video or photo images.
DOMString? MimeType
Used to set the value of mimeType.
unsigned long? imageWidth
Used to set hte value of imageWidth.
unsigned long? imageHeight
Used to set the value of imageWidth.

Error Handling

General Principles

Errors are indicated in two ways: exceptions and objects passed to error callbacks. Both forms of error reporting must provide an object of type RecordingError. An exception must be thrown in the following cases:

In all other cases, an error object must be provided to the failure callback. The error name in the object provided must be picked from the RecordingErrorName enums. After raising the error, the UA must raise a dataavailable event, containing any data that it has gathered, and then a recordingdone event. The UA may set platform-specific limits, such those for the minimum and maximum Blob size that it will support, or the number of Tracks it will record at once. It must signal a fatal error if these limits are exceeded. If a non-fatal error occurs during recording, the UA should raise a recordingwarning event, with data indicating the nature of the problem, and continue recording.

RecordingError

readonly attribute RecordingErrorEnum name
A string representing the type of the error.
readonly attribute DOMString? message
A human readable description of the error. This string MAY vary between different user agents.
RecordingErrorEnum name
Used to set the name attribute.
DOMString? message
Used to set the message attribute.

RecordingErrorNameEnum

OUT_OF_MEMORY
The UA has exhaused the available memory. User agents SHOULD provide as much additional information as possible in the message attribute.
ILLEGAL_STREAM_MODIFICATION
A modification to the stream has occurred that makes it impossible to continue recording. An example would be the addition of a Track while recording is occurring. User agents SHOULD provide as much additional information as possible in the message attribute.
"OTHER_RECORDING_ERROR"
Used for an fatal error other than those listed above. User agents SHOULD provide as much additional information as possible in the message attribute.

RecordingExceptionEnum

INVALID_STATE
The function was called on a MediaRecorder that is an invalid state, or a state in which the function is not allowed to be executed.
INVALID_MEDIASTREAM_TRACK_ID
The argument provided is not the ID of any MediaStreamTrack belonging to the MediaRecorder's stream.
UNSUPPORTED_OPTION
The UA cannot provide the codec or recording option that has been requested.

Event summary

The following additional events fire on MediaRecorder objects:

Event name Interface Fired when...
start Event The UA has started recording data on the MediaStream.
stop Event The UA has stopped recording data on the MediaStream.
dataavailable BlobEvent The UA generates this even to return data to the application. The 'data' attribute of this event contains a Blob of recorded data.
photo BlobEvent The UA uses this event to return a photo to the application (as a Blob). The 'data' attribute of this event contains a Blob of recorded snapshot data.
pause Event The UA has paused recording data on the MediaStream.
resume MediaSteamEvent The UA has resumed recording data on the MediaStream.
mutetrack MediaSteamTrackEvent The MediaRecorder has muted recording on a Track.
unmutetrack MediaSteamTrackEvent The MediaRecorder has unmuted recording on a Track.
recordingerror RecordingError A fatal error has occurred and the UA has stopped recording. More detailed error information is available in the 'message' attribute.
warning CustomEvent A problem has occurred, but the UA has not stopped recording. More detailed information is available in the 'message' attribute. [Not clear if we need this event.]

A. Open Issues

  1. Do we need an MTI format?
  2. Do we need a "setSyncPoint()" operator and a "syncpoint" signal, so that the client can tell the recorder to insert a point at which a recording can be broken up (typically a new I-frame)?
  3. Do we need to ask the user's permission before we record?