W3C home > Mailing lists > Public > public-media-capture@w3.org > December 2011

Generic media recording proposal

From: Travis Leithead <travis.leithead@microsoft.com>
Date: Thu, 8 Dec 2011 17:50:21 +0000
To: "public-media-capture@w3.org" <public-media-capture@w3.org>
Message-ID: <9768D477C67135458BF978A45BCF9B38381C517F@TK5EX14MBXW602.wingroup.windeploy.ntdev.microsoft.com>
What is the group's thoughts on creating a recorder interface that could be used for more than just MediaStreams? For example, it seems like it might be neat to be able to record a <video> or <audio> tag, or record video from an animating <canvas>?

If this sounds like an interesting scenario for future extension, I would suggest that the "recording" capability be logically separated from the MediaStream. 

I would propose something akin to the FileReader, but for recording:

// encodeType required here, but could be made optional if the type is left to UA discretion
[Constructor(DOMString encodeType, optional DOMString quality)]
interface MediaEncoder {
    // similar to canPlayType() of HTMLMediaElement
    static boolean canRecordType(DOMString type);
    readonly attribute DOMString encodingType; // Reports the constructor's value
    readonly attribute DOMString encodingQuality; // Reports the constructor's encoding quality (or the default)
    // encoder state
    readonly attribute DOMString readyState; // "idle", "encoding", "error"
    readonly attribute DOMError error;  // Reports the error state.
    // data source
    void setSource(MediaStream source);  // The source of the stream, including access to related tracks, etc.
    // Could add more overloads here in the future...e.g.:
    // void setSource(HTMLVideoElement source);
    readonly attribute any src; // null or reference to the current source (MediaStream)

    // encoder controls
    void start(); // async start encoding request
    void stop(); // async end request
    // encoded result
    readonly attribute Stream stream; // suitable for using StreamReader to collect into a Blob if desired
    // Support [encoder] media events
    [TreatNonCallableAsNull] attribute Function? onload;
    [TreatNonCallableAsNull] attribute Function? onstart;
    [TreatNonCallableAsNull] attribute Function? onstop;
    [TreatNonCallableAsNull] attribute Function? onbufferempty;
    [TreatNonCallableAsNull] attribute Function? onchange;
    [TreatNonCallableAsNull] attribute Function? onerror;

* load - fired after a new src is established, and the encoder has finished analyzing the media stream and prepared its internal state
* start - fired when encoded data is available in the [internal] buffer after the call to start() has been made (the stream result will then have data available)
* stop - fired after the stop() request was made and the encoder stopped processing new data from the src (data may still be in the buffer waiting to be read by the stream sink)
* bufferempty - fired when the MediaRecorder's buffer is empty (i.e., the stream sink has finished reading all the data from the [internal] recorded buffer)
* change - fired when the recording source is changed (via setSource() before load fires)
* error - fired for a variety of possible reasons, such as calling stop() before start(), attempting to assign a new setSource() while encoding is in progress, incompatible mutations to the MediaStream's tracks, etc.

The above proposal is built around audio/video capture, but might be extended for photo/snapshot use too. 

Received on Thursday, 8 December 2011 17:51:22 UTC

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