We propose adding a configuration property on the VideoTrack and AudioTrack interfaces to allow authors to read media file properties as determined by the User Agent.
There are a number of web APIs which ingest specific data about video and audio codecs and formats. MediaCapabilites, for one example, lets a page query whether a given VideoConfiguration or AudioConfiguration are supported. WebCodecs, for another, allows a page to construct a VideoFrame with a particular VideoColorSpace. WebAudio, for a third, allows a page to create an AudioContext with a specific sample rate, and attach a media element via MediaElementSourceNode. However, the mechanism by which a page is supposed to determine the properties of a particular piece of media exist only outside the web platform.
A page would like to implement something akin to YouTube’s “Stats for Nerds” panel. It may not have the ability to know the media’s configuration out-of-band.
A page allows users to upload content from their own device, but the page would like to do on-device pre-flighting that the media’s codecs were supported by the upload service.
A page would like to create an AudioContext whose sampleRate matches the sample rate of a particular piece of audio content.
A page would like to validate that its encoder and muxer generates media whose VideoColorSpace is correctly parsed by the browser.
The HTMLMediaElement’s VideoTrack and AudioTrack objects would vend a new attribute VideoTrackConfiguration and AudioTrackConfiguration.
dictionary VideoTrackConfiguration {
DOMString codec;
unsigned long long bitrate;
double framerate;
unsigned long width;
unsigned long height;
VideoColorSpace colorSpace;
};
dictionary AudioTrackConfiguration {
DOMString codec;
unsigned long long bitrate;
unsigned long sampleRate;
unsigned long numberOfChannels;
};
partial interface VideoTrack {
readonly attribute VideoTrackConfiguration configuration
};
partial interface AudioTrack {
readonly attribute AudioTrackConfiguration configuration
};
VideoTrackConfiguration’s colorSpace object, in particular, is borrowed from WebCodecs.
The VideoTrackList and AudioTrackList objects would have a new event handler onconfigurationchange, which would fire a TrackEvent event referencing the VideoTrack or AudioTrack whose configuration has changed.
partial interface VideoTrackList {
attribute EventHandler onconfigurationchange;
};
partial interface AudioTrackList {
attribute EventHandler onconfigurationchange;
};
The attributes exposed through VideoTrackConfiguration and AudioTrackConfiguration should be filled with values as understood by the browser. For example, if in the absence of explicit color space information the browser assumes BT.709, it should return “bt709“ in the configuration’s colorSpace.primaries rather than null.
Due to the risk of leaking information across security origins, the VideoTrackConfiguration and AudioTrackConfiguration objects for media elements whose media data is CORS cross-origin should be empty.