Re: HTML5 support for track metadata

On Tue, 2014-03-04 at 20:17 +0000, Bob Lund wrote:
No. I am proposing extending HTML5 support for the 3rd option, i.e. Using inBandMetadataTrackDispatchType for metadata tracks and kind for all other tracks.
I don't see anywhere on the wiki saying what the format will actually be. Presumably it would be the same?

Why that option though? I took a look at the pros and cons<https://www.w3.org/community/inbandtracks/wiki/Main_Page>, and I disagree with several of them.

  *   inBandMetadataTrackDispatchType isn't implemented in any UA's I'm aware of (neither are the other two options).
  *   Changing the HTML5 spec isn't a big deal, because the TextTrack spec is in flux, and no one has implemented in-band metadata yet. Getting it right is more important than changing some attributes.
  *   The use-case for all three is exactly the same, but it's listed as a pro for inBandMetadataTrackDispatchType and a con for In-band Metadata Attribute.
  *   Accessing the data isn't significantly easier with one or another:


// In-band Metadata TextTrack
binaryData = track.activeCues[0].data;

// In-band Metadata Attribute
binaryData = track.metadata;

// inBandMetadataTrackDispatchType
binaryData = base64DecToArr(track.inBandMetadataTrackDispatchType.split(" ")[1])


For inBandMetadataTrackDispatchType, converting from a base64 string to an ArrayBuffer is fairly complicated, but Mozilla provides the code for a base64DecToArr() function<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Base64_encoding_and_decoding>, so that's what I used above. Presumably people will copy and paste that code, or something similar, into their projects.
I'm somewhat biased, because I've been writing patches to support the In-band Metadata TextTrack alternative<https://bugs.webkit.org/show_bug.cgi?id=122001>, but from my perspective the pros and cons are:
Alternative     Pros    Cons
In-band Metadata TextTrack

  *   Allows JS to look at previous and future metadata (possibly not useful?).
  *   Allows us to keep the data as binary.
  *   DataCue was created for this purpose.


  *   Mixing metadata with cues is possibly inelegant.
  *   Possible complexity regarding old cues.


In-band Metadata Attribute

  *   Metadata as an attribute seems more elegant.
  *   Allows us to keep the data as binary.


inBandMetadataTrackDispatchType

  *   Metadata as an attribute seems more elegant.


  *   Need to encode binary as a string.


If we go with one of the attributes, we'll want to add some sort of "change" event to TextTrack, but I don't consider that to be a con, since it's trivial.