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Re: A new proposal for how to deal with text track cues

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Fri, 14 Jun 2013 18:30:39 +1000
Message-ID: <CAHp8n2nJwS9at-e2Typ31gkKniLkBMKDukkdK=3B8TMqZ8Ak2w@mail.gmail.com>
To: Bob Lund <B.Lund@cablelabs.com>
Cc: public-html <public-html@w3.org>
On Thu, Jun 13, 2013 at 2:22 AM, Bob Lund <B.Lund@cablelabs.com> wrote:
> On 6/11/13 11:11 PM, "Silvia Pfeiffer" <silviapfeiffer1@gmail.com> wrote:
>>Hi all,
>>The model in which we have looked at text tracks (<track> element of
>>media elements) thus far has some issues that I would like to point
>>out in this email and I would like to suggest a new way to look at
>>tracks. This will result in changes to the HTML and WebVTT specs and
>>has an influence on others specifying text track cue formats, so I am
>>sharing this information widely.
>>Current situation
>>Text tracks provide lists of timed cues for media elements, i.e. they
>>have a start time, an end time, and some content that is to be
>>interpreted in sync with the media element's timeline.
>>WebVTT is the file format that we chose to define as a serialisation
>>for the cues (just like audio files serialize audio samples/frames and
>>video files serialize video frames).
>>The means in which we currently parse WebVTT files into JS objects has
>>us create objects of type WebVTTCue. These objects contain information
>>about any kind of cue that could be included in a WebVTT file -
>>captions, subtitles, descriptions, chapters, metadata and whatnot.
>>The WebVTTCue object looks like this:
>>enum AutoKeyword { "auto" };
>>[Constructor(double startTime, double endTime, DOMString text)]
>>interface WebVTTCue : TextTrackCue {
>>           attribute DOMString vertical;
>>           attribute boolean snapToLines;
>>           attribute (long or AutoKeyword) line;
>>           attribute long position;
>>           attribute long size;
>>           attribute DOMString align;
>>           attribute DOMString text;
>>  DocumentFragment getCueAsHTML();
>>There are attributes in the WebVTTCue object that relate only to cues
>>of kind captions and subtitles (vertical, snapToLines etc). For cues
>>of other kinds, the only relevant attribute right now is the text
>>This works for now, because cues of kind descriptions and chapters are
>>only regarded as plain text, and the structure of the content of cues
>>of kind metadata is not parsed by the browser. So, for cues of kind
>>descriptions, chapters and metadata, that .text attribute is
>>The consequence
>>As we continue to evolve the functionality of text tracks, we will
>>introduce more complex other structured content into cues and we will
>>want browsers to parse and interpret them.
>>For example, I expect that once we have support for speech synthesis
>>in browsers [1], cues of kind descriptions will be voiced by speech
>>synthesis, and eventually we want to influence that speech synthesis
>>with markup (possibly a subpart of SSML [2] or some other simpler
>>markup that influences prosody).
>>Since we have set ourselves up for parsing all cue content that comes
>>out of WebVTT files into WebVTTCue objects, we now have to expand the
>>WebVTTCue object with attributes for speech synthesis, e.g. I can
>>imagine cue settings for descriptions to contain a field called
>>"channelMask" to contain which audio channels a particular cue should
>>be rendered into with values being center, left, right.
>>Another example is that eventually somebody may want to introduce
>>ThumbnailCues that contain data URLs for images and may have a
>>"transparency" cue setting. Or somebody wants to formalize
>>MidrollAdCues that contain data URLs for short video ads and may have
>>a "skippableAfterSecs" cue setting.
>>All of these new cue settings would end up as new attributes on the
>>WebVTTCue object. This is a dangerous design path that we have taken.
>>[2] http://www.w3.org/TR/speech-synthesis/#S3.2
>>Problem analysis
>>What we have done by restricting ourselves to a single WebVTTCue
>>object to represent all types of cues that come from a WebVTT file is
>>to ignore that WebVTT is just a serialisation format for cues, but
>>that cues are the ones that provide the different types of timed
>>content to the browser. The browser should not have to care about the
>>serialisation format. But it should care about the different types of
>>content that a track cue could contain.
>>For example, it is possible that a WebVTT caption cue (one with all
>>the markup and cue settings) can be provided to the browser through a
>>WebM file or through a MPEG file or in fact (gasp!) through a TTML
>>file. Such a cue should always end up in a WebVTTCue object (will need
>>a better name) and not in an object that is specific to the
>>serialisation format.
>>What we have done with WebVTT is actually two-fold:
>>1. we have created a file format that serializes arbitrary content
>>that is time-synchronized with a media element.
>>2. and we have created a simple caption/subtitle cue format.
>>That both are called "WebVTT" is the cause of a lot of confusion and
>>not a good design approach.
>>The solution
>>We thus need to distinguish between cue formats in the browser and not
>>between serialisation formats (we don't distinguish between different
>>image formats or audio formats in the browser either - we just handle
>>audio samples or image pixels).
> I agree with the analysis. This is precisely the issue I ran into with
> metadata text track cues for in-band MPEG2-TS, i.e. Other metadata in the
> serialization format actually defined the type of the Cue objects carried
> within the serialization container. The sole reason the metadata track
> dispatch solution was requested was to deal with the problem that the Cue
> format is not defined by @kind and serialization format.

I believe it would be useful to replace the metadata track dispatch
attribute with something more generic that the browser can use for any
serialisation format that provides a hint about the format in which
the cue content is provided. I think it would be a logical

>>Once a WebVTT file is parsed into a list of cues, the browser should
>>not have to care any more that the list of cues came from a WebVTT
>>file or anywhere else. It's a list of cues with a certain type of
>>content that has a parsing and a rendering algorithm attached.
> Other serialization formats should be supported by the solution. I'm not
> saying the proposal doesn't extend to other formats, but this should be a
> requirement. TTML (and variants) are obvious alternatives for
> serialization formats. Existing formats, e.g. MPEG4 and MPEG-2 PS, also
> define serialization methods for in-band text track cues.

I was indeed considering all these other formats, too, as I was
thinking about the solution of this problem. In fact, looking at this
bigger picture is what motivated this proposal.

For example, I expect that there will be a need for IE to specify a
TTMLCaptionCue object that contains specific settings of TTML caption
cues and expects as its content captions in TTML format.

>>Spec consequences
> An analysis of serialization formats besides WebVTT (see comment above)
> should be done before concluding that these are the only spec consequences.

This research was done even before starting to specify WebVTT, see
http://wiki.whatwg.org/wiki/Timed_track_formats (and other pages in
that wiki) . It resulted in an understanding of the role of
serialisation formats and of cues. This is why I am suggesting to make
cues more explicit content types.

With this understanding, it is possible for any new cue format that a
browser wants to support to either find a mapping to an existing cue
object or introduce a new Cue object type into the hierarchy.

For example, MP4 chapters may be extracted as specified here:
http://code.google.com/p/subler/wiki/ChapterTextFormat - these can
simply map into the proposed ChapterCue as plain text chapers. In
contrast, GPAC 3GPP chapters e.g.
http://sourceforge.net/p/gpac/discussion/287547/thread/4ffdaf01 may
need some extra positioning and styling information that may require
creation of specific 3GPPChaperCue objects, even though a browser
could decide to simply ignore that extra information and still only
map it to ChapterCue objects.

While my goal is to make browsers have to deal with as few as
necessary objects for different types of cues, I am fully aware that
different cue types require different handling. That was the
motivation for proposing this change.

Hope that clarifies it some more.

Best Regards,

>>What needs to change in the specs to deal with this different approach
>>to text tracks is not hard to deduct.
>>Firstly, there are consequences on the WebVTT spec.
>>I suggest we rename WebVTTCue [1] to VTTCaptionCue and allow such cues
>>only on tracks of kind={caption, subtitle}.
>>Also, we separate out the WebVTT serialisation format syntax
>>specification from the cue syntax specification [2] and introduce
>>separate parsers [3] for the different cue syntax formats.
>>The rendering section [4] has already started distinguishing between
>>cue rendering for chapters and for captions/subtitles. This will
>>easily fit with the now separated cue syntax formats.
>>We will then introduce a ChapterCue which adds a .text attribute and a
>>constructor onto AbstractCue for cues (in WebVTT or from elsewhere)
>>that are interpreted as chapters and have their own rendering
>>Similarly, we introduce a DescriptionCue which adds a .text attribute
>>and a constructor onto AbstractCue and we define a rendering algorithm
>>that makes use of the new speech synthesis API [5].
>>Similarly, we introduce a MetadataCue which adds a .content attribute
>>and a constructor onto AbstractCue with no rendering algorithm.
>>I think these new cue objects would even make more sense being defined
>>in HTML including their rendering algorithms rather than in the WebVTT
>>spec, because they are generic and we don't want chapters to be
>>rendered differently just because they have originated from a
>>different serialisation format.
>>[1] http://dev.w3.org/html5/webvtt/#webvtt-api
>>[2] http://dev.w3.org/html5/webvtt/#syntax
>>[3] http://dev.w3.org/html5/webvtt/#parsing
>>[4] http://dev.w3.org/html5/webvtt/#rendering
>>Secondly, there are consequences for the TextTrackCue object hierarchy
>>in the HTML spec.
>>I suggest we rename TextTrackCue [6] to AbstractCue (or just Cue). It
>>is simply the abstract result of parsing a serialisation of cues (e.g.
>>a WebVTT file) into its individual cues.
>>Similarly TextTrackCueList [7] should be renamed to CueList and should
>>be a cue list of only one particular type of cue. Thus, the parsing
>>and rendering algorithm in use for all cues in a CueList is fixed.
>>Also, a CueList of e.g. ChapterCues should only be allowed to be
>>attached to a track of kind=chapters, etc.
>>Doing this will make WebVTT and the TextTrack API extensible for new
>>cue formats, such as cues in SSML format, or ThumbnailCues, or
>>MidrollAdCues or whatnot else we may see necessary in the future.
>>This may look like a lot of changes, but it's really just some
>>renaming and an introduction of a small number of semantically clean
>>new objects. I'm happy to prepare the patches for the WebVTT and
>>HTML5.1 specs if this is agreeable.
>>Feedback welcome.
Received on Friday, 14 June 2013 08:31:27 UTC

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