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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 14:54:12 +1000
Message-ID: <CAHp8n2=CY3xE626L3xfYrjFgKmoCM7sV5o8t6hM+tAM+jA=jSQ@mail.gmail.com>
To: Pierre-Anthony Lemieux <pal@sandflow.com>
Cc: public-html <public-html@w3.org>
On Fri, Jun 14, 2013 at 1:50 AM, Pierre-Anthony Lemieux
<pal@sandflow.com> wrote:
> Hi Silvia,
> I like the idea of making the HTML cue interface independent from the
> underlying serialization format, and move discussions on the latter to
> the TTWG, as suggested by others.

So you agree that this group should rename TextTrackCue to AbstractCue
(or just Cue) and TextTrackCueList to CueList?

> In fact, along the same lines, I would move paragraphs [a] and [b]
> (see below) of Section 4.8.9 to the WebVTT specification. I think this
> would remove the last normative provisions tied to a specific
> serialization format.

You may be looking at HTML5.0. HTML5.1 doesn't contain these any more.

I would indeed suggest that we adjust HTML5.0 to contain the same text
as HTML5.1 for tracks.

> Hope it makes sense.


> Best,
> -- Pierre
> [a] If the element's track URL identifies a WebVTT resource, and the
> element's kind attribute is not in the metadata state, then the WebVTT
> file must be a WebVTT file using cue text. [WEBVTT]
> [b] Furthermore, if the element's track URL identifies a WebVTT
> resource, and the element's kind attribute is in the chapters state,
> then the WebVTT file must be both a WebVTT file using chapter title
> text and a WebVTT file using only nested cues. [WEBVTT]
> On Tue, Jun 11, 2013 at 10:11 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
>> attribute.
>> 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
>> sufficient.
>> 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.
>> [1] https://dvcs.w3.org/hg/speech-api/raw-file/tip/speechapi.html#tts-section
>> [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).
>> 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.
>> Spec consequences
>> ==================
>> 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
>> algorithm.
>> 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
>> [5] https://dvcs.w3.org/hg/speech-api/raw-file/tip/speechapi.html#tts-section
>> 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.
>> [6] http://www.w3.org/html/wg/drafts/html/master/single-page.html#texttrackcue
>> [7] http://www.w3.org/html/wg/drafts/html/master/single-page.html#texttrackcuelist
>> 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.
>> Regards,
>> Silvia.
Received on Friday, 14 June 2013 04:54:59 UTC

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