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

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Thu, 13 Jun 2013 21:56:34 -0700
Message-ID: <CAF_7JxDgz5gi81J00ege5aC0J+6pw7K80VWUSXzSorHtAZp8vA@mail.gmail.com>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Cc: public-html <public-html@w3.org>
> You may be looking at HTML5.0. HTML5.1 doesn't contain these any more.

I pulled these two paragraphs from [1], which is HTML 5.1 nightly, right?

[1] http://www.w3.org/html/wg/drafts/html/master/single-page.html

-- Pierre

On Thu, Jun 13, 2013 at 9:54 PM, Silvia Pfeiffer
<silviapfeiffer1@gmail.com> wrote:
> 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.
>
> Indeed.
> Thanks,
> Silvia.
>
>
>> 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 04:57:22 UTC

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