Re: [blink-dev] WebVTT vs TTML Features

(bcc: blink-dev, cc: public-texttracks)

Hi Glenn,

I'm moving the discussion to public-texttracks@ because I think these are
good points that should generally be debated and eventually extend WebVTT
to support some of them, if needed by caption authors.

Victor


On Tue, Nov 26, 2013 at 3:39 PM, Glenn Adams <glenn@chromium.org> wrote:

>
>
>
> On Tue, Nov 26, 2013 at 3:25 PM, Silvia Pfeiffer <silviapf@chromium.org>wrote:
>
>>
>> Have they tried to convert from TTML to WebVTT for presentation in
>> browsers? Since all major browsers now support WebVTT, it would the
>> path of least pain. It would also help to find out which TTML features
>> cannot be presented in WebVTT. You might find that to be a very small
>> set.
>>
>
> I expect that greater than 50% of TTML features aren't translatable into
> WebVTT. For example, TTML1 makes use of 24 style properties [1], all based
> on CSS or SVG properties (in most cases identically defined). Of these 24,
> the following 10 *cannot* be expressed in whole or part by WebVTT content:
>
>    - backgroundColor
>    - display
>    - displayAlign
>    - extent
>    - origin
>    - overflow
>    - padding
>    - showBackground
>    - wrapOption
>    - zIndex
>
> The following can be expressed, but *not* *in a WebVTT file*, only in a
> CSS stylesheet associated with the page in which the WebVTT HTML/CSS
> presentation will be rendered:
>
>    - color
>    - fontFamily
>    - fontSize
>    - fontStyle
>    - fontWeight
>    - lineHeight
>    - opacity
>    - textDecoration
>    - textOutline
>    - visibility
>
> Support for the following TTML (CSS) properties require mutating the text
> to insert or modify explicit bidi control codes:
>
>    - direction
>    - unicodeBidi
>
> So nearly half (ten) of the style properties do not translate at all or
> only in part, and ten other style properties require use of separate style
> sheets that have to be delivered independently from the related WebVTT file.
>
> Overally, TTML1 defines 114 features [2], 69 of which are related to the
> above 24 style properties. I fully expect that more than half of these
> features are not encodable or translatable to WebVTT, or if they are, then
> have the added disadvantage of having to maintain a separate CSS style
> sheet containing rules that apply to specific VTT files.
>
> [1]
> http://www.w3.org/TR/2013/REC-ttml1-20130924/#styling-attribute-vocabulary
> [2] http://www.w3.org/TR/2013/REC-ttml1-20130924/#feature-designations
>
>