WebVTT vs TTML Features

[Glenn Adams]

On Tue, Nov 26, 2013 at 3:25 PM, Silvia Pfeiffer <silv...@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

[Peter Beverloo]

I believe the purpose of WebVTT is not to be compatible with TTML, but instead provide a more generalized model for various other text track formats.  Several of the features you mention are nice to have, but it is important to have data on how frequently they are used to assess their importance.  For example, background colors and z-indices aren't features I (consciously) remember seeing a lot while watching video with subtitles.

Since Blink doesn't seem interested in having a TTML implementation, this thread may yield more productive results on the WebVTT or general text track mailing lists.

Thanks,

Peter

[Adam Barth]

This discussion is off-topic for this list.  Please take this discussion elsewhere.

Adam

[Adam Barth]

Peter emailed me off-list and correctly pointed out that my tone was overly harsh.  I'd like to apologize for that.

Adam

On Tue, Nov 26, 2013 at 9:22 AM, Adam Barth <aba...@chromium.org> wrote:

This discussion is off-topic for this list.  Please take this discussion elsewhere.

[Victor Cărbune]

(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