Timed tracks

This page contains notes for the development of the first version of timed track features in HTML.

See also use cases for timed tracks rendered over video by the UA, use cases for API-level access to timed tracks.

Requirements

Subtitle/Caption/Karaoke File Format

Structure

• multiple voices
• per-segment time in/out cues
• inline time cues for karaoke
• bidi, newlines, ruby, italics
• position

Positioning

• vertical: top/middle/bottom/% (default bottom)
• horizontal: left/center/right/% (default center)
• display modes: replace previous text, scroll previous text up and add to bottom
• multiple voices placed in adjacent places would need to automatically stack so they don't overlap
• multiple segments with overlapping times would need to be stacked so they don't overlap

(Percentage positions would work like background-position in CSS.)

Formatting

Inline

• text should be bidi-aware
• some cases use ruby
• some cases use italics

Global

• color of background/text/outline is needed for readability on different types of video.
• webfonts is needed to provide high quality subtitles in some non-Latin languages (e.g. Chinese where a suitable font is unlikely to be available even on Chinese computer systems).
• providing a pseudo-element to style each voice would likely be sufficient for authors who want overall formatting control (this would also allow user overrides conveniently)

Audio Descriptions File Format

• Just timing and text?

HTML

• an API and UI for exposing what timed tracks exist and selectively enabling/disabling them

• format for external subtitles/captions
• format for external audio descriptions
• some mechanism for text in the page to be used instead of external files, for subtitles/captions or audio descriptions
• an API to allow a segment to be dynamically inserted into the rendering on the fly

• an API for exposing what the currently relevant segments of each timed track are
• a way to hook into this mechanism to advance slides

• native rendering of subtitles
• native rendering of audio descriptions
• native rendering of multiple audio or video tracks, to allow pre-recorded audio descriptions to be mixed in and sign language video to be overlaid
• a way to hook into this to manually render timed tracks

Architecture

<img src="http://docs.google.com/drawings/pub?id=1GR6Pzq0GY2n1sx_ZjDfuICM2LnXxLVxzvyl4kuQy-48&w=640&h=480">

Declaring timed tracks

Each timed track is either enabled, in which case it is downloaded, triggers events, and if appropriate is rendered by the user agent; or disabled, in which case it does nothing.

Each timed track has a kind which is one of:

• for visual display (subtitles, captions, translations), enabled based on user preferences, shows in video playback area
• for audio playback (audio descriptions), enabled based on user preferences, renders as audio
• for navigation (chapter titles), enabled by default, shows in UA UI
• for off-video display (lyrics), disabled by default in this version, not shown by UA
• for metadata (slide timings, annotation data for app-rendered annotations), enabled by default, not shown by UA

Tracks that are for visual display or audio playback have additionally a user-facing label and a language.

Tracks that are for visual display have an additional boolean indicating if they include sound effects and speaker identification (intended for the deaf, hard of hearing, or people with sound muted) or not (i.e. translations intended for people with audio enabled but who cannot understand the language, or karaoke lyrics).

Each timed track associated with a media resource, like the media resource itself, can have multiple sources.

Each source for a timed track has:

• URL
• type

The media resource can also imply certain timed tracks based on data in the media resource.

The script can also add "virtual" timed tracks dynamically.

Other minor things

We need to make sure that media playback is paused until all enabled timed tracks are locally available.