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Data model

Overall

The canonical model closely follows the TTML 2 data model, as constrained by the IMSC 1.1 Text Profile specification. This includes both the static structure of the model as well as temporal, layout and style processing. The objective is for a valid IMSC 1.1 Text Profile document to be mapped into a canonical model instance such that presenting instance results in the same outout as the input IMSC document.

The canonical model is specified in ttconv.model.

The class hierarchy of the canonical model is summarized the following figure:

  ContentDocument
    : Region* Body?

  Body
    : Div*
  
  Div
    : (P | Div)*
  
  P
    : (Span | Ruby | Br)*
  
  Span
    : (Span | Br | Text)*
  
  Ruby
    : Rb? Rt?
    | Rb? Rp Rt? Rp
    | Rbc Rtc Rtc?
  
  Rbc
    : Rb*
  
  Rtc
  : Rt*
  | Rp Rt* Rp

  Rb, Rt, Rp
    : Span*

where:

  • the ContentDocument class corresponds to the tt element
  • the Body, Div, P, Span, Br and Region classes corresponds to the TTML content element of the same name, respectively
  • the Text class corresponds to a TTML text node
  • the Ruby, Rt, Rb, Rtc, Rbc, Rp classes correspond to the TTML span element with the computed value of tts:ruby attribute specified in the following table
Canonical model class Computed value of tts:ruby attribute
Ruby container
Rt text
Rb base
Rtc textContainer
Rbc baseContainer
Rp delimiter

Basic operation

The canonical model allows content elements (instances of ttconv.model.ContentElement) to be arranged in a hierarchical structures (using the ttconv.model.ContentElement.push_child() and ttconv.model.ContentElement.remove_child()) that are associated with a single document (using the ttconv.model.ContentElement.set_doc() method with an instance of ttconv.model.ContentDocument).

Divergences with the TTML data model

Initial values

The TTML initial elements are accessed using the ContentDocument.set_initial_value() and ContentDocument.get_initial_value() method.

Styling

Style properties are access using the ContentElement.get_style() and ContentElement.set_style() methods.

The style properties themselves are defined in ttconv.style_properties, where the lower camel case names used in TTML are replaced by their equivalent in upper camel case. Deprecated style properties, e.g. tts:zIndex are not supported.

Only inline styling of content elements is supported, and neither referential sytling nor chained referential styling nor nested styling are supported.

Metadata

TTML metadata elements are not supported.

Timing

Only parallel time container semantics are supported and temporal offsets are expressed as fractions.Fraction instances in seconds. As a result, the following parameters are not supported: ttp:frameRate, ttp:frameRateMultiplier, ttp:subFrameRate, ttp:tickRate.

Writer module can express temporal offsets in units of ticks, frames, etc. as demanded by the output format or configured by the user.

The dur timing attribute is not supported.

Lengths

Extent, origin and position lengths can be expressed in c, %, rh, rw and px units.