ISSUE-283: Deterministic text wrapping and presentation

Deterministic Presentation

Deterministic text wrapping and presentation

State:
OPEN
Product:
TTML2
Raised by:
Nigel Megitt
Opened on:
2013-10-10
Description:
There's a complex interaction between lineHeight, fontSize, overflow and wrapOption that determines, for the font that the display processor chooses, how much text will fit on a line and whether any text that doesn't fit overflows or is truncated. This creates a problem for document authors if they can not be certain of the metrics of the font used to present their content.

The goal from an audience perspective is that the on-screen text is readable and complete. Nobody wants missing words (that could change the editorial meaning) or text that is visible but unreadable.

TTML offers little by way of solution to this real world problem at the moment. The IMSC submission presents a 'reference font' mechanism, which should be considered. Is there anything more that we can do natively in TTML to allow deterministic rendering to be defined at the point of authoring?

Raising this issue for discussion at TPAC.

Note that there are related issues (to be filed separately) around lineHeight=normal being related to the height of the text actually flowed onto a line (is it? or is it related to the descendent elements of the <p>?) and being set to a percentage of the font size - should it be 100%, 120%, 125% etc. for compatibility with CSS etc.
Related Actions Items:
Related emails:
  1. {minutes} TTWG Meeting 2015-02-05 (from nigel.megitt@bbc.co.uk on 2015-02-05)
  2. {agenda} TTWG Meeting 2015-02-05 (from nigel.megitt@bbc.co.uk on 2015-02-04)
  3. {agenda} TTWG Meeting 2015-01-29 (from nigel.megitt@bbc.co.uk on 2015-01-28)
  4. {agenda} TTWG Meeting 2015-01-22 (from nigel.megitt@bbc.co.uk on 2015-01-21)
  5. {agenda} TTWG Meeting 2015-01-15 (from nigel.megitt@bbc.co.uk on 2015-01-14)
  6. TTML Minutes for 11/11/13 (from nigel.megitt@bbc.co.uk on 2013-11-21)
  7. Re: Clarification needed regarding tts:overflow (from nigel.megitt@bbc.co.uk on 2013-11-05)
  8. {minutes} TTML Meeting of 24/10/13 (from glenn@skynav.com on 2013-10-24)
  9. RE: TTML Agenda for 24/10/13 (from mdolan@newtbt.com on 2013-10-23)
  10. TTML Agenda for 24/10/13 (from nigel.megitt@bbc.co.uk on 2013-10-23)
  11. {minutes} TTML Meeting of 17/10/13 (from glenn@skynav.com on 2013-10-17)
  12. TTML Agenda for 17/10/13 (from Sean.Hayes@microsoft.com on 2013-10-17)
  13. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-11)
  14. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from nigel.megitt@bbc.co.uk on 2013-10-11)
  15. Re: ISSUE-284: value of lineHeight [TTML2] (from glenn@skynav.com on 2013-10-11)
  16. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-11)
  17. RE: ISSUE-284: value of lineHeight [TTML2] (from mdolan@newtbt.com on 2013-10-11)
  18. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from pal@sandflow.com on 2013-10-11)
  19. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from nigel.megitt@bbc.co.uk on 2013-10-11)
  20. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-11)
  21. RE: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from John.Birch@screensystems.tv on 2013-10-11)
  22. Re: ISSUE-284: value of lineHeight [TTML2] (from glenn@skynav.com on 2013-10-11)
  23. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from pal@sandflow.com on 2013-10-11)
  24. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-11)
  25. ISSUE-284: value of lineHeight [TTML2] (from sysbot+tracker@w3.org on 2013-10-11)
  26. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from nigel.megitt@bbc.co.uk on 2013-10-11)
  27. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-11)
  28. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from nigel.megitt@bbc.co.uk on 2013-10-11)
  29. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-11)
  30. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-11)
  31. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from pal@sandflow.com on 2013-10-10)
  32. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from pal@sandflow.com on 2013-10-10)
  33. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-10)
  34. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from pal@sandflow.com on 2013-10-10)
  35. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-10)
  36. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from pal@sandflow.com on 2013-10-10)
  37. Re: ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from glenn@skynav.com on 2013-10-10)
  38. ISSUE-283 (Deterministic Presentation): Deterministic text wrapping and presentation [TTML2] (from sysbot+tracker@w3.org on 2013-10-10)

Related notes:

We have discussed this many times in the past, going back to 2003, and within CSS and XSL-FO, it is similarly a known problem.

The only way to obtain interoperable deterministic line breaks is:

OPTION 1 to manually break the line using <br/> and specify wrapOption='noWrap'

or

OPTION 2 require every presentation processor to support at least one concretely specified font, with effectively identical metrics on every platform, *and* require every presentation processor to support at least one concrete line break implementation, with a way for the author to express that algorithm must be used;

or

OPTION 3 require support for downloadable fonts and at least one specifiable, universally supported line break implementation;

or

OPTION 4 use only image based captions, where rendering is done once during authoring.

Comments

OPTION 1 - May lead to region overflow (and possible clipping)
OPTION 2 - Difficult to specify concrete collection of fonts that serves all of Unicode, or at least the subset of Unicode used in regional caption/subtitle text.
OPTION 3 - Probably best option in theory, most likely solution would require support for (1) OpenType fonts delivered by WOFF, (2) freetype font rasterizer, and (3) ICU implementation of UAX14.
OPTION 4 - Makes timed "text" rather pointless, unless both image and text formats delivered together.

Glenn Adams, 10 Oct 2013, 19:49:38

Nigel: P1

Philippe Le Hégaret, 24 Oct 2013, 15:49:55

Related to ISSUE-294

Nigel Megitt, 5 Nov 2013, 11:26:07

[pal]: TPAC 2013 PROPOSAL: add informative text (e.g. to Section 9.4) on controlling line breaks (see also issue-273 on downloadable fonts)

11 Nov 2013, 07:19:14

Propose no action. TTML2 now has downloadable font support, which at least ensures the same metrics. However, absent a normative and concrete line layout and line breaking algorithm, interoperability is still not achievable. Authors that wish to guarantee visual interoperability can use image based rendering in combination with capability based resource selection, e.g., choosing among alternate renderings in DASH, etc.

Glenn Adams, 14 Jan 2015, 06:40:05

[nigel]: [meeting 2015-02-05] Resolution: add an editorial note to guide authors and implementors to §11.3.2 Line Layout.

5 Feb 2015, 15:56:30

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David Singer <singer@apple.com>, Nigel Megitt <nigel.megitt@bbc.co.uk>, Chairs, Thierry Michel <tmichel@w3.org>, Philippe Le Hégaret <plh@w3.org>, Staff Contacts
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