ISSUE-339: Allow the use of #overflow

allow #overflow

Allow the use of #overflow

State:
CLOSED
Product:
TTML IMSC 1.0
Raised by:
Frans de Jong
Opened on:
2014-08-27
Description:
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Request
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Section 5.8 (Common Constraints->Features)

It shall be possible to use the #overflow feature.

Needed change

Now:
#overflow -> SHALL NOT be used

Proposed
#overflow->MAY be used


If this editorial change is not feasible the second (but worse option) would be to add a requirement that a processor should not stop processing the document even if it contains the unsupported tts:overflow attribute.


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Background
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In IMSC the use of overflow is not allowed. It is not defined if a processor is allowed to reject a document that uses tts:overflow.

The assumed behaviour in IMSC is tts:overflow="hidden". As this is the TTML initial value and "visible" is not allowed, the feature is prohibited entirely.

In EBU-TT-D it is specified that

If the value of this attribute is “visible", then content
should not be clipped. If the value is hidden, then content that
goes outside of the affected region should be clipped and is not
visible

Further it is added in an informative note:

Note: Setting the feature to “visible” does not
guarantee that content that overflows the region will be
presented, e.g. if the content would need to overflow the root
container region.

Therefore if the default behaviour is applied (content that overflows is hidden) the processor behaviour will still be EBU-TT-D compliant and acceptable. But a processor shall not reject the document and shall process it.

It would be better to enable the feature to make IMSC more consistent. As the implied presentation behaviour is not a strict requirement for a presentation processor it should not cause any problem for any existing processor that has not implemented this feature yet.
Related Actions Items:
Related emails:
  1. Re: ISSUE-339 (allow #overflow): Allow the use of #overflow [TTML IMSC 1.0] (from pal@sandflow.com on 2014-09-24)
  2. {agenda} TTWG Meeting 25/9/2014 (from nigel.megitt@bbc.co.uk on 2014-09-24)
  3. Re: ACTION-329: Revisit issue-339 to investigate potential resolution options (from glenn@skynav.com on 2014-09-23)
  4. RE: ACTION-329: Revisit issue-339 to investigate potential resolution options (from dejong@ebu.ch on 2014-09-23)
  5. {minutes} Geneva F2F day 2 17/09/2014 (from nigel.megitt@bbc.co.uk on 2014-09-17)
  6. ACTION-328: Revisit issue-339 to investigate potential resolution options (from sysbot+tracker@w3.org on 2014-09-17)
  7. ACTION-329: Revisit issue-339 to investigate potential resolution options (from sysbot+tracker@w3.org on 2014-09-17)
  8. Re: {minutes} TTWG Meeting 11/9/2014 (from silviapfeiffer1@gmail.com on 2014-09-13)
  9. Re: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339 (from nigel.megitt@bbc.co.uk on 2014-09-12)
  10. Re: {minutes} TTWG Meeting 11/9/2014 (from nigel.megitt@bbc.co.uk on 2014-09-12)
  11. Re: {minutes} TTWG Meeting 11/9/2014 (from silviapfeiffer1@gmail.com on 2014-09-12)
  12. Re: {minutes} TTWG Meeting 11/9/2014 (from silviapfeiffer1@gmail.com on 2014-09-12)
  13. Re: {minutes} TTWG Meeting 11/9/2014 (from ckennedy@apple.com on 2014-09-11)
  14. Re: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339 (from John.Birch@screensystems.tv on 2014-09-11)
  15. Re: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339 (from pal@sandflow.com on 2014-09-11)
  16. Re: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339 (from nigel.megitt@bbc.co.uk on 2014-09-11)
  17. Re: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339 (from pal@sandflow.com on 2014-09-11)
  18. {minutes} TTWG Meeting 11/9/2014 (from nigel.megitt@bbc.co.uk on 2014-09-11)
  19. Re: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339 (from John.Birch@screensystems.tv on 2014-09-11)
  20. IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339 (from pal@sandflow.com on 2014-09-10)
  21. Re: {agenda} TTWG Meeting 11/9/2014 (from glenn@skynav.com on 2014-09-10)
  22. {agenda} TTWG Meeting 11/9/2014 (from nigel.megitt@bbc.co.uk on 2014-09-10)
  23. {agenda} TTWG Meeting 4/9/2014 (from nigel.megitt@bbc.co.uk on 2014-09-03)
  24. ISSUE-339 (allow #overflow): Allow the use of #overflow [TTML IMSC 1.0] (from sysbot+tracker@w3.org on 2014-08-27)

Related notes:

Result from ACTION-329:

1. Intention of EBU-TT-D usage of tts:overflow.

Presentation of text is the primary goal of the EBU-TT-D use cases; whilst unexpected overflow of text is undesirable, in a chain that can not be fully constrained in regards to fonts and text layout it is considered unavoidable. It is considered a worse option to clip overflowed text and potentially change the editorial meaning as read by the user than to permit presentation engines to try to display the text.

2. Interpretation of TTML1 tts:overflow wording.

EBU interpreted the text of TTML 1 SE tts:overflow as follows:
• The region size is unaffected by overflow.
• Wrapping of text, when wrapOption=“wrap” is bounded by the size of the region.
• Line areas flowed into a region may be affected by overflow, and they may be rendered outside the region if tts:overflow=“visible”.
• We expect presentation processors to clip all content outside the region area if tts:overflow=“hidden”.
• The default behaviour if tts:overflow is unspecified is to process the document according to the initial value “hidden”, which could in some cases, that we are unable to control, result in some text being hidden.

3. Interpretation of IMSC 1 current wording.

EBU interprets the prohibition of tts:overflow from IMSC 1 documents as a directive that the initial value of tts:overflow, “hidden”, applies and that processors are therefore expected to clip text that happens to fall outside the region.

4. Mitigation approaches.

EBU would support a recommendation that documents should be authored assuming the ‘worst case scenario’, i.e. with strict clipping of content that falls out of region areas, regardless of the value of tts:overflow in the document.

EBU proposes a change to IMSC 1 to make clear that presentation processors shall be permitted to display text that overflows the region, indicated within documents by use of tts:overflow=“visible”:

“NOTE: Presentation processors are permitted to display text that overflows the region if tts:overflow=“visible”. “

In order to permit those processors to show overflowed text, the value “visible” must be permitted within IMSC 1 documents. This change requires that the #overflow feature be changed to MAY be permitted in IMSC 1, as requested in Issue-339.

As a minimum, even in the absence of any functionality to display or indeed clip overflowed text, presentation processors should be permitted to process documents that have a tts:overflow attribute set either to “hidden” or “visible”.

EBU would support an erratum to TTML1SE that clarifies that the intended meaning of the tts:overflow wording is as stated in 2. above. Specifically it must be clear that, for the purposes of drawing region backgrounds and calculating wrapping, the region size is unaffected by the value of tts:overflow.

4. Impact on TTML2.

EBU does not consider tts:overflow a complete solution to the rendering and presentation problems that may occur, and
may propose, or would support in principle, overflow avoidance strategies, for example copy-fit (aka shrink to fit) algorithms, or attribute-controlled direction for processors to adjust region origin and size to maintain the presentation of text within a region.

Frans de Jong, 23 Sep 2014, 10:40:12

Addressed at https://dvcs.w3.org/hg/ttml/rev/82c18c2fe212

Pierre-Anthony Lemieux, 24 Sep 2014, 22:40:09

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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>, Atsushi Shimono <atsushi@w3.org>, Staff Contacts
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