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ISSUE-339 (allow #overflow): Allow the use of #overflow [TTML IMSC 1.0]

From: Timed Text Working Group Issue Tracker <sysbot+tracker@w3.org>
Date: Wed, 27 Aug 2014 20:28:50 +0000
Message-Id: <E1XMjpq-0002Bk-Bn@stuart.w3.org>
To: public-tt@w3.org
ISSUE-339 (allow #overflow): Allow the use of #overflow [TTML IMSC 1.0]

http://www.w3.org/AudioVideo/TT/tracker/issues/339

Raised by: Frans de Jong
On product: TTML IMSC 1.0

---------------------------------------------------------------
Request
---------------------------------------------------------------
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.
Received on Wednesday, 27 August 2014 20:28:52 UTC

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