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ISSUE-215 (negative extent): Negative extent lengths should be prohibited. [TTML 1.0]

From: Timed Text Working Group Issue Tracker <sysbot+tracker@w3.org>
Date: Sun, 26 May 2013 19:05:49 +0000
Message-Id: <E1UggGL-0006tS-8o@tibor.w3.org>
To: public-tt@w3.org
ISSUE-215 (negative extent): Negative extent lengths should be prohibited. [TTML 1.0]


Raised by: Glenn Adams
On product: TTML 1.0

After further research, I believe the language I added to TTML10SE 8.2.7 [1] is misleading:

"where, if either width or height is negative or zero, then extent must be considered zero in both inline and block progression dimensions."

[1] https://dvcs.w3.org/hg/ttml/raw-file/default/ttml10/spec/ttaf1-dfxp.html#style-attribute-extent

Looking further into both XSL-FO and CSS2, one finds explicit statements:

"Negative values for 'height' are illegal." [2][3]
"Negative values for 'width' are illegal." [4][5]

[2] http://www.w3.org/TR/2006/REC-xsl11-20061205/#height
[3] http://www.w3.org/TR/2008/REC-CSS2-20080411/visudet.html#propdef-height
[4] http://www.w3.org/TR/2006/REC-xsl11-20061205/#width
[5] http://www.w3.org/TR/2008/REC-CSS2-20080411/visudet.html#propdef-width

Since we defined tts:extent as a derivation of XSL-FO height/width [6], these prohibitions already effectively apply. However, to ward off mis-interpretations by readers, I suggest we add similar language proscribing negative extent lengths to make this clear. Note that no change will be made to the XSD, since we use xs:string as the data type, which means this is a semantic validity rule that applies to the PSVI.

[6] https://dvcs.w3.org/hg/ttml/raw-file/default/ttml10/spec/ttaf1-dfxp.html#attribute-vocab-derivation-table
Received on Sunday, 26 May 2013 19:05:51 UTC

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