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[ttml2] Consider relabelling vw and vh units

From: Nigel Megitt via GitHub <sysbot+gh@w3.org>
Date: Thu, 15 Sep 2016 17:21:56 +0000
To: public-tt@w3.org
Message-ID: <issues.opened-177235581-1473960115-sysbot+gh@w3.org>
nigelmegitt has just created a new issue for 
https://github.com/w3c/ttml2:

== Consider relabelling vw and vh units ==
TTML2 introduces the excellent new units `vw`and `vh`, which are 
relative to the width and height of the root container region. However
 this definition could be perceived as clashing with the CSS 
definition of the identically named units, which are relative to the 
viewport. This may cause (at least) two implementation difficulties:

1. In the case of a root container region that is not coincident with 
a related media object (or the media player) an implementation may 
assume that the root container region is in fact coincident and 
calculate sizes incorrectly.

2. A naïve translation to HTML and CSS for rendering on a media player
 may not translate the units at all and assume that for TTML rendering
 purposes the viewport is identical to a host HTML page's  viewport, 
resulting in incorrect sizes except when viewing in full screen mode.

This is a case of something that is not exactly _wrong_ as is, but may
 encourage or lead to poor implementations. I would propose nudging 
implementors towards checking the specification more carefully by 
renaming these units `rw` and `rh` to make it clear that they are 
related to the root container region and not the viewport.

I would also propose the addition of an editorial note describing how 
to convert `rw` to `vw` and and `rh` to `vh` respectively given a root
 container region whose size is defined relative to a media player 
window and whose media player window size is known relative to the 
viewport.

Please view or discuss this issue at 
https://github.com/w3c/ttml2/issues/181 using your GitHub account
Received on Thursday, 15 September 2016 17:22:03 UTC

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