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Re: BBC submission on Safe Crop Area

From: Glenn Adams <glenn@skynav.com>
Date: Sun, 18 Sep 2016 09:34:59 -0600
Message-ID: <CACQ=j+eGGGbmOh_WxP8YiWGeG+OeeHB_Jvv32GvmVAHyuVLmfQ@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: W3C Public TTWG <public-tt@w3.org>
Rereading your original submission, and have a few comments/questions:

Scale the root container region to the display size and lose positional
> accuracy, and risk introducing line breaks and/or overflow scenarios.

I would not expect a change in line breaks/overflow, since here "scale"
means scale the formatted image of the root container, and not change the
root container dimensions and re-layout accordingly.

Visible Rendering Region

Regarding use of the term *Related Media Object Region* (RMOR), TTML1
defines the extent of the *Root Container Region* (RCR) as follows:

   1. If @tts:extent absent on tt:tt, then EXTENT(RCR) = EXTENT(RMOR)
   2. otherwise, EXTENT(RCR) = @tts:extent

Also, TTML1 effectively:

   - defines RCR to be coincident with document coordinate space (DCS);
   - defines RMOR to be coincident with document coordinate space (DCS);
   - thus RCR and RMOR are coincident;
   - notes that (in the XSL-FO mapping) that the XSL-FO page-viewport-area
   corresponds with the RCR;
   - appears to assume that *Pixel Aspect Ratio* (PAR) of *Related Media
   Object* (RMO) corresponds with PAR of DCS;

Further, the relationship between the *Root Media Object* (RMO) and the
RMOR appears to be wholly defined by the *Document Processing Context* (DPC).
So, for example, EXTENT(RMOR) and EXTENT(RMO) [in DCS] may be the same or
different. If different, then EXTENT(RMOR) may be presumed to be contained
in EXTENT(RMO) although this is not stated or implied.

should fully enclose all Regions that are referenced within the content in
> the whole sequence of document instances

Why the *should*? Isn't this an author choice? Better to simply say that
safe crop area must (or should) not change over time (for a given binding
of a set of TTML files and a RMO).

Presentation Processors shall display the TTML content that falls within
> the safe crop area. Presentation Processors should use best endeavours to
> maintain text size and positional alignment of TTML content within the safe
> crop area relative to the Related Video Object.

I don't know how to interpret "maintain text size and positional alignment
...". What does that mean for a processor?

On Thu, Jun 9, 2016 at 8:08 AM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:

> Please see attached BBC submission for a Safe Crop Area semantic for TTML.
> Kind regards,
> Nigel
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Received on Sunday, 18 September 2016 15:35:49 UTC

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