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RE: Re: TTML and aspect ratio

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Thu, 31 Jan 2013 01:58:31 +0000
To: David Ronca <dronca@netflix.com>, "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <E9A92BD0A4FC934EB7935470A46D15241F55C9D0@DB3EX14MBXC323.europe.corp.microsoft.com>
we have issue 179 related to this namely that :

"we need to clarify the pixel behaviour; in particular we should explain that where extent is used on the tt element this effectively *defines* the size of a pixel in the sense that the root area extent is still mapped to a video overlay and divided into 'logical pixels'. If such an extent is not defined on the tt element, then the px measure should probably not be used (or even expressly forbidden)"

In this interpretation it does not imply that the extent covers some specific hardware pixels, but that the region is to be counted as having x by y pixels; so that the px measure when used within the TTML document has some meaning independent of the hardware. If this is coupled with square aspect ratio pixels (the default), then by definition the root would have the specified aspect ratio.

-----Original Message-----
From: David Ronca [mailto:dronca@netflix.com] 
Sent: 30 January 2013 17:33
To: public-tt@w3.org
Subject: Re: TTML and aspect ratio

> ok, but now i'm back to why not just use tts:extent, since the AR of 
> the root container is nothing more than extent(width) / extent(height)?

Because tts:extent specifies exact pixel container area that the client must use.  If a video starts at 480p, and the tts:extent="640,480", then when the video adaptive-switches  to 720p, and/or 1080p, the caption area (as a percentage of the video) will be reduced, and move towards the top-left.  

We need a caption presentation model that is completely independent of the video.  All positioning must be relative, and the client must know whether to present the captions in a 4:3 area of the video or a 16:9 area.  

David
Received on Thursday, 31 January 2013 01:59:34 GMT

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