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RE: Re: Re: TTML and aspect ratio

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Thu, 31 Jan 2013 16:26:45 +0000
To: David Ronca <dronca@netflix.com>, "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <E9A92BD0A4FC934EB7935470A46D15241F55CFAC@DB3EX14MBXC323.europe.corp.microsoft.com>
TTML was designed to be more general than simply providing captions for video content; including scenarios as Glenn states where there is in fact no associated media. So we cannot have a global mechanism for this.

SDP-US however is intended to work for TV type captioning and does make the stipulations that you indicate in that 
only percentage positioning is used, the default extent is exactly that of the video and therefore it will adopt the aspect ratio of the displayed video, and it is centered, since it covers 100% of the video. Within that default root container you can position your drawing regions using % and the content will 'work' even in the case that the video is played back with a different aspect ratio than the one the content was authored to.

SDP UP however cannot guarantee exact picture registration in the case where the video is not presented at its default aspect ratio, as there is no universal agreed mechanism for presenting 4:3 video content in a 16:9 window or vice-versa. What we do stipulate is that whatever mechanism is used, the caption root container should be 100% aligned to the presented video.

We will be addressing the SMPTE request to include additional metadata to indicate the intended aspect ratio, however even if we do that, this would not address the actual onscreen formatting issue.

I believe that this scenario does add weight to the case for having tools to position drawing regions where the registration point is other than the top left; and this is open issue 21 http://www.w3.org/AudioVideo/TT/tracker/issues/21.

If you feel there is something that we could add to the next version of TTML to better support your use case, I would encourage you to join the working group meetings and use the team wiki 
http://www.w3.org/wiki/TTML/ChangeProposalIndex  to author a change proposal to explain your concepts more fully.

Sean TTML WG Chair.

-----Original Message-----
From: David Ronca [mailto:dronca@netflix.com] 
Sent: 31 January 2013 06:46
To: public-tt@w3.org
Subject: Re: Re: TTML and aspect ratio

>The implementer of the client provides it. The implementer might ask 
>the user, or the system platform might define. Or the TTML client might 
>be embedded in a larger context (e.g., an HTML5 browser) that provides 
>its own mechanism for defining where to present captions.

 A proprietary model will not work.  We need a method to define a consistent layout for *any* well behaved client.  Consider two scenarios:  1) we implement a client to present the W3C-tt, 2) we pass the W3C-tt to a client that will do the presentation (i.e. game console, etc.).  For both of these scenarios, the client *must* present the captions in a proper and consistent way.  This is our use case, and it does not work if TT presentation is based on an undefined "external authoring context".  

We believe that our use case can be accomplished as follows:
1) Cell or percentage positioning
2) default <tt tts:extent>
3) The addition of root container aspect ratio (along with a default behavior for centering the root container in the video window rule).
Received on Thursday, 31 January 2013 16:30:48 UTC

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