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RE: ISSUE-317 (IMSC should not require frame alignment): IMSC should not require frame alignment [TTML IMSC 1.0]

From: Michael Dolan <mdolan@newtbt.com>
Date: Thu, 22 May 2014 08:42:15 -0700
To: "'Timed Text Working Group'" <public-tt@w3.org>
Message-ID: <00e901cf75d4$69954320$3cbfc960$@newtbt.com>
No, that's not sufficient.  It must be possible to composite the text in the
exact same time space domain as the related (coded) video. Unambiguous frame
alignment is absolutely required.  What happens after that is a decoder
problem.  

If you also want to attempt to provide hints about alignment to display
formats, or in other applications video frame sync is not important, that's
OK.  But that does not relax the requirement for the ability to align with
the coded video. And in order to do that, the math must be prescribed.

	Mike

-----Original Message-----
From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk] 
Sent: Thursday, May 22, 2014 8:34 AM
To: Michael Dolan; 'Timed Text Working Group'
Subject: Re: ISSUE-317 (IMSC should not require frame alignment): IMSC
should not require frame alignment [TTML IMSC 1.0]

On 22/05/2014 15:40, "Michael Dolan" <mdolan@newtbt.com> wrote:

>This is a complex topic and absolutely required to provide coded 
>frame-level text/video sync.

I don't believe that frame-level text/video sync is the requirement though
- the text needs to be synced against media time, and so does the video, and
so does the audio.

> 
>
>It is, I believe, impossible for an author to enable sync to display 
>frames.

I think that's an academic point - what's needed is for the author to
specify times as precisely as she/he is able to, and the processor to honour
those as closely as it can. The frame rate of the video that the author is
creating captions for can not always be guaranteed in the workflow to be the
same as the frame rate of the video being played back with those captions.
I'm arguing that the processor and display combination should try to honour
the authored times as accurately as possible independently of the encoded
video frame rate for playback.

Nigel

>
>	Mike
>
>-----Original Message-----
>From: Timed Text Working Group Issue Tracker 
>[mailto:sysbot+tracker@w3.org]
>Sent: Thursday, May 22, 2014 3:20 AM
>To: public-tt@w3.org
>Subject: ISSUE-317 (IMSC should not require frame alignment): IMSC 
>should not require frame alignment [TTML IMSC 1.0]
>
>ISSUE-317 (IMSC should not require frame alignment): IMSC should not 
>require frame alignment [TTML IMSC 1.0]
>
>http://www.w3.org/AudioVideo/TT/tracker/issues/317
>
>Raised by: Nigel Megitt
>On product: TTML IMSC 1.0
>
>IMSC 1.0 4.4 [1] currently requires temporal quantisation of media 
>times to frame display times. This rule comes into play when times are 
>not expressed in frames, and therefore the same document may apply to a 
>range of related media objects covering different frame rates. In the 
>case when frames are used the document can only be displayed alongside 
>media of the same frame rate so there's no need for the frame alignment
expression.
>
>This approach prevents implementations from changing caption display at 
>screen refresh rate quantisation and enforces quantisation based on the 
>encoded video frame rate. This means that if a low frame rate video is 
>provided, e.g. quarter rate which could be around 6 frames per second, 
>the effective word reading rate may be increased to the point where 
>text becomes hard to read.
>
>Consider a streaming environment in which there is enough network 
>capacity to provide audio and captions but the video experience is 
>badly
>impacted: in this case it must be permitted that the implementation 
>continue to present captions alongside the audio regardless of the 
>frames of video that are displayed.
>
>I propose a solution to this problem that implementations SHALL display 
>captions as temporally close to the media time specified as the display 
>device permits, independent of video frame rate.
>
>Note that where frames are used in media time expressions this reduces 
>to exactly the current behaviour.
>
>[1]
>https://dvcs.w3.org/hg/ttml/raw-file/ea1a92310a27/ttml-ww-profiles/ttml
>-ww
>-profiles.html#synchronization
>
>
>
>
Received on Thursday, 22 May 2014 15:42:50 UTC

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