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

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Fri, 23 May 2014 16:11:12 +0000
To: Pierre-Anthony Lemieux <pal@sandflow.com>
CC: Timed Text Working Group <public-tt@w3.org>
Message-ID: <CFA52ED7.1E467%nigel.megitt@bbc.co.uk>
Hi Pierre,

It would certainly help to explain the model more clearly, along the lines
that you've outlined. The specific proposal wouldn't help though, since
the precise timing information would have been lost at the point of
temporal quantisation and could not be regenerated later.

For example this suggests a chain such as:

1. IMSC Document authored against video at 30fps.
2. Video encoded for distribution at 7.5fps.
3. Receiving system must align the resolved TTML time expressions with the
7.5fps 'quanta' prior to display, as per the rule in the current spec.

Even if the display device refreshes at 60fps it would be forbidden from
using the original timings because the spec references the encoded video.

What I'm trying to get to is a solution that is permitted (actually
encouraged) to align with display frames as late as possible while losing
minimal information. In some real world systems that's unavoidably earlier
than the display, but we shouldn't use the lowest common denominator to
set the rule for all implementations.

Kind regards,

Nigel


On 23/05/2014 16:33, "Pierre-Anthony Lemieux" <pal@sandflow.com> wrote:

>Hi Nigel,
>
>IMSC 1.0 4.4 [1] refers to synchronization with the related video
>object against which the timed text content is delivered, not
>synchronization to the displayed frame rate by the
>terminal/UA/device/display/TV. In other words, if a
>terminal/UA/device/display/TV chooses to alter the video frame rate of
>the related video object it receives (for whatever reason), then I
>expect it will accordingly alter the timed text display (perhaps along
>the lines of what is suggested below), with the knowledge that the
>timed text was authored according to the constraints of Section 4.4.
>
>Would a note to that effect help?
>
>Thanks,
>
>-- Pierre
>
>On Thu, May 22, 2014 at 3:19 AM, Timed Text Working Group Issue
>Tracker <sysbot+tracker@w3.org> wrote:
>> 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-w
>>w-profiles.html#synchronization
>>
>>
>>
Received on Friday, 23 May 2014 16:12:13 UTC

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