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

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Fri, 23 May 2014 09:13:43 -0700
Message-ID: <CAF_7JxDxpCfNzTWz43pbS-xnsD5+sGgG60p2a5KqZ3XXGC=FMQ@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Timed Text Working Group <public-tt@w3.org>
Hi Nigel,

> 2. Video encoded for distribution at 7.5fps.

Can you point to actual use case/deployment for this?


-- Pierre

On Fri, May 23, 2014 at 9:11 AM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
> 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?
>>-- 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
>>> 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]
Received on Friday, 23 May 2014 16:14:33 UTC

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