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

From: Glenn Adams <glenn@skynav.com>
Date: Sat, 24 May 2014 09:53:28 +0900
Message-ID: <CACQ=j+fXkYYmdXNCUy6UmD9AodHkDGgQXOWVkKu7rHixk7TKaw@mail.gmail.com>
To: Michael Dolan <mdolan@newtbt.com>
Cc: Timed Text Working Group <public-tt@w3.org>
On Fri, May 23, 2014 at 1:39 AM, Michael Dolan <mdolan@newtbt.com> wrote:

> -----Original Message-----
> From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
> Sent: Thursday, May 22, 2014 8:50 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]
> You can achieve unambiguous frame alignment in the current IMSC draft spec
> by specifying frames in the time expression, so I think your requirement is
> already met.
> MD>>  This is simply not true, especially for non-integer framerates.
>  There
> is a *lengthy* reflector exchange on this topic, maybe a year ago which
> resulted in changes to TTML1SE.  Perhaps that will help.  You'll be glad to
> hear it is simpler for European (integer) framerates. :-)

Note that since we support expressing time as arbitrary precision fractions
of frames, i.e., using the *offset-time* syntax [1] with metric 'f' and
fraction component.

We also support using the 't' metric with fractions, so, e.g., one could
author a document using ttp:tickRate of 90000 or 27000000 to synchronize
with PTS or PCR granularity (or fractions thereof).


How this is time is synchronized to a related media object that uses a
different frame or tick rate is another issue, but I think there is no
problem in authoring against a known related media object.

> I disagree that removing the requirement to align unambiguously with frames
> takes the timings outside the time space domain as the related video. In
> fact they are both in a time domain arbitrarily defined as media time that
> the presentation system manages. They're just expressing times with
> different levels of precision.
> MD>>  I am afraid it is not that simple. You seem to be missing: 1) (above)
> the problem that alignment to specific frames of coded video is ambiguous
> (without the added spec language in TTML1SE); and 2) text is only visible
> in
> today's decoders for the integral period in which the coded video frame(s)
> are visible - that is, you cannot in the general case actually make the
> text
> appear and disappear on other than coded video frame boundaries.  I suppose
> in some futuristic decoder/display architecture (and a connector other than
> HDMI), it might be possible to truly have text appear and disappear
> unrelated to the coded video object frame alignment, but I'm not aware of
> any equipment today where that is possible.  I'm not saying we should
> forbid
> sub-frame sync (and we do not as far as I know).  I am saying at a minimum
> we need to crisply support alignment with the coded video frame. If one
> wants to venture into the world of subframe visual sync, that's perfectly
> fine, but doesn't help the existing designs where that is not possible.
> Since I don't recognise the requirement for text display time alignment
> with
> encoded video frames please could you describe where this requirement comes
> from?
> MD>> See above.  Is the problem that ISMC *requires* that it work only this
> way and forbids subframe sync?  If so, we should correct it.
> Kind regards,
> Nigel
> On 22/05/2014 16:42, "Michael Dolan" <mdolan@newtbt.com> wrote:
> >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 ยง4.4 [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/ttm
> >>l
> >>-ww
> >>-profiles.html#synchronization
> >>
> >>
> >>
> >>
> >
> >
Received on Saturday, 24 May 2014 00:54:18 UTC

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