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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: Wed, 28 May 2014 23:20:00 -0700
Message-ID: <CAF_7JxD2Ji2Lx6_PM_wcGypQ_Udh9nTgg+PnAYmANb9Fw2XSVg@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Timed Text Working Group <public-tt@w3.org>
Hi Nigel,

As I understand it, the motivation to map media time expression to
video frames is to allow authors to unambiguously determine on which
frame of the related video object a subtitle will appear. I would
think this is particularly important on scene boundaries.

This unambiguous determination is not possible if the time expression
mapping depends on the display frame rate, which is not known at time
of authoring and largely arbitrary.

I would expect the timed text content to remain usable in the current
approach as long as the duration of intermediate synchronic documents
duration remains larger than the frame duration. Is there a use case
where this would systematically not be the case?

In general, without more information on the use case below (e.g. what
is the minimum frame rates/why not use reduced resolution instead, how
frequently will this frame rate reduction happen, is IMSC1 even
applicable/desirable...), I am reluctant to suggest a change to an
approach that appears to be working for users.

Best,

-- Pierre

On Tue, May 27, 2014 at 4:07 AM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
> Hi Pierre,
>
>>>Yes, they are able to reduce frame rate as an available
>>>option for adapting to network conditions.
>>
>>Is the BBC doing this? If not, who? Just trying to fully understand
>>the use case.
>
> You're hitting my limits on how much I can freely discuss on this forum.
> Reduced frame rate distribution options are likely to be used in the way
> I've described by organisations you've heard of within the lifetime of
> IMSC1. Knowing who won't enhance the use case further.
>
> Kind regards,
>
> Nigel
>
>
>
> On 23/05/2014 17:46, "Pierre-Anthony Lemieux" <pal@sandflow.com> wrote:
>
>>Hi Nigel,
>>
>>> Yes, they are able to reduce frame rate as an available
>>> option for adapting to network conditions.
>>
>>Is the BBC doing this? If not, who? Just trying to fully understand
>>the use case.
>>
>>Thanks,
>>
>>-- Pierre
>>
>>On Fri, May 23, 2014 at 9:24 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>>wrote:
>>> On 23/05/2014 17:20, "Pierre-Anthony Lemieux" <pal@sandflow.com> wrote:
>>>
>>>>> Yes - it's likely that adaptive streaming distribution mechanisms
>>>>> will do this.
>>>>
>>>>Do they do this today?
>>>
>>> Yes, they are able to reduce frame rate as an available option for
>>> adapting to network conditions.
>>>
>>> Kind regards,
>>>
>>> Nigel
>>>
>>>>
>>>>Best,
>>>>
>>>>-- Pierre
>>>>
>>>>On Fri, May 23, 2014 at 9:19 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>>>>wrote:
>>>>> Hi Pierre,
>>>>>
>>>>>>Hi Nigel,
>>>>>>
>>>>>>> 2. Video encoded for distribution at 7.5fps.
>>>>>>
>>>>>>Can you point to actual use case/deployment for this?
>>>>>
>>>>> Yes - it's likely that adaptive streaming distribution mechanisms will
>>>>>do
>>>>> this. Profiles for this purpose that go down to e.g. 25/4=6.25 fps.
>>>>>are
>>>>> likely to be used for mobile devices for example. I assume that
>>>>>something
>>>>> similar will result from a starting point of 30fps.
>>>>>
>>>>> Kind regards,
>>>>>
>>>>> Nigel
>>>>>
>>>>>>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?
>>>>>>>>
>>>>>>>>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/
>>>>>>>>>tt
>>>>>>>>>ml
>>>>>>>>>-w
>>>>>>>>>w-profiles.html#synchronization
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>
>>>
>>>
>>>
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Received on Thursday, 29 May 2014 06:20:52 UTC

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