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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 09:46:17 -0700
Message-ID: <CAF_7JxAWjKpBT9oqsLZJvUa=4oSS0codQ90emAkAVKY+7HpPuQ@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Timed Text Working Group <public-tt@w3.org>
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.


-- 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
>>-- Pierre
>>On Fri, May 23, 2014 at 9:19 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>>> 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
>>> 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
>>> 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>
>>>>> Hi Pierre,
>>>>> It would certainly help to explain the model more clearly, along the
>>>>> that you've outlined. The specific proposal wouldn't help though,
>>>>> 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
>>>>> 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
>>>>> using the original timings because the spec references the encoded
>>>>> 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
>>>>> minimal information. In some real world systems that's unavoidably
>>>>> than the display, but we shouldn't use the lowest common denominator
>>>>> set the rule for all implementations.
>>>>> Kind regards,
>>>>> Nigel
>>>>> On 23/05/2014 16:33, "Pierre-Anthony Lemieux" <pal@sandflow.com>
>>>>>>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
>>>>>>>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
>>>>>>>media of the same frame rate so there's no need for the frame
>>>>>>> This approach prevents implementations from changing caption display
>>>>>>>screen refresh rate quantisation and enforces quantisation based on
>>>>>>>encoded video frame rate. This means that if a low frame rate video
>>>>>>>provided, e.g. quarter rate which could be around 6 frames per
>>>>>>>the effective word reading rate may be increased to the point where
>>>>>>>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
>>>>>>>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
>>>>>>>captions as temporally close to the media time specified as the
>>>>>>>device permits, independent of video frame rate.
>>>>>>> Note that where frames are used in media time expressions this
>>>>>>>to exactly the current behaviour.
>>>>>>> [1]
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Received on Friday, 23 May 2014 16:47:06 UTC

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