Re: VTT edge margins: we need to know current practice. PLEASE RESPOND If you implement a VTT UA (even in a polyfill)

I still think it's relevant, but would be better in the issue tracker. We
should indeed remove the "aesthetic" part of the spec, Nigel is correct
that it has no place in a standard. And John's input about 50px left and
right as well as 240px left and right is quite a surprise - that's much
bigger a no-caption area than I expected.

Best Regards,
Silvia.

On 17 Nov. 2017 5:25 am, "David Singer" <singer@apple.com> wrote:

This is an interesting discussion, but my question is a little more
specific:

If you implement the algorithm in the VTT specification, you are asked to
assign a percentage value to edge_margin. What value do you assign in your
implementation?


> On Nov 16, 2017, at 5:16 , John Birch <John.Birch@screensystems.tv> wrote:
>
> Hi Nigel,
>
> CIL…
>
> Best,
> John
>
> John Birch | Strategy and Business Development Manager | Screen
> Main Line : +44 1473 831700 | Ext: 2208 | Direct Dial: +44 1473 834532
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>
> Visit us at
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> From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
> Sent: 16 November 2017 11:28
> To: John Birch <John.Birch@screensystems.tv>
> Cc: singer@apple.com; public-texttracks@w3.org; TTWG <public-tt@w3.org>
> Subject: Re: VTT edge margins: we need to know current practice. PLEASE
RESPOND If you implement a VTT UA (even in a polyfill)
>
> John,
>
> If I understand correctly those values are relative to the encoded video
rather than the video viewport.  Is that correct?
> Yes, they are expressed in video pixels in the base video resolution. I’m
not quite sure what you mean by encoded video and video viewport (perhaps
you are referring to overscan – which is not really relevant in broadcast
anymore?) We (at Screen) do not have any practical concept of a separate
canvas for subtitles… we tend to think in terms of the effective changes to
the underlying video pixels. In fact this is quite a relevant point in
terms of subtitle creation… the traditional practise is to lock the
subtitle pixels over the obscured video pixels… because subtitlers during
the creative process traditionally have a responsibility to avoid important
on screen action in the video, or to avoid lower thirds, or to place
subtitle text into areas of the video specifically left ‘black’ for
subtitles etc. The concept of a subtitle canvas or viewport that is an
independent layer in the presentation over the underlying video has
introduced new concepts (and complexity) into subtitling J
>
> Just to reiterate my concern here, since I'm not sure how well it came
over in the minutes and issue notes: by defining an inset area relative to
the video viewport only the ability to position subtitles and captions
relative to the video is removed. Specifying some expected or normal values
for an inset within the viewport area does not resolve this, though it
could help to set some expectations that might slightly reduce the problem,
on a good day with a following wind.
>
> By the way, stating an aesthetic motivation for not allowing caption
areas to sit against the viewport edge is a subjective point that should be
in the realm of the caption author not the specification. There are likely
to be exceptions that should not be prohibited.
>
> I agree… and the original motivations for subtitle placement away from
the boundary were NOT primarily aesthetic, they were practical. In the
first instance early broadcast could not guarantee the presentation of
pixels near the edges of the transmitted video signal (the overscan issue)…
more recently, with the introduction of HD and the use of ‘eye tracking’
research, it has been ‘determined’ that a more centred presentation
enhances the reception (the readability) of the subtitles on larger display
devices – so this placement is still a practical consideration rather than
an aesthetic one.
>
> Nigel
>
> --
> Nigel Megitt
> Executive Product Manager, BBC Design & Engineering
> Telephone: +44 (0)3030807996
> BC2 C1 Broadcast Centre, Media Village, 201 Wood Lane, London W12 7TP
>
> On 14 Nov 2017, at 16:28, John Birch <John.Birch@screensystems.tv> wrote:
>
> By my calculation for SD that is ~7%... [50/720 * 100]
> As previously for HD there are practical reasons (not just aesthetic
reasons) for placing the margins closer to the centre of the wider picture.
>
> I suspect there is no scope within your algorithm for a sensitivity to
the rendering area aspect ratio - in which case a value of #7% would also
work... but result in an atypical presentation (from a broadcasters
perspective :)
>
> Best regards,
> John
>
>
>
> John Birch | Strategy and Business Development Manager | Screen
> Main Line : +44 1473 831700 | Ext: 2208 |  Direct Dial: +44 1473 834532
> Mobile: +44 7919 558380 |  Fax : +44 1473 830078
> John.Birch@screensystems.tv
>
> Visit us at
> BVE, Excel London, Stand N25, 27 Feb - 1 Mar
>
>
> PBefore printing, think about the environment
>
> -----Original Message-----
> From: singer@apple.com [mailto:singer@apple.com]
> Sent: 14 November 2017 16:08
> To: John Birch <John.Birch@screensystems.tv>
> Cc: public-texttracks@w3.org; TTWG <public-tt@w3.org>
> Subject: Re: VTT edge margins: we need to know current practice. PLEASE
RESPOND If you implement a VTT UA (even in a polyfill)
>
> Hi John
>
> it really is the exact value used in the algorithm in 6.1 for this step
that I am looking for:
>
> "       • Let edge margin be a user-agent-defined horizontal length,
expressed as a percentage of the width of the video’s rendering area, which
will be used to define a margin at the left and right edges of the video
into which this cue will not be placed. In situations with overscan, this
margin should be sufficient to place the cue within the title-safe area. In
the absence of overscan, this value should be picked for aesthetics (to
avoid text being aligned precisely on the left or right edge of the video,
which can be ugly)."
>
>
> On Nov 14, 2017, at 5:30 , John Birch <John.Birch@screensystems.tv> wrote:
>
> Hi David,
>
> As an information point... in our systems for the generation of subtitles
in DVB bitmap format or for open (burnt in) insertion we use the following
starting values for subtitle 'margins'.
> N.B. These are often subsequently adjusted (tweaked) on a channel by
channel basis to suit local requirements.
>
> For SD resolution...
> 50 pixels left and right
>
> For HD resolution...
> 240 pixels left and right
>
> I hope this is of use...
> N.B. the HD values are intended to create a centred 4:3 style
presentation. Even though the screen has a wider aspect ratio in HD,
subtitles still tend to be maintained in a 4:3 presentation window in the
screen centre to minimise eye movements - but this is up to the
broadcasters... some choose to push the left and right margins closer to
the screen edges. Our system, as is typical practise in subtitling,
justifies to the set margins, rather than using absolute pixel positioning
for the subtitles.
>
> Best regards,
> John Birch
>
>
>
> John Birch | Strategy and Business Development Manager | Screen Main
> Line : +44 1473 831700 | Ext: 2208 |  Direct Dial: +44 1473 834532
> Mobile: +44 7919 558380 |  Fax : +44 1473 830078
> John.Birch@screensystems.tv
>
> Visit us at
> BVE, Excel London, Stand N25, 27 Feb - 1 Mar
>
>
> PBefore printing, think about the environment
>
> -----Original Message-----
> From: David Singer [mailto:singer@mac.com]
> Sent: 13 November 2017 23:35
> To: public-texttracks@w3.org; TTWG <public-tt@w3.org>
> Subject: VTT edge margins: we need to know current practice. PLEASE
> RESPOND If you implement a VTT UA (even in a polyfill)
>
> This sentence is problematic giving each UA a free hand:
> "In the absence of overscan, this value should be picked for aesthetics
(to avoid text being aligned precisely on the left or right edge of the
video, which can be ugly)."
>
> Resolution at TPAC: add a default edge margin instead of the above
> sentence ? (find out what the different UAs have implemented right
> now)
>
>
> * * * *
>
> Please let us know what you actually implement. We would like to
> settle on a recommended or even mandated value for edge margin.  If we
> all actually have the same value, we can choose that and not cause
> disruption, so KNOWING what EVERYONE does kinda makes sure you’re not
> accidentally disadvantaged…
>
> PLEASE tell us….
>
> David Singer
>
> singer@mac.com
>
>
>
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>
> David Singer
> Manager, Software Standards, Apple Inc.
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David Singer
Manager, Software Standards, Apple Inc.