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

Well as originally stated these are our default ‘startling’ values. They fairly closely approximate to the typical Teletext start and end of row positions (although these were never formally defined).

It should also be appreciated that subtitling presentation has been evolved to a ‘style’ that maximises the utility of the service when used dynamically over a video presentation, and thus may appear less than truly aesthetic when looked at as a static still frame capture! :-)

Best regards,
John

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On 16 Nov 2017, at 20:24, Silvia Pfeiffer <silviapfeiffer1@gmail.com<mailto:silviapfeiffer1@gmail.com>> wrote:

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<mailto: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<mailto:John.Birch@screensystems.tv>> wrote:
>
> Hi Nigel,
>
> CIL…
>
> Best,
> John
>
> John Birch | Strategy and Business Development Manager | Screen
> Main Line : +44 1473 831700<tel:%2B44%201473%20831700> | Ext: 2208 | Direct Dial: +44 1473 834532<tel:%2B44%201473%20834532>
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> From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>]
> Sent: 16 November 2017 11:28
> To: John Birch <John.Birch@screensystems.tv<mailto:John.Birch@screensystems.tv>>
> Cc: singer@apple.com<mailto:singer@apple.com>; public-texttracks@w3.org<mailto:public-texttracks@w3.org>; TTWG <public-tt@w3.org<mailto: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<tel:%2B44%20%280%293030807996>
> 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<mailto: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<tel:%2B44%201473%20831700> | Ext: 2208 |  Direct Dial: +44 1473 834532<tel:%2B44%201473%20834532>
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>
> -----Original Message-----
> From: singer@apple.com<mailto:singer@apple.com> [mailto:singer@apple.com<mailto:singer@apple.com>]
> Sent: 14 November 2017 16:08
> To: John Birch <John.Birch@screensystems.tv<mailto:John.Birch@screensystems.tv>>
> Cc: public-texttracks@w3.org<mailto:public-texttracks@w3.org>; TTWG <public-tt@w3.org<mailto: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<mailto: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
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>
> -----Original Message-----
> From: David Singer [mailto:singer@mac.com<mailto:singer@mac.com>]
> Sent: 13 November 2017 23:35
> To: public-texttracks@w3.org<mailto:public-texttracks@w3.org>; TTWG <public-tt@w3.org<mailto: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<mailto:singer@mac.com>
>
>
>
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>
> David Singer
> Manager, Software Standards, Apple Inc.
>
>
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David Singer
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