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Re: Issue-224 3D approach - disparity rather than (translation and condition)

From: Glenn Adams <glenn@skynav.com>
Date: Tue, 20 Jan 2015 07:35:39 -0700
Message-ID: <CACQ=j+fWMZTd-EVwEC=YaRTLhg3tPg6G+LJmWHgg5z1RRgzNVw@mail.gmail.com>
To: John Birch <John.Birch@screensystems.tv>
Cc: Nigel Megitt <nigel.megitt@bbc.co.uk>, TTWG <public-tt@w3.org>
On Tue, Jan 20, 2015 at 3:53 AM, John Birch <John.Birch@screensystems.tv>
wrote:

> Thanks Nigel,
>
> I have to admit that, although I can see how Glenn's proposal would work,
> I would have reservations about a double decode approach.
>

It is not decoded twice, just rendered twice.


> For example, what happens in a double decode approach if the second
> document did not decode (e.g. the conditional resulted in an invalid
> document),


conditionalization is not at syntactic level, so this is not possible
without the entire document being invalid


> or the second decode inadvertently had a side effect (e.g. the subtitle
> was clipped against a region boundary?).
>

not possible, as translation applies to region, not content in region


>
> The approach of a separate disparity value, that could be animatable, seem
> IMHO preferable.


i disagree; it is overly specialized and eventually maps to translation in
any case


> Decode once, then shift the resulting graphic as necessary (where each
> graphic gets shifted half the disparity in opposite directions).
> In fact that's another issue for the conditional approach... Doc A would
> have to be left eye, Doc B would have to be right eye position.... but then
> neither document would work correctly as a non-stereoscopic document.
>

nope, study the example under tts:translate; if stereoscopic imaging is
applied, then region is still well placed


> With a disparity property, ignoring the property in a non-stereoscopic
> render would result in the correct intended positioning.
>

as does the approach shown in the example


>
> Best regards,
> John
>
> John Birch | Strategic Partnerships 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 | www.screensystems.tv |
> https://twitter.com/screensystems
>
> Visit us at
> BVE, Excel London 24-26 February 2015 Stand No. N19
>
> P Before printing, think about the environment-----Original Message-----
> From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
> Sent: 20 January 2015 10:46
> To: John Birch; TTWG; Glenn Adams
> Subject: Re: Issue-224 3D approach - disparity rather than (translation
> and condition)
>
> Thanks John, good point.
>
> <length> is permitted to be a real number, either as a percentage or
> expressed in one of the length units. I agree that it is important that any
> implementation must use sub-pixel rendering to achieve a good audience
> experience.
>
> Kind regards,
>
> Nigel
>
>
> On 20/01/2015 10:43, "John Birch" <John.Birch@screensystems.tv> wrote:
>
> >Hi Nigel,
> >
> >Be advised that, as per the DVB specification, to achieve good
> >positioning in 3D space, sub pixel offsets are necessary.
> >This is particularly important if the disparity is animated (i.e. if
> >the subtitle is moved to follow an on screen object).
> >Quantisation of disparity to a single pixel level leads to perceivable
> >jumps in the subtitle depth which is extremely disconcerting to a viewer.
> >
> >It is dependent upon display (e.g. cinema or TV screen) and viewer! but
> >we have found that a 1/10 pixel difference is easily discernible.
> >
> >Best regards,
> >John
> >
> >John Birch | Strategic Partnerships 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 |
> >www.screensystems.tv | https://twitter.com/screensystems
> >
> >Visit us at
> >BVE, Excel London 24-26 February 2015 Stand No. N19
> >
> >P Before printing, think about the environment-----Original
> >Message-----
> >From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
> >Sent: 20 January 2015 10:22
> >To: TTWG; Glenn Adams
> >Subject: Issue-224 3D approach - disparity rather than (translation and
> >condition)
> >
> >Glenn,
> >
> >I see you have created update
> >https://dvcs.w3.org/hg/ttml/rev/abebbd0a303b
> >to address issue-224, for 3D disparity. It looks as though the approach
> >you've taken is to allow the same document to be processed twice, once
> >for the left image and once for the right image for a stereoscopic
> >display, and to allow translation to be specified, being dependent on a
> >parameter and using the condition attribute.
> >
> >Can I propose an alternate way to achieve stereoscopic object placement
> >that may be more amenable to simple, i.e. single pass, processing? This
> >would be to add a tts:disparity style attribute, whose value would be a
> ><length>, positive or negative. This would be inherited and animatable,
> >and apply to region, div or p (possibly a span too). Positive values
> >imply that the image is behind the plane of display and negative values
> >imply that the image is in front of the plane of display.
> >
> >For example see [1] §4.2.1. Following the references, this seems to be
> >how it's done in DVB [2].
> >
> >[1] ETSI TS 101 600 C1.1.1 (2012-05)
> >http://www.etsi.org/deliver/etsi_ts/101600_101699/101600/01.01.01_60/ts
> >_10
> >1
> >600v010101p.pdf
> >[2] ETSI EN 300 743 V1.4.1 (2011-10)
> >http://www.etsi.org/deliver/etsi_en/300700_300799/300743/01.04.01_60/en
> >_30
> >0
> >743v010401p.pdf
> >
> >A good description from [2] (p. 34) is:
> >
> >> Disparity is the difference between the horizontal positions of a
> >>pixel representing the same point in space in the right and left views
> >>of a plano-stereoscopic image. Positive disparity values move the
> >>subtitle objects enclosed by a subregion away from the viewer whilst
> >>negative values move them towards the viewer. A value of zero places
> >>the objects enclosed by that subregion in the plane of the display
> >>screen.
> >
> >
> >And from a little further down:
> >
> >> A positive disparity shift value for example of +7 will result in a
> >>shift of 7 pixels to the left in the left subtitle subregion image and
> >>a shift of 7 pixels to the right in the right subtitle subregion image.
> >>A negative disparity shift value of -7 will result in a shift of 7
> >>pixels to the right in the left subtitle subregion image and a shift
> >>of
> >>7 pixels to the left in the right subtitle subregion image. Note that
> >>the actual disparity of the displayed subtitle is therefore double the
> >>value of the disparity shift values signalled in the disparity integer
> >>and/or fractional fields […]
> >
> >Kind regards,
> >
> >Nigel
> >
> >
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>
> This message may contain confidential and/or privileged information. If
> you are not the intended recipient you must not use, copy, disclose or take
> any action based on this message or any information herein. If you have
> received this message in error, please advise the sender immediately by
> reply e-mail and delete this message. Thank you for your cooperation.
> Screen Subtitling Systems Ltd. Registered in England No. 2596832.
> Registered Office: The Old Rectory, Claydon Church Lane, Claydon, Ipswich,
> Suffolk, IP6 0EQ
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Received on Tuesday, 20 January 2015 14:36:26 UTC

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