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

From: John Birch <John.Birch@screensystems.tv>
Date: Tue, 20 Jan 2015 14:58:29 +0000
To: Glenn Adams <glenn@skynav.com>
CC: Nigel Megitt <nigel.megitt@bbc.co.uk>, TTWG <public-tt@w3.org>
Message-ID: <0981DC6F684DE44FBE8E2602C456E8AB016C146F89@SS-IP-EXMB-01.screensystems.tv>
Hi Glenn,

CIL

BR,
John

John Birch | Strategic Partnerships Manager | Screen
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P Before printing, think about the environment

From: Glenn Adams [mailto:glenn@skynav.com]
Sent: 20 January 2015 14:36
To: John Birch
Cc: Nigel Megitt; TTWG
Subject: Re: Issue-224 3D approach - disparity rather than (translation and condition)



On Tue, Jan 20, 2015 at 3:53 AM, John Birch <John.Birch@screensystems.tv<mailto: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.
Unnecessary to *fully* render twice – the resulting images *must* be identical (they *must* only differ in position). But this could be an implementation optimisation or a specified recommended approach to implementation.
But surely the document must still be processed twice using the parameter for the condition set first to “stereoLeft” and then *again* with the condition set to “stereoRight”?
How can you only process the document once? Or are you proposing that the implementation speculatively executes both conditions?

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
Validation of a document that uses conditionalization (by which I mean – gives the intended result – not meets the schema) would seem to be more onerous than a document that does not – since all combinations of condition values must be tested?

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
Fair enough. Although testing that a region does not translate out of root area (and so clips its content) would still be necessary. But that holds however you do it ☺.


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
I agree it is always translation ☺ How do you animate conditionalization? Is that possible?

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
Is see that, since neither condition is met so no translation is applied.

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
Accepted.


Best regards,
John

John Birch | Strategic Partnerships 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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P Before printing, think about the environment-----Original Message-----
From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk<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<mailto: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<tel:%2B44%201473%20834532> Mobile : +44
>7919 558380 | Fax : +44 1473 830078 John.Birch@screensystems.tv<mailto:John.Birch@screensystems.tv> |
>www.screensystems.tv<http://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<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

Received on Tuesday, 20 January 2015 14:59:01 UTC

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