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

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Tue, 20 Jan 2015 11:22:35 +0000
To: John Birch <John.Birch@screensystems.tv>, TTWG <public-tt@w3.org>, "Glenn Adams" <glenn@skynav.com>
Message-ID: <D0E3EA5B.194AC%nigel.megitt@bbc.co.uk>
On 20/01/2015 11:15, "John Birch" <John.Birch@screensystems.tv> wrote:

>Agree.
>
>+1 last point.
>
>Although the problem described exists for all 3D objects, not just the
>subtitles!
>A disparityOffset that is generally applied is interesting, but it might
>better be implemented as a disparityScaling factor. In fact I think that
>it might be a useful feature as a viewer preference in a display!

I don't follow why a multiplier is better than an offset here, given that
the task is to move the text objects slightly toward the video. As I
understand it, that's achieved by adding a negative number. The value of
that negative number could be parameterised based on a viewer preference,
which would be achieved using a condition test.

>
>Best,
>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 11:11
>To: John Birch; TTWG; Glenn Adams
>Subject: Re: Issue-224 3D approach - disparity rather than (translation
>and condition)
>
>On 20/01/2015 10:53, "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.
>>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), or the second decode inadvertently had a side effect (e.g.
>>the subtitle was clipped against a region boundary?).
>
>Indeed. Or if strange authoring resulted in the two decodes being
>completely different, and not left eye/right eye views at all.
>
>
>>The approach of a separate disparity value, that could be animatable,
>>seem IMHO preferable. Decode once, then shift the resulting graphic as
>>necessary (where each graphic gets shifted half the disparity in
>>opposite directions).
>
>Yes - actually the DVB way of doing it is to shift the whole disparity
>value, i.e. the total disparity is 2x the value. That's a minor detail
>though.
>
>
>>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.
>>With a disparity property, ignoring the property in a non-stereoscopic
>>render would result in the correct intended positioning.
>
>Agreed (both points).
>
>
>A colleague has just pointed out to me that ideally, for comfort, 3D
>subtitles/captions need to be placed some fixed distance in front of
>whatever object they relate to, but the comfortable offset in disparity
>for that fixed distance is variable, and is dependent on screen size and
>viewer distance. For cinema screens, that's a problem because some of the
>audience is near the screen and some are far - the experience has been
>that only those about half way back get a good experience, given that
>there's only one screen. For environments with small numbers of audience
>members, for example televisions in living rooms, there's some hope of
>getting it right though. I would add to my proposal we should allow for a
>single per-document disparity offset value, to be added to the disparity
>value of the content. Then a condition could be used to set different
>offsets based on viewer preference, for example.
>
>So the disparity to be used when rendering any single piece of content
>would be (tts:disparity + tts:disparityOffset).
>
>Kind regards,
>
>Nigel
>
>
>>
>>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/t

>>>s
>>>_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/e

>>>n
>>>_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
>>>
>>>
>>>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
>>
>>
>>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
>
>
>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 11:23:06 UTC

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