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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 11:35:48 +0000
To: John Birch <John.Birch@screensystems.tv>, TTWG <public-tt@w3.org>
Message-ID: <D0E3EE06.194CA%nigel.megitt@bbc.co.uk>
On 20/01/2015 11:33, "John Birch" <John.Birch@screensystems.tv> wrote:

>A multiplier gives you the ability to compress the 3D effect (or extend
>it).
>The problem for close viewers is that the disparity length is a greater
>angle on the eye relative to a more distant viewer (i.e. the 3D effect
>becomes telegraphed when you are closer to the screen - [and conversely
>it becomes 'flatter' when you are further away]).

As you suggest, that's a problem to solve for the entire presentation,
video included, not something specific to the captions/subtitles.

>
>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 11:23
>To: John Birch; TTWG; Glenn Adams
>Subject: Re: Issue-224 3D approach - disparity rather than (translation
>and condition)
>
>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.
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>>>>sender immediately by reply e-mail and delete this message. Thank you
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>>>>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
>
>
>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 11:36:19 UTC

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