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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 18:25:19 +0000
To: Pierre-Anthony Lemieux <pal@sandflow.com>
CC: Glenn Adams <glenn@skynav.com>, TTWG <public-tt@w3.org>
Message-ID: <8EA762FD-F316-4C7C-BB21-5D9F3068D0FE@bbc.co.uk>



> On 20 Jan 2015, at 18:15, Pierre-Anthony Lemieux <pal@sandflow.com> wrote:
> 
> Hi Nigel et al.,
> 
>> are there pre-existing implementations that take
>> this approach of direct translation with conditional offset values?
> 
> Issue-224 was motivated by a SMPTE liaison (SEPT 2012) and references
> D-Cinema subtitles (SMTPE ST 428-7). In the latter, rendering of
> subtitles to left- and right-eye stereoscopic images is controlled
> using an attribute ("ZPosition") that specifies the disparity (as a
> percentage of the root container) between left- and right-eye images.
> 
> """When present, the Zposition attribute shall provide a value that
> specifies the horizontal distance between the “left eye” image center
> and the “right eye” image center - in order to generate a stereoscopic
> effect."""
> 
> Minimally, I would think that the approach selected by TTWG should
> support the D-Cinema approach, which is implemented.

Thanks Pierre,

That sounds exactly coincident with my proposal for disparity. 

Nigel

> 
> Best,
> 
> -- Pierre
> 
>> On Tue, Jan 20, 2015 at 8:43 AM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
>> From: Glenn Adams <glenn@skynav.com> Date: Tuesday, 20 January 2015 14:37
>> 
>> 
>> 
>> On Tue, Jan 20, 2015 at 3:22 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
>> wrote:
>>> 
>>> 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.
>> 
>> 
>> I discussed this thoroughly with Pierre before publishing this approach, and
>> we are both in agreement that it can handle the requirements. So that's what
>> I'm going with.
>> 
>> 
>> I don't disagree that an author can, with care, craft a document that will
>> display stereoscopically with the correct characteristics using this
>> technique, however "can handle" is not equal to "best way to express this
>> information".
>> 
>> Pierre,  are there pre-existing implementations that take this approach of
>> direct translation with conditional offset values? 3D subtitles using a
>> single disparity value are in common usage as per the links I sent (now
>> below).
>> 
>> 
>>> 
>>> 
>>> 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_101
>>> 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_300
>>> 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
>> 
Received on Tuesday, 20 January 2015 18:25:52 UTC

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