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

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Tue, 20 Jan 2015 10:15:09 -0800
Message-ID: <CAF_7JxAY0R81yR0cE4HWwtzLmBfDQ3_z67CpRtFgVW179=kigA@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Glenn Adams <glenn@skynav.com>, TTWG <public-tt@w3.org>
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.

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 18:15:59 UTC

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