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

From: Glenn Adams <glenn@skynav.com<mailto: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<mailto: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