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

From: John Birch <John.Birch@screensystems.tv>
Date: Tue, 20 Jan 2015 10:43:05 +0000
To: Nigel Megitt <nigel.megitt@bbc.co.uk>, TTWG <public-tt@w3.org>, "Glenn Adams" <glenn@skynav.com>
Message-ID: <0981DC6F684DE44FBE8E2602C456E8AB016C1466EC@SS-IP-EXMB-01.screensystems.tv>
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

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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/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


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Received on Tuesday, 20 January 2015 10:43:37 UTC

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