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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 10:46:17 +0000
To: John Birch <John.Birch@screensystems.tv>, TTWG <public-tt@w3.org>, "Glenn Adams" <glenn@skynav.com>
Message-ID: <D0E3E220.19436%nigel.megitt@bbc.co.uk>
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

Kind regards,


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 Birch | Strategic Partnerships Manager | Screen
>Main Line : +44 1473 831700 | Ext : 2208 | Direct Dial : +44 1473 834532
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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
>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)

>[2] ETSI EN 300 743 V1.4.1 (2011-10)

>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
>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,
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Received on Tuesday, 20 January 2015 10:46:48 UTC

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