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

On Mon, Jan 26, 2015 at 7:45 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
wrote:

>  Thanks Glenn,
>
>  I like it (as you might have guessed!). A couple of follow-up points:
>
>  1. Do we still need "usesStereo" for anything?
>

it is still useful metadata


>
>  2. We should define that, relative to a value of 'zero', positive values
> should result in the image appearing further from the viewer than the plane
> of the display, and negative values should result in the image appearing
> closer to the viewer. This is ambiguous right now.
>

actually, i need to disallow negative values; the value is the horizontal
disparity value, not depth; so it is up to author to choose desired
disparity


>
>
>  Plus a note to ourselves:
>
>  The disparity approach fits matches the request from SMPTE as well as
> what's specified by ETSI/DVB. Where those latter differ is that in the case
> of the SMPTE Digital Cinema spec the value is halved, and the left and
> right image each offset by that half-value; whereas in the case of DVB the
> value is applied equally both to the left and right image, resulting in
> double the apparent disparity compared to the same value in the SMPTE spec.
> As long as we're clear, which I think we are right now ("… is evenly
> divided along the horizontal axis between left and right stereoscopic
> images."), there's no problem.
>
>  Kind regards,
>
>  Nigel
>
>
>   From: Glenn Adams <glenn@skynav.com>
> Date: Sunday, 25 January 2015 21:50
> To: Nigel Megitt <nigel.megitt@bbc.co.uk>
> Cc: TTWG <public-tt@w3.org>
> Subject: Re: Issue-224 3D approach - disparity rather than (translation
> and condition)
>
>   I have decided to submit to the apparent preference for specifying
> disparity directly, and have consequently changed from tts:translate to
> tts:disparity in [1].
>
>  [1] https://dvcs.w3.org/hg/ttml/rev/9b8dc79e4004
>
> 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.
>>
>> 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
>> <http://www.etsi.org/deliver/etsi_ts/101600_101699/101600/01.01.01_60/ts_101600v010101p.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
>> <http://www.etsi.org/deliver/etsi_en/300700_300799/300743/01.04.01_60/en_300743v010401p.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
>>
>>
>