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Re: [imsc] ittp:aspectRatio: SAR or DAR?

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Mon, 27 Oct 2014 15:36:17 -0700
Message-ID: <CAF_7JxAeJRrizsDoLWWvr+snuPnd8G9Vimbq5YaSzhGRvhEhvA@mail.gmail.com>
To: Glenn Adams <glenn@skynav.com>
Cc: TTWG <public-tt@w3.org>
> This is a problem because coordinates expressed in pixel units in TTML documents are,
> due the existence of the ttp:pixelAspectRatio property, implicitly expressing pixels in the
> storage rather than display coordinate space.

ttp:pixelAspectRatio does not apply when "px" metrics are used
anywhere in the document.

It sounds like ttp:pixelAspectRatio applies to 'the aspect ratio of
non-square pixels'.

Best,

-- Pierre

On Mon, Oct 27, 2014 at 3:18 PM, Glenn Adams <glenn@skynav.com> wrote:
> IMSC defines ittp:aspectRatio by making reference to "a rectangular area
> within the related media object" and "the ratio of width to height of the
> rectangular area". However, the definition does not distinguish between
> Storage Aspect Ratio (as stored) (SAR) [2] and Display Aspect Ratio  (as
> displayed) (DAR) [3]. These two possibly distinct ratios are related by the
> following formula:
>
> DAR = SAR * PAR
>
> where PAR = Pixel Aspect Ratio (4), where, in TTML documents, the PAR is
> equal to the computed value of the ttp:pixelAspectRatio parameter property.
>
> My question is whether the definition of ittp:aspectRatio refers to SAR or
> DAR? Further, if it refers to DAR and the related media object employs
> non-square pixels, then there may be a problem in that IMSC excludes
> specifying ttp:pixelAspectRatio. This is a problem because coordinates
> expressed in pixel units in TTML documents are, due the existence of the
> ttp:pixelAspectRatio property, implicitly expressing pixels in the storage
> rather than display coordinate space.
>
> [1]
> https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml-ww-profiles/ttml-ww-profiles.html#ttp-aspectRatio
> [2] http://en.wikipedia.org/wiki/Aspect_ratio_(image)#Distinctions
> [3] http://en.wikipedia.org/wiki/Display_aspect_ratio
> [4] http://en.wikipedia.org/wiki/Pixel_aspect_ratio
Received on Monday, 27 October 2014 22:37:06 UTC

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