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[imsc] ittp:aspectRatio: SAR or DAR?

From: Glenn Adams <glenn@skynav.com>
Date: Mon, 27 Oct 2014 16:18:33 -0600
Message-ID: <CACQ=j+fj1a8YXwF89=cNTU9WFS-QMrVeF1O+uhGahKsbfYVgrg@mail.gmail.com>
To: TTWG <public-tt@w3.org>
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:


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.

[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 22:19:20 UTC

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