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RE: Re: TTML and aspect ratio

From: David Ronca <dronca@netflix.com>
Date: Thu, 31 Jan 2013 04:20:59 +0000
To: "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <F7DC5134CE82754A878475EEBA450B3B30EECC@EXMB105.corp.netflix.com>
>we have issue 179 related to this namely that :

>"we need to clarify the pixel behaviour; in particular we should explain that where extent is used on the tt element this effectively *defines* 
>the size of a pixel in the sense that the root area extent is still mapped to a video overlay and divided into 'logical pixels'. If such an extent is 
>not defined on the tt element, then the px measure should probably not be used (or even expressly forbidden)"

> In this interpretation it does not imply that the extent covers some specific hardware pixels, but that the region is to be counted as having x by y 
> pixels; so that the px measure when used within the TTML document has some meaning independent of the hardware. If this is coupled with square 
> aspect ratio pixels (the default), then by definition the root would have the specified aspect ratio.

Let see if I am understanding this.  If I have a 1080p video display, and 1080p content, <tt tts:extent="640px,480px"> does not mean to display captions in a 640x480 window, it means to divide the 1920x1080 pixels into 640x480 logical pixels for mapping captions?  Further, by using <tt ttp:pixelAspectRatio="1,1", the client would not stretch the caption window horizontally, but would work within a 4:3 region of the video window and the logical pixels would map to 1440x1080 physical pixels? 
Received on Thursday, 31 January 2013 04:21:27 UTC

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