Re: TTML and aspect ratio

>For the TTML author, either SAR pixels or DAR pixels may be used for
>coordinates. If SAR is used, then in this case, the author would need to
>specify ttp:pixelAspectRatio='10 11' on the tt element. So, for example,
>the author might specify:


We use an adaptive streaming model where a given client will get a set of streams at various resolutions, so using tts:extent on the root container could be ambiguous.  How should a client behave if the root container <tt tts:extent='640px 480px'> and the video is 1080p?  At least the inverse (480p with <tt tts:extent='1920px 1080px'>, the client could assume that the extent should be scaled down.


> So, an author could specify the outer root container extent using pixels

Because we have a very large amount of content, we contract with many captioning vendor.  In order to protect our assets, we provide these vendors with a very low quality, low resolution asset to work with.  Hence, we the caption author will set <tt tts:extent='320px 240px'>.  How should a client displaying 1080p interpret this?

What we need is a way to specify the caption presentation in relative terms, so that any client can present the captions in a consistent and well defined way regardless of the display size or the video frame size.