Re: RE: Re: Re: TTML and aspect ratio

> Required by whom?

It is our understanding of the new FCC requirements, that all devices must present a given caption in a consistent manner.  If the author uses CEA-608 or any format that works on a 4:3 presentation area, then by definition, the presentation area is a region that is 100% of the video height, and H*4/3 width, and this window is centered over the video.  This is crucial because the actual screen location of the position 10%,10% is different for 4:3 and 16:9 presentation area.  This is independent of the display aspect ratio of the video.  If a client knows the presentation AR, and percentage or cell positioning is used, then the captions can be scaled and presented to fit in the video window properly without any use of pixels or "external context".