[imsc] Root Container Region and Safe-Title Area (#568)

adam-gol has just created a new issue for https://github.com/w3c/imsc:

== Root Container Region and Safe-Title Area ==
@mikedo and I were discussing (my alleged) lack of specificity in the standards about whether the IMSC1 document is or is not required/recommended to keep captions within the safe title area or not.  I cannot find clear text which indicates that an IMSC1 document should only place text within the safe title area, and I do find indication that it need not.  

There's a suggestion that perhaps IMF makes this requirement/recommendation.  I don't think it does.  Below, a set of standards that seem pretty clear that the root container region be the whole visible screen, but nowhere does it limit the use of that region to be the safe-title portion of it.  

Constraining to the US, and constraining to CTA-608/708, the FCC makes requirements about how much text the receiver is required to be capable of displaying **within the safe-title area**, and it's clear that 608/708 captions are authored expecting that any screen area addressable using the 608/708 mechanisms is within the safe-title area.   In an imsc1 context, where most (television) captioning is initially authored in 608 and then converted, it seems possible that the conversions process is aware that the 608/708 area is smaller than the root container region and specifically makes allowances for that (by producing imsc1/ttml that avoids the area outside the safe title area).  **However, I can't find that as a requirement** (or even a recommendation) for 608/708 conversions anywhere.  Additionally, for imsc1/ttml-native content, there's similarly to requirement that I can find (and it's difficult to even infer it the way you can with 608/708 converted content).  Therefore, t's difficult to both comply with an FCC expectation that captions are only displayed inside the safe title area and that imsc1/ttml is capable of causing captions to be rendered outside the safe title are.

Help?  (Or is this better directed to a SMPTE audience?)



ST 2067-2:2020
**5.4.7 Mapping of the Root Container Region** 
The Root Container Region of the Document Instance shall be mapped to the Display Rectangle (defined by SMPTE ST 377-1) of the Main Image Virtual Track.

ST 377-1:2019
**G.1.3 Display Rectangle**
The Display Rectangle shall be the rectangular region which is visible in the display device

So therefore: “ The Root Container Region of the Document Instance shall be mapped to the rectangular region which is visible in the display device.”

**ttml1:**
Root Container Region
A logical region that establishes a coordinate system into which Document Instance content regions are placed and optionally clipped.

**8.2.14 tts:origin**
The tts:origin attribute is used to specify the x and y coordinates of the origin of a region area with respect to the origin of the Root Container Region.

It seems clear to me:  tts:origin of (0% 0%) is the upper left of the visible area of the display, not the upper left of the safe title area.  It’s similarly clear that ttml1 doesn’t describe constraints with respect to the safe area on where within the Root Container Region regions may be positioned.

Additionally, imsc1.0.1 explicitly requires (in most cases) "the Root Container Region of a Document Instance SHALL be mapped to the image frame in its entirety."






Please view or discuss this issue at https://github.com/w3c/imsc/issues/568 using your GitHub account


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Received on Wednesday, 14 October 2020 17:19:08 UTC