Consider aligning initial values of tts:position and tts:origin.

[cconcolato]

Per discussion during Jan. 2018 F2F, for backwards compatibility with TTML1.

[css-meeting-bot]

The Working Group just discussed Consider aligning initial values of tts:position and tts:origin. ttml2#551, and agreed to the following resolutions:

• RESOLUTION: Make the initial value of tts:position "top left" and remove the text defining behaviour of position when specified on the tt element.

The full IRC log of that discussion

<nigel> Topic: Consider aligning initial values of tts:position and tts:origin. ttml2#551
<nigel> github: https://github.com//issues/551
<nigel> Cyril: I added it to the agenda but expected some internal feedback and have not had any
<nigel> .. yet so I propose to defer it. Does the group have any position on this issue?
<nigel> Glenn: If we defer it we end up with two different initial values as apply to region.
<nigel> Pierre: This is not a big fix.
<nigel> Nigel: Are we all agreed that the initial value of position should be "top left" to align with
<nigel> .. the initial value of origin, for regions?
<nigel> Glenn: I think it makes more sense to have "center center" but it introduces the disparity.
<nigel> .. Since users can add an initial element it is easy to change so I'm happy with making
<nigel> .. it "top left".
<nigel> +1
<nigel> Nigel: Do we need to qualify this for regions and have a different treatment for the root element?
<nigel> Glenn: There is some normative language in the spec that applies it to tt but on reviewing
<nigel> .. it recently I noticed it was not applicable to tt, so there is that discrepancy to deal with -
<nigel> .. does it apply to tt or not? Does it position the root container region within a positioning
<nigel> .. area such as the related media object region or some other positioning area.
<nigel> .. There's also the applicability of it to backgroundImage and image with respect to the
<nigel> .. extent of the padding area. With regard to the question of initial value semantics we
<nigel> .. need to have an answer that can apply to all contexts of use but we may actually end
<nigel> .. up saying there is an exception based on the content of use.
<nigel> Nigel: It doesn't apply to image because that has the tts:backgroundPosition attribute.
<nigel> .. So I propose if we want a position to apply to tt then we call it something different.
<nigel> Glenn: I don't like that idea - there are three similar position things in the spec. It is possible I admit.
<nigel> Nigel: We possibly do not have the requirement to explicitly position the root container
<nigel> .. region anyway.
<nigel> Glenn: IMSC1 makes the normative statement of positioning the root container region
<nigel> .. centered relative to the related media. That's where this feature comes from.
<nigel> Nigel: Does anyone need to specify any option other than "center center"?
<nigel> Glenn: There's a concept we started to discuss one time - frequently I find in letterboxing,
<nigel> .. especially in some of the really wide formats, the captioning is in the letter box area
<nigel> .. outside the active video. Right now other than using position we don't have any way
<nigel> .. to achieve that. We did not thoroughly discuss this issue.
<nigel> Nigel: For now can we agree on position for region and open a different issue for positioning
<nigel> .. the root container region?
<nigel> Glenn: The easiest thing to do is remove the two paragraphs and a note beneath the Percentage Based Positioning
<nigel> .. diagram under tts:position, after reviewing if there's anything that applies to region.
<nigel> .. If anything does apply to region I would leave that present.
<nigel> Cyril: Regarding the initial value I'm fine. Re the use on the tt element, how does this
<nigel> .. relate to the alignment we discussed with the related media object that is also in IMSC 1.1?
<nigel> .. Is it configurable in IMSC 1.1?
<nigel> Pierre: No the root container region cannot be configured within IMSC 1.1. Maybe a container
<nigel> .. like ISOBMFF might allow it to be specified.
<nigel> PROPOSAL: Make the initial value of tts:position "top left" and remove the text defining behaviour of position when specified on the tt element.
<nigel> RESOLUTION: Make the initial value of tts:position "top left" and remove the text defining behaviour of position when specified on the tt element.
<nigel> Glenn: Where we remove functionality I have been marking as ttml.next. That applies here.