tts:position should be allowed on region only

[palemieux]

To be consistent with tts:origin, tts:position should be

• allowed only on region only
• use px units or "percentage" only

See also w3c/ttml2#710

[nigelmegitt]

There's a wider question of whether we want to allow inline origin|position and extent on content elements, and use the TTML2 implicit region animation semantics. Just raising because we haven't discussed it. I don't particularly want to introduce additional unnecessary flexibility for authors here to achieve what can already be done, but recognising that the practice is already in use (albeit not in IMSC) there's at least some argument in favour. To be clear, my preference is not to allow them.

[css-meeting-bot]

The Working Group just discussed tts:position should be allowed on region only imsc#366, and agreed to the following:

• RESOLUTION: We will not support `#region-implied-animation ` in IMSC 1.1.
• RESOLUTION: Apply the same constraints that exist on `tts:origin` to `tts:position`.

The full IRC log of that discussion

<nigel> Topic: tts:position should be allowed on region only imsc#366
<nigel> github: https://github.com//issues/366
<nigel> Pierre: My impression is that we have trained people not to use origin and extent on content
<nigel> .. elements. Certainly in IMF and ATSC we have trained people not to do it.
<nigel> .. imsc.js and TTPE and EBU-TT-D does not support it, so in fact I think the trend has
<nigel> .. been the opposite, that we have reiterated that you cannot do it. We have been saying
<nigel> .. that for the past 2 years and I'm finally getting to the point where I'm not seeing it anymore.
<nigel> Nigel: Now I really want to make sure it is on the at risk list for TTML2 because I really
<nigel> .. don't want to support it.
<nigel> Pierre: Based on my experience in imsc.js the feature really breaks the way TTML was
<nigel> .. designed. Unless someone stands up to say they really want it I don't think we should do it.
<nigel> Nigel: It is not on the at risk list for TTML2 CR2.
<nigel> Pierre: TTPE doesn't support it in TTML1, I'm not sure if it supports it in TTML2.
<nigel> Nigel: Does anyone want to take the action to raise an issue to add the
<nigel> .. `#region-implied-animation ` to the at risk list for TTML2?
<nigel> Pierre: I think it's reasonable to add it.
<nigel> Nigel: If we can close this meeting early then I'll go ahead and do that.
<nigel> .. Going back to the issue you want to only allow position on region? Is that only on text profile?
<nigel> Pierre: The same restrictions on tts:origin should be placed on tts:position, whatever those are.
<nigel> Nigel: Don't you want position on image?
<nigel> Pierre: You put it on the region that the image is in.
<nigel> Nigel: And you only allow one image per region?
<nigel> Pierre: Correct, just like in IMSC1.
<nigel> RESOLUTION: We will not support `#region-implied-animation ` in IMSC 1.1.
<nigel> Nigel: We also have another resolution to do what the issue requests.
<nigel> RESOLUTION: Apply the same constraints that exist on `tts:origin` to `tts:position`.