[ttml2] Interaction between tts:writingMode and tts:direction on <p> (#1211)

palemieux has just created a new issue for https://github.com/w3c/ttml2:

== Interaction between tts:writingMode and tts:direction on <p>  ==
TTML2 specifies:

_When applied to a p element, the computed value of `tts:direction` explicitly establishes the paragraph level as specified by [UAX9], ยง4.3, Higher Level Protocol HL1._

It is unclear how this interacts with `tts:writingMode`. Specifically:

- are there any cases where `tts:direction` and `tts:writingMode` should conflict on `<p>`? In other words, does specifying `tts:direction="ltr"` ever make sense if `tts:writingDirection="rl`? CSS does not allow this inconsistency.

- what does `tts:direction` mean when the IPD is vertical? [UAX #9, clause 6.2](http://www.unicode.org/reports/tr9/tr9-37.html#Vertical_Text) implies that `tts:direction` should affect character rotation, but, again, this is ignored by CSS.

- the [Direction003.html test case](https://github.com/w3c/imsc-tests/blob/master/imsc1/ttml/direction/Direction003.ttml) implies that specifying `tts:direction="rtl"` on `<p>` changes the IPD.

- XSL specifies that `If the "direction" property is explicitly specified on the same formatting object the value of the "direction" property will override the inline-progression-direction set by the "writing-mode".`

I suggest considering the following clarification:

_when specified on `<p>`,  `tts:direction` sets the inline progression direction, overriding that set by `tts:writingDirection`. In the case where the block progression direction is horizontal, `tts:direction="ltr"` specifies a top-to-bottom inline progression direction whereas `tts:direction="rtl"` specifies a bottom-to-top inline progression direction._




Please view or discuss this issue at https://github.com/w3c/ttml2/issues/1211 using your GitHub account


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Received on Saturday, 1 August 2020 21:45:30 UTC