Using heuristics to determine the direction.

[r12a]

This is a comment from the i18n WG, however we recognise that it comes late in the process for 2nd Edition, so we recommend that if it is not fixable before your transition, you consider it when you start work on the 3rd Edition.

10.2.12 tts:direction
https://w3c.github.io/ttml2/index.html#style-attribute-direction

There doesn't seem to be a way to indicate the base direction using heuristics when tts:direction is not used. We would like to see this feature added to TTML.

[skynavga]

Is this a proposal to add an auto value to tts:direction? If so, then please provide a reference to a normative specification for such usage.

[r12a]

It's not a request to add auto to tts:direction (though that may or may not come after we've put some more thought into it). It's saying that if tts:direction isn't used and the text is in a RTL script, it may be possible to rescue what could otherwise produce poor results on display.

[skynavga]

Ah, in that case, then we don't need to do anything substantive, since we already assume that the UAX #9 algorithm is part of the implementation and does exactly what this issue is suggesting as the default. Indeed, one can probably deduce this fro the chain of referenced XSL-FO semantics, though I haven't attempted to do so recently. BTW, I know that TTPE does indeed implement this behavior for its BIDI support for M.E. languages. Probably at most, we might add an informative note.