[ttml2] Incorporate CSS advances into TTML vertical text handling

r12a has just created a new issue for https://github.com/w3c/ttml2:

== Incorporate CSS advances into TTML vertical text handling ==
10.2.52 tts:writingMode
http://w3c.github.io/ttml2/spec/ttml2.html#style-attribute-writingMode

10.2.46 tts:textOrientation
http://w3c.github.io/ttml2/spec/ttml2.html#style-attribute-textEmphasis

I think it would be useful to make the text in this section more closely resemble the approach in CSS, recent developments for which, i believe, have fixed some issues related to vertical text handling which apply to the model described for TTML.

CSS has the following values for vertical writing modes:
vertical-rl (equivalent to tbrl)
vertical-lr (equivant to tblr)
sideways-rl
sideways-lr

The latter two values are intended for use with 'horizontal scripts' when one wants to set them vertically (usually alongside something). They are useful additions because they effectively rotate the progression of lines in one direction or the other, which is what you typically need for such use cases. This affects where the start and end points lie, and how text will wrap to the next line, as well as the default orientation of the characters. For example, for sideways-lr, start is at the bottom of the vertical line, Latin characters would lie on their left side, and lines would wrap when they reach the top of the vertical line. These are things that don't happen with vertical-lr.

I believe there are some technical advantages to this approach, but also it will help authors if the models for TTML and CSS are similar.

Perhaps there will be some resistance because vertical text values such as sidewaysLeft and sidewaysRight existed previously in TTML, i'm not sure. If so, and therefore things can't be removed, then i suggest that sideways-rl and sideway-lr values be added, and the user be advised to use those rather than the sidewaysLeft and sidewaysRight values of tts:textOrientation.

(The TTML model also doesn't adopt all of the values used in XSL, and so the -rl and -lr parts of tb-rl and tb-lr don't have equivalents in the vertical text values. In CSS the inline direction of the block is not indicated by an explicit -rl/-lr on the container. That is set by the direction property. I'm guessing that you don't want to go there.)

Please view or discuss this issue at https://github.com/w3c/ttml2/issues/277 using your GitHub account

Received on Monday, 13 March 2017 19:58:13 UTC