Layout-dependent feature subsetting

This thought occurred to me today after Skef’s mention of the palt 
feature. Possibly an agenda item for the next meeting, or that can be 
resolved in email if it is already something that is taken into account 
in the protocol.

We have talked about default script shaping features, and about 
discretionary, user-activated features, and now about possible 
user-selected variants from within alternate substitutions (GSUB lookup 
type 3). I am wonder, though, about layout-dependent features, i.e. 
features that are off by default but that a layout engine may invoke for 
specific types of text configuration, e.g. vertical versus horizontal 
layout of Chinese. I do not know enough about CJK layout to understand 
all the ways in which the various OTL features that Adobe registered are 
used, but my understanding is that a lot of the CJK-specific features 
are dependent on text layout. This implies the possibility of subsetting 
determined by higher-level text layout settings.

Another, more esoteric example—but one I am more familiar with—would be 
use of rtlm/rtla features triggered by Unicode formatting control 
characters for right-to-left text, e.g. to apply boustrophedon layout to 
Greek inscriptional transcription. As with the CJK examples, this is an 
example of discretionary (non-default) substitutions being selected 
/indirectly/ by a document creator via a higher level layout setting.

In this example, even-numbered lines run right-to-left and 
non-symmetrical glyphs use rtla flipped forms (in red):

Have we accounted in IFT for layout features that are off-by-default but 
dependent on layout rather than direct activation by the user?

J.


-- 

John Hudson
Tiro Typeworks Ltdwww.tiro.com

Tiro Typeworks is physically located on islands
in the Salish Sea, on the traditional territory
of the Snuneymuxw and Penelakut First Nations.

__________

EMAIL HOUR
In the interests of productivity, I am only dealing
with email towards the end of the day, typically
between 4PM and 5PM. If you need to contact me more
urgently, please use other means.