[mpeg-OTspec] Vertical ligatures

[John Daggett]

Eric Muller wrote:

> I think that what John is looking for is an explicit and reliable
> indication of the relative positions of glyphs. Peter Constable
> mentioned to me this possibility: currently the resolution of
> feature tags to actual lookups takes into account an OT script and
> an OT language; it could also include another parameter, the
> relative position. Thus, ('liga', script, language, horizontal_lr)
> could resolve to lookups that forms the usual "fi" ligature for the
> run <f, i>, while ('liga', script, language, horizontal_rl) could
> resolve to lookups that form the "fi" ligature for the run <i, f>,
> while ('liga', script, language, vertical-lr) could resolve to
> lookups that forms the Kazuraki vertical ligatures.
> 
> There are multiple ways to incorporate this in OT: one way is to add
> one stage to the current mapping, another is to have feature tags
> resolve to lookups just as they do today, but to have flags on
> lookups that say under which conditions they should be applied. In
> fact, I have the intuition that the rtl flag was intended precisely
> for that use, and was later "deprecated" to its current noop status
> when everybody forgot the problem it was intended to solve.

This makes a lot of sense to me.  Adding a stage seems like the right
approach but I don't see how you could restructure the tables in a
backward-compatible way.  As for the flags approach, is there a way to
define flags that would be ignored in older OpenType layout engines?

Regards,

John Daggett
Mozilla Japan