What does it mean to apply a feature?

[simoncozens]

Describe:

• How the run is processed (first applied lookup ends processing at each glyph position)
• Matching of pre- and post- context
• The effect of lookup flags

etc.

[mikeday]

It would be great to have a spec for this, but note that there are some unresolved issues about how nested contextual substitutions are actually supposed to work:

https://github.com/yeslogic/allsorts/issues/25

[n8willis]

Describe:

* How the run is processed (first applied lookup ends processing at each glyph position)

* Matching of pre- and post- context

* The effect of lookup flags

etc.

My knee-jerk reaction is that, while this is stuff that needs to be specified, it is also stuff that is more fundamental to implementing OpenType, at the basic, "non-shaping" (not OTL) level. That is, it would also apply to all the shaping-irrelevant features, up to and including cv and ss. So it seems like this is not exactly the place it should live.

@simoncozens Do you disagree?

[simoncozens]

My understanding of shaping is that it is precisely the process of applying OpenType lookups to a run of glyphs! So processing cvXX and ssXX features is also shaping. (Another way to say this is that shaping is what a shaping engine does.)

Lookup flags, pre- and post-context etc. only make sense in the context of OTL lookups. Where else would you put a description of them?

[n8willis]

Well, words can have nuances of meaning in addition to totally separate definitions. The way we're using it in this repo, "shaping" refers to script-support features and algorithms, whereas we're intentionally not getting into user-preference-level features. A la Adam's classification list. So there's occasional references to non-OTL features, but we've only barely grazed them — such as locl and ccmp — and that's where the HarfBuzz hb-ot-shape-complex-* code incorporates them. Not going anywhere near aalt, dtls, frac, rand, or a host of other registered features.

I guess I think that the details of how the context and flags work feels more like it should live in or by the specification of GSUB / GPOS and lookups themselves.

There's certainly a line to draw somewhere, right? We definitely do not want to duplicate info.

[simoncozens]

That's fine. If it's out of scope for what you're doing, I've cover it in CommonType. It just needs to be written down somewhere, and if someone else was planning to do it I wouldn't have to. ;-)