Support public.skipExportGlyphs lib key for storing export flags

[madig]

#515.

Handling this well in a oh-please-not-more-technical-debt kind of way is tricky.

There are three ways to signal that a glyph should not be exported:

1. The old com.schriftgestaltung.Glyphs.Export glyph lib key
2. The new UFO-level public.skipExportGlyphs lib key
3. The new Designspace-level public.skipExportGlyphs lib key

ufo2ft will look into the Designspace lib key when using the compile*FromDS functions and into the UFO-level lib keys with the other compile* functions. Writing only to the Designspace-level lib key may complicate e.g. fontmake, which uses glyphsLib.to_designspace to convert a .glyphs file into DS + UFOs, but uses compile* or compile*FromDS depending on what the user specifies on the CLI.

Proposed behavior:

• ufo2glyphs (to_glyphs):
  • When reading a Designspace file, honor only the old glyph lib key and the Designspace-level lib key
  • When reading loose UFOs, construct a Designspace and read the UFO-level public.skipExportGlyphs lib key, but error out when the lists don't match or some have one and others don't
• glyphs2ufo (to_ufos, to_designspace)
  • Write UFO and Designspace-level public.skipExportGlyphs lib keys.

[anthrotype]

sgtm

[madig]

Must check interaction with the "Keep Glyphs", "Export Glyphs" and "Remove Glyphs" (and others?) parameters...

[madig]

We only support "Keep Glyphs" currently and it turns out it causes feature subsetting on export from Glyphs.app. public.skipExportGlyphs does not, currently, and as such can't be used to adapt "Keep Glyphs".

[anthrotype]

I also had to revert this, because fontmake --subset option relies on the individual glyph Export flags and works slightly differently than then new skipExportGlyphs feature as implemented in ufo2ft. Specifically, the latter decomposes component glyphs that are marked as non-export, whereas fontmake (which uses fonttools subsetter to subset after compilation, not on the source files) keeps the components that are marked as non-export without decomposing the composite glyphs that reference them.
We need to keep supporting the old behavior and make the new one opt-in behind some flag.

[madig]

new one opt-in behind some flag

In fontmake or glyphsLib?

[aaronbell]

Hi. Did this ever get implemented in another way? Or did it sort of fall by the wayside?

[aaronbell]

(I should mention that I am able to work around it by manually updating the designspace file as part of the export process, but it'd be nice not to have to do that :) )

[madig]

glyphs2ufo --write-public-skip-export-glyphs ...

[aaronbell]

Aha! Wish I'd spotted that sooner 😅