Glyph Alternates in OpenType (-ish?) fonts

[kellyo-cricut]

I’m trying to query for alternates in Zapfino, a macOS system font, using harfbuzz 7.2.0. Looking at the font data (using fontdrop.io) doesn’t detect any OpenType features, but it does show a GSUB table that has a few OT-looking features for glyph alternates like aalt, calt, etc. When I query for font features using hb_ot_layout_table_get_feature_tags does not find any features. Calling hb_ot_layout_collect_lookups not specifying any scripts, languages, or features does not return any indices. Calling hb_ot_layout_lookup_get_glyph_alternates with indices found in the GSUB table does not return any alternates for a glyph id that has mappings inside the GSUB table.

Most of the text/font world is quite new to me, so apologies if I’m missing something obvious here. Is this a case of some oddly formed font data? I’m able to shape using alternate glyphs by id using the solution from this discussion, but I’m at a loss as to how to retrieve the IDs of glyphs that are mapped in one of the GSUB features. TextEdit offers alternates for those glyphs, so I’m hoping there is a way to query for those with harfbuzz. Thanks!

[behdad]

The version of Zapfino I have access to doesn't have GSUB at all. So I can't help. Maybe show us a ttx dump of the GSUB table?

[kellyo-cricut]

Ah- I'm not seeing a GSUB table using that tool:

I'm unsure of what fontdrop.io is doing, but the same file is suggesting that there is a GSUB table with mappings:

Im assuming ttx is correct here. Knowing that, is there another way to query for alternate glyphs? I'm trying to get the same alternates that TextEdit does here:

[behdad]

I'm unsure of what fontdrop.io is doing, but the same file is suggesting that there is a GSUB table with mappings:

I don't get any OT features in fontdrop for the font I have.

Im assuming ttx is correct here. Knowing that, is there another way to query for alternate glyphs? I'm trying to get the same alternates that TextEdit does here:

I'm afraid we don't implement that API for AAT fonts. I'm not even sure which table stores those. Probably the Zapf table? @ebraminio

[behdad]

Where does it show GSUB?

[behdad]

Ah, I see. In Data.

[kellyo-cricut]

Looking at some of the dumps from ttx, it looks like that mapping data is in the morx table. Is that still part of AAT that harfbuzz doesn't search?

[behdad]

HarfBuzz applies morx but doesn't query it no. In general that's a very tricky problem since morx contains state-machines, not regular tables.

[khaledhosny]

Looks like Zapf table would have such data indeed, see https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6Zapf.html (Structures For Groups of Glyphs section)