Short version: Is it possible for Glyphs to open an otf made in another program, maintain all the glyphs and alternates already in the file, add some code to effect substitutions, and then save it without it effecting any special kerning that was already in the otf?
Long version: I made a typeface of my handwriting using Fontself. Each glyph has 5 alternate versions to allow for variation to look more natural. I used Fontself because I liked its smart spacing and kerning (with so many alternate glyphs I didn’t want to hand kern all the variations). However, Fontself doesn’t have any coding ability.
Based on this post Features, part 3: advanced contextual alternates | Glyphs it looks like Glyphs can do the coding side to cycle through alternate glyphs, but I’m unclear if it can import a pre-existing OTF, add some code but not alter kerning information.
I’ve tried doing this in fontforge, but it seems like when I open the font in fontforge it loses all my alternate glyphs.
Also for context I’m brand new to all things making fonts, and know nothing about coding outside of the previous link and reading this https://opentypecookbook.com/
Opening an OTF will never be 100% lossless, but sometimes it is good enough. Kerning and alternative glyphs are kept when opening a font in Glyphs. You can try yourself, there is a free trial for the first 30 days of using Glyphs that has all features.
If you need help with any specific feature code questions, feel free to ask!
Also make sure to save your imported font as a .glyphs file. So that from that point on you have a proper source file with all your new edits.
Otherwise, each time you open your OTF, Glyphs will uncompile that with whatever the GlyphsApp’s current uncompile capabilities are.
And just so I know what to look out for, are there certain things that are more likely to not come through unchanged when uncompiling an otf?
Things that are not (or cannot be) recovered from an OTF are names internal to the font source (master names, kerning group names, lookup names, …) and feature code will be imported quite differently to how it would look in a source file (different organization of classes, lookups, features).
I think for a font create with Fontself, most of the above should not apply, so it should be fine to open the OTF. If anything is missing, you can send us the OTF file and we’ll see if/how it can be recovered.
Also, any organization that would happen in a source file (color labels, tags, custom parameters, export settings, …) are not part of the exported OTF, so once you opened an OTF in Glyphs, it’s best to save the file as a .glyphs source file, as Mark described.