I have an existing font (.ttf or as dumped .ttx) that I want to extend with additional GSUB rules. I wanted to work with an AFDKO feature file, so I created a UFO, and filled a features.fea with my desired substitutions. The UFO has dummy placeholder glyphs as the existing font glyphs should be used. The UFO compiles nicely and the GSUB rules are working.
I am now looking for a way to properly merge this with the existing font, which already has a GSUB table.
What I tried:
- Dumping the new GSUB table and use ttx to merge it in the existing font (doesn't work, existing GSUB gets overwritten instead of merged)
- Manually transplanting the new GSUB table, updating indexes etc. with a simple script. Works, but feels hacky.
What I think might work but feels too nasty to actually do:
- Dump the existing .ttf to UFO, add new GSUB rules, compile UFO back to .ttf.
Any advice on a cleaner way?