@anthrotype What do you recommend for InstructionCompiler in the end? Mixin, functions, merge into OutlineTTFCompiler?

Thank you @anthrotype! I will check take a look as soon as possible, hopefully this afternoon.

Yes, it does support variable fonts. In my usage scenario, all master UFOs contain identical fpgm, prep, and glyf programs. The cvts are usually different in each master, of course. fontTools.varLib will build a cvar from the different cvts, and copy the fpgm, prep and glyf programs when that's the case.

It should also be enough if the default master contains the fpgm, prep, and glyf programs, but I need to check again.

I see, thanks for confirming. If we don't have one already, we should add a test that compiles a VF with hinting and confirms that varLib generates the cvar deltas

It should also be enough if the default master contains the fpgm, prep, and glyf programs, but I need to check again.

Confirmed.

I've run the latest version on a couple of fonts, and the resulting fonts are identical. Haven't looked at the code yet.

I see, thanks for confirming. If we don't have one already, we should add a test that compiles a VF with hinting and confirms that varLib generates the cvar deltas

It's tested in fontTools.varLib when building from TTF masters (fonttools/fonttools#1069). Or do you mean we should a test here for building from UFO?

@anthrotype I've had a look at your changes. LGTM.

yeah we could/should add a test here as well as a sort of integration test for variable+hinted builds

I've added TrueType data to the integration test. I stumbled over fonttools/fonttools#3036 while preparing the expected ttx data, but I fixed it for now by converting the input to fromAssembly() to a list.