Hi, I'm making variable fonts with FOSSs(free open source softwares).
i use fontforge(both GUI and python api to design and prepare masters), notepad++, fonttools(name records modification, format conversion, ttx,...), fontmake(generate variable fonts and instances) and python scripts(regex, automation,...).
there is 2 problems about interpolation of masters, which are related to varlib(a part of fontmake) limitations:
1- anisotropic interpolation(multiply a number to x and y delta values) it's usefull to contrast regulations, especially in sans serif fonts.
2- Extrapolation(going out of designspace borders mathematically): fontforge has this ability but it's not working for opentype GPOS(mark anchors coordinates, kerning values,...).
apparently MutatorMath with ufoprocessor could handle these problems but i couldn't find how...
Could anyone please help me to find a solution or working examples?(i know Glyphs, fontlab 7, robofont(?), FontCreator(?),... have these abilities, I'm searching for opensource solutions).
Extrapolation is not possible in OpenType, but if you use FEZ to author your GPOS data (cutting edge stuff) you can interpolate it.
However, whatever software you use, if you are dedicated to using libre software, you'll need to start contributing to the software's development, because no libre applications yet exist.
[FontLab 7 does this really well. It is possible in the Font Info / Axes panel to define space outside the limits of the axis extreme masters and extrapolate into that space. All the same data that is affected by interpolation within the design space is extrapolated outside it.]