there is a well-known piece of free software for this called "metafont" (also the original name for the concept here being neologized "variable font"—a "metafont" is an infinite set of fonts produced by an algorithm from input variables) but it doesn't have a gui and can't import woff or truetype
a variable font is a more generic term than a metafont. it’s a bit pedantic but while both have parameters, a metafont is usually parametrically generated based on its concept of a pen, whereas a variable font usually has different parameters that have been defined by manual bezier drawings. I don’t remember if metafont itself can produce a variable font but people have used metapost to go to SVG and then UFO (font source) from there
in metafont (the software) you can define a metafont (a font-valued function) in terms of bézier outlines as well as paths stroked with pens; the relevant metafont commands are 'fill' in the first case and 'draw' (rather than postscript's 'stroke') in the second. it doesn't have an equivalent of postscript's 'eofill'. chapter 13 of the metafontbook describes these commands
i don't know if there's a reasonable way to export any standard vector format from metafont; i think there originally wasn't
I think you may be missing the point. Metafonts have absolutely nothing to do with "variable fonts". It's a completely different branch of font technology. You cannot produce a variable font as OP intends using metafont.
metafonts, as implemented by the metafont program, and variable fonts are literally exactly the same thing: a parameterized font which produces an infinite set of fonts as you vary some parameters
i think what you mean to say is that the metafont program isn't compatible with truetype or opentype, but i kind of already explained that in the comment you are replying to
that page opens by saying it's a generic term, not the specific implementation of the concept in opentype:
> A variable font (VF) is a font file that is able to store a continuous range of design variants.
this definition is the same thing as the definition of a metafont, and clearly includes the metafonts implemented by the program named metafont. it seems like the authors of the page didn't know about the program named metafont, though, so they mistakenly think apple invented it
It does not claim it's a generic term. The whole page is about variable fonts as derived from "Apple's TrueType GX font variations"; one sentence taken out of context doesn't change that. If you search the internet for variable fonts, you only find people talking about this technology. Parameterized fonts as in metafont have never been called variable fonts and trying to claim otherwise is just creating unnecessary confusion.
