Consider transition to harfbuzz subsetter for static instantiation

[Chris Simpkins]

The harfbuzz subsetter is a compiled C++ lib that supports variable to static font instantiation. This appears to be exposed through the uharfbuzz Python bindings and it should be significantly faster.

https://github.com/harfbuzz/uharfbuzz/blob/da2e6d5a2c4607276378c84f0ba1fd3bf362beed/src/uharfbuzz/_harfbuzz.pyx#L387

so, I think that we could do something along the lines of the following:

import uharfbuzz

with open(fontpath, "rb") as fontbin:
        hb_face = uharfbuzz.Face(fontbin.read())

hb_font = uharfbuzz.Font(hb_face)

variation_axis_defs = {
    "opsz": "10",
    "wght": "700",
    "GRAD": "-25",
}

hb_font.set_variations(variation_axis_defs)

# TODO
# how do you write binary back to disk? 

It isn't clear to me based on a quick read through of the hb source what level of feature support it has relative to the pure Python varLib.mutator and varLib.instancer libs.

Questions

• What are the differences (if any) relative to fontTools.varLib.mutator? fonttools/fonttools#2248 (reply in thread)

[Chris Simpkins]

Per Cosimo, instancing is not yet supported fonttools/fonttools#2248 (reply in thread)

But it is planned. Something to keep an eye on. Will leave this open.

[Chris Simpkins]

D/W Behdad and his feedback was that it is best to continue to base this application on fontTools for now. The support in harfbuzz is not mature at the moment and the fonttools implementation is likely to remain the default in the near - mid term. Will leave this open and follow.