Static To Variable

static-to-variable

Turn a set of static font weights (separate OTF/TTF files) into one variable font with a weight axis, using the static-to-variable CLI. It rebuilds the glyphs so they interpolate, builds the font with fontmake, and writes the OpenType metadata.

• IS: authoring an stv.config.json, running the CLI to produce a variable font from static files, and previewing it.
• IS NOT: drawing or hinting glyphs, editing an existing variable font, or hand-writing fontTools/fontmake scripts (this CLI wraps that).

Install

npm install -g static-to-variable

Needs Node >= 24.11, Python >= 3.11, and uv. Outside a repo checkout the CLI provisions its own Python engine on first build (needs uv on PATH). Run static-to-variable doctor to confirm the environment.

Workflow

- [ ] 1. Read each static file's weight (OS/2 usWeightClass)
- [ ] 2. Write stv.config.json (axis + one donor + one master per weight)
- [ ] 3. static-to-variable build --config stv.config.json
- [ ] 4. Verify the output font (fvar axis, named instances, interpolate a mid value)
- [ ] 5. Preview over HTTP with a wght slider

1. Read the weights, don't guess them

The axis position for each file comes from its OS/2.usWeightClass. Read it; static families often use non-standard ranges (e.g. Operator Mono is 275-400, not 100-900):

uv run --with fonttools python -c "import sys; from fontTools.ttLib import TTFont; [print(f, TTFont(f)['OS/2'].usWeightClass) for f in sys.argv[1:]]" *.otf

2. Author stv.config.json

static-to-variable init scaffolds a starter. In an interactive terminal it scans the folder for .ttf/.otf files, reads their real weights and names, and writes a config that builds as-is; in a non-TTY shell (the usual agent case) it writes a static template to edit. Every path is relative to the config file. One donor and one master per weight; the master's location.wght must match that file's weight, and its donorId must reference a donor id. Exactly one master needs "default": true. It is strict JSON: no comments or trailing commas. source need not exist yet (it is bootstrapped from the default-master donor).

{
  "version": 3,
  "id": "myfamily",
  "family": {
    "name": "My Family VF",
    "version": "1.000",
    "vendor": "MYCO",
    "designer": "Name",
    "designerUrl": "https://example.com",
    "vendorUrl": "https://example.com"
  },
  "axes": [
    {
      "tag": "wght",
      "name": "Weight",
      "minimum": 100,
      "default": 400,
      "maximum": 900,
      "namedInstances": { "100": "Thin", "400": "Regular", "900": "Black" }
    }
  ],
  "styles": {
    "roman": {
      "italic": false,
      "source": "build/myfamily.glyphs",
      "output": "build/myfamily-vf.ttf",
      "donors": [
        {
          "id": "thin",
          "name": "Thin",
          "path": "donors/MyFamily-Thin.otf",
          "location": { "wght": 100 }
        },
        {
          "id": "regular",
          "name": "Regular",
          "path": "donors/MyFamily-Regular.otf",
          "location": { "wght": 400 }
        },
        {
          "id": "black",
          "name": "Black",
          "path": "donors/MyFamily-Black.otf",
          "location": { "wght": 900 }
        }
      ],
      "masters": [
        { "name": "Thin", "donorId": "thin", "location": { "wght": 100 } },
        {
          "name": "Regular",
          "donorId": "regular",
          "location": { "wght": 400 },
          "default": true
        },
        { "name": "Black", "donorId": "black", "location": { "wght": 900 } }
      ]
    }
  },
  "output": {
    "dir": "build",
    "releaseDir": "build/release",
    "formats": ["ttf", "woff2"]
  }
}

Use 2-5 masters. More masters follow the design more closely but demand more glyphs interpolate cleanly. A minimal axis is just the two extremes plus a default.

3. Build

static-to-variable build --config stv.config.json    # rebuild, normalize, build
static-to-variable release --config stv.config.json  # plus finalized metadata + WOFF2

build bootstraps a .glyphs source from the default-master donor when source doesn't exist, rebuilds every master from the donors so they share one structure, then builds the font. Read the run output: reconstructed glyphs were made interpolatable; ai-pending / frozen glyphs could not reconcile (a genuine topology difference across weights) and are held to one master, so they render correctly but don't vary. A handful is normal for independently-drawn statics.

4. Verify

uv run --with fonttools python -c "
from fontTools.ttLib import TTFont; from fontTools.varLib.instancer import instantiateVariableFont; import copy
f=TTFont('build/myfamily-vf.ttf'); print('axes', [(a.axisTag,a.minValue,a.defaultValue,a.maxValue) for a in f['fvar'].axes])
instantiateVariableFont(copy.deepcopy(f), {'wght': (f['fvar'].axes[0].minValue+f['fvar'].axes[0].maxValue)/2}); print('interpolates OK')"

5. Preview

file:// blocks @font-face, so serve over HTTP. Minimal previewer with a live weight slider:

<style>
  @font-face {
    font-family: VF;
    src: url("build/myfamily-vf.ttf");
  }
  body {
    font-family: VF;
    font-variation-settings: "wght" var(--w, 400);
  }
</style>
<input
  type="range"
  min="100"
  max="900"
  value="400"
 
/>
<h1>The quick brown fox 0123456789</h1>

python3 -m http.server 8137   # then open http://localhost:8137/preview.html

To show every glyph, read the font's cmap and render one cell per codepoint, each using the variable font so the slider reweights them all at once.

Commands

build, release, split, and doctor take --json for a machine-readable summary on stdout (progress goes to stderr). Exit codes are 0 ok, 1 failure, 2 usage (bad flag, missing or invalid config), 3 environment. Configs are validated against the JSON schema before any work starts.

Gotchas

• Axis positions are the files' real usWeightClass. Setting location.wght to round numbers that don't match the files produces a wrong or collapsed axis.
• Exactly one master must be "default": true. The bootstrapped source and the fallback for frozen glyphs both come from it.
• The source file does not need to exist for a brand-new family; it is synthesized from the default-master donor. Point source/output inside dir.
• Standalone mode provisions a uv venv on first build (a one-time delay) and caches it; doctor shows mode: standalone vs checkout. It needs uv present.
• Independently-drawn statics won't all interpolate. Glyphs with different contour counts or point structure across weights get frozen; that is the tool working, not an error. Check the underweight list in the build output.
• Known quirk (current versions): the default named instance (e.g. Regular) can ship with a blank name record; the instance still exists in fvar.