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fontforge

Design & creation

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Updated 10-26 12:22

About this skill

FontForge Control

FontForge is driven headless through its embedded Python (fontforge -lang=py -script): each task is one short-lived process that opens a font, changes it, and writes new files. Nothing opens on screen and nothing keeps running. Verified against FontForge 20251009 on macOS (the binary lives inside FontForge.app/Contents/Resources/opt/local/bin/, not on PATH).

There is no live bridge: FontForge has no remote-control interface into a running GUI. Work on files. If the user wants to look at the result, open -a FontForge out.sfd.

All helpers run through one entry point, python3 scripts/ff.py, which finds the binary, filters FontForge's banner noise, applies a timeout and turns a Python traceback into a non-zero exit:

  • scripts/ff.py — check (version, Python, real WOFF2 test), run SCRIPT.py ARGS… for your own FontForge scripts, specimen FONT -o x.png --text … to render a PNG, and the four helpers below.
  • ff.py iconfont DIR --family NAME [--map map.json|csv] -o x.sfd -o x.otf -o x.woff2 [--css x.css] — ff_iconfont.py: SVG folder → icon font. Unmapped icons get PUA code points from U+E000. Every glyph is cleaned and given one advance; --fit center --padding N normalises icons drawn on inconsistent canvases.
  • ff.py subset FONT -o x.woff2 --text "…" | --text-file page.txt | --unicodes U+0020-007E — ff_subset.py: keeps exactly those characters (+ .notdef, space), unlinks accented composites so nothing renders blank, keeps names and kerning between kept glyphs.
  • ff.py validate FONT [--fix -o fixed.sfd -o fixed.otf] [--close-open] — ff_validate.py: per-glyph problems decoded from the validation bitmask, plus non-integer points; --fix repairs and re-validates.
  • ff.py inspect FONT [--cmap] [--names] [--glyph H] [--pairs AV,To] — ff_inspect.py: JSON read-back of names, metrics, glyph and kerning counts, features, validation failures, and the kerning value of given pairs.
  • references/python-api.md — the API calls, generate flags, lookups/kerning, selection, and the full validation bitmask table.
  • references/recipes.md — metrics + kerning + names on an existing font, SVG letters → glyphs, batch conversion, UFO round trip, composites, feature files.
  • references/gotchas.md — the traps found on this version (SVG import bugs, glyph names overriding code points, stale name-table entries, additive kerning lookups) and the security posture. Read it before writing your own script.

Workflow

  1. Check the install: python3 scripts/ff.py check. Exit 127 means FontForge is missing — suggest the app from fontforge.org or brew install fontforge (the Homebrew cask is disabled).
  2. Pick the helper or write a script. The four helpers cover icon fonts, subsetting, validation and read-back. For anything else — metrics, kerning, renaming, conversion, glyph surgery — write a small FontForge Python script (start from recipes.md) and run it with ff.py run.
  3. Keep a source. Save SFD (or UFO) next to the binaries; generate OTF/TTF/WOFF2 from it. A binary font is an output, not something to keep editing.
  4. Never overwrite the input font. Write to a new path, and tell the user which file is which.
  5. Verify before you report done:
    • ff.py inspect OUT --cmap --names — glyph count, code points, family/version.
    • ff.py inspect OUT --pairs AV,To --glyph H — kerning and metrics you changed.
    • ff.py validate OUT — exit 0 means no problems left.
    • ff.py specimen OUT -o specimen.png --text "…" and look at the PNG: blank boxes, shifted or giant icons, and missing accents show up there and nowhere else.

Diagnosing blank or missing glyphs

When a character renders as an empty box or loses its accent, check in this order with ff.py inspect FONT --cmap --glyph á --glyph a --glyph acute:

  1. Not encoded — the character is missing from cmap (glyph exists under another code point, or an AGL name pulled it elsewhere).
  2. Broken composite — references names a glyph that is not in the font (after a subset or a merge); unlink before removing bases, or add the base back.
  3. Empty outline — contours: 0 with a non-zero width; an import or a subset left the glyph empty.
  4. Outline problems — ff.py validate FONT: open contours and wrong direction can render as holes or nothing at all in some rasterizers. Then render ff.py specimen with the failing text to confirm the fix.

Rules that save a round trip

  • Imported SVG glyphs have width 0 — set glyph.width (the iconfont helper does).
  • A glyph name from the Adobe Glyph List beats the code point (plus lands on U+002B, not your PUA slot). Check names with fontforge.unicodeFromName(name).
  • Changing names on an existing OTF/TTF: set the properties and the sfnt_names entries (Version, UniqueID, Family, Fullname, PostScriptName), plus font.sfntRevision for head.fontRevision, or the old version and ID ship. Respect the OFL Reserved Font Name when renaming.
  • Changing kerning in a font that already kerns: put an override subtable first in the existing kern lookup. A new lookup adds to the old values instead of replacing them.
  • Validation fixes run in order removeOverlap → correctDirection → addExtrema → round, and never removeOverlap on an open contour (it mangles the glyph). Report open contours rather than silently closing them unless the user agrees.
  • Subsetting: features whose glyphs were removed are dropped (FontForge warns per glyph; the helper counts the warnings). Kerning among the kept glyphs survives. If the user needs every layout feature intact on a web subset, mention pyftsubset.

Reporting

Tell the user what was produced (paths, formats, sizes), what changed (glyph count, code points, names, pairs), and what the verification showed — the inspect numbers, remaining validation problems (or none), and the specimen. Say plainly what FontForge dropped (ignored tables, invalidated lookups) and anything left for manual work, such as open contours.