CorneaDev Mono

A monospaced font built as a tool: a utility typeface for software development, optimized for readability at 10–16pt — particularly for developers whose close-up vision isn't what it used to be. Grotesque (Helvetica-flavored) skeleton with deliberate serif cues only where they disambiguate.

Why it looks the way it does

CorneaDev Mono is designed for one reader: a developer with presbyopia — the age-related loss of near focus that sets in around 40. The screen image isn't cropped, it's blurred, so the whole design problem is keeping letters distinguishable when the picture is slightly out of focus. Every unusual-looking choice follows from that:

• Open apertures (the "funny" a, e, c) — closed openings fill in and smear into neighbours under blur and crowding; open ones hold their identity. This is the single best-supported decision in the font.
• Disambiguated confusables (0/O, 1/l/I/|, 8/B, rn/m, …) — the look-alikes that cause real bugs, shaped per-glyph rather than uniformly.
• Selective serifs on the troublemakers only (I, foot of l, i/j) — serifs help where stroke contrast is uniform (as in a monospace) and at the vertical extremes of letters; they go nowhere else.
• Tall x-height + generous, even spacing — the lever that actually buys blur tolerance at a given point size; monospace's roomy spacing is a bonus for degraded reading.
• Mid-range weight; italic as the weakest channel — heavy strokes close counters and work against the open-aperture strategy, and slant is the hardest style for these eyes, so the italic stays gentle and leans on weight/colour.

Two companion documents in docs/ go deeper:

• Legibility Assessment & Design Rationale — a research-grounded evaluation mapping each decision to its evidence basis and a confidence rating, with honest caveats and references.
• Plain-Language Guide — the same reasoning without the type-design vocabulary.

Design decisions

Decision	Choice
Advance	600/1000 em, strictly monospaced (all glyphs, ligatures = exact cell multiples)
x-height	540 (tall) — legibility from large lowercase bodies, not from width
Zero	Dotted (default; --zero slashed available)
1 / l / I / |	1 = flag + foot serif; l = curved tail; I = top/bottom serifs; | = full-height bar
i / j	Top-left flag serif + foot, distinct from l
Brackets/braces	Exaggerated: extra tall, long flanges, prominent brace beak
Backtick vs quote	Backtick heavy and clearly slanted; quotes vertical
Ligatures	Core set via liga+calt: -> <- => == === != !== >= <= && || :: ...
Weights	Regular (400) + Bold (700), same advance, generated from shared parametric skeletons
Italic	True italic (~10° slope) with single-story a and cursive descending f; box-drawing, blocks and Powerline stay upright so cells still connect
Format	TTF (UPM 1000, quadratic outlines via cu2qu)

Deferred: small caps.

Coverage

• Full ASCII (U+0020–007E)
• Latin-1 Supplement (U+00A0–00FF): composed diacritics, Ð ð Þ þ ß Æ æ Ø ø, currency, ordinals, superscripts, fractions
• Greek (U+0391–03C9; identical capitals alias Latin glyphs)
• Common math: − ≈ ≠ ≤ ≥ ∞ √ (plus ± × ÷ from Latin-1)
• General punctuation: – — ‘ ’ “ ” • … and arrows ← ↑ → ↓
• Box Drawing (U+2500–257F) and Block Elements (U+2580–259F), spanning the full line box so cells connect in terminals
• Powerline separators/symbols (U+E0A0–E0A2, E0B0–E0B3)

Installing

Grab the .ttf files from dist/ (or a tagged release), then:

• macOS — double-click each .ttf and click Install Font, or copy them to ~/Library/Fonts.
• Windows — right-click each .ttf and choose Install, or copy them to C:\Windows\Fonts.
• Linux — copy them to ~/.local/share/fonts and run fc-cache -f.

Then set your editor/terminal font to CorneaDev Mono (use the Bold weight for bold cues and Italic for italics — IDEs pick these up automatically). Don't have it yet? Build from source below.

Building

python3 -m venv .venv
.venv/bin/pip install -r requirements.txt      # fonttools, pillow, ttfautohint-py

.venv/bin/python build_font.py                 # builds dist/*.ttf + specimen.png
.venv/bin/python build_font.py --help          # all options
.venv/bin/python build_font.py --zero slashed --seven crossbar
.venv/bin/python build_font.py --family "My Mono" --weights regular
.venv/bin/python build_font.py --no-ligatures

Every build regenerates dist/specimen.png (shown above): a full specimen sheet — character set, disambiguation and ligature showcase, a syntax-highlighted code sample (keywords in Bold, comments in Italic), a weight comparison, and a 10–16pt size waterfall. Judge legibility in that waterfall, at target sizes, not in the large display rows.

To redesign or re-render the specimen poster without rebuilding the fonts:

.venv/bin/python -m cornea.specimen            # re-renders dist/specimen.png

For close inspection of individual glyphs:

.venv/bin/python debug_render.py --size 72                       # full sheet
.venv/bin/python debug_render.py --text "O0o 1lI|" --size 96     # custom row
.venv/bin/python debug_render.py --font dist/CorneaDevMono-Bold.ttf

Code layout

• build_font.py — CLI entry point
• cornea/params.py — per-weight design parameters (stems, x-height, etc.)
• cornea/geometry.py — path primitives (stems, bars, rings, arc bands); overlapping positive contours merge under nonzero winding, no booleans
• cornea/glyphs.py — one builder per glyph, registered with its codepoint; new coverage = new builders registering into BUILDERS
• cornea/ligatures.py — ligature glyphs + generated FEA feature code
• cornea/builder.py — fontTools assembly (tables, metrics, monospace flags)
• cornea/specimen.py — specimen sheet rendering (poster + size waterfall; runnable standalone via python -m cornea.specimen)

License

CorneaDev Mono is licensed under the SIL Open Font License 1.1 — free to use, study, modify, and redistribute (including bundled with software); it just can't be sold on its own and must stay under the OFL. CorneaDev Mono is a Reserved Font Name: if you distribute a modified version, give it a different name.