Pretext

Pretext Text Effects

Generate browser pages powered by Pretext (@chenglou/pretext)—a pure-arithmetic text measurement library that bypasses DOM layout reflow entirely. Pretext measures text with proportional font precision using canvas measureText, enabling effects impossible with CSS alone.

Companion library: opentype.js ([email protected]) — direct font binary parsing for per-glyph SVG path rendering, Bezier control point access, kerning tables, and variable font axes. Pretext handles line breaking; opentype.js handles what happens inside each glyph. Together they unlock effects neither can achieve alone.

Output is always a single self-contained HTML file. No build step, no framework, runs in any modern browser.

Quick Start

Describe a text effect. Claude picks the right Pretext pattern and generates a complete HTML file.

Examples:

• /pretext fluid smoke ASCII art with gold characters on black
• /pretext chat bubbles that shrinkwrap tighter than CSS
• /pretext calligram — the word "ocean" shaped like a wave
• /pretext editorial layout with text flowing around draggable circles
• /pretext masonry grid of shower thoughts with instant height prediction
• /pretext glyph path art — SVG letterforms with stroke animation
• /pretext animated dragon cursor that pushes text aside as it moves
• /pretext illuminated manuscript with live vine growth reflow
• /pretext variable font wave — per-character weight animation
• /pretext glyph morph — letterform interpolation from A to Z
• /pretext letterbox gallery of "BEACON" in Playfair Display italic on dark background
• /pretext glyph-mask calligram — letter R filled with lorem ipsum in Georgia

Do NOT Use This Skill When

• Simple CSS text-shadow, text-stroke, or gradient text effects
• CSS Shapes level 1 (shape-outside on floated elements) for static layouts
• Basic @keyframes text animation (fade, slide, typewriter)
• Monospace ASCII art (no proportional measurement needed)
• SVG <text> without per-glyph control
• Rich text editing (contenteditable, ProseMirror, TipTap)
• PDF generation or markdown rendering

These are all achievable without Pretext or opentype.js.

Effect Categories

Pretext-Only Effects

Category	Pretext APIs Used	When to Use
Height Prediction	prepare + layout	Accordion, masonry, virtualized lists — anywhere you need text height without DOM reads
Shrinkwrap	walkLineRanges + binary search	Chat bubbles, tooltips, labels — finding the exact tightest width for multiline text
Obstacle Routing	layoutNextLine (variable width)	Text flowing around images, logos, draggable orbs — editorial layouts
Animated Obstacles	layoutNextLine + carveTextLineSlots	Moving creatures/orbs that displace text at 60fps — slot-carving fills BOTH sides of obstacle
Typographic ASCII	prepareWithSegments (char measurement)	Fluid simulations, 3D wireframes, particle systems rendered as proportional characters
Calligrams	prepareWithSegments + SDF	Words rendered as shapes using their own letters — hearts, stars, spirals
Glyph-Mask Calligrams	Canvas measureText + getImageData pixel mask	Any font glyph as calligram shape — fill a large letter with small text using pixel-mask technique (no SDF needed, no opentype.js)
Letterbox Gallery	Glyph-mask + per-letter <canvas> grid	Each character in a string gets its own canvas with text fill, cursor displacement, and independent interaction
Multi-column Editorial	All rich APIs combined	Magazine-style layouts with headline fitting, pull quotes, drop caps, column flow

Pretext + opentype.js Effects

Category	APIs Used	When to Use
Glyph Path Art	opentype glyph.getPath()	SVG letterforms with fill/stroke/control point modes, stroke-draw animation
Text on Path	opentype getPath() + arc-length sampling	Per-glyph placement along Bezier curves with tangent rotation
Variable Font Animation	opentype font.tables.fvar.axes + CSS font-variation-settings	Per-character weight/width waves, breathe, ripple, cascade effects
Glyph Morphing	opentype paths + flubber interpolate()	Letterform interpolation between glyphs with contour-aware morphing
Outline Calligrams	Pretext layoutNextLine + opentype glyph mask	Text fills the interior of a large glyph's actual contour (not SDF approximation)
Illuminated Manuscript	All Pretext + opentype combined	Living medieval pages: wet ink, vine reflow, capital inflation, aging, erasure poetry

Concept-to-Effect

Every design decision derives from the concept. Do not default—derive.

Choose the API tier from the effect complexity:

• Static height only → prepare() + layout() (fastest, opaque)
• Need line text/positions → prepareWithSegments() + layoutWithLines()
• Need per-line width variation → layoutNextLine() (iterator, variable width per line)
• Need aggregate geometry without strings → walkLineRanges() (no string materialization)
• Need individual character widths → prepareWithSegments() on single chars

Choose the rendering target from the output type:

• DOM elements (.line { position: absolute }) → editorial, accordion, masonry
• Canvas 2D (ctx.fillText) → calligrams, some ASCII art
• HTML spans with inline styles → typographic ASCII (weight/style/opacity per character)

Architecture

Pretext CDN → prepare/prepareWithSegments → layout/layoutNextLine → line positions
                                                                          ↓
opentype.js CDN → font.parse(buffer) → glyph.getPath() ──────────> SVG <path> elements
                                                                          ↓
                                                              requestAnimationFrame (if animated)
                                                              resize handler (always)

Two-library split: Pretext decides WHERE text goes (line breaking, obstacle routing). opentype.js decides HOW each glyph looks (SVG paths, Bezier curves, contour data). Use Pretext alone for DOM-positioned text. Add opentype.js when you need per-glyph SVG rendering, path animation, or glyph contour access.

CDN Imports

<!-- Pretext (ESM, required) -->
<script type="module">
import { prepare, layout, prepareWithSegments, layoutWithLines, walkLineRanges, layoutNextLine, clearCache } from 'https://esm.sh/@chenglou/[email protected]'
</script>

<!-- opentype.js (UMD, optional — only for glyph path effects) -->
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/opentype.min.js"></script>

<!-- Font binary for opentype.js (Inter .woff — confirmed working) -->
<!-- const FONT_URL = 'https://cdn.jsdelivr.net/npm/@fontsource/[email protected]/files/inter-latin-400-normal.woff' -->

<!-- flubber (optional — only for glyph morphing) -->
<!-- <script src="https://cdn.jsdelivr.net/npm/[email protected]/build/flubber.min.js"></script> -->

Import only the functions you need. Pin the version. opentype.js cannot parse .woff2 — use .woff or .ttf only.

Composition Parameters

All Effects

Parameter	Default	Notes
Font	'18px Georgia, Palatino, serif'	Never use system-ui — unreliable with Pretext
Line height	28 (px)	Must match CSS line-height for height prediction
Background	#08080e or #f5f1ea	Dark or light — never pure black #000

Typographic ASCII

Parameter	Range	Default
Font size	10–18px	14
Font family	serif preferred	Georgia, Palatino, "Times New Roman", serif
Charset	printable ASCII	.,:;!+-=*#@%&a-zA-Z0-9
Weights	1–3	[300, 500, 800]
Styles	normal, italic	both
Opacity levels	6–10	10 CSS classes .a1–.a10

Calligrams

Parameter	Range	Default
Canvas size	200–800px	400
Char density	6–24px	14
Shapes	heart, circle, star, wave, spiral	heart
Animation	spring entrance	springK: 0.08, damping: 0.75

Editorial / Obstacle Routing

Parameter	Range	Default
Columns	1–4	2
Column gap	20–60px	40
Gutter	30–80px	48
Orb count	1–6	5
Orb radius	30–120px	65–110
hPad (horizontal padding around obstacle)	8–20px	14
vPad (vertical padding)	2–8px	4
MIN_SLOT_WIDTH	30–80px	50

opentype.js — Glyph Path Effects

Parameter	Range	Default
Font URL	.woff or .ttf only	@fontsource/[email protected]
Font size	10–200px	14 (body), 42 (display)
Stroke-dashoffset draw speed	30–150ms per glyph	80ms
Wet ink amplitude	0.5–3px	1.8
Wet ink decay (tau)	800–3000ms	1500
Glyph morph easing	linear, ease-in-out, spring	ease-in-out

Glyph-Mask Calligrams / Letterbox Gallery

Parameter	Range	Default
Canvas size (per letter)	200–1000px	500
Fill font size	8–24px	11
Fill model	self, lorem, alphabet, custom	lorem
Fill case	upper, lower, mixed	upper
Fill columns	1–4	1
Grid columns (gallery)	2–6	3
Cursor displacement radius	50–250px	100
Cursor displacement force	10–50	35
Displacement damping	0.85–0.98	0.94

opentype.js + flubber — Glyph Morphing

Parameter	Range	Default
Morph duration	300–2000ms	800
Flubber maxSegmentLength	5–20	10
Contour strategy	equal, 1-to-many, many-to-1	auto-detect

References

Working on...	Load
Full API surface, types, caveats	references/api-reference.md
Typographic palette, brightness, ASCII grid	references/typographic-ascii.md
Column flow, obstacles, headline fitting, slot-carving, animated physics	references/obstacle-routing.md
SDF shapes, proportional spacing, animation	references/calligram-shapes.md
Pixel-mask technique, glyph-mask calligrams, cursor displacement, letterbox gallery	references/calligram-shapes.md (Pixel-Mask section)
opentype.js + Pretext integration patterns	references/opentype-integration.md
Working accordion template	assets/templates/height-prediction.html
Working bubbles template	assets/templates/shrinkwrap-bubbles.html
Working editorial template	assets/templates/editorial-engine.html
Working ASCII art template	assets/templates/typographic-ascii.html
Working calligram template	assets/templates/calligram.html
Working letterbox gallery template	assets/templates/letterbox-gallery.html
GlyphKit demos (6 working demos, local)	~/Desktop/Programming/glyphkit/demos/ (machine-specific)

Anti-Patterns

Pretext

• Never use system-ui as font — canvas and DOM can resolve different fonts on macOS
• Never use React/Vue/framework — vanilla JS + HTML only
• Never omit <meta name="viewport"> — proportional measurement depends on correct device pixels
• Never use setInterval for animation — always requestAnimationFrame
• Never skip the resize handler — text layout depends on container width
• Never call prepare() inside the animation loop — it's the expensive one-time pass. Cache it.
• Never omit document.fonts.ready.then(...) — measure after fonts load, not before
• Never use pure black (#000000) — use rich off-blacks (#06060a, #08080e, #0a0a0c)
• Never build line strings when you only need geometry — use walkLineRanges instead of layoutWithLines
• Never position text with CSS flow — use position: absolute and place lines from Pretext coordinates
• Never pick one side of an obstacle when the obstacle is in the middle — use carveTextLineSlots to fill both sides
• Never create/destroy DOM elements per frame — use element pooling (syncPool)

opentype.js

• Never use .woff2 files — opentype.js cannot parse them. Use .woff or .ttf only
• Never use opentype.load() — use fetch().then(r => r.arrayBuffer()).then(opentype.parse) for better error handling
• Never render glyphs at absolute positions then try to move them — render at (0,0) and position via <g transform="translate(x,y)">
• Never forget font.unitsPerEm — the scale factor is fontSize / font.unitsPerEm
• Never call glyph.getPath() inside a tight loop without caching — cache the pathData string, regenerate only when position changes
• Never skip kerning — always check font.getKerningValue(glyph, nextGlyph) * scale between adjacent glyphs
• Never use Google Fonts TTF CDN URLs (they return 404 for programmatic access) — use @fontsource via jsdelivr

Post-Generation QA

python3 ~/.claude/skills/pretext/scripts/validate_pretext.py output.html

Checklist:

1. ESM import from esm.sh/@chenglou/pretext present
2. <script type="module"> tag
3. Named font declaration (no system-ui)
4. <meta name="viewport"> present
5. Window resize handler
6. document.fonts.ready awaited before measurement
7. requestAnimationFrame loop (for animated effects)
8. Touch event handlers (for interactive effects)
9. No setInterval for animation
10. No framework imports