Pretext

Pretext Creative Demos

Overview

@chenglou/pretext is a 15KB zero-dependency TypeScript library by Cheng Lou (React core, ReasonML, Midjourney) for DOM-free multiline text measurement and layout. It does one thing: given (text, font, width), return the line breaks, per-line widths, per-grapheme positions, and total height — all via canvas measurement, no reflow.

That sounds like plumbing. It is not. Because it is fast and geometric, it is a creative primitive: you can reflow paragraphs around a moving sprite at 60fps, build games whose level geometry is made of real words, drive ASCII logos through prose, shatter text into particles with exact per-grapheme starting positions, or pack shrink-wrapped multiline UI without any getBoundingClientRect thrash.

This skill exists so the agent can make cool demos with it — the kind people post to X. See pretext.cool and chenglou.me/pretext for the community demo corpus.

When to Use

Use when the user asks for:

• A "pretext demo" / "cool pretext thing" / "text-as-X"
• Text flowing around a moving shape (hero sections, editorial layouts, animated long-form pages)
• ASCII-art effects using real words or prose, not monospace rasters
• Games where the playfield / obstacles / bricks are made of text (Tetris-from-letters, Breakout-of-prose)
• Kinetic typography with per-glyph physics (shatter, scatter, flock, flow)
• Typographic generative art, especially with non-Latin scripts or mixed scripts
• Multiline "shrink-wrap" UI (smallest container width that still fits the text)
• Anything that would require knowing line breaks before rendering

Don't use for:

• Static SVG/HTML pages where CSS already solves layout — just use CSS
• Rich text editors, general inline formatting engines (pretext is intentionally narrow)
• Image → text (use ascii-art / ascii-video skills)
• Pure canvas generative art with no text role — use p5js

Creative Standard

This is visual art rendered in a browser. Pretext returns numbers; you draw the thing.

• Don't ship a "hello world" demo. The hello-orb-flow.html template is the starting point. Every delivered demo must add intentional color, motion, composition, and one visual detail the user didn't ask for but will appreciate.
• Dark backgrounds, warm cores, considered palette. Classic amber-on-black (CRT / terminal) works, but so do cold-white-on-charcoal (editorial) and desaturated pastels (risograph). Pick one and commit.
• Proportional fonts are the point. Pretext's whole vibe is "not monospaced" — lean into it. Use Iowan Old Style, Inter, JetBrains Mono, Helvetica Neue, or a variable font. Never default sans.
• Real source/text, not lorem ipsum. The corpus should mean something. Short manifestos, poetry, real source code, a found text, the library's own README — never lorem ipsum.
• First-paint excellence. No loading states, no blank frames. The demo must look shippable the instant it opens.

Stack

Single self-contained HTML file per demo. No build step.

<script type="module">
import {
  prepare, layout,                   // use-case 1: simple height
  prepareWithSegments, layoutWithLines,  // use-case 2a: fixed-width lines
  layoutNextLineRange, materializeLineRange, // use-case 2b: streaming / variable width
  measureLineStats, walkLineRanges,  // stats without string allocation
} from "https://esm.sh/@chenglou/pretext@0.0.6";
</script>

Pin the version. @0.0.6 at time of writing — check npm for the latest if demo behavior is off.

The Two Use Cases

Almost everything reduces to one of these two shapes. Learn both.

Use-case 1 — measure, then render with CSS/DOM

const prepared = prepare(text, "16px Inter");
const { height, lineCount } = layout(prepared, 320, 20);

You still let the browser draw the text. Pretext just tells you how tall the box will be at a given width, without a DOM read. Use for:

• Virtualized lists where rows contain wrapping text
• Masonry with precise card heights
• "Does this label fit?" dev-time checks
• Preventing layout shift when remote text loads

Keep font and letterSpacing exactly in sync with your CSS. The canvas ctx.font format (e.g. "16px Inter", "500 17px 'JetBrains Mono'") must match the rendered CSS, or measurements drift.

Use-case 2 — measure and render yourself

const prepared = prepareWithSegments(text, FONT);
const { lines } = layoutWithLines(prepared, 320, 26);
for (let i = 0; i < lines.length; i++) {
  ctx.fillText(lines[i].text, 0, i * 26);
}

This is where the creative work lives. You own the drawing, so you can:

• Render to canvas, SVG, WebGL, or any coordinate system
• Substitute per-glyph transforms (rotation, jitter, scale, opacity)
• Use line metadata (width, grapheme positions) as geometry

For variable-width-per-line flow (text around a shape, text in a donut band, text in a non-rectangular column):

let cursor = { segmentIndex: 0, graphemeIndex: 0 };
let y = 0;
while (true) {
  const lineWidth = widthAtY(y);  // your function: how wide is the corridor at this y?
  const range = layoutNextLineRange(prepared, cursor, lineWidth);
  if (!range) break;
  const line = materializeLineRange(prepared, range);
  ctx.fillText(line.text, leftEdgeAtY(y), y);
  cursor = range.end;
  y += lineHeight;
}

This is the most important pattern in the whole library. It's what unlocks "text flowing around a dragged sprite" — the demo that went viral on X.

Helpers worth knowing

• measureLineStats(prepared, maxWidth) → { lineCount, maxLineWidth } — the widest line, i.e. multiline shrink-wrap width.
• walkLineRanges(prepared, maxWidth, callback) — iterate lines without allocating strings. Use for stats/physics over graphemes when you don't need the characters.
• @chenglou/pretext/rich-inline — the same system but for paragraphs mixing fonts / chips / mentions. Import from the subpath.

Demo Recipe Patterns

The community corpus (see references/patterns.md) clusters into a handful of strong patterns. Pick one and riff — don't invent a new category unless asked.

See templates/donut-orbit.html and templates/hello-orb-flow.html for working single-file starters.

Workflow

1. Pick a pattern from the table above based on the user's brief.
2. Start from a template:
   • templates/hello-orb-flow.html — text reflowing around a moving orb (reflow-around-obstacle pattern)
   • templates/donut-orbit.html — advanced example: measured ASCII logo obstacles, draggable wire sphere/cube, morphing shape fields, selectable DOM text, and dev-only controls
   • write_file to a new .html in /tmp/ or the user's workspace.
3. Swap the corpus for something intentional to the brief. Real prose, 10-100 sentences, no lorem.
4. Tune the aesthetic — font, palette, composition, interaction. This is the work; don't skip it.
5. Verify locally:

   cd <dir-with-html> && python3 -m http.server 8765
   # then open http://localhost:8765/<file>.html
   

6. Check the console — pretext will throw if prepareWithSegments is called with a bad font string; Intl.Segmenter is available in every modern browser.
7. Show the user the file path, not just the code — they want to open it.

Performance Notes

• prepare() / prepareWithSegments() is the expensive call. Do it once per text+font pair. Cache the handle.
• On resize, only rerun layout() / layoutWithLines() — never re-prepare.
• For per-frame animations where text doesn't change but geometry does, layoutNextLineRange in a tight loop is cheap enough to do every frame at 60fps for normal-length paragraphs.
• When rendering ASCII masks per frame, keep a cell buffer (Uint8Array/typed arrays), derive measured per-row obstacle spans from the cells or projected geometry, merge spans, then feed those spans into layoutNextLineRange before drawing text.
• Keep visual animation and layout animation coupled. If a sphere morphs into a cube, tween both the rendered cell buffer and the obstacle spans with the same value; otherwise the demo looks painted-on instead of physically reflowed.
• For fades, prefer layer opacity over changing glyph intensity or obstacle scale. Put transient ASCII sprites on their own canvas and fade the canvas with CSS/GSAP opacity so geometry does not appear to shrink.
• Canvas ctx.font setting is surprisingly slow; set it once per frame if font doesn't vary, not per fillText call.

Common Pitfalls

1. Drifting CSS/canvas font strings. ctx.font = "16px Inter" measured, but CSS says font-family: Inter, sans-serif; font-size: 16px. Fine if Inter loads. If Inter 404s, CSS falls back to sans-serif and measurements drift by 5-20%. Always preload the font or use a web-safe family.

2. Re-preparing inside the animation loop. Only layout* is cheap. Re-calling prepare every frame will tank perf. Keep the prepared handle in module scope.

3. Forgetting Intl.Segmenter for grapheme splits. Emoji, combining marks, CJK — "é".split("") gives you two chars. Use new Intl.Segmenter(undefined, { granularity: "grapheme" }) when sampling individual visible glyphs.

4. break: 'never' chips without extraWidth. In rich-inline, if you use break: 'never' for an atomic chip/mention, you must also supply extraWidth for the pill padding — otherwise chip chrome overflows the container.

5. Using @chenglou/pretext from unpkg with TypeScript-only entry. Use esm.sh — it compiles the TS exports to browser-ready ESM automatically. unpkg will 404 or serve raw TS.

6. Monospace fallbacks silently erasing the whole point. Users seeing monospace-looking output often have a CSS font-family that fell through to monospace. Verify the actual rendered font via DevTools.

7. Skipping rows vs adjusting width when flowing around a shape. If the corridor on this row is too narrow to fit a line, skip the row (y += lineHeight; continue;) rather than passing a tiny maxWidth to layoutNextLineRange — pretext will return one-grapheme lines that look broken.

8. Shipping a cold demo. The default first-paint looks tutorial-grade. Add: vignette, subtle scanline, idle auto-motion, one carefully chosen interactive response (drag, hover, scroll, click). Without these, "cool pretext demo" lands as "intern repro of the README."

Verification Checklist

• Demo is a single self-contained .html file — opens by double-click or python3 -m http.server
• @chenglou/pretext imported via esm.sh with pinned version
• Corpus is real prose, not lorem ipsum, and matches the demo's concept
• Font string passed to prepare matches the CSS font exactly
• prepare() / prepareWithSegments() called once, not per frame
• Dark background + considered palette — not the default white canvas
• At least one interactive response (drag / hover / scroll / click) or idle auto-motion
• Tested locally with python3 -m http.server and confirmed no console errors
• 60fps on a mid-tier laptop (or graceful degradation documented)
• One "extra mile" detail the user didn't ask for

Reference: Community Demos

Clone these for inspiration / patterns (all MIT-ish, linked from pretext.cool):

• Pretext Breaker — breakout with word-bricks — github.com/rinesh/pretext-breaker
• Tetris × Pretext — github.com/shinichimochizuki/tetris-pretext
• Dragon animation — github.com/qtakmalay/PreTextExperiments
• Somnai editorial engine — github.com/somnai-dreams/pretext-demos
• Bad Apple!! ASCII — github.com/frmlinn/bad-apple-pretext
• Drag-sprite reflow — github.com/dokobot/pretext-demo
• Alarmy editorial clock — github.com/SmisLee/alarmy-pretext-demo

Official playground: chenglou.me/pretext — accordion, bubbles, dynamic-layout, editorial-engine, justification-comparison, masonry, markdown-chat, rich-note.