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 Hermes 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/[email protected]";
</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.

1. Swap the corpus for something intentional to the brief. Real prose, 10-100 sentences, no lorem.
2. Tune the aesthetic — font, palette, composition, interaction. This is the work; don't skip it.
3. Verify locally:

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

1. Check the console — pretext will throw if prepareWithSegments is called with a bad font string; Intl.Segmenter is available in every modern browser.
2. 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.

1. 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.

1. 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.

1. `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.

1. 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.

1. 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.

1. 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.

1. 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.