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omd-cjk-type-shader — 먹 번짐: CJK display type as sumi ink on hanji
Ink soaking into paper is the one image Korean, Japanese and Chinese writing share. This skill makes a big CJK headline read like it was printed or brushed in sumi on hanji. On a fast skim it should look like a calligraphy poster, not a filter.
How it works. The heading is ordinary DOM text in the ink color, readable and selectable from the first frame. The skill traces every grapheme where the browser laid it out and builds three fields from that mask: distance outside the strokes, distance inside them, and local ink coverage. A WebGL2 layer drawn over the text then adds only what real ink does:
- wet → dry: the stroke starts glossy-dark with a soft bloom around it, and the bloom shrinks back as the ink dries;
- pooling: ink collects darker along the stroke rim and at stroke ends and convex corners (low local coverage);
- kasure: paper fibers show through the middle of thick strokes once dry;
- nijimi: capillary feathering runs along a few paper fibers at some edges, chosen by low-frequency noise, never as an even ring around every glyph;
- paper: a procedural hanji texture (SVG turbulence, no image) on the page.
It plays once when the heading enters the viewport, then the render loop stops. Line wrapping, keep-all, font swaps and resizes are re-traced.
Working example: examples/demo.html (single file, Korean + Japanese + Traditional Chinese). Proof: scripts/verify.mjs.
1. When to use / when not
Use when:
- a Korean, Japanese or Chinese display headline (≥ 40px; hero, launch title, campaign line, section opener) should feel inked, calligraphic, "printed on hanji / washi / 宣紙";
- the brand's ink and paper colors are known (DESIGN.md foreground/canvas, or the heading's computed
color and the page background).
Don't use for:
- body text, labels, buttons, nav, anything below ~32px (pooling and feathering need stroke width to read);
- Latin-only headlines (it works, but the cultural point is gone);
- more than one or two headings per page. Boldness in one place (tf-shaders rule);
- light text on dark paper. The effect darkens ink, so it needs a paper that ink reads on.
2. The effect and its two parameters
| Param | Range | Default | What it does |
|---|
bleed | 0–1 | 0.7 | How far the capillary feathering travels along fibers (up to 0.12em × bleed, at the edges noise picks) and how big the wet bloom gets. 0.4 = barely there, 0.8 = a wet brush. |
duration | s | 2.6 | Time from wet to dry. The bloom peaks early and shrinks; pooling and fiber show-through come in as it dries. Under 1.5s it reads as a flash, over 4s as a loading state. |
Everything else is fixed on purpose. Ink = the heading's computed color, darkened to 25% for pools; paper = the page background color. Text ink should be a sumi grey-black, not pure black (the demo uses #3a3530 on #f3eee4, about 10:1), so the darker pools have room to show. One signature, not a control panel.
3. The contract that keeps text text
- The heading is a normal
<h1>/<h2> with its real text. inkBleed() wraps the text in .ink-text and inserts an aria-hidden="true", pointer-events: none layer over it (z-index 2). The layer only darkens, lightens or extends what is already there. It never replaces or re-draws the glyphs, and with the layer removed the heading is still complete.
- The headline paints on the first frame. No
opacity: 0, no clip, no wait for the shader. tf-shaders measured LCP going to 7.8s when hero copy waited for a shader.
- Selection, copy, find-in-page, translation, screen readers and SEO see plain text.
verify.mjs checks that selecting the heading returns its full text in every pass.
- Leave ~0.4em of clear space around the heading (the demo uses
margin-bottom: 0.4em). Feathering and bloom extend up to ~0.3em past the glyphs.
- Never draw CJK headlines with a canvas/
Text shader component. Canvas text loses selection, accessibility, indexing and correct CJK shaping (tf-shaders "Don't" list).
4. CJK typesetting (applies before the shader)
From /hangul (HG rules) for Korean; ja/zh scoped separately so Korean rules don't leak:
.ink:lang(ko) { word-break: keep-all; overflow-wrap: anywhere; /* HG-10, scoped to :lang(ko) */
letter-spacing: -0.02em; /* HG-7: display never below -0.03em */
line-height: 1.3; } /* HG-5: wrapping display >= 1.3 */
.ink:lang(ja), .ink:lang(zh-Hant) {
line-break: strict; /* kinsoku: no small kana / closing marks at line start */
word-break: keep-all; overflow-wrap: anywhere; /* break only at <wbr> phrase marks you place */
letter-spacing: 0; line-height: 1.4; }
:lang(ko), :lang(ja), :lang(zh) { font-synthesis: none; } /* no fake bold/italic on CJK */
- Mark Japanese/Chinese phrase boundaries with
<wbr> (BudouX-style: 墨がにじむように、<wbr>名前が<wbr>紙に染みこむ。). In headless Chrome 149, word-break: auto-phrase computed correctly but did not produce phrase breaks in this demo, so don't rely on it.
- Korean: line breaks only at spaces between 어절, never syllable by syllable. Hand-place display line breaks when the copywriter cares (tf-shaders
cjk-type.md).
- Set
lang on the heading (ko, ja, zh-Hant/zh-Hans). It picks the right glyph variants for shared Han code points and scopes the rules above.
- Fonts: use a face that ships the weight you set (HG-12). The demo uses Noto Serif KR / JP / TC 700 from Google Fonts, which loads only the glyph slices the page uses. A myeongjo/mincho/song face suits ink. Pretendard works too. Trace after
document.fonts.ready so the field follows the real face, not the fallback.
5. Library, license, budget
Library: @paper-design/shaders (vanilla ShaderMount), pinned 0.0.81, WebGL2. Decision record (verified 2026-10-13 from the npm tarballs' LICENSE files, npm metadata and the GitHub license API):
| @paper-design/shaders 0.0.81 | shaders 4.0.4 (shaders.com) |
|---|
| License | Apache-2.0 (LICENSE + NOTICE "Paper Shaders, Copyright 2026 Paper") | MIT (© 2026 Shader Effects Inc.); dep typegpu 0.12.3 MIT (Software Mansion) |
| Redistribution in a skill/template | allowed; keep NOTICE if you vendor code | allowed |
| Runtime | WebGL2: every current desktop and mobile browser | WebGPU only, no fallback (Chrome/Edge, Android 12+ Chrome, Safari 26+, recent Firefox) |
| JS for this effect | 17.5 KiB min / 5.3 KiB gzip (esbuild, ShaderMount only); CDN per-file +esm 16.4 KiB / 5.0 KiB gzip | 2,535 KiB min / 710 KiB gzip (esbuild, Shader+MeshGradient from shaders/react) |
| Telemetry | none (its only fetch() reads a local blob's content type) | on by default: samples 5% of sessions to shaders.com/api/telemetry; pass disableTelemetry |
| Custom GLSL + textures | yes (ShaderMount(el, frag, uniforms), image uniforms) | via its component graph |
The effect needs a custom fragment shader and one texture, which is exactly what ShaderMount is, at 1/130th of the WebGPU runtime's weight and with twice the browser reach. The demo imports Paper from jsDelivr (pinned, per-file build) and does not vendor it, so no NOTICE copy is needed. If you vendor Paper's files, copy its NOTICE alongside. If a project already ships shaders, this effect still runs on Paper. Don't load both runtimes on one page.
Budget for one heading:
- JS: 5.0–5.3 KiB gzip library + 5.3 KiB gzip of effect code (shader, two distance transforms, coverage blur, tracing; measured unminified with comments).
- GPU: one quad, one texture lookup and 7 value-noise calls per pixel, over the heading box plus a ≤0.32em pad,
minPixelRatio: 1, capped at 1920×1080 px. It runs for duration and then setSpeed(0) stops the rAF loop. Paper also pauses it offscreen and in hidden tabs.
- CPU: per layout change (font swap, resize, re-wrap; debounced 120ms), two Euclidean distance transforms (outside and inside the strokes) and one separable box blur, at half resolution above 64px type.
- Paper texture: an inline SVG
feTurbulence data URI on html (~600 bytes), no image request.
- Fonts are the real cost. CJK faces are large. Load only the display weight, as unicode-range slices.
6. Fallback and reduced motion
- No WebGL2 (probe
canvas.getContext('webgl2') before mounting; ShaderMount throws if it can't get a context or link the program): no layer is inserted, the heading gets .ink-static (a faint CSS text-shadow). The heading is already in ink on textured paper, so it is complete without the shader. verify.mjs forces this path by stubbing getContext.
prefers-reduced-motion: reduce: mount at the dried frame with speed 0. The reader gets the final inked state as one still frame, with no motion and no rAF loop.- Shader compile failure (one happened while building this: a precision mismatch with Paper's vertex shader) takes the same
.ink-static path. In your own GLSL, declare uniform mediump vec2 u_resolution;, because the precision must match Paper's vertex shader.
7. Verify on a real GPU
next build or a passing lint proves nothing for shaders (tf-shaders rule). Run:
npm i -D playwright-core && npx playwright-core install chromium # a fresh Playwright Chromium, not your own Chrome
node <skill>/scripts/verify.mjs path/to/page.html ./ink-verify --frames 36
It launches headless Chromium with --enable-gpu --ignore-gpu-blocklist --use-angle=metal (macOS) and runs three passes: GPU, reduced-motion, no-WebGL. Then it:
- records the WebGL renderer string, so you know whether a real GPU or SwiftShader drew the frames;
- steps the shader clock deterministically and saves frames (feed them to ffmpeg for the demo GIF/MP4);
- proves rendering with pixels: shader-on vs shader-off inside the heading box, and first frame vs dried frame;
- checks that the selected text equals the heading text, the layer is
aria-hidden + pointer-events: none, reduced motion is still, and no-WebGL leaves no canvas.
Measured on 2026-10-13 (headless Chromium 149, ANGLE Metal Renderer: Apple M5 Pro, not SwiftShader): 10.2% of heading-box pixels changed by the shader, wet ≠ dry (5.9% differ), all checks passed. Look at the frames anyway, at full size: a misaligned field (wrong baseline) shows as pooling offset from the strokes, and a feathering setting that is too strong reads as fur. Only your eyes catch those.
8. Next.js / React port
Keep the same function and lifecycle in a 'use client' leaf: useEffect(() => { let h; document.fonts.ready.then(async () => { const { ShaderMount } = await import('@paper-design/shaders'); h = await inkBleed(ref.current, { bleed, duration }); }); return () => h?.mount.dispose(); }, []). Render the heading server-side as plain text, so the shader only decorates it after hydration. Don't use @paper-design/shaders-react's <ShaderMount> here: it doesn't give you the WebGL2 probe, the fallback class, or control over when the field is re-traced.
9. Attribution
The budget-first framing, the fallback-is-design rule, the first-frame-LCP rule, 어절-not-syllable reveals, keep-all, and the real-GPU screenshot loop come from the owner's tf-shaders experiment (skill/shader-landing/SKILL.md, references/cjk-type.md, scripts/shoot.mjs, RESEARCH.md, 2026-10). Korean type values come from OmD's /hangul skill. The distance transform is Felzenszwalb & Huttenlocher's linear-time EDT, written fresh here.
Don't
- Don't hide the heading until the shader is ready. The text is the content; the ink is the frame.
- Don't animate syllable by syllable, and don't split Hangul into per-character spans for a reveal.
- Don't run the loop forever. A dried ink effect costs nothing; a looping one drains batteries.
- Don't put two runtimes (Paper +
shaders) on one page for one heading.
- Don't feather every edge or draw a uniform halo. An even ring around each glyph reads as a sticker, not ink; feathering belongs to a few edges.
- Don't use pure black text ink. Pools need a darker value to show, and sumi is grey-black anyway.
- Don't push
bleed until the fibers read as fur. Check the dried frame at your real font size.