Lens
A square glass lens you drag over a printed picture. Inside it the picture is magnified with a barrel bulge (the middle more than the edges), the rim carries a faint chromatic fringe, and the lens tilts into its direction of travel on springs.
Code
1. Install
# No packages beyond React.
2. Files
"use client";
// Lens: a square glass lens you drag over a printed picture. Inside it the picture is magnified with a barrel bulge
// (the middle more than the edges), the rim carries a faint chromatic fringe, and the lens tilts into its direction of
// travel on springs. Arrow keys move it too.
import { useEffect, useRef, useState, type CSSProperties, type KeyboardEvent as RKE, type PointerEvent as RPE } from "react";
export type LensCue = "glass-grab" | "glass-place";
export type LensImage = {
/** Image URL. */
src: string;
/** Description of the picture, read out by screen readers. */
alt: string;
};
export type LensProps = {
/** The picture under the glass. The default is Teseria's demo art (/art/tide.svg), which only exists in Teseria: pass your own. Cropped to fill the square. */
image?: LensImage;
/** Caption printed over the picture, bottom left (first line). Empty to hide. */
title?: string;
/** Second caption line, smaller. Empty to hide. */
subtitle?: string;
/** Side of the square picture in px. */
size?: number;
/** Side of the lens in px. */
lensSize?: number;
/** Sound hook: "glass-grab" on grab, "glass-place" on release. */
onSound?: (cue: LensCue) => void;
className?: string;
style?: CSSProperties;
};
export const DEFAULT_LENS_IMAGE: LensImage = { src: "/art/tide.svg", alt: "Tide" };
const clamp = (n: number, a: number, b: number) => Math.min(b, Math.max(a, n));
type Spr = { p: number; v: number };
const spr = (p = 0): Spr => ({ p, v: 0 });
const stepSpr = (s: Spr, target: number, k: number, d: number, dt: number) => {
s.v += (target - s.p) * k * dt;
s.v *= d ** dt;
s.p += s.v * dt;
return s.p;
};
const still = () => typeof window !== "undefined" && !!window.matchMedia?.("(prefers-reduced-motion: reduce)").matches;
/** True while the node is near the viewport and the tab is visible; starts false. */
function useOnScreen(ref: { current: Element | null }) {
const [near, setNear] = useState(false);
const [tab, setTab] = useState(true);
useEffect(() => {
const el = ref.current;
if (!el) return;
if (typeof IntersectionObserver === "undefined") return setNear(true);
const io = new IntersectionObserver((es) => setNear(es[es.length - 1].isIntersecting), { rootMargin: "160px" });
io.observe(el);
return () => io.disconnect();
}, [ref]);
useEffect(() => {
const f = () => setTab(!document.hidden);
f();
document.addEventListener("visibilitychange", f);
return () => document.removeEventListener("visibilitychange", f);
}, []);
return near && tab;
}
/** rAF loop (dt in 16.67ms frames), StrictMode-safe. */
function useLoop(fn: (dt: number) => void, on: boolean) {
const ref = useRef(fn);
ref.current = fn;
useEffect(() => {
if (!on) return;
let raf = 0;
let last = 0;
const f = (t: number) => {
const dt = last ? clamp((t - last) / 16.67, 0, 2.5) : 1;
last = t;
ref.current(dt);
raf = requestAnimationFrame(f);
};
raf = requestAnimationFrame(f);
return () => cancelAnimationFrame(raf);
}, [on]);
}
export function Lens({
image = DEFAULT_LENS_IMAGE,
title = "TIDE · NO. 03",
subtitle = "low water 04:12 · 0.6 m",
size = 260,
lensSize = 92,
onSound,
className,
style,
}: LensProps) {
const S = size, L = lensSize, COLS = 24;
const root = useRef<HTMLDivElement>(null);
const stage = useRef<HTMLDivElement>(null);
const lens = useRef<HTMLDivElement>(null);
const glass = useRef<HTMLCanvasElement>(null);
const base = useRef<HTMLCanvasElement>(null);
const src = useRef<HTMLCanvasElement | null>(null);
const st = useRef({ x: spr(S * 0.55), y: spr(S * 0.5), px: S * 0.55, py: S * 0.5, held: false, tx: spr(0), ty: spr(0) });
const live = useOnScreen(root);
useEffect(() => {
let dead = false;
const img = new Image();
img.onload = () => {
if (dead) return;
const dpr = Math.min(2, window.devicePixelRatio || 1);
const c = document.createElement("canvas");
c.width = S * dpr; c.height = S * dpr;
const ctx = c.getContext("2d")!;
// cover-fit; a 3:4 portrait becomes S wide and S*4/3 tall, centred
const k = Math.max(S / (img.naturalWidth || S), S / (img.naturalHeight || S));
const w = (img.naturalWidth || S) * k, h = (img.naturalHeight || S) * k;
ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
ctx.drawImage(img, (S - w) / 2, (S - h) / 2, w, h);
// a fine printed dot screen and a caption: soft art alone hides magnification, these show the bulge
ctx.fillStyle = "rgba(255,255,255,.42)";
for (let y = 7; y < S; y += 13) for (let x = 7; x < S; x += 13) { ctx.beginPath(); ctx.arc(x, y, 1.1, 0, Math.PI * 2); ctx.fill(); }
ctx.fillStyle = "rgba(255,255,255,.92)";
ctx.font = "500 11px system-ui, sans-serif";
if (title) ctx.fillText(title, 16, S - 34);
ctx.font = "400 10px system-ui, sans-serif";
ctx.fillStyle = "rgba(255,255,255,.7)";
if (subtitle) ctx.fillText(subtitle, 16, S - 19);
src.current = c;
const b = base.current;
if (b) { b.width = c.width; b.height = c.height; b.getContext("2d")!.drawImage(c, 0, 0); }
};
img.src = image.src;
return () => { dead = true; img.onload = null; src.current = null; };
}, [image.src, S, title, subtitle]);
// a new lens or picture size keeps the lens inside the frame
useEffect(() => {
const s = st.current;
s.px = clamp(s.px, L / 2, S - L / 2);
s.py = clamp(s.py, L / 2, S - L / 2);
}, [S, L]);
useLoop((dt) => {
const s = st.current;
stepSpr(s.x, s.px, 0.22, 0.7, dt);
stepSpr(s.y, s.py, 0.22, 0.7, dt);
const tilt = still() ? 0 : 1;
stepSpr(s.tx, clamp(s.x.v * 1.1, -14, 14) * tilt, 0.2, 0.7, dt);
stepSpr(s.ty, clamp(-s.y.v * 1.1, -14, 14) * tilt, 0.2, 0.7, dt);
const el = lens.current;
if (el) el.style.transform = `translate(${(s.x.p - L / 2).toFixed(2)}px, ${(s.y.p - L / 2).toFixed(2)}px) rotateX(${s.ty.p.toFixed(2)}deg) rotateY(${s.tx.p.toFixed(2)}deg)`;
const cv = glass.current, sc = src.current;
if (!cv || !sc) return;
const dpr = Math.min(2, window.devicePixelRatio || 1);
if (cv.width !== L * dpr) { cv.width = L * dpr; cv.height = L * dpr; }
const ctx = cv.getContext("2d")!;
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.clearRect(0, 0, cv.width, cv.height);
const magY = 1.5, cw = L / COLS;
for (let c = 0; c < COLS; c++) {
const u = ((c + 0.5) / COLS) * 2 - 1;
const mag = 1.62 - 0.42 * u * u; // barrel: the middle magnifies more than the edges
const lx0 = c * cw;
const sx = s.x.p + (lx0 - L / 2) / mag;
const sw = cw / mag;
const sy = s.y.p - L / 2 / magY;
const sh = L / magY;
ctx.drawImage(sc, sx * dpr, sy * dpr, sw * dpr, sh * dpr, lx0 * dpr, 0, cw * dpr, L * dpr);
}
}, live);
const aim = (e: RPE) => {
const s = st.current;
const r = stage.current!.getBoundingClientRect();
s.px = clamp(e.clientX - r.left, L / 2, S - L / 2);
s.py = clamp(e.clientY - r.top, L / 2, S - L / 2);
};
const down = (e: RPE) => {
st.current.held = true;
aim(e);
stage.current!.setPointerCapture(e.pointerId);
onSound?.("glass-grab");
};
const move = (e: RPE) => {
if (st.current.held) aim(e);
};
const up = () => {
if (!st.current.held) return;
st.current.held = false;
onSound?.("glass-place");
};
const key = (e: RKE) => {
const s = st.current;
const d = 16;
if (e.key === "ArrowLeft") s.px = clamp(s.px - d, L / 2, S - L / 2);
else if (e.key === "ArrowRight") s.px = clamp(s.px + d, L / 2, S - L / 2);
else if (e.key === "ArrowUp") s.py = clamp(s.py - d, L / 2, S - L / 2);
else if (e.key === "ArrowDown") s.py = clamp(s.py + d, L / 2, S - L / 2);
else return;
e.preventDefault();
};
return (
<div ref={root} className={`len-root${className ? " " + className : ""}`} style={{ width: S, height: S, ...style }}>
<div ref={stage} className="len-stage" tabIndex={0} onPointerDown={down} onPointerMove={move} onPointerUp={up} onPointerCancel={up} onKeyDown={key} role="img" aria-label={`${image.alt}, with a lens you can drag; arrow keys move it`}>
<canvas ref={base} className="len-pic" style={{ width: S, height: S }} />
<div ref={lens} className="len-lens" style={{ width: L, height: L }}>
<canvas ref={glass} className="len-glass" style={{ width: L, height: L }} />
<span className="len-edge" />
</div>
</div>
</div>
);
}
/* Lens. Prefix len-. */
.len-root, .len-root * { box-sizing: border-box; }
.len-root {
position: relative; display: grid; place-items: center; margin: 0 auto; touch-action: none; user-select: none; -webkit-user-select: none;
color: var(--ts-fg, #14161b); font-family: var(--ts-font, system-ui, sans-serif);
}
.len-stage { position: absolute; inset: 0; border-radius: var(--ts-r-inner); overflow: hidden; cursor: grab; perspective: 600px; }
.len-stage:active { cursor: grabbing; }
.len-stage:focus { outline: none; }
.len-stage:focus-visible { outline: 2px solid var(--ts-focus, #3446eb); outline-offset: 2px; }
.len-pic { position: absolute; inset: 0; width: 100%; height: 100%; object-fit: cover; display: block; pointer-events: none; }
.len-lens { position: absolute; left: 0; top: 0; border-radius: 18px; overflow: hidden; transform-style: preserve-3d; pointer-events: none; }
.len-glass { position: absolute; inset: 0; display: block; }
.len-edge { position: absolute; inset: 0; border-radius: 18px; box-shadow: inset 0 0 0 1.5px rgba(255, 255, 255, .55), inset 1.5px 0 0 1.5px rgba(255, 70, 60, .35), inset -1.5px 0 0 1.5px rgba(70, 120, 255, .35), 0 0 0 1px rgba(0, 0, 0, .18); }
/*
* Teseria tokens — the only colours, fonts and corner radii a block reads.
*
* Every name sits under --ts- so it never collides with your app's own variables. Include this file once, or point
* the names at your theme (e.g. --ts-accent: var(--brand)). Light by default; dark follows [data-theme="dark"] on
* any ancestor, or the OS setting unless [data-theme="light"] is set.
*
* Surfaces page → raised (cards, popovers) · sunken (wells, tracks) · tint-1..3 (neutral fills, light to deep)
* Text fg → secondary → tertiary → muted → faint · fg-inverse sits on an fg-coloured fill
* Lines border · edge (the box-shadow a filled surface uses instead of an outline: soft in light, nothing in
* dark; composes with other shadows)
* Signals accent · focus · success · warning · danger
* Corners controls are always pills · r-panel (dialogs, sheets, cards) · r-pop (menus, popovers, toasts) · r-inner
* (thumbnails, inner cards); a row inside a panel uses the panel radius minus the panel padding
*/
:root {
--ts-font: Inter, ui-sans-serif, system-ui, -apple-system, "Segoe UI", sans-serif;
--ts-r-panel: 24px;
--ts-r-pop: 18px;
--ts-r-inner: 12px;
--ts-page: #ffffff;
--ts-raised: #ffffff;
--ts-sunken: #f3f2f0;
--ts-tint-1: #f5f4f2;
--ts-tint-2: #ecebe8;
--ts-tint-3: #e3e1dd;
--ts-fg: #161514;
--ts-fg-rgb: 22, 21, 20;
--ts-fg-secondary: #3a3836;
--ts-fg-tertiary: #585552;
--ts-fg-muted: #6d6a66;
--ts-fg-faint: #89857f;
--ts-fg-inverse: #ffffff;
--ts-border: rgba(22, 21, 20, 0.1);
--ts-edge: 0 0 0 1px rgba(22, 21, 20, 0.07), 0 1px 2px rgba(22, 21, 20, 0.05);
--ts-accent: #3446eb;
--ts-focus: #3446eb;
--ts-success: #13804a;
--ts-warning: #a9650b;
--ts-danger: #df474e;
}
:root[data-theme="dark"] {
--ts-page: #0b0a09;
--ts-raised: #24221f;
--ts-sunken: #0d0c0b;
--ts-tint-1: #131211;
--ts-tint-2: #1b1a18;
--ts-tint-3: #2b2926;
--ts-fg: #f6f5f3;
--ts-fg-rgb: 246, 245, 243;
--ts-fg-secondary: #bab8b4;
--ts-fg-tertiary: #9e9b97;
--ts-fg-muted: #8e8b86;
--ts-fg-faint: #72706c;
--ts-fg-inverse: #0b0a09;
--ts-border: rgba(246, 245, 243, 0.1);
--ts-edge: 0 0 #0000;
--ts-accent: #3446eb;
--ts-focus: #7f8cff;
--ts-success: #3ccf80;
--ts-warning: #efa130;
--ts-danger: #df474e;
}
@media (prefers-color-scheme: dark) {
:root:not([data-theme="light"]) {
--ts-page: #0b0a09;
--ts-raised: #24221f;
--ts-sunken: #0d0c0b;
--ts-tint-1: #131211;
--ts-tint-2: #1b1a18;
--ts-tint-3: #2b2926;
--ts-fg: #f6f5f3;
--ts-fg-rgb: 246, 245, 243;
--ts-fg-secondary: #bab8b4;
--ts-fg-tertiary: #9e9b97;
--ts-fg-muted: #8e8b86;
--ts-fg-faint: #72706c;
--ts-fg-inverse: #0b0a09;
--ts-border: rgba(246, 245, 243, 0.1);
--ts-edge: 0 0 #0000;
--ts-accent: #3446eb;
--ts-focus: #7f8cff;
--ts-success: #3ccf80;
--ts-warning: #efa130;
--ts-danger: #df474e;
}
}
3. Usage
import { Lens } from "./Lens";
import "./Lens.css";
export default function Demo() {
return <Lens />;
}