Aurora

/**
 * Aurora — Flowing color waves inspired by the northern lights.
 * Category: backgrounds | Animate: true | Interactive: false | Complexity: low
 */
export default function aurora(ctx, state) {
  const LAYERS = 4;
  const SPEEDS = [0.0004, 0.0006, 0.0005, 0.0003];
  const COLORS = [
    [0, 200, 180], // teal
    [80, 120, 255], // blue
    [160, 60, 240], // purple
    [0, 240, 140], // green
  ];
  const AMPLITUDES = [0.18, 0.14, 0.16, 0.12];
  const Y_OFFSETS = [0.35, 0.45, 0.55, 0.4];

  return {
    frame({ width, height, time, reducedMotion }) {
      const t = reducedMotion ? 0 : time;

      for (let l = 0; l < LAYERS; l++) {
        const [r, g, b] = COLORS[l];
        const yBase = height * Y_OFFSETS[l];
        const amp = height * AMPLITUDES[l];
        const speed = SPEEDS[l] * (l % 2 === 0 ? 1 : -1);

        const gradient = ctx.createLinearGradient(0, yBase - amp * 2, 0, yBase + amp * 2);
        gradient.addColorStop(0, `rgba(${r},${g},${b},0)`);
        gradient.addColorStop(0.4, `rgba(${r},${g},${b},0.18)`);
        gradient.addColorStop(0.6, `rgba(${r},${g},${b},0.22)`);
        gradient.addColorStop(1, `rgba(${r},${g},${b},0)`);

        ctx.beginPath();
        ctx.moveTo(0, yBase);

        for (let x = 0; x <= width; x += 4) {
          const y =
            yBase +
            amp *
              Math.sin(x * 0.005 + t * speed * 1000) *
              Math.cos(x * 0.003 + t * speed * 700 + l);
          ctx.lineTo(x, y);
        }

        ctx.lineTo(width, height);
        ctx.lineTo(0, height);
        ctx.closePath();

        ctx.fillStyle = gradient;
        ctx.fill();
      }
    },
  };
}

<canvas id="canvas-effect" style="width:100%;height:400px;display:block" aria-hidden="true"></canvas>
<script>
/**
 * mountCanvas — Minimal runtime for Canvas effects.
 *
 * Handles HiDPI scaling, ResizeObserver, RAF loop, pointer tracking,
 * reduced-motion, and cleanup. The effect only implements draw logic.
 *
 * @param {HTMLCanvasElement} canvas
 * @param {(ctx: CanvasRenderingContext2D, state: object) => object} createEffect
 * @param {object} [options]
 * @returns {() => void} destroy function
 */
function mountCanvas(canvas, createEffect, options = {}) {
  const ctx = canvas.getContext('2d', { alpha: true });
  if (!ctx) throw new Error('2D context not available');

  const opts = {
    animate: true,
    clear: true,
    maxDpr: 2,
    pauseWhenHidden: true,
    respectReducedMotion: true,
    ...options,
  };

  const reducedMotion =
    opts.respectReducedMotion && window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  const state = {
    canvas,
    ctx,
    width: 0,
    height: 0,
    pixelWidth: 0,
    pixelHeight: 0,
    dpr: 1,
    time: 0,
    delta: 0,
    frame: 0,
    playing: false,
    reducedMotion,
    pointer: { x: 0, y: 0, inside: false, down: false },
  };

  let rafId = 0;
  let lastTime = 0;
  let disposed = false;

  function measure() {
    const rect = canvas.getBoundingClientRect();
    const w = Math.max(1, rect.width);
    const h = Math.max(1, rect.height);
    const dpr = Math.min(window.devicePixelRatio || 1, opts.maxDpr);
    const pw = Math.round(w * dpr);
    const ph = Math.round(h * dpr);

    if (canvas.width !== pw || canvas.height !== ph) {
      canvas.width = pw;
      canvas.height = ph;
    }

    state.width = w;
    state.height = h;
    state.pixelWidth = pw;
    state.pixelHeight = ph;
    state.dpr = dpr;
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
  }

  function clear() {
    ctx.clearRect(0, 0, state.width, state.height);
  }

  const effect = createEffect(ctx, state) || {};

  function onResize() {
    measure();
    effect.resize?.(state);
    if (!opts.animate || reducedMotion) {
      if (opts.clear) clear();
      effect.frame?.(state);
    }
  }

  function loop(now) {
    if (disposed) return;
    if (opts.pauseWhenHidden && document.hidden) {
      rafId = requestAnimationFrame(loop);
      return;
    }

    state.delta = lastTime ? (now - lastTime) / 1000 : 0;
    state.time = now / 1000;
    state.frame += 1;
    lastTime = now;

    if (opts.clear) clear();
    effect.frame?.(state);

    rafId = requestAnimationFrame(loop);
  }

  function onPointerMove(e) {
    const rect = canvas.getBoundingClientRect();
    state.pointer.x = e.clientX - rect.left;
    state.pointer.y = e.clientY - rect.top;
    if (!opts.animate || reducedMotion) {
      if (opts.clear) clear();
      effect.frame?.(state);
    }
  }
  function onPointerEnter() {
    state.pointer.inside = true;
  }
  function onPointerLeave() {
    state.pointer.inside = false;
    state.pointer.down = false;
  }
  function onPointerDown() {
    state.pointer.down = true;
  }
  function onPointerUp() {
    state.pointer.down = false;
  }

  const ro = new ResizeObserver(onResize);
  ro.observe(canvas);

  canvas.addEventListener('pointermove', onPointerMove);
  canvas.addEventListener('pointerenter', onPointerEnter);
  canvas.addEventListener('pointerleave', onPointerLeave);
  canvas.addEventListener('pointerdown', onPointerDown);
  window.addEventListener('pointerup', onPointerUp);

  measure();

  if (opts.animate && !reducedMotion) {
    state.playing = true;
    rafId = requestAnimationFrame(loop);
  } else {
    effect.frame?.(state);
  }

  return function destroy() {
    disposed = true;
    state.playing = false;
    cancelAnimationFrame(rafId);
    ro.disconnect();
    canvas.removeEventListener('pointermove', onPointerMove);
    canvas.removeEventListener('pointerenter', onPointerEnter);
    canvas.removeEventListener('pointerleave', onPointerLeave);
    canvas.removeEventListener('pointerdown', onPointerDown);
    window.removeEventListener('pointerup', onPointerUp);
    effect.destroy?.(state);
  };
}

/**
 * Aurora — Flowing color waves inspired by the northern lights.
 * Category: backgrounds | Animate: true | Interactive: false | Complexity: low
 */
const __effect = function aurora(ctx, state) {
  const LAYERS = 4;
  const SPEEDS = [0.0004, 0.0006, 0.0005, 0.0003];
  const COLORS = [
    [0, 200, 180], // teal
    [80, 120, 255], // blue
    [160, 60, 240], // purple
    [0, 240, 140], // green
  ];
  const AMPLITUDES = [0.18, 0.14, 0.16, 0.12];
  const Y_OFFSETS = [0.35, 0.45, 0.55, 0.4];

  return {
    frame({ width, height, time, reducedMotion }) {
      const t = reducedMotion ? 0 : time;

      for (let l = 0; l < LAYERS; l++) {
        const [r, g, b] = COLORS[l];
        const yBase = height * Y_OFFSETS[l];
        const amp = height * AMPLITUDES[l];
        const speed = SPEEDS[l] * (l % 2 === 0 ? 1 : -1);

        const gradient = ctx.createLinearGradient(0, yBase - amp * 2, 0, yBase + amp * 2);
        gradient.addColorStop(0, `rgba(${r},${g},${b},0)`);
        gradient.addColorStop(0.4, `rgba(${r},${g},${b},0.18)`);
        gradient.addColorStop(0.6, `rgba(${r},${g},${b},0.22)`);
        gradient.addColorStop(1, `rgba(${r},${g},${b},0)`);

        ctx.beginPath();
        ctx.moveTo(0, yBase);

        for (let x = 0; x <= width; x += 4) {
          const y =
            yBase +
            amp *
              Math.sin(x * 0.005 + t * speed * 1000) *
              Math.cos(x * 0.003 + t * speed * 700 + l);
          ctx.lineTo(x, y);
        }

        ctx.lineTo(width, height);
        ctx.lineTo(0, height);
        ctx.closePath();

        ctx.fillStyle = gradient;
        ctx.fill();
      }
    },
  };
}

const canvas = document.getElementById('canvas-effect');
mountCanvas(canvas, __effect, { animate: true, clear: true });
</script>