Pluggable canvas visualizer

<div class="pcv-panel">
	<div class="pcv-controls" role="group" aria-label="Visualization mode">
		<button type="button" class="pcv-btn is-on" data-pcv-mode="bars">Bars + peak hold</button>
		<button type="button" class="pcv-btn" data-pcv-mode="scope">Oscilloscope</button>
		<button type="button" class="pcv-btn" data-pcv-mode="vu">VU needles</button>
		<button type="button" class="pcv-btn" data-pcv-mode="matrix">Dot matrix</button>
		<button type="button" class="pcv-btn" data-pcv-mode="radial">Radial</button>
		<button type="button" class="pcv-btn" data-pcv-mode="ambience">Ambience</button>
		<button type="button" class="pcv-btn" id="pcv-pause">Pause</button>
	</div>
	<div class="pcv-wrap"><canvas id="pcv-canvas"></canvas></div>
</div>

Colors come from shared theme tokens — --surface, --ink, --border, --accent and friends — so this CSS carries no palette of its own. Use Runnable file to copy the tokens along with it.

.pcv-panel { display: flex; flex-direction: column; gap: 0.75rem; }

.pcv-controls { display: flex; flex-wrap: wrap; gap: 0.5rem; }

.pcv-btn {
	font: inherit;
	font-size: 0.9375rem;
	padding: 0.4rem 0.8rem;
	border: 1px solid var(--border);
	border-radius: 6px;
	background: var(--surface);
	color: var(--ink);
	cursor: pointer;
}

.pcv-btn.is-on {
	background: var(--accent);
	border-color: var(--accent);
	color: var(--accent-ink);
}

.pcv-btn:disabled { opacity: 0.5; cursor: not-allowed; }

.pcv-wrap {
	background: var(--ground);
	border: 1px solid var(--border);
	border-radius: var(--radius);
	overflow: hidden;
}

#pcv-canvas { display: block; width: 100%; height: 240px; }

(function () {
	var canvas = document.getElementById('pcv-canvas');
	var context = canvas.getContext('2d');
	var root = document.documentElement;
	var reducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

	var binCount = 64;
	var spectrum = new Uint8Array(binCount);
	var waveform = new Uint8Array(256);
	var peaks = new Float32Array(binCount);
	var needle = { position: 0, velocity: 0 };
	var mode = 'bars';
	var running = true;
	var clock = 0;
	var frameCount = 0;

	// Deterministic value noise. A real analyser's hiss is genuinely random, but
	// a seeded hash keeps a screenshot of frame N identical between runs, which
	// is what makes this reproducible in a visual test.
	function hashNoise(first, second) {
		var mixed = Math.sin(first * 127.1 + second * 311.7) * 43758.5453;
		return mixed - Math.floor(mixed);
	}

	// A synthetic signal generator so the pattern needs no audio permission and
	// no media file. It has the same shape a real analyser node would hand back:
	// a frequency-magnitude array and a time-domain waveform array, both 0-255.
	function synthesize() {
		clock += 0.016;
		frameCount += 1;
		for (var bin = 0; bin < binCount; bin += 1) {
			var tilt = Math.pow(1 - bin / binCount, 1.6);
			var beat = 0.55 + 0.45 * Math.sin(clock * 3.1 + bin * 0.08);
			var slow = 0.6 + 0.4 * Math.sin(clock * 0.7 + bin * 0.02);
			var noise = hashNoise(bin, frameCount) * 0.22;
			spectrum[bin] = Math.max(0, Math.min(255, (tilt * beat * slow + noise) * 235));
		}
		for (var sample = 0; sample < waveform.length; sample += 1) {
			var phase = (sample / waveform.length) * Math.PI * 2;
			var value =
				Math.sin(phase * 3 + clock * 5) * 0.5 +
				Math.sin(phase * 7 + clock * 2) * 0.25 +
				Math.sin(phase * 13 + clock * 9) * 0.12 +
				(hashNoise(sample, frameCount) - 0.5) * 0.06;
			waveform[sample] = Math.max(0, Math.min(255, 128 + value * 110));
		}
	}

	// Read the palette from the live custom properties, not a fixed table, so
	// every mode follows a theme switch without the visualizer knowing it happened.
	// No hex fallback: an unset token is a bug in the host page, and silently
	// standing in for it would hide exactly that bug.
	function palette() {
		var styles = getComputedStyle(root);
		return {
			ground: styles.getPropertyValue('--ground').trim(),
			accent: styles.getPropertyValue('--accent').trim(),
			dim: styles.getPropertyValue('--dim').trim(),
			ink: styles.getPropertyValue('--ink').trim()
		};
	}

	function barHeights(count) {
		var out = new Float32Array(count);
		for (var index = 0; index < count; index += 1) {
			var from = Math.floor((index * binCount) / count);
			var to = Math.max(from + 1, Math.floor(((index + 1) * binCount) / count));
			var total = 0;
			for (var bin = from; bin < to; bin += 1) total += spectrum[bin];
			out[index] = total / (to - from) / 255;
		}
		return out;
	}

	// Every mode is a plain (frame) => void that paints the *whole* canvas,
	// background included. The host never clears between frames -- that single
	// rule is what makes a persistence mode (see "ambience") possible at all,
	// and it lets a consuming app register a replacement mode under an existing
	// key without forking this file.
	var modes = {
		bars: function (frame) {
			var colors = frame.colors,
				width = frame.width,
				height = frame.height;
			context.fillStyle = colors.ground;
			context.fillRect(0, 0, width, height);
			var count = 48,
				gap = 3;
			var barWidth = (width - gap * (count - 1)) / count;
			var heights = barHeights(count);
			for (var index = 0; index < count; index += 1) {
				var value = heights[index];
				// Peak hold: the marker only falls, and only at a fixed decay rate.
				peaks[index] = Math.max(value, Math.max(peaks[index] - 0.015, 0));
				var x = index * (barWidth + gap);
				context.fillStyle = colors.accent;
				context.fillRect(x, height - value * height, barWidth, value * height);
				context.fillStyle = colors.ink;
				context.fillRect(x, height - peaks[index] * height - 2, barWidth, 2);
			}
		},
		scope: function (frame) {
			var colors = frame.colors,
				width = frame.width,
				height = frame.height;
			context.fillStyle = colors.ground;
			context.fillRect(0, 0, width, height);
			context.strokeStyle = colors.accent;
			context.lineWidth = 2;
			context.beginPath();
			for (var sample = 0; sample < waveform.length; sample += 1) {
				var x = (sample / (waveform.length - 1)) * width;
				var y = height / 2 + ((waveform[sample] - 128) / 128) * (height / 2 - 8);
				if (sample === 0) context.moveTo(x, y);
				else context.lineTo(x, y);
			}
			context.stroke();
		},
		vu: function (frame) {
			var colors = frame.colors,
				width = frame.width,
				height = frame.height;
			context.fillStyle = colors.ground;
			context.fillRect(0, 0, width, height);
			var total = 0;
			for (var sample = 0; sample < waveform.length; sample += 1) {
				var normalized = (waveform[sample] - 128) / 128;
				total += normalized * normalized;
			}
			var rootMeanSquare = Math.sqrt(total / waveform.length);
			// Spring-damper integrated once per frame, clamped just above 1 so the
			// needle can overshoot into the red without leaving the dial.
			var stiffness = 0.18,
				damping = 0.3;
			var acceleration = stiffness * (rootMeanSquare - needle.position) - damping * needle.velocity;
			needle.velocity += acceleration;
			needle.position = Math.max(0, Math.min(1.05, needle.position + needle.velocity));
			[0.28, 0.72].forEach(function (share, dialIndex) {
				var centerX = width * share;
				var centerY = height * 0.86;
				var radius = Math.min(width * 0.2, height * 0.72);
				context.strokeStyle = colors.dim;
				context.lineWidth = 2;
				context.beginPath();
				context.arc(centerX, centerY, radius, Math.PI * 1.17, Math.PI * 1.83);
				context.stroke();
				context.strokeStyle = colors.accent;
				context.lineWidth = 4;
				context.beginPath();
				context.arc(centerX, centerY, radius, Math.PI * 1.66, Math.PI * 1.83);
				context.stroke();
				var wobble = dialIndex === 0 ? 1 : 0.88;
				var angle = Math.PI * 1.17 + Math.PI * 0.66 * Math.min(1, needle.position * wobble);
				context.strokeStyle = colors.ink;
				context.lineWidth = 2.5;
				context.beginPath();
				context.moveTo(centerX, centerY);
				context.lineTo(
					centerX + Math.cos(angle) * radius * 0.94,
					centerY + Math.sin(angle) * radius * 0.94
				);
				context.stroke();
			});
		},
		matrix: function (frame) {
			var colors = frame.colors,
				width = frame.width,
				height = frame.height;
			context.fillStyle = colors.ground;
			context.fillRect(0, 0, width, height);
			var columns = 40,
				rows = 14;
			var heights = barHeights(columns);
			var cellWidth = width / columns,
				cellHeight = height / rows;
			var dot = Math.min(cellWidth, cellHeight) * 0.42;
			for (var column = 0; column < columns; column += 1) {
				var lit = heights[column] * rows;
				for (var row = 0; row < rows; row += 1) {
					var fromBottom = rows - 1 - row;
					var intensity =
						fromBottom < Math.floor(lit) ? 1 : fromBottom === Math.floor(lit) ? lit % 1 : 0;
					context.globalAlpha = intensity > 0 ? 0.25 + intensity * 0.75 : 0.18;
					context.fillStyle = intensity > 0 ? colors.accent : colors.dim;
					context.beginPath();
					context.arc(
						column * cellWidth + cellWidth / 2,
						row * cellHeight + cellHeight / 2,
						dot,
						0,
						Math.PI * 2
					);
					context.fill();
				}
			}
			context.globalAlpha = 1;
		},
		radial: function (frame) {
			var colors = frame.colors,
				width = frame.width,
				height = frame.height;
			context.fillStyle = colors.ground;
			context.fillRect(0, 0, width, height);
			var centerX = width / 2,
				centerY = height / 2;
			var radius = Math.min(width, height) * 0.46;
			var inner = 0.42;
			var count = 72;
			var heights = barHeights(count);
			context.strokeStyle = colors.accent;
			context.lineWidth = Math.max(2, (Math.PI * 2 * radius * inner) / count - 2);
			for (var index = 0; index < count; index += 1) {
				var angle = -Math.PI / 2 + (index / count) * Math.PI * 2;
				var reach = inner + heights[index] * (1 - inner);
				context.beginPath();
				context.moveTo(centerX + Math.cos(angle) * radius * inner, centerY + Math.sin(angle) * radius * inner);
				context.lineTo(centerX + Math.cos(angle) * radius * reach, centerY + Math.sin(angle) * radius * reach);
				context.stroke();
			}
		},
		ambience: function (frame) {
			var colors = frame.colors,
				width = frame.width,
				height = frame.height;
			// Persistence: a near-opaque wash over the previous frame instead of a
			// clear, so the trail decays instead of vanishing. globalAlpha is used
			// rather than an rgba() string built from the token, because a palette
			// entry is as likely to be a CSS custom property as a hex value, and
			// splicing an alpha channel into an arbitrary custom property silently
			// parses to nothing.
			context.globalAlpha = 0.06;
			context.fillStyle = colors.ground;
			context.fillRect(0, 0, width, height);
			context.globalAlpha = 1;
			// The trace itself has to stay a theme color too: a hue cycled on its
			// own timeline, independent of the palette, can land on a color close
			// to the ground's own lightness and read as nearly invisible.
			context.strokeStyle = colors.accent;
			context.shadowBlur = 12;
			context.shadowColor = colors.accent;
			context.lineWidth = 2;
			context.beginPath();
			for (var sample = 0; sample < waveform.length; sample += 1) {
				var x = (sample / (waveform.length - 1)) * width;
				var y = height / 2 + ((waveform[sample] - 128) / 128) * (height / 2 - 10);
				if (sample === 0) context.moveTo(x, y);
				else context.lineTo(x, y);
			}
			context.stroke();
			context.shadowBlur = 0;
		}
	};

	// Size the backing buffer to the element's CSS size times the device pixel
	// ratio, then remap the context so every draw function above still works in
	// plain CSS pixels.
	function sizeCanvas() {
		var ratio = window.devicePixelRatio || 1;
		var rect = canvas.getBoundingClientRect();
		if (rect.width === 0 || rect.height === 0) return false;
		var targetWidth = Math.round(rect.width * ratio);
		var targetHeight = Math.round(rect.height * ratio);
		// Assigning canvas.width or canvas.height resets and clears the whole
		// bitmap even when set to its current value (the HTML spec requires the
		// reset "whenever the width and height attributes are set, whether to a
		// new value or to the previous value"). Calling this every frame was
		// clearing the canvas every frame, which is exactly what the host-never-
		// clears rule above forbids -- it silently broke the ambience mode's
		// persistence trail. Only touch the bitmap, and only reapply the
		// transform, when the target size has actually changed.
		if (canvas.width !== targetWidth || canvas.height !== targetHeight) {
			canvas.width = targetWidth;
			canvas.height = targetHeight;
			context.setTransform(ratio, 0, 0, ratio, 0, 0);
		}
		return true;
	}

	function paintFrame() {
		// An element with no layout yet (display: none, not yet attached) reports
		// a zero-size rect. Painting into that would divide by zero downstream in
		// every mode above, so the frame is skipped rather than guarded per mode.
		if (!sizeCanvas()) return;
		synthesize();
		var rect = canvas.getBoundingClientRect();
		modes[mode]({ width: rect.width, height: rect.height, colors: palette() });
	}

	function tick() {
		if (running) paintFrame();
		requestAnimationFrame(tick);
	}

	function resetScratchState() {
		// Scratch state belongs to the mode that wrote it. Wiping it on every
		// switch is what stops a peak-hold trail or a needle's velocity from
		// bleeding into a mode that never produced it.
		peaks.fill(0);
		needle.position = 0;
		needle.velocity = 0;
		context.clearRect(0, 0, canvas.width, canvas.height);
	}

	Array.prototype.forEach.call(document.querySelectorAll('[data-pcv-mode]'), function (button) {
		button.addEventListener('click', function () {
			mode = button.getAttribute('data-pcv-mode');
			resetScratchState();
			Array.prototype.forEach.call(document.querySelectorAll('[data-pcv-mode]'), function (other) {
				other.classList.toggle('is-on', other === button);
			});
			if (reducedMotion) paintFrame();
		});
	});

	var pauseButton = document.getElementById('pcv-pause');
	if (reducedMotion) {
		// A single static frame stands in for the animation loop entirely --
		// there is nothing running for a pause control to pause.
		pauseButton.disabled = true;
		pauseButton.textContent = 'Static frame';
		paintFrame();
	} else {
		pauseButton.addEventListener('click', function () {
			running = !running;
			pauseButton.textContent = running ? 'Pause' : 'Resume';
		});
		requestAnimationFrame(tick);
	}
})();

Paste this into an agent to rebuild the pattern from scratch.