perf: cache post-geometry pixels for faster adjustments

Split processing into geometry phase (crop/resize) and adjustment phase.
Cache the post-geometry pixels keyed by image fingerprint + crop/resize config.
When only adjustments change, skip expensive decode/crop/resize operations.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/lib/processing/photonWorker.ts

@@ -4,6 +4,15 @@ import type { Device, PipelineConfig } from '../types';
 
 let photon: typeof import('@silvia-odwyer/photon') | null = null;
 
+// Cache for post-crop/resize data to speed up adjustment-only changes
+type CachedGeometry = {
+	pixels: Uint8ClampedArray;
+	width: number;
+	height: number;
+};
+const geometryCache: Map<string, CachedGeometry> = new Map();
+const MAX_CACHE_SIZE = 10; // Keep last 10 unique geometry states
+
 async function initPhoton() {
 	if (photon) return photon;
 	const module = await import('@silvia-odwyer/photon');
@@ -12,6 +21,38 @@ async function initPhoton() {
 	return photon;
 }
 
+function buildGeometryCacheKey(
+	imageData: ArrayBuffer,
+	device: Device,
+	config: PipelineConfig
+): string {
+	// Use image size + first 1000 bytes as fingerprint
+	const view = new Uint8Array(imageData);
+	const fingerprint = Array.from(view.slice(0, 1000)).join(',');
+	const imageKey = `${imageData.byteLength}:${fingerprint}`;
+
+	// Include geometry-affecting config
+	const cropKey = config.crop.enabled
+		? `crop:${config.crop.mode}:${JSON.stringify(config.crop.region ?? null)}`
+		: 'crop:off';
+	const resizeKey = config.resize.enabled ? `resize:${device.width}x${device.height}` : 'resize:off';
+
+	return `${imageKey}|${cropKey}|${resizeKey}`;
+}
+
+function pruneCache(): void {
+	if (geometryCache.size > MAX_CACHE_SIZE) {
+		// Remove oldest entries (first in map)
+		const keysToDelete = Array.from(geometryCache.keys()).slice(
+			0,
+			geometryCache.size - MAX_CACHE_SIZE
+		);
+		for (const key of keysToDelete) {
+			geometryCache.delete(key);
+		}
+	}
+}
+
 function getPhoton() {
 	if (!photon) throw new Error('Photon not initialized');
 	return photon;
@@ -185,6 +226,21 @@ async function processImage(
 	await initPhoton();
 	const p = getPhoton();
 
+	// Check geometry cache first
+	const cacheKey = buildGeometryCacheKey(imageData, device, config);
+	const cached = geometryCache.get(cacheKey);
+
+	let geometryPixels: Uint8ClampedArray;
+	let geometryWidth: number;
+	let geometryHeight: number;
+
+	if (cached) {
+		// Use cached post-geometry data - skip expensive crop/resize
+		geometryPixels = new Uint8ClampedArray(cached.pixels);
+		geometryWidth = cached.width;
+		geometryHeight = cached.height;
+	} else {
+		// Run geometry phase: decode → crop → resize
 		let img;
 		try {
 			const { pixels, width, height } = await decodeImage(imageData);
@@ -256,6 +312,35 @@ async function processImage(
 				img = resized;
 			}
 
+			// Extract pixel data after geometry phase
+			const imgData = getImageData(img);
+			geometryPixels = imgData.data;
+			geometryWidth = imgData.width;
+			geometryHeight = imgData.height;
+
+			// Cache this geometry result
+			geometryCache.set(cacheKey, {
+				pixels: new Uint8ClampedArray(geometryPixels),
+				width: geometryWidth,
+				height: geometryHeight
+			});
+			pruneCache();
+		} finally {
+			try {
+				img.free();
+			} catch {
+				/* ignore */
+			}
+		}
+	}
+
+	// Adjustment phase: work on a copy of geometry pixels
+	let data = geometryPixels;
+	const width = geometryWidth;
+	const height = geometryHeight;
+	let img = fromImageData(data, width, height);
+
+	try {
 		// Adjustments
 		if (config.brightness.enabled && config.brightness.value !== 0) {
 			p.adjust_brightness(img, Math.round(config.brightness.value * 0.5));
@@ -275,8 +360,7 @@ async function processImage(
 
 		// Get pixel data for manual operations
 		const imgData = getImageData(img);
-		let { data } = imgData;
+		data = imgData.data;
-		const { width, height } = imgData;
 
 		// Auto-levels
 		if (config.autoLevels.enabled) {