perf(P6B.T7): WebP thumbnails for image attachments (320px max, thumb-first render)

apps/desktop/src/lib/imageCompress.ts

@@ -55,3 +55,51 @@ function renameToWebp(original: string): string {
 export async function compressImages(files: File[]): Promise<File[]> {
   return Promise.all(files.map((f) => compressImage(f)));
 }
+
+// Bandwidth threshold for thumb generation. Below ~50KB the WebP overhead
+// of a fresh re-encode can exceed the original; not worth a second upload.
+const THUMB_SKIP_BELOW_BYTES = 50 * 1024;
+const THUMB_MAX_DIM = 320;
+const THUMB_QUALITY = 0.7;
+// Animated formats lose motion when redrawn onto a Canvas, so we skip them
+// and let the receiver render the full file. GIF is the dominant case; the
+// rest stay too (apng/animated-webp).
+const THUMB_ANIMATED_MIME = /^image\/(gif|apng)$/;
+
+// Generates a small WebP preview thumb (max 320×320) from an image file.
+// Used by the send path so each image attachment can ship a tiny inline
+// preview alongside the encrypted full blob. Returns `null` when:
+//  - the input isn't an image,
+//  - the input is animated (GIF/APNG — would lose motion),
+//  - the input is already small enough that a thumb wouldn't save bandwidth,
+//  - OffscreenCanvas / createImageBitmap aren't available, or
+//  - decode/encode threw (corrupt input).
+// The caller treats `null` as "skip thumb" and uploads only the full blob.
+export async function generateWebPThumb(
+  file: File,
+  maxDim: number = THUMB_MAX_DIM,
+): Promise<Blob | null> {
+  if (!file.type.startsWith('image/')) return null;
+  if (THUMB_ANIMATED_MIME.test(file.type)) return null;
+  if (file.size < THUMB_SKIP_BELOW_BYTES) return null;
+  if (typeof createImageBitmap !== 'function') return null;
+  if (typeof OffscreenCanvas !== 'function') return null;
+  try {
+    const bitmap = await createImageBitmap(file);
+    const ratio = Math.min(maxDim / bitmap.width, maxDim / bitmap.height, 1);
+    const w = Math.max(1, Math.round(bitmap.width * ratio));
+    const h = Math.max(1, Math.round(bitmap.height * ratio));
+    const canvas = new OffscreenCanvas(w, h);
+    const ctx = canvas.getContext('2d');
+    if (!ctx) {
+      bitmap.close();
+      return null;
+    }
+    ctx.drawImage(bitmap, 0, 0, w, h);
+    bitmap.close();
+    return await canvas.convertToBlob({ type: 'image/webp', quality: THUMB_QUALITY });
+  } catch (err: unknown) {
+    console.warn('generateWebPThumb failed', err);
+    return null;
+  }
+}