Give a background something to cover with

- The stand-in backgrounds were the app's own gradients, which are overlay
  scrims: measured, not one pixel in either is opaque, and where they look
  dark they are transparent. As a background their dark half was simply
  absent, which is what showed on the slow path.
- They are now laid on the canvas colour they are painted over and kept as
  JPEG, which cannot carry transparency at all.
- The same hole was waiting for anyone's own picture: a canvas starts
  transparent, WebP keeps an alpha channel, and a file small enough to keep
  as it was never passed through either. Every picture now goes through an
  opaque ground, whatever its size.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/livekit/backgroundEffects.ts

@@ -5,8 +5,8 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import desktopGradient from "../graphics/desktop-gradient.png?url";
-import mobileGradient from "../graphics/mobile-gradient.png?url";
+import indoorStandIn from "../graphics/background-indoor-standin.jpg?url";
+import outdoorStandIn from "../graphics/background-outdoor-standin.jpg?url";
 
 /** How much to blur, when the chosen effect is blur. */
 export const blurRadius = 15;
@@ -17,11 +17,18 @@ export interface ShippedBackground {
 }
 
 // EXPLORATION SHORTCUT (S1): the two shipped backgrounds, one indoor and one
-// outdoor, do not exist yet. These stand-ins are existing gradient assets, so
-// the pipeline can be exercised and measured. Never port this.
+// outdoor, do not exist yet. These stand-ins are the app's own gradients, laid
+// on the canvas colour they are painted over and kept as JPEG.
+//
+// Not a detail: those gradients are overlay scrims, and measured, not one
+// pixel in either is opaque — where they look dark they are transparent. Used
+// directly as a background their dark half was simply absent, which is what
+// showed on the slow path. Flattening them is what makes them a picture rather
+// than a veil, and JPEG cannot carry transparency at all, so it cannot come
+// back. Never port this.
 export const shippedBackgrounds: ShippedBackground[] = [
-  { id: "indoor", imagePath: desktopGradient },
-  { id: "outdoor", imagePath: mobileGradient },
+  { id: "indoor", imagePath: indoorStandIn },
+  { id: "outdoor", imagePath: outdoorStandIn },
 ];
 
 export type BackgroundEffect =

src/livekit/backgroundImages.ts

@@ -83,16 +83,24 @@ export async function prepareImage(file: Blob): Promise<Blob> {
   }
 
   try {
+    // Every picture goes through the canvas, not only the oversized ones: a
+    // background covers what is behind it, so it has to be opaque, and a
+    // picture with transparency in it — a logo, a screenshot with rounded
+    // corners — would otherwise be stored with its holes and drawn with them.
+    // Passing small files straight through is what kept them.
     const longest = Math.max(bitmap.width, bitmap.height);
-    if (longest <= maxStoredEdge) return file;
-
-    const scale = maxStoredEdge / longest;
+    const scale = Math.min(1, maxStoredEdge / longest);
     const canvas = new OffscreenCanvas(
       Math.round(bitmap.width * scale),
       Math.round(bitmap.height * scale),
     );
     const context = canvas.getContext("2d");
     if (!context) throw new UnusableImage("undecodable");
+    // The ground the picture is laid on, so nothing it does not cover is a
+    // hole. Black rather than white: an unfilled corner reads as the frame's
+    // own edge rather than as a lamp.
+    context.fillStyle = "black";
+    context.fillRect(0, 0, canvas.width, canvas.height);
     context.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
     return await canvas.convertToBlob({ type: "image/webp", quality: 0.9 });
   } finally {