Revert to premultiplied alpha on black (original preprocessing)

app.py

@@ -376,8 +376,7 @@ def preprocess_image(input: Image.Image) -> Image.Image:
     bbox = center[0] - size // 2, center[1] - size // 2, center[0] + size // 2, center[1] + size // 2
     output = output.crop(bbox)  # type: ignore
     output = np.array(output).astype(np.float32) / 255
-    alpha = output[:, :, 3:4]
-    output = output[:, :, :3] * alpha + (1 - alpha)  # composite on white
+    output = output[:, :, :3] * output[:, :, 3:4]
     output = Image.fromarray((output * 255).astype(np.uint8))
     return output