Restore premultiplied alpha preprocessing (matches training data)

app.py

@@ -375,10 +375,9 @@ def preprocess_image(input: Image.Image) -> Image.Image:
     size = int(size * 1)
     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 = np.array(output)
-    alpha = output_np[:, :, 3]
-    output_np[:, :, :3][alpha < 0.05 * 255] = [0, 0, 0]
-    output = Image.fromarray(output_np[:, :, :3])
+    output = np.array(output).astype(np.float32) / 255
+    output = output[:, :, :3] * output[:, :, 3:4]
+    output = Image.fromarray((output * 255).astype(np.uint8))
     return output