fix mask creation

app.py

@@ -42,7 +42,11 @@ def prepare_image_and_mask(
         fill="white",
     )
     mask = Image.new("RGB", image.size, "black")
-    mask = ImageOps.expand(mask, border=(0, 20, 0, 0), fill="white")
+    mask = ImageOps.expand(
+        mask,
+        border=(padding_left, padding_top, padding_right, padding_bottom),
+        fill="white",
+    )
     return background, mask