diff --git a/config.yaml b/config.yaml
index 3f6f4ab..8e03137 100644
--- a/config.yaml
+++ b/config.yaml
@@ -1,15 +1,7 @@
 operators:
-  nian:
-    common_name: placeholder
-    preview: preview.jpg
-    project_json: project.json
-    release_folder: ./release/{name}/
-    source_folder: ./operator/{name}/extracted/
-    target_folder: ./operator/{name}/processed/
-    title: Nian
-    use_skel: true
   chen:
     common_name: placeholder
+    fallback_name: char_1012_skadi2_2
     preview: preview.jpg
     project_json: project.json
     release_folder: ./release/{name}/
@@ -17,8 +9,19 @@ operators:
     target_folder: ./operator/{name}/processed/
     title: Ch'en the Holungday
     use_skel: true
+  nian:
+    common_name: placeholder
+    fallback_name: char_1012_skadi2_2
+    preview: preview.jpg
+    project_json: project.json
+    release_folder: ./release/{name}/
+    source_folder: ./operator/{name}/extracted/
+    target_folder: ./operator/{name}/processed/
+    title: Nian
+    use_skel: true
   skadi:
     common_name: dyn_illust_char_1012_skadi2
+    fallback_name: char_1012_skadi2_2
     preview: preview.jpg
     project_json: project.json
     release_folder: ./release/{name}/
diff --git a/lib/alpha_composite.py b/lib/alpha_composite.py
index 6e2c34a..e9a936b 100644
--- a/lib/alpha_composite.py
+++ b/lib/alpha_composite.py
@@ -22,6 +22,9 @@ class AlphaComposite:
     def __open_image(self):
         self.image = Image.open(self.image_path)
         self.mask = Image.open(self.mask_path)
+        if self.image.size != self.mask.size:
+            # resize mask
+            self.mask = self.__resize(self.mask, self.image.size[0], self.image.size[1])
         self.loaded_image = self.image.load()
         self.loaded_mask = self.mask.load()
 
@@ -29,4 +32,8 @@ class AlphaComposite:
         for y in range(self.mask.size[1]):
             for x in range(self.mask.size[0]):
                 self.loaded_image[x, y] = (self.loaded_image[x, y][0], self.loaded_image[x, y][1], self.loaded_image[x, y][2], self.loaded_mask[x, y][2])
-        self.image.save(pathlib.Path.cwd().joinpath(self.save_path))
\ No newline at end of file
+        self.image.save(pathlib.Path.cwd().joinpath(self.save_path))
+
+    # resize image
+    def __resize(self, image, width: int, height: int):
+        return image.resize((width, height))
diff --git a/lib/builder.py b/lib/builder.py
index 27cd23a..6c24f90 100644
--- a/lib/builder.py
+++ b/lib/builder.py
@@ -21,12 +21,16 @@ class Builder:
             self.__release_file(operator_name)
         return
 
+    def stop(self):
+        return
+
     def build_assets(self, operator_name):
 
         use_skel = self.config["operators"][operator_name]["use_skel"]
         source_path = self.config["operators"][operator_name]["source_folder"].format(name=operator_name)
         target_path = self.config["operators"][operator_name]["target_folder"].format(name=operator_name)
         common_name = self.config["operators"][operator_name]["common_name"]
+        fallback_name = self.config["operators"][operator_name]["fallback_name"]
         file_paths = dict(
             json=target_path + common_name + ".json",
             atlas=target_path + common_name + ".atlas",
@@ -62,13 +66,19 @@ class Builder:
                 ),
                 daemon=True,
             )
+            fallback_thread = threading.Thread(
+                target=AlphaComposite, 
+                args=(source_path + fallback_name, target_path + "../fallback"),
+                daemon=True,
+            )
 
             ar_thread.start()
             skeleton_binary_thread.start()
+            fallback_thread.start()
             ar_thread.join()
             atlas_base64_thread.start()
 
-            # alpha composite
+            # alpha composite for live 2d assets
             for item in ar.images:
                 alpha_composite_thread = threading.Thread(
                     target=AlphaComposite, 
@@ -121,10 +131,12 @@ class Builder:
                 json_base64_thread.start()
                 json_base64_thread.join()
 
+            # join remaining threads
             for thread in png_to_base64_threads:
                 thread.join()
 
             atlas_base64_thread.join()
+            fallback_thread.join()
             
             jsonContent = prefix + str(data)
             with open(operator_file, "w") as f:
diff --git a/lib/config.py b/lib/config.py
index be6684f..95aa248 100644
--- a/lib/config.py
+++ b/lib/config.py
@@ -18,6 +18,7 @@ class Config:
                 source_folder=str,
                 target_folder=str,
                 common_name=str,
+                fallback_name=str,
                 release_folder=str,
                 project_json=str,
                 preview=str,