diff --git a/assert_mipmap.py b/assert_mipmap.py
new file mode 100644
index 0000000..2500c4e
--- /dev/null
+++ b/assert_mipmap.py
@@ -0,0 +1,15 @@
+from PIL import Image
+import sys
+from twiddle import npot
+
+total = 0
+
+in_files = sys.argv[1:]
+for in_file in in_files:
+    with Image.open(in_file) as im:
+        width, height = im.size
+        assert npot(width) == npot(width), width
+        assert npot(height) == npot(height), height
+        if width != height:
+            print(width, height, in_file)
+        total += width * height