diff --git a/visualizations/animation.py b/visualizations/animation.py
index 08529b1e..4ebf96ff 100644
--- a/visualizations/animation.py
+++ b/visualizations/animation.py
@@ -23,17 +23,19 @@
 
 
 def build_mask(s: int, margin: float = 2., dtype=torch.float32):
-    mask = torch.zeros(1, 1, s, s, dtype=dtype)
+    mask = torch.ones(1, 1, s, s, dtype=dtype)
     c = (s - 1) / 2
     t = (c - margin / 100. * c) ** 2
     sig = 2.
-    for x in range(s):
-        for y in range(s):
-            r = (x - c) ** 2 + (y - c) ** 2
-            if r > t:
-                mask[..., x, y] = np.exp((t - r) / sig ** 2)
-            else:
-                mask[..., x, y] = 1.
+    y, x = np.ogrid[:s, :s]
+    r = (x - c) ** 2 + (y - c) ** 2
+    # r > t
+    outer_mask = ((t - r) / sig ** 2)
+    outer_mask = outer_mask ** (r > t)  # To prevent overflow
+    outer_mask = (r > t) * np.exp(outer_mask)
+    # r <= t
+    inner_mask = (r <= t)
+    mask = mask * outer_mask + mask * inner_mask
     return mask