diff --git a/3dgs.py b/3dgs.py
index 3289a45..5e75a42 100644
--- a/3dgs.py
+++ b/3dgs.py
@@ -158,7 +158,7 @@ def preprocess(
 
             # compute radius, by finding eigenvalues of 2d covariance
             # transfrom point from NDC to Pixel
-            mid = 0.5 * (cov[0] + cov[1])
+            mid = 0.5 * (cov[0] + cov[2])
             lambda1 = mid + sqrt(max(0.1, mid * mid - det))
             lambda2 = mid - sqrt(max(0.1, mid * mid - det))
             my_radius = ceil(3 * sqrt(max(lambda1, lambda2)))