diff --git a/utils/mcxsvmc.m b/utils/mcxsvmc.m
index 26cab2bf..ae36a531 100644
--- a/utils/mcxsvmc.m
+++ b/utils/mcxsvmc.m
@@ -153,6 +153,9 @@
 %% computing total area and normal vector for each boundary voxel
 areas = elemvolume(iso.vertices, iso.faces);
 normals = surfacenorm(iso.vertices, iso.faces);
+if(isoctavemesh)
+    normals = -normals;
+end
 centroids = meshcentroid(iso.vertices, iso.faces);
 
 totalarea = zeros(size(vol));