diff --git a/VolZmapGraphics.cpp b/VolZmapGraphics.cpp
index 5dc1c13..d8a9683 100644
--- a/VolZmapGraphics.cpp
+++ b/VolZmapGraphics.cpp
@@ -1993,8 +1993,8 @@ VolZmap::ExtMarchingCubes( int nBlock, VoxelContainer& vVox) const
 
                  // calcolo SVD 
                   SvdDecomposer svd( dMatrixN, Eigen::ComputeThinU | Eigen::ComputeThinV) ;
-                  auto dMatrixV = svd.matrixV() ;
-                  auto dSingularValue = svd.singularValues() ;
+                  const auto& dSingularValue = svd.singularValues() ;
+                  const auto& dMatrixV = svd.matrixV() ;
 
                  // Se la feature è un edge annullo il valore singolare minore.
                   if ( nFeatureType == EDGE) {