diff --git a/BBox3d.cpp b/BBox3d.cpp
index 17c9f02..1e8b0a1 100644
--- a/BBox3d.cpp
+++ b/BBox3d.cpp
@@ -74,6 +74,24 @@ BBox3d::Add( const Point3d& ptP)
        m_ptMax.z = ptP.z ;
 }
 
+//----------------------------------------------------------------------------
+void
+BBox3d::Add( double dX, double dY, double dZ)
+{
+   if ( dX < m_ptMin.x)
+       m_ptMin.x = dX ;
+   if ( dY < m_ptMin.y)
+       m_ptMin.y = dY ;
+   if ( dZ < m_ptMin.z)
+       m_ptMin.z = dZ ;
+   if ( dX > m_ptMax.x)
+       m_ptMax.x = dX ;
+   if ( dY > m_ptMax.y)
+       m_ptMax.y = dY ;
+   if ( dZ > m_ptMax.z)
+       m_ptMax.z = dZ ;
+}
+
 //----------------------------------------------------------------------------
 void
 BBox3d::Add( const BBox3d& b3B)
@@ -130,7 +148,7 @@ BBox3d::GetCenterExtent( Point3d& ptCenter, Vector3d& vtExtent) const
    if ( ! IsValid())
       return false ;
    ptCenter = 0.5 * ( m_ptMin + m_ptMax) ;
-   vtExtent = 0.5 * ( m_ptMin - m_ptMax) ;
+   vtExtent = 0.5 * ( m_ptMax - m_ptMin) ;
    return true ;
 }