diff --git a/demos/opengl/trackball.cpp b/demos/opengl/trackball.cpp
index 77ac790..7c2da8e 100644
--- a/demos/opengl/trackball.cpp
+++ b/demos/opengl/trackball.cpp
@@ -23,7 +23,7 @@
   {
     Vector3f axis = mLastPoint3D.cross(newPoint3D).normalized();
     float cos_angle = mLastPoint3D.dot(newPoint3D);
-    if ( internal::abs(cos_angle) < 1.0 )
+    if ( std::abs(cos_angle) < 1.0 )
     {
       float angle = 2. * acos(cos_angle);
       if (mMode==Around)