diff --git a/demos/opengl/trackball.h b/demos/opengl/trackball.h
new file mode 100644
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+++ b/demos/opengl/trackball.h
@@ -0,0 +1,42 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2008 Gael Guennebaud <gael.guennebaud@inria.fr>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef EIGEN_TRACKBALL_H
+#define EIGEN_TRACKBALL_H
+
+#include <Eigen/Geometry>
+
+class Camera;
+
+class Trackball
+{
+  public:
+
+    enum Mode {Around, Local};
+
+    Trackball() : mpCamera(0) {}
+
+    void start(Mode m = Around) { mMode = m; mLastPointOk = false; }
+
+    void setCamera(Camera* pCam) { mpCamera = pCam; }
+
+    void track(const Eigen::Vector2i& newPoint2D);
+
+  protected:
+
+    bool mapToSphere( const Eigen::Vector2i& p2, Eigen::Vector3f& v3);
+
+    Camera* mpCamera;
+    Eigen::Vector3f mLastPoint3D;
+    Mode mMode;
+    bool mLastPointOk;
+
+};
+
+#endif // EIGEN_TRACKBALL_H