// Specify positions of all clipping planes
    //  - this is called from WorldView.draw()
    //  - it calls placeClipPlane() below 6 times -- once for each
    //    side of the view volume
    public void placeClipPlanes(GL gl)
    {
	gl.glPushMatrix();
        gl.glTranslated(-Camera.tx(), -Camera.ty(), -Camera.tz());
        gl.glRotated(-Camera.roll(), 0.0, 0.0, 1.0);
        gl.glRotated(-Camera.yaw(), 0.0, 1.0, 0.0);
        gl.glRotated(-Camera.pitch(), 1.0, 0.0, 0.0);

	placeClipPlane(gl, 5, new Point3d(Camera.right(),Camera.bottom(),Camera.near()),
			      new Point3d(Camera.right(),Camera.top(),Camera.near()),
			      new Point3d(Camera.left(),Camera.top(),Camera.near()));

	gl.glPopMatrix();
    }

    // Specify position of one particular clipping plane given 3
    // points on the plane ordered counter-clockwise
    void placeClipPlane(GL gl, int plane, Point3d p1, Point3d p2, Point3d p3)
    {
        // Plane equation Ax + By + Cz + D stored as [A,B,C,D]
        double[] eqn = new double[4];

        // Compute the plane equation from the 3 points -- fill in eqn[]
	//
	Vector3d v1 = new Vector3d(p1);
	Vector3d v2 = new Vector3d(p3);
	v1.sub(p2);
	v2.sub(p2);

	Vector3d normal = new Vector3d();
	normal.cross(v1,v2);
	normal.normalize();

	eqn[0]=normal.x;
	eqn[1]=normal.y;
	eqn[2]=normal.z;
	eqn[3]=-(p1.x*normal.x+p1.y*normal.y+p1.z*normal.z);

	System.out.println(" " + p1.z);

        // Specify the clipping plane
        gl.glClipPlane(plane, eqn, 0);
    }