void Camera :: setModelViewMatrix(void)
{ // load modelview matrix with existing camera values
	float m[16];
	Vector3 eVec(eye.x, eye.y, eye.z); // a vector version of eye 
	m[0] =  u.x; m[4] =  u.y; m[8]  =  u.z;  m[12] = -eVec.dot(u);
	m[1] =  v.x; m[5] =  v.y; m[9]  =  v.z;  m[13] = -eVec.dot(v);
	m[2] =  n.x; m[6] =  n.y; m[10] =  n.z;  m[14] = -eVec.dot(n);
	m[3] =  0;   m[7] =  0;   m[11] =  0;    m[15] = 1.0;
	glMatrixMode(GL_MODELVIEW);
	glLoadMatrixf(m); // load OpenGL’s modelview matrix
}
void Camera:: set(Point3 Eye, Point3 look, Vector3 up)
{	// create a modelview matrix and send it to OpenGL
	eye.set(Eye); // store the given eye position
n.set(eye.x - look.x, eye.y - look.y, eye.z - look.z); // make n
	u.set(up.cross(n)); // make u = up X n
	n.normalize(); u.normalize(); // make them unit length
	v.set(n.cross(u));  // make v =  n X u
	setModelViewMatrix(); // tell OpenGL 
}