Question

My instructor says they can be used interchangeably to affect the projection matrix. He argues that gluLookAt() is preferably used because of its ‘relative simplicity due to its ability to define the viewing angle’. Is this true? I’ve seen code examples using both gluLookAt() and glFrustum() and I’m wondering why would the programmmer mix them.

Like in the cube.c example of the redbook appears:

void display(void)
{
  glClear(GL_COLOR_BUFFER_BIT);
  glColor3f(1.0, 1.0, 1.0);
  glLoadIdentity(); 

  gluLookAt(0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
  **//why isn't this a call to glFrustum?** 
  glScalef(1.0, 2.0, 1.0); 
  glutWireCube(1.0);
  glFlush();
}

void reshape(int w, int h)
{

  glViewport(0, 0, (GLsizei) w, (GLsizei) h);
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  glFrustum(-1.0, 1.0, -1.0, 1.0, 1.5, 20.0); 
  //why isn't this a call a call to gluLookAt()? 
  glMatrixMode(GL_MODELVIEW);
}

Answer 1

Both glFrustum and gluLookAt perform simple matrix multiplication. Check the man pages for equations for those matrices:

gluLookAt

glFrustum

Both of those can be replaced by a glMultMatrix* call.

The most important difference is that glFrustum is used most of the time to establish a perspective projection matrix (used internally by gluPerspective), and gluLookAt is a convenience method for specifying model-view matrices (usually: implementing a camera).