little practice of two viewports

Practice of "Try this" in Red-Book chapter 3.

Target: 
Modify an existing program so that an object is drawn twice, in different Viewports.

Answer:

void reshape (int w, int h)
{
   glClear (GL_COLOR_BUFFER_BIT);
   glColor3f (1.0, 1.0, 1.0);
   
   glViewport (0, 0, (GLsizei) w/2, (GLsizei) h);
   glMatrixMode (GL_PROJECTION);
   glLoadIdentity ();
   glFrustum (-1.0, 1.0, -1.0, 1.0, 1.5, 20.0);
   glMatrixMode (GL_MODELVIEW);
   glLoadIdentity (); 
   gluLookAt (0.0, 0.0, 3.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
   glScalef (1.0, 2.0, 1.0);      /* modeling transformation */ 
   glutWireCube (1.0);

   glViewport ((GLsizei) w/2, 0, (GLsizei) w/2, (GLsizei) h);
   glMatrixMode (GL_PROJECTION);
   glLoadIdentity ();
   glFrustum (-1.0, 1.0, -1.0, 1.0, 1.5, 20.0);
   glMatrixMode (GL_MODELVIEW);
   glLoadIdentity (); 
   gluLookAt (0.0, 0.0, 3.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
   glScalef (1.0, 2.0, 1.0);      /* modeling transformation */ 
   glutWireCube (1.0);

    glFlush ();
}