为什么不同视区下显示的效果不一样？

strong

void display(void)
{
   glClear (GL_COLOR_BUFFER_BIT);
   glColor3f (0.9, 0.8, 0.9);

   

   //glMatrixMode(GL_MODELVIEW);
   glLoadIdentity ();             /* clear the matrix */
           /* viewing transformation  */
   
   gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
       /* modeling transformation */
   
   
   //glTranslatef(0.0,0.0,-5.0);
   //glRotatef(45.0,0.0,1.0,0.0);
   glScalef (1.0, 2.0, 1.0);

   
   glMatrixMode (GL_PROJECTION);
   glLoadIdentity ();
   //glFrustum (-1.0, 1.0, -1.0, 1.0, 1.5, 20.0);
   gluPerspective(75.0,1.0,1.5,20.0);
   
   glViewport(0,0,100,100);
   glutWireCube (1.0);
   
   
   glViewport(100,0,200,100);
   glutWireCube(1.0);

   glViewport(200,0,300,100);
   glutWireCube(1.0);
   
   glFlush ();

}