   why the teapot not reversal?
  the glOrtho function :if zNear is negative then the near clipping-plane( also observing plane) is behind the viewpoint and the object we see will be upside-down. but my demo contract with it.  here, the viewpoint is (0.0, 0.0, 5.0),so i set the near clipping-plane's z=6, but the result is same as the ones that set the near clipping-plane's z=-0.5..  why not upside-down? thank youvoid glOrtho(  GLdouble left,  GLdouble right,  GLdouble bottom,  GLdouble top,  GLdouble zNear,  GLdouble zFar);**************my demo********************void display(void){   glClear (GL_COLOR_BUFFER_BIT);   glColor3f (1.0, 1.0, 1.0);   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);   glutWireTeapot(0.5);//the object to display   glFlush ();}void reshape (int w, int h){   glViewport (0, 0, (GLsizei) w, (GLsizei) h);    glMatrixMode (GL_PROJECTION);   glLoadIdentity ();   glOrtho(-1.0,1.0,-1.0,1.0,-6.0,20.0);   glMatrixMode (GL_MODELVIEW);}[Edited by - sunrisefe on September 17, 2007 3:23:46 AM]
  i am on the way to studying the OPENGL programing, so i meet many problems ,i hope some warm-hearted people would give me some suggestion and help. thank you all.   