#include <GL/glut.h>

   GLint winWidth = 600, winHeight = 600;   //  Initial display-window size.

   GLfloat x0 = 100.0, y0 = 50.0, z0 = 50;  //  Viewing-coordinate origin.
   GLfloat xref = 50.0, yref = 50.0, zref = 0.0;  //  Look-at point.
   GLfloat Vx = 0.0, Vy = 1.0, Vz = 0.0;          //  View up vector.

   /*  Set coordinate limits for the clipping window:  */
   GLfloat xwMin = -40.0, ywMin = -60.0, xwMax = 40.0, ywMax = 60.0;

   /*  Set positions for near and far clipping planes:  */
   GLfloat dnear = 25.0, dfar = 125.0;

   void init (void)
   {
      glClearColor (1.0, 1.0, 1.0, 0.0);

      glMatrixMode (GL_MODELVIEW);
      gluLookAt (x0, y0, z0, xref, yref, zref, Vx, Vy, Vz);

      glMatrixMode (GL_PROJECTION);
      glFrustum (xwMin, xwMax, ywMin, ywMax, dnear, dfar);
   }

   void displayFcn (void)
   {
      glClear (GL_COLOR_BUFFER_BIT);

      /*  Set parameters for a square fill area.  */
      glColor3f (0.0, 1.0, 0.0);         //  Set fill color to green.
      glPolygonMode (GL_FRONT, GL_FILL);
      glPolygonMode (GL_BACK, GL_LINE);  //  Wire-frame back face.
      glBegin (GL_QUADS);
         glVertex3f (0.0, 0.0, 0.0);
         glVertex3f (100.0, 0.0, 0.0);
         glVertex3f (100.0, 100.0, 0.0);
         glVertex3f (0.0, 100.0, 0.0);
      glEnd ( );

      glFlush ( );
   }

   void reshapeFcn (GLint newWidth, GLint newHeight)
   {
      glViewport (0, 0, newWidth, newHeight);

      winWidth = newWidth;
      winHeight = newHeight;
   }

   void main (int argc, char** argv)
   {
      glutInit (&argc, argv);
      glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
      glutInitWindowPosition (50, 50);
      glutInitWindowSize (winWidth, winHeight);
      glutCreateWindow ("Perspective View of A Square");

      init ( );
      glutDisplayFunc (displayFcn);
      glutReshapeFunc (reshapeFcn);
      glutMainLoop ( );
   }