#include <GL/glut.h>
   #include <math.h>
   #include <stdlib.h>

   const double TWO_PI = 6.2831853;

   /*  Initial display-window size.  */
   GLsizei winWidth = 400, winHeight = 400;  
   GLuint regHex;

   class screenPt {
   public:
     GLint x, y;
   };

   static void init (void)
   {
      screenPt hexVertex, circCtr;
      GLdouble theta;
      GLint k;

      /*  Set circle center coordinates.  */
      circCtr.x = winWidth / 2;           
      circCtr.y = winHeight / 2;

      glClearColor (1.0, 1.0, 1.0, 0.0);   //  Display-window color = white.

      /*  Set up a display list for a red regular hexagon.
       *  Vertices for the hexagon are six equally spaced
       *  points around the circumference of a circle.
       */
      regHex = glGenLists (1);   //  Get an identifier for the display list.
      glNewList (regHex, GL_COMPILE);
         glColor3f (1.0, 0.0, 0.0);   //  Set fill color for hexagon to red.
         glBegin (GL_POLYGON);
            for (k = 0; k < 6; k++) {
               theta = TWO_PI * k / 6.0;
               hexVertex.x = circCtr.x + 150 * cos (theta);
               hexVertex.y = circCtr.y + 150 * sin (theta);
               glVertex2i (hexVertex.x, hexVertex.y);
            }
         glEnd ( );
      glEndList ( );
   }

   void regHexagon (void)
   {
      glClear (GL_COLOR_BUFFER_BIT);

      glCallList (regHex);

      glFlush ( );
   }

   void winReshapeFcn (int newWidth, int newHeight)

   {
      glMatrixMode (GL_PROJECTION);
      glLoadIdentity ( );
      gluOrtho2D (0.0, (GLdouble) newWidth, 0.0, (GLdouble) newHeight);

      glClear (GL_COLOR_BUFFER_BIT);
   }

   void main (int argc, char** argv)
   {
      glutInit (&argc, argv);
      glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
      glutInitWindowPosition (100, 100);
      glutInitWindowSize (winWidth, winHeight);
      glutCreateWindow ("Reshape-Function & Display-List Example");

      init ( );
      glutDisplayFunc (regHexagon);
      glutReshapeFunc (winReshapeFcn);

      glutMainLoop ( );
   }