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

   GLsizei winWidth = 400, winHeight = 300;   // Initial display-window size.
   GLint edgeLength = 10;                  // Initial edge length for square.

   void init (void)
   {
      glClearColor (0.0, 0.0, 1.0, 1.0)   // Set display-window color to blue.

      glMatrixMode (GL_PROJECTION);
      gluOrtho2D (0.0, 200.0, 0.0, 150.0);
   }

   void displayFcn (void)
   {
      glClear (GL_COLOR_BUFFER_BIT);          //  Clear display window.

      glColor3f (1.0, 0.0, 0.0);              //  Set fill color to red.
   }

   void winReshapeFcn (GLint newWidth, GLint newHeight)
   {
      /*  Reset viewport and projection parameters  */
      glViewport (0, 0, newWidth, newHeight);
      glMatrixMode (GL_PROJECTION);
      glLoadIdentity ( );
      gluOrtho2D (0.0, GLdouble (newWidth), 0.0, GLdouble (newHeight));

      /*  Reset display-window size parameters.  */
      winWidth  = newWidth;
      winHeight = newHeight;
   }

   /*  Display a red square with a selected edge-length size.  */
   void fillSquare (GLint button, GLint action, GLint xMouse, GLint yMouse)
   {
      GLint x1, y1, x2, y2;

      /*  Use left mouse button to select a position for the
       *  lower-left corner of the square.
       */
      if (button == GLUT_LEFT_BUTTON && action == GLUT_DOWN)
      {
         x1 = xMouse;
         y1 = winHeight - yMouse;
         x2 = x1 + edgeLength;
         y2 = y1 + edgeLength;
         glRecti (x1, y1, x2, y2);
      }
      else
         if (button == GLUT_RIGHT_BUTTON)   //  Use right mouse button to quit.
            exit (0);

      glFlush ( );
   }

   /*  Use keys 2, 3, and 4 to enlarge the square.  */
   void enlargeSquare (GLubyte sizeFactor, GLint xMouse, GLint yMouse)
   {
      switch (sizeFactor)
      {
         case '2':
            edgeLength *= 2;
            break;
         case '3':
            edgeLength *= 3;
            break;
         case '4':
            edgeLength *= 4;
            break;
         default:
            break;
      }
   }

   /*  Use function keys F2 and F4 for reduction factors 1/2 and 1/4.  */
   void reduceSquare (GLint reductionKey, GLint xMouse, GLint yMouse)

   {
      switch (reductionKey)
      {
         case GLUT_KEY_F2:
            edgeLength /= 2;
            break;
         case GLUT_KEY_F3:
            edgeLength /= 4;
            break;
         default:
            break;
      }
   }

   void main (int argc, char** argv)
   {
      glutInit (&argc, argv);
      glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
      glutInitWindowPosition (100, 100);
      glutInitWindowSize (winWidth, winHeight);
      glutCreateWindow ("Display Squares of Various Sizes");

      init ( );
      glutDisplayFunc (displayFcn);
      glutReshapeFunc (winReshapeFcn);
      glutMouseFunc (fillSquare);
      glutKeyboardFunc (enlargeSquare);
      glutSpecialFunc (reduceSquare);

      glutMainLoop ( );
   }