#include <GL/glut.h>

   GLsizei winWidth = 400, winHeight = 300;   // Initial display-window size.

   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 point color to red.
      glPointSize (3.0);                 //  Set point size to 3.0.
   }

   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;
   }

   void plotPoint (GLint x, GLint y)
   {
      glBegin (GL_POINTS);
         glVertex2i (x, y);
      glEnd ( );
   }

   void mousePtPlot (GLint button, GLint action, GLint xMouse, GLint yMouse)
   {
      if (button == GLUT_LEFT_BUTTON && action == GLUT_DOWN)
         plotPoint (xMouse, winHeight - yMouse);

      glFlush ( );
    }

    void main (int argc, char** argv)
    {
       glutInit (&argc, argv);
       glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
       glutInitWindowPosition (100, 100);
       glutInitWindowSize (winWidth, winHeight);
       glutCreateWindow ("Mouse Plot Points");

       init ( );
       glutDisplayFunc (displayFcn);
       glutReshapeFunc (winReshapeFcn);
       glutMouseFunc (mousePtPlot);

       glutMainLoop ( );
   }