glutReshapeFunc

hi ike,
 I just typed exactly this sample program. I understand all, except for glutReshapeFunc. Whenever i resize the window, it will invoke winReshapeFcn. However, i don't see how winReshapeFcn work.  please explain. Thanks

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#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
{

      private:
            GLint x, y;

      public :
            /* Default Constructor : initializes coordinate position to (0,0) */
            screenPt()
            {
                  x = y = 0;
            }
            void setCoords (GLint xCoord, GLint yCoord)
            {
                  x = xCoord;
                  y = yCoord;
            }
            GLint getx() const
            {
                  return x;
            }
            GLint gety() const
            {
                  return y;
            }
};

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

      /* set circle center coordinates */
      circCtr.setCoords (winWidth/2, winHeight/2);
      glClearColor (1.0, 1.0, 1.0, 0.0); // display-windows 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.setCoords(circCtr.getx() + 150*cos(theta),
                                                      circCtr.gety()+ 150*sin(theta));
                        glVertex2i( hexVertex.getx(), hexVertex.gety());
                  }
            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();
}