In OpenGL, how can I adjust for the window being resized?

Question

I am drawing several shapes (such as circles) that are keyed off of the window height & width. Since the window always starts at a given size, they are drawn correctly,

Answer 1

I am learning opengl too. I faced this problem yesterday and I have not found the correct answer. Today, I have solve the problem. In my little program, the object changes size when resizing the window changing width or heigt. This is my code (please, forgive my errors):

#include 

void fnReshape(int ww, int hh)
{
  GLdouble r;
  if(hh == 0)
    hh = 1.0;
  if(hh > ww)
    r = (double) hh / (double) ww;
  else
    r = (double) ww / (double) hh;
  glViewport(0, 0, ww, hh);
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  if(hh > ww)
    glFrustum(-1.0, 1.0, -1.0 * r, 1.0 * r, 2.0, 200.0);
  else
    glFrustum(-1.0 * r, 1.0 * r, -1.0, 1.0, 2.0, 200.0);
}
//////////////////////////////////////////////////////////////////
void fnDisplay(void)
{
  glClear(GL_COLOR_BUFFER_BIT);
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  glTranslatef(0., 0., -5.);
  glBegin(GL_QUAD_STRIP);
  glColor3f(0., 0., 1.0);
  glVertex3f(-1., -1., 0.);
  glVertex3f(-1., 1., 0.);
  glVertex3f(1., -1., 0.);
  glVertex3f(1., 1., 0.);
  glEnd();
  glutSwapBuffers();
}
////////////////////////////////////////////////////////////
int main(int argc, char **argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA |GLUT_DOUBLE);
    glutInitWindowSize(500, 500);
    glutCreateWindow("Sample window");
    glClearColor(1.0, 0, 0, 1.0);
    glutDisplayFunc(fnDisplay);
    glutReshapeFunc(fnReshape);
    glutMainLoop();
    return 0;
}