// -- Class Button -- 
GLvoid button::draw(GLfloat x, GLfloat y, GLfloat z, GLfloat height, GLfloat width) {
  glBegin(GL_QUADS);
    glVertex3f(x, y, z);
    glVertex3f(x+width, y, z);
    glVertex3f(x+width, y-height, z);
    glVertex3f(x, y-width, z);
  glEnd();
}

bool button::IsButtonClicked(GLfloat mouse_x, GLfloat mouse_y) {
  GLfloat x = get_coordx();
  GLfloat y = get_coordy();
  GLfloat width = get_width();
  GLfloat height = get_height();

  // mouse coord are screen coord and x,y are openGL coord, how to solve ???
  if((mouse_x >= x && mouse_y <= x+width) && (mouse_y <= y && mouse_y >= y-height))
     return true;
  return false;
}
// -----------------

int DrawGLScene(GLvoid)	{
  // ...

  // button bttn (x, y, z, height, width);
  button bttn1(8.6f, 9.63f, -1.0f, 0.8f, 3.0f);
  bttn1.draw();

  // ...
}

LRESULT CALLBACK WndProc( HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam){
  // ...

  case WM_LBUTTONDOWN:
  {
    POINTS ptClicked;
    ptClicked = MAKEPOINTS(lParam);

    if(bttn1.IsButtonClicked(ptClicked.x, ptClicked.y))
      // Do Something

    return 0;
   }

  // ...
}

case WM_LBUTTONDOWN:{  POINTS ptClicked;  ptClicked = MAKEPOINTS(lParam);  GLfloat wx=ptClicked.x, wy = ptClicked.y, wz=0.0f;  GLdouble x, y, z;  GLdouble modelview[16];  GLdouble projection[16];  GLint viewport[4];			  glGetIntegerv(GL_VIEWPORT, viewport);  glGetDoublev(GL_MODELVIEW_MATRIX,modelview);  glGetDoublev(GL_PROJECTION_MATRIX,projection);  gluUnProject(wx, wy, wz, modelview, projection, viewport, &x, &y, &z);  if(bttn_create.IsButtonClicked(x, y))    // Do Something;