kaku=30;
    dt=2*M_PAI/kaku;
    for(i=0;i<kaku;i++){
     glBegin(GL_POLYGON);
     glVertex3d(cos(dt*i),sin(dt*i),0.0);
     glVertex3d(cos(dt*(i+1)),sin(dt*(i+1)),0.0);
     glVertex3d(cos(dt*(i+1)),sin(dt*(i+1)),1.0);
     glVertex3d(cos(dt*i),sin(dt*i),1.0);
     glEnd();
     glFlush();
}