Неправильно отображается

void DrawQuad(float x1, float y1, float x2, float y2, float z)
{
      glColor3f(0.7f, 0.1f, 0.0f);
glBegin(GL_POLYGON);   // передняя
glVertex3f(x1, y1, 0.0f);
glVertex3f(x2, y1, 0.0f);
glVertex3f(x2, y2, 0.0f);
glVertex3f(x1, y2, 0.0f);
glEnd();
      if (z == 0.0f)
        return;
 
      glColor3f(0.2f, 0.1f, 0.3f);
glBegin(GL_POLYGON);   // задняя 
glVertex3f(x1, y1, -z);
glVertex3f(x2, y1, -z);
glVertex3f(x2, y2, -z);
glVertex3f(x1, y2, -z);
glEnd();
 
     glColor3f(0.2f, 0.8f, 0.3f);
glBegin(GL_POLYGON);   // боковая>>
glVertex3f(x2, y1, 0.0f);
glVertex3f(x2, y2, 0.0f);
glVertex3f(x2, y2, -z);
glVertex3f(x2, y1, -z);
glEnd();
 
     glColor3f(1.0f, 0.8f, 0.3f);
glBegin(GL_POLYGON);  // боковая<<
glVertex3f(x1, y1, 0.0f);
glVertex3f(x1, y2, 0.0f);
glVertex3f(x1, y2, -z);
glVertex3f(x1, y1, -z);
glEnd();
 
     glColor3f(0.2f, 0.8f, 1.0f);
glBegin(GL_POLYGON);  // верхняя
glVertex3f(x1, y2, 0.0f);
glVertex3f(x2, y2, 0.0f);
glVertex3f(x2, y2, -z);
glVertex3f(x1, y2, -z);
glEnd();
     glColor3f(0.2f, 1.0f, 0.3f);
glBegin(GL_POLYGON);  // нижняя
glVertex3f(x1, y1, 0.0f);
glVertex3f(x2, y1, 0.0f);
glVertex3f(x2, y1, -z);
glVertex3f(x1, y1, -z);
glEnd();
}
 
 
void Display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
    DrawQuad(0.1f, 0.1f, 0.7f, 0.3f, 0.2f);
    DrawQuad(-0.5f, 0.5f, 0.5f, 0.7f, 0.8f);
    DrawQuad(-0.9f, -0.9f, -0.7f, 0.1f, 0.9f);
 
    glFlush();
}
 
void Init()
{
    glClearColor(0.0, 0.0, 0.0, 1.0);
    glEnable(GL_DEPTH_TEST);
 
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    
    glOrtho(-1.0f, 1.0f, -1.0f, 1.0f, -1.0f, 1.0f);
 
 
    glEnable(GL_NORMALIZE);
}
 
void Keyboard(unsigned char key, int x, int y)
{
    glRotatef(3, 1.0, -0.3, 0.1);
    glutPostRedisplay();
}
int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE | GLUT_DEPTH);
    glutInitWindowSize(600, 600);
    glutInitWindowPosition(400,100);
    glutCreateWindow("Test");
 
    Init();
    glutDisplayFunc(Display);
    glutKeyboardFunc(Keyboard);
 
    glutMainLoop();
    return 0;
}