cegui中的窗口可以再次用opengl画线之类的么？

[OnlyHappy]

在cegui中的例子里面我在drawframe函数里面

void CEGuiOpenGLBaseApplication::drawFrame(void)
{
    CEGUI::System& guiSystem = CEGUI::System::getSingleton();
    // do time based updates
    int thisTime = glutGet(GLUT_ELAPSED_TIME);
    float elapsed = static_cast<float>(thisTime - d_lastFrameTime);
    d_lastFrameTime = thisTime;
    // inject the time pulse
    guiSystem.injectTimePulse(elapsed / 1000.0f);
    // update fps fields
    doFPSUpdate();

    // update logo rotation
    static float rot = 0.0f;
    d_logo_geometry->setRotation(CEGUI::Vector3(rot, 0, 0));
    rot += 180.0f * (elapsed / 1000.0f);
    if (rot > 360.0f)
        rot -= 360.0f;

    // do rendering for this frame.
    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);   // 在这里画线但是没有反应，在另外一个测试程序里面去掉cegui的相关代码，可以画出来
    glLineWidth( 2.0 );
    glBegin( GL_LINES );
    glVertex3f( .0 , .0 , .0 );
    glVertex3f( 100.0 , 100.0 , .0 );
    glEnd();
    guiSystem.renderGUI();
    glutPostRedisplay();
    glutSwapBuffers();

    // here we check the 'quitting' state and cleanup as required.
    // this is probably not the best way to do this, but since we're
    // using glut, and glutMainLoop can never return, we need some
    // way of checking when to exit.  And this is it...
    if (d_quitFlag)
    {
        CEGUI::OpenGLRenderer::destroySystem();

        // exit
        exit(0);
    }
}

[OnlyHappy]