OpenGL glBegin（GL_TRIANGLES）只是第一次工作

Question

我尝试将Qml和OpenGL联系起来，所以我举了例子openglunderqml
在示例中，我创建自己的类

class QProject1Main : public QQuickItem

void QProject1Main::handleWindowChanged(QQuickWindow *win) {
    if (win) {
        connect(win, SIGNAL(beforeRendering()), this, SLOT(paint()), Qt::DirectConnection);
        win->setClearBeforeRendering(false);
    }
}

void QProject1Main::paint() {
    qDebug() << "QProject1Main::paint()";
    qreal ratio = window()->devicePixelRatio();
    int w = int(ratio * window()->width());
    int h = int(ratio * window()->height());
    glViewport(0, 0, w, h);

   // each iteration bg color change from green to read
    glClearColor(((float)++idx)/100, 0.3, 0.3, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glLoadIdentity();
   // triangle visible only at first iteration
    glColor3f(0.5f, 0.5f, 0.5f);
    glBegin(GL_TRIANGLES);
      glVertex3f( 0.1f, 0.8f, 0.1f);
      glVertex3f(-0.8f,-0.8f, 0.1f);
      glVertex3f( 0.8f,-0.8f, 0.1f);
    glEnd();
}

我调整窗口大小来调用paint方法，看到背景颜色发生了变化但是第一次只涂了三角形（