OpenGL and frame drop

[Sfiet_Konstantin]

Hi
I'm currently working on an application that have to display pictures very quickly (1/60 s). I need to have 60 fps so I decided to use a QGLWidget and enabled VSync with

@format.setSwapInterval(1);
@

But currently there are two strange problems.

1. I have on my laptop (Lenovo X200 - Intel X4500 HD) 30FPS instead of 60. I can get 60 FPS with
   @format.setSwapInterval(0);
   @

2. More important. I sometimes have "frame drop". When I need to display a picture for the 1/60s (ie on 1 frame), sometimes it is not displayed at all. Is there any way to force drawing it or at least detect this drop ?

My testing code :

@
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

QGLWidget *widget = new QGLWidget(this);
QGLFormat format = QGLFormat();
format.setSwapInterval(0);
widget->setFormat(format);

ui->graphicsView->setViewport(widget);

m_scene = new QGraphicsScene(this);
m_scene->setBackgroundBrush(Qt::black);

ui->graphicsView->setScene(m_scene);

m_mainItem = m_scene->addRect(0, 0, 100, 100, QPen(), QBrush(Qt::blue));
m_subItem = m_scene->addRect(0, 0, 100, 100, QPen(), QBrush(Qt::red));
m_subItem->setVisible(false);

QTimer *timer = new QTimer(this);
timer->setInterval(0);
timer->start();

connect(timer, SIGNAL(timeout()), this, SLOT(slotTimeout()));

}

MainWindow::~MainWindow()
{
delete ui;
}

void MainWindow::slotTimeout()
{
// Redraw
m_counter ++;

m_subItem->setVisible(m_counter % 60 == 0);
ui->graphicsView->viewport()->update();

}
@