QPainter very slow on QGLWidget even in simple example?

[dpkay]

As far as I understood it, QPainter would be a convenient way to draw simple shapes in my GL widget without going through the hassle of individual vertices, but also without any significant loss of performance. However, in a simple rectangle test, native OpenGL still seems to be faster by a factor of 30. Am I doing something wrong?

Here's the code of my QGLWidget's paintEvent. The vector testpos contains 10'000 points as of now. I would expect no changes in the OpenGL state between the drawRect calls.

void GLWidget::paintEvent(QPaintEvent *event)
{
    makeCurrent();
    qglClearColor(Qt::black);
    setupViewport(width(), height());
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
    // draw rectangles using native OpenGL code
    // normalized coordinates
    QTime stopwatch;
    stopwatch.start();
    glColor4f(0.0f,0.0f,1.0f,1.0f);
    foreach(const QPointF * pt, testpos)
    {
        glBegin(GL_LINE_LOOP);
        glVertex2f(pt->x(), pt->y());
        glVertex2f(pt->x()+0.03, pt->y());
        glVertex2f(pt->x()+0.03, pt->y()+0.03);
        glVertex2f(pt->x(), pt->y()+0.03);
        glEnd();
    }
    qDebug() << "elapsed msec native gl: " << stopwatch.elapsed();
 
 
    // draw rectangles using QPainter
    // screen space coordinates
    stopwatch.start();
    QPainter p(this);
    p.setPen(Qt::blue);
    foreach(const QPointF * pt, testpos)
    {
        p.drawRect(pt->x()*500, pt->y()*500, 20, 20);
    }
    p.end();
    qDebug() << "elapsed msec painter: " << stopwatch.elapsed();
}

void GLWidget::paintEvent(QPaintEvent *event)
{
    makeCurrent();
    qglClearColor(Qt::black);
    setupViewport(width(), height());
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // draw rectangles using native OpenGL code
    // normalized coordinates
    QTime stopwatch;
    stopwatch.start();
    glColor4f(0.0f,0.0f,1.0f,1.0f);
    foreach(const QPointF * pt, testpos)
    {
        glBegin(GL_LINE_LOOP);
        glVertex2f(pt->x(), pt->y());
        glVertex2f(pt->x()+0.03, pt->y());
        glVertex2f(pt->x()+0.03, pt->y()+0.03);
        glVertex2f(pt->x(), pt->y()+0.03);
        glEnd();
    }
    qDebug() << "elapsed msec native gl: " << stopwatch.elapsed();


    // draw rectangles using QPainter
    // screen space coordinates
    stopwatch.start();
    QPainter p(this);
    p.setPen(Qt::blue);
    foreach(const QPointF * pt, testpos)
    {
        p.drawRect(pt->x()*500, pt->y()*500, 20, 20);
    }
    p.end();
    qDebug() << "elapsed msec painter: " << stopwatch.elapsed();
}

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The output I get is:
elapsed msec native gl: 2
elapsed msec painter: 65
elapsed msec native gl: 2
elapsed msec painter: 76
...

Omitting one or the other drawing method doesn't change anything, and it's not a measurement error. The delay scales with the number of rectangles, so it has to be a problem in the drawRect. What could be wrong here?

Best wishes,
- Dominik