How to Efficiently Draw a QImage to a Window.

[as2388]

I think we're getting a little distracted trying to optimise the system instead of the drawing.

Assuming that I could draw directly to a widget and therefore had no need for QImages or QPixmaps, I might be able to express that like this:

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

    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(update()));
    timer->start(50);
}

void MainWindow::paintEvent(QPaintEvent *event) {
    QPainter painter(this);
    painter.drawLine(QPointF(10, 10), QPointF(20, 20));
    painter.end();
}

This uses 25% CPU, which seems like a lot just to draw a short line segment. How could this simple example be optimised (if at all)?

[SGaist]

Is your application painting static content ? If so, then don't trigger update that much, there's no real need.

paintEvent will be called when needed so with your last sample, calling update every 50ms is useless and power consuming.

[as2388]

Ok, so that may not have been a great example. In reality it will be dynamic content, so I really do need a frequent screen refresh. This is a slightly more representative example:

int x = 0;
void MainWindow::paintEvent(QPaintEvent *event) {
    if (x > 500) x = 0;
    QPainter painter(this);
    painter.drawLine(QPointF(10, 10), QPointF(x++, x++));
    painter.end();
}

[SGaist]

Do you need some kind of graph ?

[as2388]

No, it's arbitrary shapes.

[SGaist]

Is there some kind of history for these shapes are do you need to re-draw them all every time ?

[as2388]

Need to redraw.

[SGaist]

The first thing I'd do is optimize the data sent e.g. do you really need to build a line of 500 points if you already know the final coordinates ?

Otherwise, maybe consider using OpenGL

[as2388]

So to confirm: there's no way to draw in Qt without invoking the relatively expensive update() first?

[SGaist]

Yes there is, do the drawing in another thread on a QImage and then trigger the update with that image. You have an example of this in the Mandelbrot example

[kshegunov]

@as2388

Yes there is, do the drawing in another thread on a QImage and then trigger the update with that image.

I'd add to @SGaist's great point - even convert the image to a QPixmap in the worker (after drawing it), and in the paint event just splat it onto the widget.