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.