How to Efficiently Draw a QImage to a Window.

[as2388]

In my case I don't think so. But even if I could I don't see how that would help - the significant computational cost seems to be in getting paintEvent() to run in the first place, not initialising, manipulating or drawing QImages.

[SGaist]

If you could that would mean you don't have to allocate a QImage each time you call paint event which in itself is already a gain.

You should also rather work on a QPixmap since you're going to draw it, QPixmap is optimized for showing image on screen while QImage is optimized for IO and direct pixel access.

What operations do you need to do exactly ?

[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.