Problem statement

I have been tinkering with my Qt drawing application, adding sliders to control zoom level. After fighting with slots and signals, I have managed to finally make the slider work. Everything was fine, except… sometimes app felt unresponsive and slow – but only when high level of zoom was set. Testing revealed paintEvent() function, which in turn calls image.scaled(), was to blame.

Details

Constructor of main class responsible for creating a window takes 2 parameters: const int width and const int height. They don’t change through life of the application and are used to set size of the window, as well as an image, which is used by all drawing functions as a canvas (I will use both “canvas” and “image” interchangeably). Since these parameters are constant, I had to find a way of zooming in the image to see drawing in greater details. So third parameter was introduced, double scale; paintEvent() and mousePressEvent() make use of it – the former to scale canvas down and the latter to correctly register mouse presses on a scaled image.

Let’s take a look at paintEvent():

// overloaded function of QWidget
void MainWindow::paintEvent(QPaintEvent*) {
  QPainter painter(this);
  painter.drawImage(0, 0, image.scaled(QSize(scale * width, scale * height)));
}

Basically, every time a widget is changed, moved or resized, update() function is called, and in turn also paintEvent(). When there are little to no changes, my function works just fine, but when we add a slider to the widget (I didn’t bother to create a separate class for it), moving slider is one of the actions that indirectly call image.scaled(). As you can guess, it is a pretty costly operation, especially if done several times a second.

Solution

It has been established frequent image scalling is a problem that is slowing down application. I can think of several solutions:

// . . .
QImage image_scaled;
// . . .

void MainWindow::redrawImage() {
  image_scaled = image.scaled(QSize(scale * width, scale * height));
  update();
}

void MainWindow::paintEvent(QPaintEvent*) {
  QPainter painter(this);
  painter.drawImage(0, 0, image_scaled);
}