Question

I’m having trouble understanding how QGraphicsItemAnimation’s setScaleAt function works. Here’s the code I’m using:

    QGraphicsItem *item=scene.addEllipse(400, 300, 100, 100, QPen(), QBrush(Qt::SolidPattern));
    QTimeLine *timeline=new QTimeLine(3000);
    timeline->setFrameRange(0, 100);
    QGraphicsItemAnimation *animation=new QGraphicsItemAnimation;
    animation->setItem(item);
    animation->setTimeLine(timeline);
    for (int i=0; i<100; i++) {
        animation->setScaleAt(i/100.0, i/100.0, i/100.0);
    }
    connect(timeline, SIGNAL(valueChanged(qreal)), animation, SLOT(setStep(qreal)));
    timeline->start();

All I want it to do is display a dot in the middle of the screen, then have it expand to a circle over 3 seconds. What this code gets me is a dot that appears in the upper left corner, then grows and simultaneously moves to the middle. I tried to compensate by setting the position at each step, but this seems pretty convoluted. There’s got to be a cleaner way to do this.

Also, the original item is displayed for a split second before the animation starts, is there any way to hide it by default, then display for the animation?

Thanks in advance.

Answer 1

This is because the origin of the transformation is in the top left corner of the item. You should use something like addEllipse(-50, -50, 100, 100) instead so that the origin is in the center. Of course, you need to position the item somewhere where it makes sense.