Question

Mission: Draw two lines with different color on one graph with automatic cliping, by adding points bit by bit.

So, what am I doing. Create class GraphWidget, inherited from QGraphicsView. Create member of QGraphicsScene. Create 2 QPainterPath instances, and add them to graphicsScene.

Then, I eventually call graphWidget.Redraw(), where call for QPainterPath.lineTo() for both instances. And I expect appearance of that lines of graphics view, but it doesn’t.

I tired from reading Qt’s doc and forums. What am I doing wrong?

Answer 1

We need to know more, what does not happen? Does the window appear at all? Are the lines not drawn? In the meantime try out this sample code if you want 🙂 Edit: updated to show updating.

#include ...

class QUpdatingPathItem : public QGraphicsPathItem {
    void advance(int phase) {
        if (phase == 0)
            return;
        int x = abs(rand()) % 100;
        int y = abs(rand()) % 100;
        QPainterPath p = path();
        p.lineTo(x, y);
        setPath(p);
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QGraphicsScene s;
    QGraphicsView v(&s);

    QUpdatingPathItem item;
    item.setPen(QPen(QColor("red")));
    s.addItem(&item);
    v.show();

    QTimer *timer = new QTimer(&s);
    timer->connect(timer, SIGNAL(timeout()), &s, SLOT(advance()));
    timer->start(1000);

    return a.exec();
}

You should get something like this:

The path in any QGraphicsPathItem can of course be updated later. You might want to keep the original painter path somewhere to avoid performance hit caused by all the path copying (I’m not sure if QPainterPath is implicitly shared…)

QPainterPath p = gPath.path();
p.lineTo(0, 42);
gPath.setPath(p);

Animation

It seems that you’re trying to do some sort of animation/on-the-fly updating. There is entire framework for this in Qt. In the simplest form you can subclass QGraphicsPathItem, reimplement its advance() slot to automatically fetch next point from motion. The only thing left to do then would be calling s.advance() with the required frequency.

http://doc.trolltech.com/4.5/qgraphicsscene.html#advance