Program crashes on QPainter::drawPixmap()

j_omega · wrote on 19 Dec 2017, 18:43

I am having problem where my Qt program crashes after calling QPainter::drawPixmap(). I have spent 2 days debugging this and have decided that I must be unintentionally abusing some feature of Qt.

A working example of this problem can be found here.

My code consists of a QML file that updates the following properteries:

Q_PROPERTY(qreal longitude READ getLongitude WRITE setLongitude NOTIFY latitudeChanged)
Q_PROPERTY(qreal latitude READ getLatitude WRITE setLatitude NOTIFY latitudeChanged)

void Map::setLongitude(qreal longitude)
{
    double diff = (this->longitude - this->pixmapCenter.longitude()) * scale;
    this->longitude = longitude;

    if (qFabs(diff) > 50)
    {
        MapTile result = updatePixmap(scale, longitude, latitude);
        pixmap = result.pixmap;
        pixmapCenter = result.center;
    }
    update();
}

void Map::setLatitude(qreal latitude)
{
    this->latitude = latitude;
}

That in turn regenerates a new Pixmap

MapTile updatePixmap(double scale, double longitude, double latitude)
{

    QPixmap myPixmap(800, 400);
    myPixmap.fill(Qt::transparent);
    QPainter painter(&myPixmap);
    painter.translate(400, 240);
    QPen pen;

    pen.setColor(Qt::white);
    pen.setWidth(1);
    painter.setPen(pen);
    QRectF boundaries(QPointF(-91.55 , 41.55) * scale,
                     QPointF(-91.45, 41.45) * scale);
    boundaries.translate(-longitude * scale, -latitude * scale);
    painter.drawRect(boundaries);
    painter.end();

    QGeoCoordinate center(latitude, longitude);
    return MapTile(myPixmap, center);
}

This new pixmap is then drawn on the screen at the appropriate location. It is important to note that the program runs fine for a few seconds before it crashes.

It crashes with a segfault error in qdrawhelper_sse2.cpp line 587.

void Map::paint(QPainter *painter)
{
    painter->translate(boundingRect().width()/2, boundingRect().height()/2);
    painter->scale(1, -1);

    painter->translate((pixmapCenter.longitude() - longitude) * scale,
                       (pixmapCenter.latitude() - latitude) * scale);
    QPoint corner(-pixmap.width()/2, -pixmap.height()/2);

    painter->drawPixmap(corner, this->pixmap);

}

Here is an image of the moment of the crash