How do you keep the width of a QPen pixel the same when scaling in a QGraphicsView

Question

I wrote a quick and nasty program to help me visualize the aspect of the project I'm working on. Although I have been working with Qt since 4.1.1, I have never had to use the QGraphics * module.

When I started playing with the program, I was working on an old computer running Windows XP and Qt4.7.0 and Visual Studio 2008. Now I just copied the file to my host computer running Windows 8. I decided to give Qt5 a shot, so I installed QtCreator with Qt5.0.2.

When I compile the same code that I created on a Qt4.7.0 / XP machine, I get a completely different result.

This is what I see in my Qt4.7.0 compilation:

and this is what I see in my Qt5.0.2 compilation:

Clearly, there is a different behavior in the picture of each border of the rectangle. Also, if I zoom in with the mouse wheel, the width of the rectangle frame becomes larger in Qt5 compilation, but remains the same (about 1 pixel) in Qt4.7 compilation.

How to change the code so that the behavior in Qt5 is the same as in Qt4.7?

Here is the full code:

#include <QApplication> #include <QGraphicsScene> #include <QGraphicsView> #include <QGraphicsRectItem> #include <QWheelEvent> #include <QSqlDatabase> #include <QSqlQuery> #include <QSqlRecord> #include <QVariant> #include <QSqlError> #include <QMessageBox> class ItemData : public QSqlRecord{ public: ItemData(QSqlRecord const &rec) : QSqlRecord(rec) {} qreal left() const { return value(0).toDouble(); } qreal top() const { return value(1).toDouble(); } qreal width() const { return value(2).toDouble() - left(); } qreal height() const { return value(3).toDouble() - top(); } QRectF rect() const { return QRectF(left(), top(), width(), height()); } QString barcode() const { return value(4).toString(); } QString msaName() const { return value(5).toString(); } QString msaDescription() const { return value(6).toString(); } QString hsaName() const { return value(7).toString(); } QString hsaDescription() const { return value(8).toString(); } }; class DSAItem : public QGraphicsRectItem{ public: DSAItem(ItemData const &data, QGraphicsItem *parent = 0) :QGraphicsRectItem(parent) { setFlags(QGraphicsItem::ItemIsSelectable); setRect(data.rect()); QString tip = "<p><b>%1</b></p><p><b>MLSA</b><br/>%2<br/>%3</p><p><b>HLSA</b><br/>%4<br/>%5</p>"; setToolTip(tip.arg(data.barcode(), data.msaName(), data.msaDescription(), data.hsaName(), data.hsaDescription())); if(data.barcode() == "1010100101" || data.barcode() == "1010100114"){ colour = QColor(Qt::red); } else { colour = QColor(Qt::yellow); } colour.setAlphaF(.5); setBrush(QBrush(colour)); } QVariant itemChange(GraphicsItemChange change, QVariant const &value){ if (change == QGraphicsItem::ItemSelectedHasChanged){ QColor c = value.toBool() ? QColor(Qt::green) : colour; c.setAlphaF(.5); setBrush(QBrush(c)); update(rect()); } return QGraphicsRectItem::itemChange(change, value); } private: QColor colour; }; class View : public QGraphicsView { public: View(QWidget *parent = 0): QGraphicsView(parent){ populateScene(); setScene(&scene); rotate(-90); scale(1.0, -1.0); setDragMode(ScrollHandDrag); } protected: void wheelEvent(QWheelEvent *e){ setTransformationAnchor(QGraphicsView::AnchorUnderMouse); double scaleFactor = 1.15; if (e->delta() > 0){ scale(scaleFactor, scaleFactor); } else { scale(1 / scaleFactor, 1 / scaleFactor); } } private: void populateScene(){ QSqlDatabase db = QSqlDatabase::addDatabase("QODBC"); //db credentials here QString errorMessage; bool ok = db.open(); if (ok){ QSqlQuery query(db); QString sql = //query string here if (query.exec(sql)){ while(query.next()){ scene.addItem(new DSAItem(query.record())); } } else { errorMessage = query.lastError().text(); } } else { errorMessage = db.lastError().text(); } if (!errorMessage.isEmpty()){ QMessageBox::critical(0, "Database Error", errorMessage); } } private: QGraphicsScene scene; }; int main(int argc, char *argv[]) { QApplication a(argc, argv); View view; view.show(); return a.exec(); } 

Answer 1

The best answer is given rpsml below this. But for historical reasons, I will leave it here.

Setting the pen width to zero will make it a “cosmetic” pen with a width of 1.

 QPen p = pen(); p.setWidth(0) setPen(p); 

Answer 2

This is a bit late, but may be helpful.

The proposed solution works fine, but limits the width of the pen. An alternative is to use the command

 p.setCosmetic(true); 

According to the Qt link ( http://qt-project.org/doc/qt-5/qpen.html#isCosmetic )

Cosmetic pens are used to draw strokes that have a constant width regardless of any transformations applied to the QPainter with which they are used.

And this also explains why setting the width to zero works:

The zero-width pen is cosmetic by default; handles with a non-zero width are not cosmetic.