Paint a compass on a GUI

[trallallero]

I need to paint a compass with a 360° rotating rectangle on a GUI.
The compass is like the one in this image:

http://lh4.ggpht.com/_HH5byQu0gx4/S7MSIrpKCRI/AAAAAAAAAbU/tYJ257z2SAQ/s800/Compass.JPG

The only part that needs to be painted is the rectangle ...
Is there some quick way to do that or do I have to paint it by myself with a QGraphicsView object ?

[Lykurg]

Use a normal Widget and do all the stuff in its paint event. Draw background picture and then rotate the painter and draw the rect. You don't need graphics view for that!

[trallallero]

Ok thanks, but how can I rotate the painter ?

[Lykurg]

QPainter::rotate() or you use a more complex QPainter::setTransform().

[trallallero]

Uhm, it's not so clear ...
I have a GUI with a lot of objects like buttons that will not rotate, of course.
Inside this GUI I put a QFrame ? QWidget ? something that has a painter, I guess.
Then I draw the rect (how ?) and rotate the painter ...

I have experience with QGraphicsView but not with drawing in normal QWidgets.

[Lykurg]

It is basic subclassing!
class Compass : public QWidget
{
Compass(QWidget* parent = 0) : QWidget(parent) {}
protected:
void paintEvent(QPaintEvent* event)
{
Q_UNUSED(event);
QPainter p(this);
// do your paintings here
}
};

Then you also have to provide a proper size hint method.

[trallallero]

Maybe I've got it :)
I've found this example: http://cartan.cas.suffolk.edu/qtdocs/widgets-analogclock.html

[trallallero]

Yes, it's clear now.
Thanks a lot :)

[trallallero]

Everything works now, but I have a problem ...
I have a QLabel 160x160 as a widget where I draw an ellipse 160x160 and a rectangle 160x20 in the center.
So I do the following to rotate the rectangle:



QTransform transform;
transform.translate(80, 80);
transform.rotate(m_RotateAngle);
p.setTransform(transform);


but the rectangle moves around the center, it doesn't rotate.
Any suggestion ?

[Lykurg]

First you have to translate the painter! See here:
#include <QtGui>

class Compass : public QWidget
{
public:
Compass(QWidget* parent = 0) : QWidget(parent)
{
rect = QRect(0, 0, 100, 100);
}
QSize sizeHint() const
{
return rect.size();
}
protected:
void paintEvent(QPaintEvent* event)
{
Q_UNUSED(event);
QPainter p(this);
p.setRenderHint(QPainter::Antialiasing, true);
p.drawEllipse(rect);

p.save();
p.translate(rect.center());
p.rotate(15);
p.drawRect(QRect(-20, -10, 40, 20));
p.restore();
}
private:
QRect rect;
};

int main(int argc, char* argv[])
{
QApplication a(argc, argv);
Compass c;
c.show();
return a.exec();
}

[trallallero]

Ah ok, I had to change the rectangle coordinates.
That's perfect now, thanks again ;)