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:



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
  }
};

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
  }
};

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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...alogclock.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);

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

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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();
}

#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();
}

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[trallallero]

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