drawArc in the rectangle issues

Attachment 12335Attachment 12336Attachment 12336

my program will get the startpoints(x,y) and calculate width and height when you draw on the scene, the attached photo shows it.
the problem is i want to draw a perfect arc in the middle of rectangle as the sketch one attached.
my code as below:

Code:

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void ResizableRectMarker::paintArc1(int x, int y, int width, int height, QPainter* painter)
{
  double radius = sqrt(pow(0.5*width,2)+pow(0.5*height,2));
  double step = sqrt(pow(radius,2)-pow(0.5*width,2));
 
  QPointF topLeft(x,y);
  QPointF bottomRight(x+width,y+height);
  QRectF rect(topLeft, bottomRight);
 
  int startAngle = atan2(0.5*width,step)* 180 / M_PI * 16;
  int spanAngle = 2*startAngle;
  qDebug() << "start angle:" << startAngle << "|" << "spanAngle:" << spanAngle;
  painter->drawArc(rect, startAngle, spanAngle);
}

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the problem is the the arc drawn is not in the center, and also if the rectangle is not a square, the arc is wired.
any solution for it.

newbie here..

It is not entirely clear what your objective is.

Your drawing suggests that you want a circle arc that intersects the rectangle on its left and right side in the middle, the screenshot and code indicates an ellipse arc that fits into the rectangle.

Cheers,
_

I think this is what you want

Code:

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class ArcWidget : public QWidget
{
protected:
    void paintEvent(QPaintEvent * event) override
    {
        QPainter painter(this);
 
        const double radius = qSqrt(qPow(width()/2.0, 2) + qPow(height()/2.0, 2));
        const int startAngle = qAtan((width()/2.0)/radius) * 180 / M_PI * 16;
        const int spanAngle = (90 - startAngle) * 2 * 16;
        const QRect rect = event->rect().adjusted(-(radius-width()/2.0), height()-radius, (radius-width()/2.0), radius);
 
        painter.drawArc(rect, startAngle, spanAngle);
    }
};
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    ArcWidget widget;
    widget.show();
 
    return app.exec();
}

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