Draw an circle from its center.

[vlibertiaux]

Hi everyone,

I am brand new to QT and I have the following question:

I would like to know how it is possible to draw a circle from its center. I am working with a class which is basically a modified QRectF and let's say that it is used to draw an ellipse on an image.
I have a boolean that indicates if the circle is to be drawn from its center and I tried to modify the paint method inherited by the class accordingly but with little to no success so far.

Here is the code I wrote, if it can be of any interest:

void MyItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    Q_UNUSED(widget);
    painter->setPen(m_pen);
    painter->setBrush(m_brush);
    QPointF center = this->rect().center();
    qreal radius = abs(this->rect().right() - this->rect().left())/2.;
    QPointF newTopLeft(center);
    newTopLeft.setX(newTopLeft.x() - radius);
    newTopLeft.setY(newTopLeft.y() - radius);
    QPointF newBottomRight(center);
    newBottomRight.setX(newBottomRight.x() + radius);
    newBottomRight.setY(newBottomRight.y() + radius);

    QRectF modified(newTopLeft, newBottomRight);
    QPointF newCenter = mapFromItem(this, center);
    modified.moveCenter(newCenter);
    this->setRect(modified);
    if(m_ellipse && m_fromCenter)
    {
        painter->save();
        painter->drawEllipse(this->rect().center(), radius, radius);
        painter->restore();
    }
    if(m_ellipse)
    {
        painter->drawEllipse(this->rect());
    }
}

Can someone help me ?

Thank you very much in advance,
V.

[mrjj]

Hi
Do you mean like animated ?

[vlibertiaux]

Yes.
The radius would change as the user would move the mouse away or towards the center point.

[J.Hilk]

@vlibertiaux do you mean something like this?

//main.cpp

#include "widget.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();

    return a.exec();
}

//Widget.h
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);
    ~Widget();

protected:
    virtual void mouseMoveEvent(QMouseEvent *event)Q_DECL_OVERRIDE;
    virtual void paintEvent(QPaintEvent *event)Q_DECL_OVERRIDE;

    QPoint m_mousPos;
};

#endif // WIDGET_H

//widget.cpp
#include "widget.h"
#include <QMouseEvent>
#include <QPainter>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    this->resize(500,500);
    this->setMouseTracking(true);
}

Widget::~Widget()
{

}

void Widget::mouseMoveEvent(QMouseEvent *event)
{
    m_mousPos = event->pos();
    update();
}

void Widget::paintEvent(QPaintEvent *event)
{
    QWidget::paintEvent(event);

    QPainter p(this);
    QRect r(0,0,m_mousPos.x(),m_mousPos.y());
    r.moveCenter(QPoint(width()/2,height()/2));

    p.drawEllipse(r);
}

[J.Hilk]

Or if you want it to be more atuned to your Mous-Position:

void Widget::paintEvent(QPaintEvent *event)
{
    QWidget::paintEvent(event);

    QPainter p(this);
    int w = m_mousPos.x() < width()/2 ? (width()/2 -m_mousPos.x())*2 : (m_mousPos.x()- width()/2)*2;
    int h = m_mousPos.y() < height()/2 ? (height()/2 -m_mousPos.y())*2 : (m_mousPos.y()-height()/2)*2;
    QRect r(0,0,w, h);
    r.moveCenter(QPoint(width()/2,height()/2));

    p.drawEllipse(r);
}

[vlibertiaux]

@J.Hilk

This is exactly what I was looking for, thank you very much !