Arc in QGraphicsScene

Question

I want to implement arc in QGraphicsScene. I want that on clicking of three points my arc should be drawn such that on clicking of three points arc is drawn where first poin

Answer 1

This sounds like it could be solved with some relatively simple math:

https://www.google.com/search?q=define%20circle%20three%20points

https://math.stackexchange.com/a/213678

https://www.khanacademy.org/math/geometry/triangle-properties/perpendicular_bisectors/v/three-points-defining-a-circle

Here is my translation of the math into Qt goodness

// m_points is a QList
// use math to define the circle
QLineF lineBC(m_points.at(1), m_points.at(2));
QLineF lineAC(m_points.at(0), m_points.at(2));
QLineF lineBA(m_points.at(1), m_points.at(0));
qreal rad = qAbs(lineBC.length()/(2*qSin(qDegreesToRadians(lineAC.angleTo(lineBA)))));

QLineF bisectorBC(lineBC.pointAt(0.5), lineBC.p2());
bisectorBC.setAngle(lineBC.normalVector().angle());

QLineF bisectorBA(lineBA.pointAt(0.5), lineBA.p2());
bisectorBA.setAngle(lineBA.normalVector().angle());

QPointF center;
bisectorBA.intersect(bisectorBC, ¢er);

qDebug() << rad << center;

QT QGraphicsScene Drawing Arc

QPainterPath* path = new QPainterPath();
path->arcMoveTo(0,0,50,50,20);
path->arcTo(0,0,50,50,20, 90);
scene.addPath(*path);

Putting all of this together into a nice little project turns into this:

https://github.com/peteristhegreat/ThreePointsCircle

main.cpp

#include "mainwindow.h"
#include 

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

    return a.exec();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include 

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include 
#include 
#include "graphicsscene.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QGraphicsView * view = new QGraphicsView;

    GraphicsScene * scene = new GraphicsScene();
    view->setScene(scene);

    view->setSceneRect(-300,-300, 300, 300);
    this->resize(600, 600);

    this->setCentralWidget(view);
}

MainWindow::~MainWindow()
{

}

graphicsscene.h

#ifndef GRAPHICSSCENE_H
#define GRAPHICSSCENE_H

#include 
#include 
#include 
#include 

class GraphicsScene : public QGraphicsScene
{
    Q_OBJECT
public:
    explicit GraphicsScene(QObject *parent = 0);
    virtual void mouseDoubleClickEvent(QGraphicsSceneMouseEvent * mouseEvent);
    virtual void mouseMoveEvent(QGraphicsSceneMouseEvent * mouseEvent);
    virtual void mousePressEvent(QGraphicsSceneMouseEvent * mouseEvent);
    virtual void mouseReleaseEvent(QGraphicsSceneMouseEvent * mouseEvent);
signals:

public slots:

private:
    QList  m_points;
};

#endif // GRAPHICSSCENE_H

graphicsscene.cpp

#include "graphicsscene.h"
#include 
#include 
#include 
#include 
#include "qmath.h"

GraphicsScene::GraphicsScene(QObject *parent) :
    QGraphicsScene(parent)
{
    this->setBackgroundBrush(Qt::gray);
}

void GraphicsScene::mouseDoubleClickEvent(QGraphicsSceneMouseEvent * mouseEvent)
{
    qDebug() << Q_FUNC_INFO << mouseEvent->scenePos();
    QGraphicsScene::mouseDoubleClickEvent(mouseEvent);
}

void GraphicsScene::mouseMoveEvent(QGraphicsSceneMouseEvent * mouseEvent)
{
    qDebug() << Q_FUNC_INFO << mouseEvent->scenePos();
    QGraphicsScene::mouseMoveEvent(mouseEvent);
}

void GraphicsScene::mousePressEvent(QGraphicsSceneMouseEvent * mouseEvent)
{
    qDebug() << Q_FUNC_INFO << mouseEvent->scenePos();
    QGraphicsScene::mousePressEvent(mouseEvent);
}

void GraphicsScene::mouseReleaseEvent(QGraphicsSceneMouseEvent * me)
{
    qDebug() << Q_FUNC_INFO << me->scenePos();
    int radius = 20;
    QGraphicsEllipseItem * ellipse = this->addEllipse(me->scenePos().x() - radius, me->scenePos().y() - radius, radius*2, radius*2);

    ellipse->setBrush(Qt::white);
    m_points.append(me->scenePos());
    if(m_points.size() == 3)
    {
        // use math to define the circle
        QLineF lineBC(m_points.at(1), m_points.at(2));
        QLineF lineAC(m_points.at(0), m_points.at(2));
        QLineF lineBA(m_points.at(1), m_points.at(0));
        qreal rad = qAbs(lineBC.length()/(2*qSin(qDegreesToRadians(lineAC.angleTo(lineBA)))));

        QLineF bisectorBC(lineBC.pointAt(0.5), lineBC.p2());
        bisectorBC.setAngle(lineBC.normalVector().angle());

        QLineF bisectorBA(lineBA.pointAt(0.5), lineBA.p2());
        bisectorBA.setAngle(lineBA.normalVector().angle());

        QPointF center;
        bisectorBA.intersect(bisectorBC, ¢er);

        qDebug() << rad << center;

        bool drawCircle = true;

        QGraphicsEllipseItem * ellipse = new QGraphicsEllipseItem(center.x() - rad, center.y() - rad, rad*2, rad*2);
        if(drawCircle)
            this->addItem(ellipse);

        // add arc
        // this->addItem(path);
        QPainterPath path;
        QLineF lineOA(center, m_points.at(0));
        QLineF lineOC(center, m_points.at(2));
        path.arcMoveTo(ellipse->boundingRect(),lineOA.angle());
        path.arcTo(ellipse->boundingRect(), lineOA.angle(), lineOC.angle() - lineOA.angle());
        QGraphicsPathItem * pathItem = new QGraphicsPathItem(path);
        pathItem->setPen(QPen(Qt::red,10));
        this->addItem(pathItem);

        if(!drawCircle)
            delete ellipse;
        m_points.clear();
    }

    QGraphicsScene::mouseReleaseEvent(me);
}