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#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QList>

struct circle {
    QPointF c;
    qreal r;
    circle(QPointF center, qreal radius): c(center), r(radius) {}
};

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);

protected:
    void paintEvent(QPaintEvent *event);

    void mousePressEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);

    void wheelEvent(QWheelEvent *event);

private:
    QList<circle> circles;
    QPointF shift;
    qreal scale;
    QPoint previous;
    bool isDragging;
};

#endif // WIDGET_H

#include "widget.h"

#include <math.h>
#include <QPainter>
#include <QMouseEvent>
#include <QWheelEvent>

circle tangent2(const circle &c1, const circle &c2, qreal r, const QPointF &p)
{
    qreal r1 = c1.r + r;
    qreal r2 = c2.r + r;
    qreal d = c1.r + c2.r;
    qreal dx = (d*d - r2*r2 + r1*r1)/(2*d);
    qreal dy = sqrt(r1*r1 - dx*dx);
    QPointF v = (c2.c - c1.c)/d;
    QPointF n(-v.y(), v.x());
    QPointF pv = p - c1.c;
    if (pv.x()*n.x() + pv.y()*n.y() < 0)
        n = -n;
    QPointF c = c1.c + dx*v + dy*n;
    return circle(c, r);
}

circle tangent3(const circle &c1, const circle &c2, const circle &c3)
{
    qreal k1 = 1/c1.r;
    qreal k2 = 1/c2.r;
    qreal k3 = 1/c3.r;
    qreal a = k1 + k2 + k3;
    qreal b = 2*sqrt(k1*k2 + k2*k3 + k3*k1);
    qreal k = qMax(qAbs(a - b), a + b);
    return tangent2(c1, c2, 1/k, c3.c);
}

Widget::Widget(QWidget *parent)
    : QWidget(parent), shift(300, 400), scale(30), isDragging(false)
{
    setAutoFillBackground(false);
    circle c1(QPointF(0, 0), 5);
    circle c2(QPointF(8, 0), 3);
    circle c3 = tangent2(c1, c2, 4, QPointF(0, -1));
    circles << c1 << c2 << c3;
    for (int i = 0; i < 6; ++i) {
        circle c = tangent3(c1, c2, c3);
        circles << c;
        c1 = c2;
        c2 = c3;
        c3 = c;
    }
}

void Widget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    painter.fillRect(0, 0, width(), height(), Qt::darkGray);
    painter.setBrush(Qt::NoBrush);
    painter.setPen(QPen(QBrush(Qt::white), 1.5));
    painter.setRenderHint(QPainter::Antialiasing, true);
    foreach(const circle &c, circles)
        painter.drawEllipse(c.c * scale + shift, c.r * scale, c.r * scale);
}

void Widget::mousePressEvent(QMouseEvent *event)
{
    if (!isDragging && event->button() == Qt::RightButton) {
        isDragging = true;
        previous = event->pos();
    }
}

void Widget::mouseReleaseEvent(QMouseEvent *event)
{
    if (isDragging && event->button() == Qt::RightButton) {
        isDragging = false;
    }
}

void Widget::mouseMoveEvent(QMouseEvent *event)
{
    if (isDragging) {
        shift += event->pos() - previous;
        previous = event->pos();
        repaint();
    }
}

void Widget::wheelEvent(QWheelEvent *event)
{
    int numDegrees = event->delta() / 8;
    int numTicks = numDegrees / 15;
    qreal factor = pow(1.2, -numTicks);
    shift += (1 - factor) * (event->pos() - shift);
    scale *= factor;
    repaint();
}

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.resize(800, 600);
    w.show();
    return a.exec();
}

void Widget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    painter.fillRect(0, 0, width(), height(), Qt::darkGray);
    painter.setClipRect(0, 0, width(), height());  // <<<<<< added
    painter.setBrush(Qt::NoBrush);
    painter.setPen(QPen(QBrush(Qt::white), 1.5));
    painter.setRenderHint(QPainter::Antialiasing, true);
    foreach(const circle &c, circles)
        painter.drawEllipse(c.c * scale + shift, c.r * scale, c.r * scale);
}

// This limitations comes from qgrayraster.c. Any higher and
// rasterization of shapes will produce incorrect results.
const int QT_RASTER_COORD_LIMIT = 32767;

PB Sample buffers: false
hijackWindow() context created for Widget(0x22ff18, name = "Widget") 1