QGraphicsPathItem example 3: Bezier quadratic lines again is a QGraphicsPathItem example. This example shows how to use QGraphicsRectItems to manipulatie QGraphicsPathItems, resulting in quadratic Bezier curves.

Technical facts

Application type(s)

Desktop application

Operating system(s) or programming environment(s)

Lubuntu 12.10 (quantal)

IDE(s):

Qt Creator 2.5.2

Project type:

GUI application

C++ standard:

C++11

Compiler(s):

G++ 4.7.2

Libraries used:

Qt: version 4.8.3 (32 bit)
STL: GNU ISO C++ Library, version 4.7.2

Qt project file: CppQGraphicsPathItemExample3.pro


QT       += core gui

QMAKE_CXXFLAGS += -std=c++11 -Wall -Wextra -Werror

TARGET = CppQGraphicsPathItemExample3

TEMPLATE = app



SOURCES += \

    qtmain.cpp \

    qtwidget.cpp \

    qtpathitem.cpp \

    qtrectitem.cpp



HEADERS += \

    qtwidget.h \

    qtpathitem.h \

    qtrectitem.h

qtmain.cpp


#include <QApplication>

#include <QDesktopWidget>

#include "qtwidget.h"



int main(int argc, char *argv[])

{

  QApplication a(argc, argv);

  QtWidget w;

  {

    //Resize the dialog and put it in the screen center

    w.setGeometry(0,0,600,400);

    const QRect screen = QApplication::desktop()->screenGeometry();

    w.move( screen.center() - w.rect().center() );

  }

  w.show();

  return a.exec();

}

qtpathitem.h


#ifndef QTPATHITEM_H

#define QTPATHITEM_H



#include <QGraphicsPathItem>



struct QtRectItem;



struct QtPathItem : public QGraphicsPathItem

{

  QtPathItem(

    const QtRectItem * const from,

    const QtRectItem * const mid,

    const QtRectItem * const to,

    QGraphicsItem *parent = 0, QGraphicsScene *scene = 0);



  protected:

  void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);



  private:

  const QtRectItem * const m_from;

  const QtRectItem * const m_mid;

  const QtRectItem * const m_to;

};



#endif // QTPATHITEM_H

qtpathitem.cpp


#include <cassert>

#include <QPainter>

#include "qtpathitem.h"

#include "qtrectitem.h"



QtPathItem::QtPathItem(

  const QtRectItem * const from,

  const QtRectItem * const mid,

  const QtRectItem * const to,

  QGraphicsItem *parent, QGraphicsScene *scene)

  : QGraphicsPathItem(parent,scene),

    m_from(from),

    m_mid(mid),

    m_to(to)

{

  assert(!(flags() & QGraphicsItem::ItemIsMovable) );

  assert(!(flags() & QGraphicsItem::ItemIsSelectable) );

}



void QtPathItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)

{

  QPainterPath path;

  path.moveTo(m_from->pos());

  //Line must go _though_ mid pos, instead of using it as a virtual hinge point

  //Solution:

  // - define point p as the middle between from and to

  // - define point q as the mirror point of q, using mid_pos as a mirror

  const QPointF p((m_from->pos() + m_to->pos()) / 2.0);

  const double dx = m_mid->pos().x() - p.x();

  const double dy = m_mid->pos().y() - p.y();

  const QPointF q(p.x() + dx + dx, p.y() + dy + dy);

  path.quadTo(q,m_to->pos());

  this->setPath(path);



  QGraphicsPathItem::paint(painter,option,widget);

}

qtrectitem.h


#ifndef QTRECTITEM_H

#define QTRECTITEM_H



#include <QGraphicsRectItem>



struct QtRectItem : public QGraphicsRectItem

{

  QtRectItem(QGraphicsItem *parent = 0, QGraphicsScene *scene = 0);

};



#endif // QTRECTITEM_H

qtrectitem.cpp


#include "qtrectitem.h"



QtRectItem::QtRectItem(QGraphicsItem *parent, QGraphicsScene *scene)

 : QGraphicsRectItem(parent,scene)

{

  this->setFlags(

      QGraphicsItem::ItemIsSelectable

    | QGraphicsItem::ItemIsMovable);



  const double length = 8;

  this->setRect(-length/2.0,-length/2.0,length,length);

}

qtwidget.h


#ifndef QTWIDGET_H

#define QTWIDGET_H



#include <QGraphicsView>



///The widget holding the items

struct QtWidget : public QGraphicsView

{

  QtWidget(QWidget *parent = 0);

};



#endif // QTWIDGET_H

qtwidget.cpp


#include <cassert>

#include <QGraphicsScene>

#include "qtrectitem.h"

#include "qtpathitem.h"

#include "qtwidget.h"



QtWidget::QtWidget(QWidget *parent)

  : QGraphicsView(new QGraphicsScene,parent)

{

  const int n_items = 18;

  std::vector<QtRectItem *> rects;



  for (int i=0; i!=n_items; ++i)

  {

    const double angle = 2.0 * M_PI * (static_cast<double>(i) / static_cast<double>(n_items));

    const double x1 =  std::sin(angle) * 100.0;

    const double y1 = -std::cos(angle) * 100.0;

    QtRectItem * const rect = new QtRectItem;

    rect->setPos(x1,y1);

    scene()->addItem(rect);

    rects.push_back(rect);

  }

  for (int i=0; i<n_items-2; i+=3)

  {

    assert(i + 2 < n_items);

    QtPathItem * const item = new QtPathItem(

      rects[(i+0) % n_items],

      rects[(i+1) % n_items],

      rects[(i+2) % n_items]);

    scene()->addItem(item);

  }

}

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(C++) QGraphicsPathItem example 3: Bezier quadratic lines again

Technical facts

Qt project file: CppQGraphicsPathItemExample3.pro

qtmain.cpp

qtpathitem.h

qtpathitem.cpp

qtrectitem.h

qtrectitem.cpp

qtwidget.h

qtwidget.cpp