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

 

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.

 

• View a screenshot of 'CppQGraphicsPathItemExample3' (png)
• Download the Qt Creator project 'CppQGraphicsPathItemExample3' (zip)

 

 

 

 

 

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

 

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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

 

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#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

 

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#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

 

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#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

 

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#ifndef QTRECTITEM_H #define QTRECTITEM_H #include <QGraphicsRectItem> struct QtRectItem : public QGraphicsRectItem {   QtRectItem(QGraphicsItem *parent = 0, QGraphicsScene *scene = 0); }; #endif // QTRECTITEM_H

 

 

 

 

 

qtrectitem.cpp

 

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#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

 

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#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

 

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#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);   } }