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# Canvas Example

This example shows a QCanvas and some QCanvasItems in action. You can do a lot more with QCanvas than we show here, but the example provides a taste of what can be done.

Header file:

    #ifndef EXAMPLE_H
    #define EXAMPLE_H

    #include <qpopupmenu.h>
    #include <qmainwindow.h>
    #include <qintdict.h>
    #include <qcanvas.h>

    class BouncyLogo : public QCanvasSprite {
        void initPos();
        void initSpeed();
    public:
        BouncyLogo(QCanvas*);
        void advance(int);
        int rtti() const;
    };

    class FigureEditor : public QCanvasView {
        Q_OBJECT

    public:
        FigureEditor(QCanvas&, QWidget* parent=0, const char* name=0, WFlags f=0);
        void clear();

    protected:
        void contentsMousePressEvent(QMouseEvent*);
        void contentsMouseMoveEvent(QMouseEvent*);

    signals:
        void status(const QString&);

    private:
        QCanvasItem* moving;
        QPoint moving_start;
    };

    class Main : public QMainWindow {
        Q_OBJECT

    public:
        Main(QCanvas&, QWidget* parent=0, const char* name=0, WFlags f=0);
        ~Main();

    public slots:
        void help();

    private slots:
        void aboutQt();
        void newView();
        void clear();
        void init();

        void addSprite();
        void addCircle();
        void addHexagon();
        void addPolygon();
        void addSpline();
        void addText();
        void addLine();
        void addRectangle();
        void addMesh();
        void addLogo();
        void addButterfly();

        void enlarge();
        void shrink();
        void rotateClockwise();
        void rotateCounterClockwise();
        void zoomIn();
        void zoomOut();
        void mirror();
        void moveL();
        void moveR();
        void moveU();
        void moveD();

        void print();

        void toggleDoubleBuffer();

    private:
        QCanvas& canvas;
        FigureEditor *editor;

        QPopupMenu* options;
        QPrinter* printer;
        int dbf_id;
    };

    #endif

Implementation:

    #include <qdatetime.h>
    #include <qmainwindow.h>
    #include <qstatusbar.h>
    #include <qmessagebox.h>
    #include <qmenubar.h>
    #include <qapplication.h>
    #include <qpainter.h>
    #include <qprinter.h>
    #include <qlabel.h>
    #include <qimage.h>
    #include <qprogressdialog.h>
    #include "canvas.h"

    #include <stdlib.h>

    // We use a global variable to save memory - all the brushes and pens in
    // the mesh are shared.
    static QBrush *tb = 0;
    static QPen *tp = 0;

    class EdgeItem;
    class NodeItem;

    class EdgeItem: public QCanvasLine
    {
    public:
        EdgeItem( NodeItem*, NodeItem*, QCanvas *canvas );
        void setFromPoint( int x, int y ) ;
        void setToPoint( int x, int y );
        static int count() { return c; }
        void moveBy(double dx, double dy);
    private:
        static int c;
    };

    static const int imageRTTI = 984376;

    class ImageItem: public QCanvasRectangle
    {
    public:
        ImageItem( QImage img, QCanvas *canvas );
        int rtti () const { return imageRTTI; }
        bool hit( const QPoint&) const;
    protected:
        void drawShape( QPainter & );
    private:
        QImage image;
        QPixmap pixmap;
    };

    ImageItem::ImageItem( QImage img, QCanvas *canvas )
        : QCanvasRectangle( canvas ), image(img)
    {
        setSize( image.width(), image.height() );

    #if !defined(Q_WS_QWS)
        pixmap.convertFromImage(image, OrderedAlphaDither);
    #endif
    }

    void ImageItem::drawShape( QPainter &p )
    {
    // On Qt/Embedded, we can paint a QImage as fast as a QPixmap,
    // but on other platforms, we need to use a QPixmap.
    #if defined(Q_WS_QWS)
        p.drawImage( int(x()), int(y()), image, 0, 0, -1, -1, OrderedAlphaDither );
    #else
        p.drawPixmap( int(x()), int(y()), pixmap );
    #endif
    }

    bool ImageItem::hit( const QPoint &p ) const
    {
        int ix = p.x()-int(x());
        int iy = p.y()-int(y());
        if ( !image.valid( ix , iy ) )
            return FALSE;
        QRgb pixel = image.pixel( ix, iy );
        return qAlpha( pixel ) != 0;
    }

    class NodeItem: public QCanvasEllipse
    {
    public:
        NodeItem( QCanvas *canvas );
        ~NodeItem() {}

        void addInEdge( EdgeItem *edge ) { inList.append( edge ); }
        void addOutEdge( EdgeItem *edge ) { outList.append( edge ); }

        void moveBy(double dx, double dy);

        //    QPoint center() { return boundingRect().center(); }
    private:
        QPtrList<EdgeItem> inList;
        QPtrList<EdgeItem> outList;
    };

    int EdgeItem::c = 0;

    void EdgeItem::moveBy(double, double)
    {
        //nothing
    }

    EdgeItem::EdgeItem( NodeItem *from, NodeItem *to, QCanvas *canvas )
        : QCanvasLine( canvas )
    {
        c++;
        setPen( *tp );
        setBrush( *tb );
        from->addOutEdge( this );
        to->addInEdge( this );
        setPoints( int(from->x()), int(from->y()), int(to->x()), int(to->y()) );
        setZ( 127 );
    }

    void EdgeItem::setFromPoint( int x, int y )
    {
        setPoints( x,y, endPoint().x(), endPoint().y() );
    }

    void EdgeItem::setToPoint( int x, int y )
    {
        setPoints( startPoint().x(), startPoint().y(), x, y );
    }

    void NodeItem::moveBy(double dx, double dy)
    {
        QCanvasEllipse::moveBy( dx, dy );

        QPtrListIterator<EdgeItem> it1( inList );
        EdgeItem *edge;
        while (( edge = it1.current() )) {
            ++it1;
            edge->setToPoint( int(x()), int(y()) );
        }
        QPtrListIterator<EdgeItem> it2( outList );
        while (( edge = it2.current() )) {
            ++it2;
            edge->setFromPoint( int(x()), int(y()) );
        }
    }

    NodeItem::NodeItem( QCanvas *canvas )
        : QCanvasEllipse( 6, 6, canvas )
    {
        setPen( *tp );
        setBrush( *tb );
        setZ( 128 );
    }

    FigureEditor::FigureEditor(
            QCanvas& c, QWidget* parent,
            const char* name, WFlags f) :
        QCanvasView(&c,parent,name,f)
    {
    }

    void FigureEditor::contentsMousePressEvent(QMouseEvent* e)
    {
        QPoint p = inverseWorldMatrix().map(e->pos());
        QCanvasItemList l=canvas()->collisions(p);
        for (QCanvasItemList::Iterator it=l.begin(); it!=l.end(); ++it) {
            if ( (*it)->rtti() == imageRTTI ) {
                ImageItem *item= (ImageItem*)(*it);
                if ( !item->hit( p ) )
                     continue;
            }
            moving = *it;
            moving_start = p;
            return;
        }
        moving = 0;
    }

    void FigureEditor::clear()
    {
        QCanvasItemList list = canvas()->allItems();
        QCanvasItemList::Iterator it = list.begin();
        for (; it != list.end(); ++it) {
            if ( *it )
                delete *it;
        }
    }

    void FigureEditor::contentsMouseMoveEvent(QMouseEvent* e)
    {
        if ( moving ) {
            QPoint p = inverseWorldMatrix().map(e->pos());
            moving->moveBy(p.x() - moving_start.x(),
                           p.y() - moving_start.y());
            moving_start = p;
            canvas()->update();
        }
    }

    BouncyLogo::BouncyLogo(QCanvas* canvas) :
        QCanvasSprite(0,canvas)
    {
        static QCanvasPixmapArray logo("qt-trans.xpm");
        setSequence(&logo);
        setAnimated(TRUE);
        initPos();
    }

    const int logo_rtti = 1234;

    int BouncyLogo::rtti() const
    {
        return logo_rtti;
    }

    void BouncyLogo::initPos()
    {
        initSpeed();
        int trial=1000;
        do {
            move(rand()%canvas()->width(),rand()%canvas()->height());
            advance(0);
        } while (trial-- && xVelocity()==0.0 && yVelocity()==0.0);
    }

    void BouncyLogo::initSpeed()
    {
        const double speed = 4.0;
        double d = (double)(rand()%1024) / 1024.0;
        setVelocity( d*speed*2-speed, (1-d)*speed*2-speed );
    }

    void BouncyLogo::advance(int stage)
    {
        switch ( stage ) {
          case 0: {
            double vx = xVelocity();
            double vy = yVelocity();

            if ( vx == 0.0 && vy == 0.0 ) {
                // stopped last turn
                initSpeed();
                vx = xVelocity();
                vy = yVelocity();
            }

            double nx = x() + vx;
            double ny = y() + vy;

            if ( nx < 0 || nx >= canvas()->width() )
                vx = -vx;
            if ( ny < 0 || ny >= canvas()->height() )
                vy = -vy;

            for (int bounce=0; bounce<4; bounce++) {
                QCanvasItemList l=collisions(FALSE);
                for (QCanvasItemList::Iterator it=l.begin(); it!=l.end(); ++it) {
                    QCanvasItem *hit = *it;
                    if ( hit->rtti()==logo_rtti && hit->collidesWith(this) ) {
                        switch ( bounce ) {
                          case 0:
                            vx = -vx;
                            break;
                          case 1:
                            vy = -vy;
                            vx = -vx;
                            break;
                          case 2:
                            vx = -vx;
                            break;
                          case 3:
                            // Stop for this turn
                            vx = 0;
                            vy = 0;
                            break;
                        }
                        setVelocity(vx,vy);
                        break;
                    }
                }
            }

            if ( x()+vx < 0 || x()+vx >= canvas()->width() )
                vx = 0;
            if ( y()+vy < 0 || y()+vy >= canvas()->height() )
                vy = 0;

            setVelocity(vx,vy);
          } break;
          case 1:
            QCanvasItem::advance(stage);
            break;
        }
    }

    static uint mainCount = 0;
    static QImage *butterflyimg;
    static QImage *logoimg;

    Main::Main(QCanvas& c, QWidget* parent, const char* name, WFlags f) :
        QMainWindow(parent,name,f),
        canvas(c)
    {
        editor = new FigureEditor(canvas,this);
        QMenuBar* menu = menuBar();

        QPopupMenu* file = new QPopupMenu( menu );
        file->insertItem("&Fill canvas", this, SLOT(init()), CTRL+Key_F);
        file->insertItem("&Erase canvas", this, SLOT(clear()), CTRL+Key_E);
        file->insertItem("&New view", this, SLOT(newView()), CTRL+Key_N);
        file->insertSeparator();
        file->insertItem("&Print...", this, SLOT(print()), CTRL+Key_P);
        file->insertSeparator();
        file->insertItem("E&xit", qApp, SLOT(quit()), CTRL+Key_Q);
        menu->insertItem("&File", file);

        QPopupMenu* edit = new QPopupMenu( menu );
        edit->insertItem("Add &Circle", this, SLOT(addCircle()), ALT+Key_C);
        edit->insertItem("Add &Hexagon", this, SLOT(addHexagon()), ALT+Key_H);
        edit->insertItem("Add &Polygon", this, SLOT(addPolygon()), ALT+Key_P);
        edit->insertItem("Add Spl&ine", this, SLOT(addSpline()), ALT+Key_I);
        edit->insertItem("Add &Text", this, SLOT(addText()), ALT+Key_T);
        edit->insertItem("Add &Line", this, SLOT(addLine()), ALT+Key_L);
        edit->insertItem("Add &Rectangle", this, SLOT(addRectangle()), ALT+Key_R);
        edit->insertItem("Add &Sprite", this, SLOT(addSprite()), ALT+Key_S);
        edit->insertItem("Create &Mesh", this, SLOT(addMesh()), ALT+Key_M );
        edit->insertItem("Add &Alpha-blended image", this, SLOT(addButterfly()), ALT+Key_A);
        menu->insertItem("&Edit", edit);

        QPopupMenu* view = new QPopupMenu( menu );
        view->insertItem("&Enlarge", this, SLOT(enlarge()), SHIFT+CTRL+Key_Plus);
        view->insertItem("Shr&ink", this, SLOT(shrink()), SHIFT+CTRL+Key_Minus);
        view->insertSeparator();
        view->insertItem("&Rotate clockwise", this, SLOT(rotateClockwise()), CTRL+Key_PageDown);
        view->insertItem("Rotate &counterclockwise", this, SLOT(rotateCounterClockwise()), CTRL+Key_PageUp);
        view->insertItem("&Zoom in", this, SLOT(zoomIn()), CTRL+Key_Plus);
        view->insertItem("Zoom &out", this, SLOT(zoomOut()), CTRL+Key_Minus);
        view->insertItem("Translate left", this, SLOT(moveL()), CTRL+Key_Left);
        view->insertItem("Translate right", this, SLOT(moveR()), CTRL+Key_Right);
        view->insertItem("Translate up", this, SLOT(moveU()), CTRL+Key_Up);
        view->insertItem("Translate down", this, SLOT(moveD()), CTRL+Key_Down);
        view->insertItem("&Mirror", this, SLOT(mirror()), CTRL+Key_Home);
        menu->insertItem("&View", view);

        options = new QPopupMenu( menu );
        dbf_id = options->insertItem("Double buffer", this, SLOT(toggleDoubleBuffer()));
        options->setItemChecked(dbf_id, TRUE);
        menu->insertItem("&Options",options);

        menu->insertSeparator();

        QPopupMenu* help = new QPopupMenu( menu );
        help->insertItem("&About", this, SLOT(help()), Key_F1);
        help->setItemChecked(dbf_id, TRUE);
        menu->insertItem("&Help",help);

        statusBar();

        setCentralWidget(editor);

        printer = 0;

        init();
    }

    void Main::init()
    {
        clear();

        static int r=24;
        srand(++r);

        mainCount++;
        butterflyimg = 0;
        logoimg = 0;

        int i;
        for ( i=0; i<canvas.width() / 56; i++) {
            addButterfly();
        }
        for ( i=0; i<canvas.width() / 85; i++) {
            addHexagon();
        }
        for ( i=0; i<canvas.width() / 128; i++) {
            addLogo();
        }
    }

    Main::~Main()
    {
        delete printer;
        if ( !--mainCount ) {
            delete[] butterflyimg;
            butterflyimg = 0;
            delete[] logoimg;
            logoimg = 0;
        }
    }

    void Main::newView()
    {
        // Open a new view... have it delete when closed.
        Main *m = new Main(canvas, 0, 0, WDestructiveClose);
        qApp->setMainWidget(m);
        m->show();
        qApp->setMainWidget(0);
    }

    void Main::clear()
    {
        editor->clear();
    }

    void Main::help()
    {
        static QMessageBox* about = new QMessageBox( "Qt Canvas Example",
                "<h3>The QCanvas classes example</h3>"
                "<ul>"
                    "<li> Press ALT-S for some sprites."
                    "<li> Press ALT-C for some circles."
                    "<li> Press ALT-L for some lines."
                    "<li> Drag the objects around."
                    "<li> Read the code!"
                "</ul>", QMessageBox::Information, 1, 0, 0, this, 0, FALSE );
        about->setButtonText( 1, "Dismiss" );
        about->show();
    }

    void Main::aboutQt()
    {
        QMessageBox::aboutQt( this, "Qt Canvas Example" );
    }

    void Main::toggleDoubleBuffer()
    {
        bool s = !options->isItemChecked(dbf_id);
        options->setItemChecked(dbf_id,s);
        canvas.setDoubleBuffering(s);
    }

    void Main::enlarge()
    {
        canvas.resize(canvas.width()*4/3, canvas.height()*4/3);
    }

    void Main::shrink()
    {
        canvas.resize(canvas.width()*3/4, canvas.height()*3/4);
    }

    void Main::rotateClockwise()
    {
        QWMatrix m = editor->worldMatrix();
        m.rotate( 22.5 );
        editor->setWorldMatrix( m );
    }

    void Main::rotateCounterClockwise()
    {
        QWMatrix m = editor->worldMatrix();
        m.rotate( -22.5 );
        editor->setWorldMatrix( m );
    }

    void Main::zoomIn()
    {
        QWMatrix m = editor->worldMatrix();
        m.scale( 2.0, 2.0 );
        editor->setWorldMatrix( m );
    }

    void Main::zoomOut()
    {
        QWMatrix m = editor->worldMatrix();
        m.scale( 0.5, 0.5 );
        editor->setWorldMatrix( m );
    }

    void Main::mirror()
    {
        QWMatrix m = editor->worldMatrix();
        m.scale( -1, 1 );
        editor->setWorldMatrix( m );
    }

    void Main::moveL()
    {
        QWMatrix m = editor->worldMatrix();
        m.translate( -16, 0 );
        editor->setWorldMatrix( m );
    }

    void Main::moveR()
    {
        QWMatrix m = editor->worldMatrix();
        m.translate( +16, 0 );
        editor->setWorldMatrix( m );
    }

    void Main::moveU()
    {
        QWMatrix m = editor->worldMatrix();
        m.translate( 0, -16 );
        editor->setWorldMatrix( m );
    }

    void Main::moveD()
    {
        QWMatrix m = editor->worldMatrix();
        m.translate( 0, +16 );
        editor->setWorldMatrix( m );
    }

    void Main::print()
    {
        if ( !printer ) printer = new QPrinter;
        if ( printer->setup(this) ) {
            QPainter pp(printer);
            canvas.drawArea(QRect(0,0,canvas.width(),canvas.height()),&pp,FALSE);
        }
    }

    void Main::addSprite()
    {
        QCanvasItem* i = new BouncyLogo(&canvas);
        i->setZ(rand()%256);
        i->show();
    }

    QString butterfly_fn;
    QString logo_fn;

    void Main::addButterfly()
    {
        if ( butterfly_fn.isEmpty() )
            return;
        if ( !butterflyimg ) {
            butterflyimg = new QImage[4];
            butterflyimg[0].load( butterfly_fn );
            butterflyimg[1] = butterflyimg[0].smoothScale( int(butterflyimg[0].width()*0.75),
                    int(butterflyimg[0].height()*0.75) );
            butterflyimg[2] = butterflyimg[0].smoothScale( int(butterflyimg[0].width()*0.5),
                    int(butterflyimg[0].height()*0.5) );
            butterflyimg[3] = butterflyimg[0].smoothScale( int(butterflyimg[0].width()*0.25),
                    int(butterflyimg[0].height()*0.25) );
        }
        QCanvasPolygonalItem* i = new ImageItem(butterflyimg[rand()%4],&canvas);
        i->move(rand()%(canvas.width()-butterflyimg->width()),
                rand()%(canvas.height()-butterflyimg->height()));
        i->setZ(rand()%256+250);
        i->show();
    }

    void Main::addLogo()
    {
        if ( logo_fn.isEmpty() )
            return;
        if ( !logoimg ) {
            logoimg = new QImage[4];
            logoimg[0].load( logo_fn );
            logoimg[1] = logoimg[0].smoothScale( int(logoimg[0].width()*0.75),
                    int(logoimg[0].height()*0.75) );
            logoimg[2] = logoimg[0].smoothScale( int(logoimg[0].width()*0.5),
                    int(logoimg[0].height()*0.5) );
            logoimg[3] = logoimg[0].smoothScale( int(logoimg[0].width()*0.25),
                    int(logoimg[0].height()*0.25) );
        }
        QCanvasPolygonalItem* i = new ImageItem(logoimg[rand()%4],&canvas);
        i->move(rand()%(canvas.width()-logoimg->width()),
                rand()%(canvas.height()-logoimg->width()));
        i->setZ(rand()%256+256);
        i->show();
    }

    void Main::addCircle()
    {
        QCanvasPolygonalItem* i = new QCanvasEllipse(50,50,&canvas);
        i->setBrush( QColor(rand()%32*8,rand()%32*8,rand()%32*8) );
        i->move(rand()%canvas.width(),rand()%canvas.height());
        i->setZ(rand()%256);
        i->show();
    }

    void Main::addHexagon()
    {
        QCanvasPolygon* i = new QCanvasPolygon(&canvas);
        const int size = canvas.width() / 25;
        QPointArray pa(6);
        pa[0] = QPoint(2*size,0);
        pa[1] = QPoint(size,-size*173/100);
        pa[2] = QPoint(-size,-size*173/100);
        pa[3] = QPoint(-2*size,0);
        pa[4] = QPoint(-size,size*173/100);
        pa[5] = QPoint(size,size*173/100);
        i->setPoints(pa);
        i->setBrush( QColor(rand()%32*8,rand()%32*8,rand()%32*8) );
        i->move(rand()%canvas.width(),rand()%canvas.height());
        i->setZ(rand()%256);
        i->show();
    }

    void Main::addPolygon()
    {
        QCanvasPolygon* i = new QCanvasPolygon(&canvas);
        const int size = canvas.width()/2;
        QPointArray pa(6);
        pa[0] = QPoint(0,0);
        pa[1] = QPoint(size,size/5);
        pa[2] = QPoint(size*4/5,size);
        pa[3] = QPoint(size/6,size*5/4);
        pa[4] = QPoint(size*3/4,size*3/4);
        pa[5] = QPoint(size*3/4,size/4);
        i->setPoints(pa);
        i->setBrush( QColor(rand()%32*8,rand()%32*8,rand()%32*8) );
        i->move(rand()%canvas.width(),rand()%canvas.height());
        i->setZ(rand()%256);
        i->show();
    }

    void Main::addSpline()
    {
        QCanvasSpline* i = new QCanvasSpline(&canvas);
        const int size = canvas.width()/6;
        QPointArray pa(12);
        pa[0] = QPoint(0,0);
        pa[1] = QPoint(size/2,0);
        pa[2] = QPoint(size,size/2);
        pa[3] = QPoint(size,size);
        pa[4] = QPoint(size,size*3/2);
        pa[5] = QPoint(size/2,size*2);
        pa[6] = QPoint(0,size*2);
        pa[7] = QPoint(-size/2,size*2);
        pa[8] = QPoint(size/4,size*3/2);
        pa[9] = QPoint(0,size);
        pa[10]= QPoint(-size/4,size/2);
        pa[11]= QPoint(-size/2,0);
        i->setControlPoints(pa);
        i->setBrush( QColor(rand()%32*8,rand()%32*8,rand()%32*8) );
        i->move(rand()%canvas.width(),rand()%canvas.height());
        i->setZ(rand()%256);
        i->show();
    }

    void Main::addText()
    {
        QCanvasText* i = new QCanvasText(&canvas);
        i->setText("QCanvasText");
        i->move(rand()%canvas.width(),rand()%canvas.height());
        i->setZ(rand()%256);
        i->show();
    }

    void Main::addLine()
    {
        QCanvasLine* i = new QCanvasLine(&canvas);
        i->setPoints( rand()%canvas.width(), rand()%canvas.height(),
                      rand()%canvas.width(), rand()%canvas.height() );
        i->setPen( QPen(QColor(rand()%32*8,rand()%32*8,rand()%32*8), 6) );
        i->setZ(rand()%256);
        i->show();
    }

    void Main::addMesh()
    {
        int x0 = 0;
        int y0 = 0;

        if ( !tb ) tb = new QBrush( Qt::red );
        if ( !tp ) tp = new QPen( Qt::black );

        int nodecount = 0;

        int w = canvas.width();
        int h = canvas.height();

        const int dist = 30;
        int rows = h / dist;
        int cols = w / dist;

    #ifndef QT_NO_PROGRESSDIALOG
        QProgressDialog progress( "Creating mesh...", "Abort", rows,
                                  this, "progress", TRUE );
    #endif

        QMemArray<NodeItem*> lastRow(cols);
        for ( int j = 0; j < rows; j++ ) {
            int n = j%2 ? cols-1 : cols;
            NodeItem *prev = 0;
            for ( int i = 0; i < n; i++ ) {
                NodeItem *el = new NodeItem( &canvas );
                nodecount++;
                int r = rand();
                int xrand = r %20;
                int yrand = (r/20) %20;
                el->move( xrand + x0 + i*dist + (j%2 ? dist/2 : 0 ),
                          yrand + y0 + j*dist );

                if ( j > 0 ) {
                    if ( i < cols-1 )
                        (new EdgeItem( lastRow[i], el, &canvas ))->show();
                    if ( j%2 )
                        (new EdgeItem( lastRow[i+1], el, &canvas ))->show();
                    else if ( i > 0 )
                        (new EdgeItem( lastRow[i-1], el, &canvas ))->show();
                }
                if ( prev ) {
                    (new EdgeItem( prev, el, &canvas ))->show();
                }
                if ( i > 0 ) lastRow[i-1] = prev;
                prev = el;
                el->show();
            }
            lastRow[n-1]=prev;
    #ifndef QT_NO_PROGRESSDIALOG
            progress.setProgress( j );
            if ( progress.wasCancelled() )
                break;
    #endif
        }
    #ifndef QT_NO_PROGRESSDIALOG
        progress.setProgress( rows );
    #endif
        // qDebug( "%d nodes, %d edges", nodecount, EdgeItem::count() );
    }

    void Main::addRectangle()
    {
        QCanvasPolygonalItem *i = new QCanvasRectangle( rand()%canvas.width(),rand()%canvas.height(),
                                canvas.width()/5,canvas.width()/5,&canvas);
        int z = rand()%256;
        i->setBrush( QColor(z,z,z) );
        i->setPen( QPen(QColor(rand()%32*8,rand()%32*8,rand()%32*8), 6) );
        i->setZ(z);
        i->show();
    }

Main:

    #include <qstatusbar.h>
    #include <qmessagebox.h>
    #include <qmenubar.h>
    #include <qapplication.h>
    #include <qimage.h>

    #include "canvas.h"

    #include <stdlib.h>

    extern QString butterfly_fn;
    extern QString logo_fn;

    int main(int argc, char** argv)
    {
        QApplication app(argc,argv);

        /*
        qDebug("sizeof(QCanvasPolygonalItem)=%d",sizeof(QCanvasPolygonalItem));
        qDebug("sizeof(QCanvasText)=%d",sizeof(QCanvasText));
        qDebug("sizeof(QWidget)=%d",sizeof(QWidget));
        qDebug("sizeof(QLabel)=%d",sizeof(QLabel));
        */

        if ( argc > 1 )
            butterfly_fn = argv[1];
        else
            butterfly_fn = "butterfly.png";

        if ( argc > 2 )
            logo_fn = argv[2];
        else
            logo_fn = "qtlogo.png";

        QCanvas canvas(800,600);
        canvas.setAdvancePeriod(30);
        Main m(canvas);
        m.resize(m.sizeHint());

        qApp->setMainWidget(&m);

        m.setCaption("Qt Example - Canvas");
        if ( QApplication::desktop()->width() > m.width() + 10
             && QApplication::desktop()->height() > m.height() +30 )
            m.show();
        else
            m.showMaximized();

        m.show();
        //    m.help();
        qApp->setMainWidget(0);

        QObject::connect( qApp, SIGNAL(lastWindowClosed()), qApp, SLOT(quit()) );

        return app.exec();
    }

See also Examples.