Painting on the Desktop

The desktop demo contains three routines, each of which draws something on the desktop. It does some nice stuff with QPainter, and also demonstrates how one can treat the desktop as a widget like any other.
Implementation:
/****************************************************************************
** $Id: qt/desktop.cpp   3.3.8   edited Jan 11 14:37 $
**
** Copyright (C) 1992-2007 Trolltech ASA.  All rights reserved.
**
** This file is part of an example program for Qt.  This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/

#include <qimage.h>
#include <qbitmap.h>
#include <qpainter.h>
#include <qapplication.h>
#include <qdropsite.h>
#include <qdragobject.h>
#include <stdio.h>


static double seed = 0.353535353535;
static const int KINDA_RAND_MAX = 32767;

static int kindaRand()
{
    seed = seed*147;
    seed = seed - (double) ((int) seed);
    return (int) ( seed*(KINDA_RAND_MAX + 1) );
}

static int velocity( int i )                    // change velocity
{
    const int velmax = 15;
    const int velmin = 4;
    if ( i == 1 || i == 2 )
        i = (kindaRand()&0x7fff % velmax)/3 + velmin;
    else
        i = (kindaRand()&0x7fff % velmax) + velmin;
    return i;
}

//
// Draw polygon on desktop.
//

void poly()
{
    QWidget *d = QApplication::desktop();
    d->setBackgroundColor( Qt::white );         // white desktop

    const int maxpoints = 5;
    const int maxcurves = 8;
    static int xvel[maxpoints];
    static int yvel[maxpoints];
    int head = 0;
    int tail = -maxcurves + 2;
    QPointArray *a = new QPointArray[ maxcurves ];
    register QPointArray *p;
    QRect r = d->rect();                        // desktop rectangle

    int i;
    for ( i=0; i<maxcurves; i++ )
        a[i].resize( maxpoints );
    p = &a[0];
    for ( i=0; i<maxpoints; i++ ) {             // setup first polygon points
        p->setPoint( i, (kindaRand()&0x7fff) % r.width(),
                        (kindaRand()&0x7fff) % r.height() );
        xvel[i] = velocity(i);
        yvel[i] = velocity(i);
    }

    QPainter paint;
    paint.begin( d );                           // start painting desktop

    for ( int ntimes=0; ntimes<2000; ntimes++ ) {
        paint.setBrush( QColor(kindaRand()%360, 180, 255, QColor::Hsv) );
        paint.drawPolygon( a[head] );
        if ( ++tail >= maxcurves )
            tail = 0;

        int minx=r.left(), maxx=r.right();
        int miny=r.top(),  maxy=r.bottom();
        int x, y;
        p = &a[head];
        if ( ++head >= maxcurves )
            head = 0;
        for ( i=0; i<maxpoints; i++ ) {         // calc new curve
            p->point( i, &x, &y );
            x += xvel[i];
            y += yvel[i];
            if ( x >= maxx ) {
                x = maxx - (x - maxx + 1);
                xvel[i] = -velocity(i);
            }
            if ( x <= minx ) {
                x = minx + (minx - x + 1);
                xvel[i] = velocity(i);
            }
            if ( y >= maxy ) {
                y = maxy - (y - maxy + 1);
                yvel[i] = -velocity(i);
            }
            if ( y <= miny ) {
                y = miny + (miny - y + 1);
                yvel[i] = velocity(i);
            }
            a[head].setPoint( i, x, y );
        }
    }
    paint.end();                                // painting done
    delete[] a;
}


//
// Rotate pattern on desktop.
//

void rotate()
{
    int i;
    const int w = 64;
    const int h = 64;
    QImage image( w, h, 8, 128 );               // create image
    for ( i=0; i<128; i++ )                     // build color table
        image.setColor( i, qRgb(i,0,0) );
    for ( int y=0; y<h; y++ ) {                 // set image pixels
        uchar *p = image.scanLine(y);
        for ( int x=0; x<w; x++ )
            *p++ = (x+y)%128;
    }

    QPixmap pm;
    pm = image;                                 // convert image to pixmap
    pm.setOptimization( QPixmap::BestOptim );   // rotation will be faster

    QWidget *d = QApplication::desktop();       // w = desktop widget

    for ( i=0; i<=360; i += 2 ) {
        QWMatrix m;
        m.rotate( i );                          // rotate coordinate system
        QPixmap rpm = pm.xForm( m );            // rpm = rotated pixmap
        d->setBackgroundPixmap( rpm );          // set desktop pixmap
        d->update();                            // repaint desktop
    }
}

//
// Generates a marble-like pattern in pm.
//

void generateStone( QPixmap *pm,
                    const QColor &c1, const QColor &c2, const QColor &c3 )
{
    QPainter p;
    QPen p1 ( c1, 0 );
    QPen p2 ( c2, 0 );
    QPen p3 ( c3, 0 );

    p.begin( pm );
    for( int i = 0 ; i < pm->width() ; i++ )
        for( int j = 0 ; j < pm->height() ; j++ ) {
            int r = kindaRand();
            if ( r < KINDA_RAND_MAX / 3 )
                p.setPen( p1 );
            else if ( r < KINDA_RAND_MAX / 3 * 2 )
                p.setPen( p2 );
            else
                p.setPen( p3 );
            p.drawPoint( i,j );
        }
    p.end();
}

void drawShadeText( QPainter *p, int x, int y, const char *text,
                    const QColor &topColor, const QColor &bottomColor,
                    int sw = 2 )
{
    if ( !p->isActive() )
        return;

    p->setPen( bottomColor );
    p->drawText( x+sw, y+sw, text );
    p->setPen( topColor );
    p->drawText( x, y, text );
}

// NOTE: desktop drag/drop is experimental

class DesktopWidget : public QWidget, private QDropSite
{
public:
    DesktopWidget( const char *s, QWidget *parent=0, const char *name=0 );
   ~DesktopWidget();
    void paintEvent( QPaintEvent * );

    void dragEnterEvent( QDragEnterEvent *e )
    {
        if ( QImageDrag::canDecode(e) )
            e->accept();
    }

    void dragLeaveEvent( QDragLeaveEvent * )
    {
    }

    void dragMoveEvent( QDragMoveEvent *e )
    {
        e->accept();
    }

    void dropEvent( QDropEvent * e )
    {
        QPixmap pmp;
        if ( QImageDrag::decode( e, pmp ) ) {
            setBackgroundPixmap( pmp );
            update();
        }
    }

private:
    QPixmap *pm;
    QString text;
};

DesktopWidget::DesktopWidget( const char *s, QWidget *parent, const char *name )
    : QWidget( parent, name, WType_Desktop | WPaintDesktop),
        QDropSite(this)
{
    text = s;
    pm   = 0;
}

DesktopWidget::~DesktopWidget()
{
    delete pm;
}

void DesktopWidget::paintEvent( QPaintEvent * )
{
    QColor c1 = backgroundColor();
    QColor c2 = c1.light(104);
    QColor c3 = c1.dark(106);
    if ( !pm ) {
        pm = new QPixmap( 64, 64 );
        generateStone( pm, c1, c2, c3 );
        setBackgroundPixmap( *pm );
        update();
    }
    QRect br = fontMetrics().boundingRect( text );
    QPixmap offscreen( br.width(), br.height() );
    int x = width()/2  - br.width()/2;
    int y = height()/2 - br.height()/2;
    offscreen.fill( this, x, y );
    QPainter p;
    p.begin( &offscreen );
    drawShadeText( &p, -br.x(), -br.y(), text, c2, c3, 3 );
    p.end();
    bitBlt( this, x, y, &offscreen );
}

void desktopWidget( const char *s = "Trolltech" )
{
    DesktopWidget *t = new DesktopWidget(s);
    t->update();
    qApp->exec();
    delete t;
}

void desktopText( const char *s = "Trolltech" )
{
    const int border = 20;

    QColor c1 =  qApp->palette().inactive().background();
    QColor c2 = c1.light(104);
    QColor c3 = c1.dark(106);

    QPixmap pm(10,10);

    QPainter p;
    p.begin( &pm );
    QRect r = p.fontMetrics().boundingRect( s );
    p.end();

    int appWidth  =  qApp->desktop()->width();
    int appHeight =  qApp->desktop()->height();
    if ( r.width() > appWidth - border*2 )
        r.setWidth( appWidth - border*2 );
    if ( r.height() > appHeight - border*2 )
        r.setHeight( appHeight - border*2 );

    pm.resize( r.size() + QSize( border*2, border*2 ) );
    generateStone( &pm, c1, c2, c3 );
    p.begin( &pm );
    drawShadeText( &p, -r.x() + border, -r.y() + border, s, c2, c3 );
    p.end();

    qApp->desktop()->setBackgroundPixmap( pm );
}

//
// The program starts here.
//

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

    if ( argc > 1 ) {
        QFont f( "charter", 96, QFont::Black );
        f.setStyleHint( QFont::Times );
        app.setFont( f );
    }

    bool validOptions = FALSE;

    if ( argc == 2 ) {
        validOptions = TRUE;
        if ( strcmp(argv[1],"-poly") == 0 )
            poly();
        else if ( strcmp(argv[1],"-rotate") == 0 )
            rotate();
        else if ( strcmp(argv[1],"-troll") == 0 )
            desktopText();
        else if ( strcmp(argv[1],"-trollwidget") == 0 )
            desktopWidget();
        else
            validOptions = FALSE;
    }
    if ( argc == 3 ) {
        validOptions = TRUE;
        if ( strcmp(argv[1],"-shadetext") == 0 )
            desktopText( argv[2] );
        else if ( strcmp(argv[1],"-shadewidget") == 0 )
            desktopWidget( argv[2] );
        else
            validOptions = FALSE;
    }
    if ( !validOptions ) {
        fprintf( stderr, "Usage:\n\tdesktop -poly"
                               "\n\tdesktop -rotate"
                               "\n\tdesktop -troll"
                               "\n\tdesktop -trollwidget"
                               "\n\tdesktop -shadetext <text>"
                               "\n\tdesktop -shadewidget <text>\n" );
        rotate();
    }
    return 0;
}