Painting on the Desktop

The desktop demo contains three routines, each of which draws something on the desktop. It does some nice stuff with TQPainter, 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 TQt.  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()
{
    TQWidget *d = TQApplication::desktop();
    d->setBackgroundColor( TQt::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;
    TQPointArray *a = new TQPointArray[ maxcurves ];
    register TQPointArray *p;
    TQRect 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);
    }

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

    for ( int ntimes=0; ntimes<2000; ntimes++ ) {
        paint.setBrush( TQColor(kindaRand()%360, 180, 255, TQColor::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;
    TQImage 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;
    }

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

    TQWidget *d = TQApplication::desktop();       // w = desktop widget

    for ( i=0; i<=360; i += 2 ) {
        TQWMatrix m;
        m.rotate( i );                          // rotate coordinate system
        TQPixmap 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( TQPixmap *pm,
                    const TQColor &c1, const TQColor &c2, const TQColor &c3 )
{
    TQPainter p;
    TQPen p1 ( c1, 0 );
    TQPen p2 ( c2, 0 );
    TQPen 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( TQPainter *p, int x, int y, const char *text,
                    const TQColor &topColor, const TQColor &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 TQWidget, private TQDropSite
{
public:
    DesktopWidget( const char *s, TQWidget *parent=0, const char *name=0 );
   ~DesktopWidget();
    void paintEvent( TQPaintEvent * );

    void dragEnterEvent( TQDragEnterEvent *e )
    {
        if ( TQImageDrag::canDecode(e) )
            e->accept();
    }

    void dragLeaveEvent( TQDragLeaveEvent * )
    {
    }

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

    void dropEvent( TQDropEvent * e )
    {
        TQPixmap pmp;
        if ( TQImageDrag::decode( e, pmp ) ) {
            setBackgroundPixmap( pmp );
            update();
        }
    }

private:
    TQPixmap *pm;
    TQString text;
};

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

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

void DesktopWidget::paintEvent( TQPaintEvent * )
{
    TQColor c1 = backgroundColor();
    TQColor c2 = c1.light(104);
    TQColor c3 = c1.dark(106);
    if ( !pm ) {
        pm = new TQPixmap( 64, 64 );
        generateStone( pm, c1, c2, c3 );
        setBackgroundPixmap( *pm );
        update();
    }
    TQRect br = fontMetrics().boundingRect( text );
    TQPixmap offscreen( br.width(), br.height() );
    int x = width()/2  - br.width()/2;
    int y = height()/2 - br.height()/2;
    offscreen.fill( this, x, y );
    TQPainter 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;

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

    TQPixmap pm(10,10);

    TQPainter p;
    p.begin( &pm );
    TQRect 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() + TQSize( 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 )
{
    TQApplication app( argc, argv );

    if ( argc > 1 ) {
        TQFont f( "charter", 96, TQFont::Black );
        f.setStyleHint( TQFont::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;
}