I create a mono image with two colors: foreground color (Qt::color0) is
red, and background color (Qt::color1) is transparent.

I can save it to an image file without problem. However, when I print it
to a PDF file, the transparent background becomes all black.

This seems to be similar to the issue faced in task #242917

Example:

#include <QtGui>

class MyGraphicsView : public QGraphicsView {
public:
MyGraphicsView( QGraphicsScene * scene ) :
QGraphicsView( scene ) {
}

protected:

virtual void contextMenuEvent ( QContextMenuEvent * event ) {
QStringList al;
al << "Print..." << "Save as an image file...";

QMenu menu;
for ( QStringList::const_iterator it=al.begin(); it!=al.end(); it++ )

{ menu.addAction( (*it) ); }

QAction* action = menu.exec( event->globalPos() );
if ( action )

{ takeAction( al.indexOf( action->text() ) ); }

}

private:

void takeAction( int idx ) {
switch ( idx ) {
case 0: {
QPrinter printer( QPrinter::HighResolution );
printer.setDocName( "test" );
printer.setCreator( "test example" );
printer.setOrientation( QPrinter::Landscape );
QPrintDialog dialog( &printer );
if ( dialog.exec() )

{ QPainter painter( &printer ); scene()->render( &painter ); }

} break;
case 1: {
QString filter( "*.bmp *.jpg *.jpeg *.png *.ppm *.tiff *.xbm *.xpm" );
QString filename = QFileDialog::getSaveFileName( this, "Save as image",
"", filter );
if ( !filename.isEmpty() ) {
QRectF frame( scene()->sceneRect() );
frame.moveTo( 0., 0. );
QImage image( frame.size().toSize(), QImage::Format_RGB32 );
QPainter painter( &image );
painter.fillRect( frame, QBrush( Qt::white ) );
scene()->render( &painter );
if ( !image.save( filename ) )

{ qDebug() << "Fail to save file"; }

}
} break;
}

}

};

class MyImageItem : public QGraphicsItem {
public:

MyImageItem( QGraphicsItem* parent=0 ) :
QGraphicsItem( parent ) {
}

QRectF boundingRect() const

{ return QRectF( 0, 0, 400, 400 ); }

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

QPen pen( Qt::red );
painter->setPen( pen );

QFont font = painter->font();
font.setPixelSize( 16 );

painter->setFont( font );
painter->drawText( 30, 30, "Text behind a transparent image" );

// create a mono image
QImage image( boundingRect().size().toSize(), QImage::Format_Mono );

// set color1 to be transparent and fill the image
image.setColor( Qt::color1, Qt::transparent );
image.fill( Qt::color1 );

// set foreground color
image.setColor( Qt::color0, pen.color().rgb() );

for ( int ix = 0; ix < image.width(); ix++ )

{ image.setPixel( QPoint( ix, 1 ), Qt::color0 ); image.setPixel( QPoint( ix, image.height()/2 ), Qt::color0 ); image.setPixel( QPoint( ix, image.height() - 1 ), Qt::color0 ); }

for ( int iy = 0; iy < image.height(); iy++ )

{ image.setPixel( QPoint( 1, iy ), Qt::color0 ); image.setPixel( QPoint( image.width()/2, iy ), Qt::color0 ); image.setPixel( QPoint( image.width()-1, iy ), Qt::color0 ); }

painter->drawImage( image.rect(), image );
//painter->drawPixmap( QPoint( 0, 0 ), QBitmap::fromImage( image ) );

}

};

int main( int argc, char** argv )
{
QApplication app( argc, argv );
QGraphicsScene* scene( new QGraphicsScene );
scene->setSceneRect( QRectF( 0, 0, 450, 450 ) );

MyImageItem imgItem;
imgItem.setPos( 50, 50 );

scene->addItem( &imgItem );

// view
MyGraphicsView view( scene );
view.resize( 500, 500 );
view.show();

return app.exec();

}