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painter->setRenderHint( QPainter::Antialiasing, true )

#include <QtCore/QDebug>
#include <QtGui/QApplication>
#include <QtGui/QGraphicsItem>
#include <QtGui/QGraphicsScene>
#include <QtGui/QGraphicsView>
#include <QtOpenGL/QGLWidget>

class GraphicsItem : public QGraphicsItem
{
public:
	GraphicsItem() : QGraphicsItem() {}

	QRectF boundingRect() const
	{
		return QRectF( 0, 0, 100, 100 );
	}

	void paint( QPainter* painter, const QStyleOptionGraphicsItem*, QWidget* )
	{
		painter->setWorldMatrixEnabled( false );
		painter->setRenderHint( QPainter::Antialiasing, true );

		painter->drawLine( 0, 0, 80, 10 );

		painter->setRenderHint( QPainter::Antialiasing, false );
		painter->setWorldMatrixEnabled( true );
	}
};

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

	GraphicsItem* item = new GraphicsItem();

	QGraphicsScene scene;
	scene.addItem( item );
	scene.setSceneRect( item->boundingRect() );

	QGraphicsView view;
	view.setViewport( new QGLWidget( &view ) );
	view.setScene( &scene );

	app.setActiveWindow( &view );
	view.show();
	view.resize( 300, 300 );

	view.fitInView( scene.sceneRect() );

	return app.exec();
}