QPainter::drawPixmap with floats ?

[christophe.daudin]

I recently discovered that:
- either QPainter::drawPixmap, despite the fact that its API accepts QPointF/QRectF, doesn't handle floating point precision.
- or I'm doing something wrong.

Below, you'll find an illusation of the problem.

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsRectItem>
#include <QPainter>
 
class MyGraphicsItem : public QGraphicsRectItem
{
	public:
 
		MyGraphicsItem()
		{
			setRect(0,0,10,10);
		}
 
		void paint ( QPainter * painter, const QStyleOptionGraphicsItem * , QWidget * )
		{
			//Draw a grid of horizontal and vertical stripes. 
			//Each stripe has a thickness of 1 QPainter unit.
			painter->setPen(Qt::NoPen);
			painter->setBrush( QColor( 0,255,0,127 ) );
			for ( int i = 0 ; i < 10 ; i+=2 )
				painter->drawRect( QRect(0,i,10,1) );
 
			painter->setBrush( QColor( 255,0,0,127 ) );
			for ( int i = 0 ; i < 10 ; i+=2 )
				painter->drawRect( QRect(i,0,1,10) );
 
			//Build a colored QPixmap.
			QPixmap p(5,2);
			p.fill( QColor(0,0,255,127) );
 
			//Build a QRectF.
			QRectF r( 1.4f,1.4f,5,5 );
			//Draw my QPixmap, fitting it in the QRectF.
			painter->drawPixmap( r , p , QRectF( 0,0,p.width(),p.height() ) );
			//Problem: it behaves as: 
			//painter->drawPixmap( r.toRect() , p , QRectF( 0,0,p.width(),p.height() ) );
/*
//			Hack to get the expected behaviour.
			QRectF r1(r.x() * 10 , r.y() * 10 , r.width() * 10 , r.height() * 10 );
			painter->scale(0.1f,0.1f);
			painter->drawPixmap( r1 , p , QRectF( 0,0,p.width(),p.height() ) );
			painter->scale(10,10);
*/
		}
 
};
 
int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
 
	QGraphicsScene * scene = new QGraphicsScene();
	QGraphicsView v(scene);
 
	MyGraphicsItem* i = new MyGraphicsItem();
	i->scale(30,30);
	scene->addItem( i );
 
	v.show();
 
	return app.exec();
}

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsRectItem>
#include <QPainter>

class MyGraphicsItem : public QGraphicsRectItem
{
	public:

		MyGraphicsItem()
		{
			setRect(0,0,10,10);
		}

		void paint ( QPainter * painter, const QStyleOptionGraphicsItem * , QWidget * )
		{
			//Draw a grid of horizontal and vertical stripes. 
			//Each stripe has a thickness of 1 QPainter unit.
			painter->setPen(Qt::NoPen);
			painter->setBrush( QColor( 0,255,0,127 ) );
			for ( int i = 0 ; i < 10 ; i+=2 )
				painter->drawRect( QRect(0,i,10,1) );

			painter->setBrush( QColor( 255,0,0,127 ) );
			for ( int i = 0 ; i < 10 ; i+=2 )
				painter->drawRect( QRect(i,0,1,10) );

			//Build a colored QPixmap.
			QPixmap p(5,2);
			p.fill( QColor(0,0,255,127) );

			//Build a QRectF.
			QRectF r( 1.4f,1.4f,5,5 );
			//Draw my QPixmap, fitting it in the QRectF.
			painter->drawPixmap( r , p , QRectF( 0,0,p.width(),p.height() ) );
			//Problem: it behaves as: 
			//painter->drawPixmap( r.toRect() , p , QRectF( 0,0,p.width(),p.height() ) );
/*
//			Hack to get the expected behaviour.
			QRectF r1(r.x() * 10 , r.y() * 10 , r.width() * 10 , r.height() * 10 );
			painter->scale(0.1f,0.1f);
			painter->drawPixmap( r1 , p , QRectF( 0,0,p.width(),p.height() ) );
			painter->scale(10,10);
*/
		}

};

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

	QGraphicsScene * scene = new QGraphicsScene();
	QGraphicsView v(scene);

	MyGraphicsItem* i = new MyGraphicsItem();
	i->scale(30,30);
	scene->addItem( i );

	v.show();
		
	return app.exec();
}

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Thanks for any help !

[high_flyer]

hmm... can you post a screen shot how it looks with:

painter->drawPixmap( r , p , QRectF( 0,0,p.width(),p.height() ) );

painter->drawPixmap( r , p , QRectF( 0,0,p.width(),p.height() ) );

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and with the workaround?

it should not matter, but can you also try using all params as floats where QRectF is being used? (i.e QRectF( 0.0,0.0,p.width(),p.height() ) etc)

[christophe.daudin]

Expected behaviour (with workaround code):

Observed behaviour:


And all-floats stuff doesn't work - well, that would have been surprising, anyway.

[high_flyer]

no attachment is visible...

[christophe.daudin]

Yes, I don't know why. I put an IMG tag with an imageshack link. Well, "Right click" -> "Open image in new tab" still works ok, right ?

[high_flyer]

there is no link in the post to anything!

[christophe.daudin]

Workaround.
Bug.

[high_flyer]

Hmm... indeed interesting.
I for one don't see anything wrong with your code, so it might really be a bug.
Or some of the experts here know more about this.

I suggest you register it! - but before have a look maybe it is already a known issue.

[christophe.daudin]

That's the plan. Wait some days here for some experts. And submit it if necessary.
(there's nothing like that in the Qt bug tracker)