Strange behaviour with QPen

[franco.amato]

Hi to all,
I have to draw some lines in the paint event of an widget.
I set a pen with width = 1 but the real width of the pen is visibility bigger than 1 pixel and the width method
return 1 as value...odd

Here some code:

QPixmap temp = QPixmap( w, h ); // I draw on a pixel to cache
 
QPainter p( &temp );
p.setRenderHint( QPainter::Antialiasing, true );
p.fillRect( temp.rect(), Qt::lightGray );
 
// temporal line
QPen pen(Qt::black, 1); //<---- 1 pixel wide
pen.setStyle(Qt::SolidLine);
p.setPen(pen);
p.drawLine(0, 20, w, 20); //width of the line bigger than 1 pixel
qDebug() << "Pen width: " << pen.width(); // returns 1

QPixmap temp = QPixmap( w, h ); // I draw on a pixel to cache

QPainter p( &temp );
p.setRenderHint( QPainter::Antialiasing, true );
p.fillRect( temp.rect(), Qt::lightGray );

// temporal line
QPen pen(Qt::black, 1); //<---- 1 pixel wide
pen.setStyle(Qt::SolidLine);
p.setPen(pen);
p.drawLine(0, 20, w, 20); //width of the line bigger than 1 pixel
qDebug() << "Pen width: " << pen.width(); // returns 1

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Any idea?

Best

[boudie]

What does it look like when you don't use QPainter::Antialiasing?

And what happens when you set pen width to zero: a "cosmetic pen"? That one should always draw 1 pixel wide.

[franco.amato]

What does it look like when you don't use QPainter::Antialiasing?

And what happens when you set pen width to zero: a "cosmetic pen"? That one should always draw 1 pixel wide.

I removed the Antialiasing flag and seems solved the problem but why???

[wagmare]

your painted line will have some strange looking like a small saw tooth wave normally rather than straight .. try to draw a cross line u can see it clearly .. antialiasing is like coating or filling the scramble space ..
see the link
http://doc.trolltech.com/4.3/coordsy...iased-painting

[franco.amato]

your painted line will have some strange looking like a small saw tooth wave normally rather than straight .. try to draw a cross line u can see it clearly .. antialiasing is like coating or filling the scramble space ..
see the link
http://doc.trolltech.com/4.3/coordsy...iased-painting

Thank you for your explanation. This is why the width of my line is bigger than 1 pixel