Qt round rectangle, why corners are different?

Question

I try to draw a round rectangle with drawRoundedRect method directly in a QPixmap (no render engine involve here exept pure Qt one ...), I double check the size of the rectangle versus the size of my pixmap :

Pixmap : QSize(50, 73) 
Rectangle: QRect(0,0 48x11) 

See plenty of space ...

EDIT: some code

pixmap = QPixmap(50,73); //example size that match my case
QRectF rect(0,0,48,11);

QPainter painter(&pixmap);
painter.setRenderHint(QPainter::TextAntialiasing);
painter.setWorldMatrixEnabled(false);
painter.setPen(QPen()); //no pen
painter.setBrush(QBrush(color));
painter.drawRoundedRect(rect, 2.0, 2.0);

• I disabled world transformation ...
• I set set transformation to unity ...
• I tried several radius (1.0,2.0,3.0,4.0) ...
• I change pen width, brush color ...

But it always ends with a rectamgle with 4 diferent corners ! Like that :

I directly ouptut the pixmap to a file to be sure I wasn't scraping it during the display ... same shape.

Anyone know about Qt round rectangle with small radius ? I saw somthing about it a long time ago but I don't remenber how to deal with it !

Answer 1

It looks like you're not using anti-aliasing (i.e. the QPainter::Antialiasing render hint). This is a Qt quirk that occurs without it. From what I've seen/heard, the Qt devs aren't terribly concerned with fixing this (most people want anti-aliasing anyway).

The work-around (besides just using anti-aliasing) is to draw the rect yourself with QPainter::drawLine() and QPainter::drawArc(). You might have to play with numbers until it looks right -- straight calculations tend to come out a pixel or two off. Also, you might find that even with this method the lower right corner is never exactly the same as the other corners.

If you're feeling mildly ambitious, you could try fixing this and submitting a patch to Qt.

Update: Arc drawing results changed in Qt 5. In my experience, it's a big improvement.