Drawing directly with QPainter and drawing QPixmap give different results

Question

There's a simple class based on QQuickPaintedItem:

class PaintedItem : public QQuickPaintedItem
{
    Q_OBJECT
public:
    PaintedItem();
    void paint(QPainter *painter) override;
};

// ...

PaintedItem::PaintedItem()
{
    setRenderTarget(QQuickPaintedItem::FramebufferObject);
}

void PaintedItem::paint(QPainter *painter)
{
    painter->drawRect(0, 0, 150, 150);

    QPixmap* m_pixmap = new QPixmap(width(), height());
    m_pixmap->fill(QColor("transparent"));

    QPainter painter2(m_pixmap);
    painter2.setPen(QColor("red"));
    painter2.drawRect(0, 0, 150, 150);

    painter->drawPixmap(0, 0, *m_pixmap);
}

The paint() function just does two things: draw a rectangle directly with QPainter and draw the QPixmap containing the same rectangle. But if I set the render target as FramebufferObject in the constructor, those rectangles doesn't match for some reason. If I comment this string, everything's OK.

With FramebufferObject

Without FramebufferObject

Could you please explain me why does it happen and how to deal with it?

Answer 1

Most likely, drawing on the QQuickPaintedItem is not clipped, and you're drawing a bigger rectangle than you think you are. For historical reasons, the rectangles are one pixel wider than the size you entered. So your rectangle is 151x151 and doesn't fit into the QPixmap.