QPainter::drawPixmapFragments does not take dpi into account.

  QPixmap m_bgCache;
  QPainter* p;

  p->drawPixmap(QPointF{}, m_bgCache);

  QPainter::PixmapFragment fragment{
    0., 0.,
    0., 0.,
    (qreal)m_bgCache.width(), (qreal) m_bgCache.height(),
    1., 1.,
    0., 1.
  };

  p->drawPixmapFragments(&fragment, 1, m_bgCache);