C++ (Qt)
QPixmap pm(100, 100);
pm.fill(Qt::transparent);
 

C++ (Qt)
QPixmap pm(100, 100);
pm.fill(Qt::red);
QPainter p(&pm);
p.setCompositionMode(QPainter::CompositionMode_Source);
p.fillRect(0, 0, 50, 50, Qt::transparent);
 

C++ (Qt)
setBrush(QColor());
drawRect();

C++ (Qt)
QPainter p(&pixmap);
p.setCompositionMode(QPainter::CompositionMode_Source);
p.fillRect(rect, Qt::transparent);