#include <QtWidgets>

class MyWidget : public QWidget
{
public:
    MyWidget(QWidget *parent = 0) : QWidget(parent) {}
protected:
    void paintEvent(QPaintEvent *)
    {
        QPixmap p("test.png");
        QPainter paint(this);
        paint.drawPixmap(0,0,p);
    }
};

class MyOverlayWidget : public QWidget
{
public:
    MyOverlayWidget(QWidget *parent = 0) : QWidget(parent, Qt::WindowTransparentForInput 
        | Qt::WindowDoesNotAcceptFocus 
        | Qt::NoDropShadowWindowHint 
        | Qt::FramelessWindowHint 
        | Qt::X11BypassWindowManagerHint // Note, this is necessary on Linux. Without this flag, the manual widget resize() doesn’t seem to take effect. Removing this flag, however, does not fix the transparent rendering issue. 
        | Qt::Tool)
    {
        setAttribute(Qt::WA_TranslucentBackground);
    }
protected:
    void paintEvent(QPaintEvent *)
    {
        QPainter painter(this); 
        painter.setRenderHint(QPainter::Antialiasing); 

        painter.setCompositionMode(QPainter::CompositionMode_Clear); 
        painter.fillRect(rect(), Qt::red); // technically any color would work 
        painter.setCompositionMode(QPainter::CompositionMode_SourceOver); 
        painter.drawRect(QRect(10,10,200,200));
    }
};

int main(int argc, char **argv)
{
    QApplication::setAttribute(Qt::AA_ImmediateWidgetCreation);
    QApplication a(argc, argv);
    MyWidget w;
    w.show();
    MyOverlayWidget overlay;
    overlay.setGeometry(w.geometry());
    overlay.show();
    return a.exec();
}