Question

Is it possible to display translucent and/or irregular-shaped windows with Qt?

(I’m assuming it ultimately depends on the capabilities of the underlying GUI system, but let’s assume at least Windows XP / Mac OS X)

If so, how does one accomplish this?

Answer 1

Yes, it is possible. The key is the Qt::WA_TranslucentBackground attribute of QWidget

Here is a simple class that draws a round translucent window with a red background 50% alpha.

TranslucentRoundWindow.h:

#include <QWidget>

class TranslucentRoundWindow : public QWidget
{
    public:
        TranslucentRoundWindow(QWidget *parent = 0);
        virtual QSize sizeHint() const;

    protected:
        virtual void paintEvent(QPaintEvent *paintEvent);
};

TranslucentRoundWindow.cpp:

#include <QtGui>

#include "TranslucentRoundWindow.h"

TranslucentRoundWindow::TranslucentRoundWindow(QWidget *parent) : QWidget(parent, Qt::FramelessWindowHint)
{
    setAttribute(Qt::WA_TranslucentBackground);
}

QSize TranslucentRoundWindow::sizeHint() const
{
    return QSize(300, 300);
}

void TranslucentRoundWindow::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setPen(Qt::NoPen);
    painter.setBrush(QColor(255, 0, 0, 127));

    painter.drawEllipse(0, 0, width(), height());
}

If you want to be able to move this window with the mouse, you will have to override mousePressEvent, mouseMoveEvent and mouseReleaseEvent.