How to make a window transparent using Qt?

Question

I’m developing a college job in Qt. I would like to make the background of my application transparent. I did some research and the function I found was:

this->setWindowOpacity(0);

Meanwhile, the Qt applies opacity throughout the window. I would like the components I add, such as buttons or even drawings using the QPainter, keep their opacities normal, that is, do not want to apply opacity to the window components, only in the frame.

I’m using Debian 7.1 with the Gnome 3.4.2.

At the moment I’m having this result:

Note that the transparency applied to the button as well.

I removed some things from my application in order to facilitate the resolution of my problem. Follow the code:

Window. h

#ifndef JANELA_H
#define JANELA_H

#include <QMainWindow>
#include <QFrame>
#include <QPushButton>
#include <QWidget>
#include <QVBoxLayout>
#include <QPainter>
#include <QPen>

class Janela: public QMainWindow{
    Q_OBJECT
public:
    Janela();
    ~Janela();
    void paintEvent(QPaintEvent * event);

public:
    QFrame *frame;
    QPushButton *botao;
    int pos_x=280,pos_y=280;
};

#endif // JANELA_H

Window.cpp

#include <Janela.h>

Janela::Janela(){
    QFrame *frame = new QFrame();
    QVBoxLayout *layout = new QVBoxLayout();
    this->setWindowOpacity(0.80);
    setCentralWidget(frame);
    setWindowTitle(QString::fromUtf8("Exemplo de QFrame"));

    frame->setLineWidth(4);
    frame->setFrameStyle(QFrame::Box);
    frame->setFixedSize(300,300);
    frame->resize(300, 300);
    frame->setMinimumSize(300,300);
    frame->setMaximumSize(300,300);
    frame->setLayout(layout);

    botao = new QPushButton("Pressione-me");
    layout->addWidget(botao);

    show();
    frame->setFocus();

};

Janela::~Janela(){};

void Janela::paintEvent(QPaintEvent *event){
    QPainter painter(this);
    painter.setPen(Qt::green);
    painter.setBrush(Qt::green);
    painter.drawEllipse(10,10,10,10);
    painter.fillRect(pos_x,pos_y,10,10,Qt::red);
}

main.cpp

#include "Janela.h"
#include <QApplication>

int main(int argc, char **argv){
    QApplication app(argc, argv);

    Janela *janela = new Janela();

    return app.exec();
}

Answer 1

You can do this by activating the flag Qt::FramelessWindowHint in Janela leading.

Janela::Janela() : QMainWindow(NULL, Qt::FramelessWindowHint)

And then activate the attribute Qt::WA_TranslucentBackground to make the background invisible:

setAttribute(Qt::WA_TranslucentBackground);

The result is this:

---

To make the background translucent, and not just invisible, you add in Paintevent the background drawing you want. Example:

QPainter painter(this);

// Para usar a cor padrão de background do sistema.
QColor background_color = palette().color(QPalette::Background);
background_color.setAlpha(128); 
painter.setBrush(QBrush(background_color));

// Define uma borda.
painter.setPen(QColor("#888"));

// Desenha o novo background da janela, antes de qualquer outra coisa.
painter.drawRect(0, 0, width()-1, height()-1);

And the result is this: