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MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
...
this->setWindowFlags( Qt::FramelessWindowHint );
this->setAttribute( Qt::WA_TranslucentBackground );
this->setStyleSheet( "background:transparent;");

QPixmap backgroud_img( ":/materials/background.png" );

QImage image(backgroud_img.size(), QImage::Format_ARGB32_Premultiplied);
image.fill(Qt::transparent);
QPainter p(&image);
p.setOpacity(0.35);
p.drawPixmap(0, 0, backgroud_img);
p.end();
backgroud_img = QPixmap::fromImage(image);

ui->background->setPixmap( backgroud_img ); // background - label элемент

}

qDebug() << "created"

qDebug() << "deleted"

#ifndef NOTIFYWINDOW_H
#define NOTIFYWINDOW_H

#include <QMainWindow>
#include <QEvent>

namespace Ui {
class NotifyWindow;
}

class NotifyWindow : public QMainWindow
{
    Q_OBJECT
    
public:
    explicit NotifyWindow(QWidget *parent = 0);
    ~NotifyWindow();


private:
    Ui::NotifyWindow *ui;

    void initWindowGeomery();

};

#endif // NOTIFYWINDOW_H

#include "notifywindow.h"
#include "ui_notifywindow.h"

#include <QPainter>
#include <QDesktopWidget>
#include <QDebug>

void setAlphaPixmap(QPixmap&, double);

NotifyWindow::NotifyWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::NotifyWindow)
{
    ui->setupUi(this);

    this->setWindowFlags( Qt::FramelessWindowHint  | Qt::WindowStaysOnTopHint );
    this->setAttribute( Qt::WA_TranslucentBackground );
    this->setStyleSheet( "background:transparent;");
    ui->centralWidget->setStyleSheet( "background:transparent;" );

    QPixmap backgroud_img( ":/materials/background.png" );
    setAlphaPixmap(backgroud_img, 0.35);

    ui->background->setPixmap( backgroud_img );
    initWindowGeomery();

    qDebug() << "created()" << "\n";

}

NotifyWindow::~NotifyWindow()
{
    qDebug() << "deleted()" << "\n";
    delete ui;
}

//////////////////////////////////////////////////

void NotifyWindow::initWindowGeomery()
{
    int screenSizeX, screenSizeY, appSizeX, appSizeY;
    QDesktopWidget desk;

    appSizeX = ui->background->width();
    appSizeY = ui->background->height();
    screenSizeX = desk.width();
    screenSizeY = desk.height();
    this->move( (screenSizeX - appSizeX) - 5, 46 );
}


void setAlphaPixmap(QPixmap &Pixmap, double alpha)
{
    QImage image(Pixmap.size(), QImage::Format_ARGB32_Premultiplied);
    image.fill(Qt::transparent);
    QPainter p(&image);
    p.setOpacity(alpha);
    p.drawPixmap(0, 0, Pixmap);
    p.end();
    Pixmap = QPixmap::fromImage(image);
}

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    NotifyWindow w;
    w.show();
    
    return a.exec();
}

<RCC>
    <qresource prefix="/">
        <file>materials/background.png</file>
    </qresource>
</RCC>

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>NotifyWindow</class>
 <widget class="QMainWindow" name="NotifyWindow">
  <property name="windowModality">
   <enum>Qt::NonModal</enum>
  </property>
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>292</width>
    <height>61</height>
   </rect>
  </property>
  <widget class="QWidget" name="centralWidget">
   <widget class="QLabel" name="background">
    <property name="geometry">
     <rect>
      <x>0</x>
      <y>0</y>
      <width>292</width>
      <height>61</height>
     </rect>
    </property>
    <property name="lineWidth">
     <number>0</number>
    </property>
    <property name="text">
     <string/>
    </property>
   </widget>
  </widget>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>