grabberdialog.cpp

#include "grabberdialog.h"

#include <QDebug>
#include <QGuiApplication>
#include <QLabel>
#include <QPixmap>
#include <QPushButton>
#include <QScreen>
#include <QVBoxLayout>
#include <QWindow>

GrabberDialog::GrabberDialog(QWidget *parent)
    : QDialog(parent)
{
    setLayout(new QVBoxLayout);
    QPushButton *captureButton = new QPushButton("Capture current screen", this);
    connect(captureButton, &QPushButton::clicked, this, &GrabberDialog::grabCurrentScreen);
    layout()->addWidget(captureButton);
    image = new QLabel;
    layout()->addWidget(image);
}

void GrabberDialog::grabCurrentScreen() {
    QScreen *thisScreen = windowHandle()->screen();
    if (!thisScreen)
        thisScreen = QGuiApplication::primaryScreen();

    auto screenGeometry = thisScreen->geometry();
    qDebug() << "Capturing window at" << geometry() << "from screen" << screenGeometry << "with ratio" << thisScreen->devicePixelRatio();
    QPixmap capture = thisScreen->grabWindow(winId(), screenGeometry.x(), screenGeometry.y(), screenGeometry.width() / thisScreen->devicePixelRatio(), screenGeometry.height() / thisScreen->devicePixelRatio());
    capture = capture.scaledToWidth(capture.width() / 3, Qt::SmoothTransformation);
    image->setPixmap(capture);
    image->resize(capture.width(), capture.height());
}

grabberdialog.h

#ifndef GRABBERDIALOG_H
#define GRABBERDIALOG_H

#include <QDialog>

class QLabel;

class GrabberDialog : public QDialog
{
    Q_OBJECT

public:
    GrabberDialog(QWidget *parent = 0);

private slots:
    void grabCurrentScreen();

private:
    QLabel *image;
};

#endif // GRABBERDIALOG_H

main.cpp

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    GrabberDialog w;
    w.show();

    return a.exec();
}

screengrabber.pro

#-------------------------------------------------
#
# Project created by QtCreator 2015-08-08T10:25:16
#
#-------------------------------------------------

QT += core gui
CONFIG += c++11


greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = screengrabber
TEMPLATE = app


SOURCES += main.cpp\
        grabberdialog.cpp

HEADERS  += grabberdialog.h

screenshot QTBUG-47643.png