Screenshot capture of dual monitor in Qt 5

Question

In the context of a Qt application, I'm using the following code snippet for taking a screenshot of the full desktop:

QDesktopWidget* dw = QApplication::desktop();
QPixmap pixmap = QPixmap::grabWindow(dw->winId(), 0, 0,
                                     dw->width(), dw->height());
pixmap.save(name, "JPG", screenshot_quality);

This approach works pretty well in Linux and Windows and with dual monitor, independently of screen's resolutions; that is, it works still if the two monitors are working with different resolutions.

However, with Qt 5, I get the following run-time warning:

static QPixmap QPixmap::grabWindow(WId, int, int, int, int) is deprecated, use QScreen::grabWindow() instead. Defaulting to primary screen.

So I reviewed the Qt 5 doc and I wrote this:

QScreen * screen = QGuiApplication::primaryScreen();
QPixmap pixmap = screen->grabWindow(0);
pixmap.save(name, "JPG", screenshot_quality);

But this approach does not capture the second screen.

So I searched a little more and, according to this thread, Taking Screenshot of Full Desktop with Qt5, I designed the screenshot capture as follows:

QScreen * screen = QGuiApplication::primaryScreen();
QRect g = screen->geometry();
QPixmap pixmap = screen->grabWindow(0, g.x(), g.y(), g.width(), g.height());
pixmap.save(name, "JPG", screenshot_quality);

Unfortunately, this does not work either.

What catches my attention is that the method with Qt 4 works well. Since I imagine there must be some way to make it in Qt 5.

How can be done with Qt 5?

Johanajohanan

I've edited out the answer as it does not belong in the question, but in an answer box. If you wish to share it, please add it as an answer, otherwise I will be adding it as a community wiki. See How to treat an old question which had an answer edited into it? – Pleven

Answer 1

Naturally, QGuiApplication::primaryScreen() will give you a single screen.

You could:

• Use QList<QScreen *> QGuiApplication::screens() to get all screens associated with the application.
• Take screenshots for all of them.
• Create another blank image.
• Size it according to how you want to compose the screens.
• Manually compose them into a final image using QPainter.

QPixmap grabScreens() {
  auto screens = QGuiApplication::screens();
  QList<QPixmap> scrs;
  int w = 0, h = 0, p = 0;
  foreach (auto scr, screens) {
    QPixmap pix = scr->grabWindow(0);
    w += pix.width();
    if (h < pix.height()) h = pix.height();
    scrs << pix;
  }
  QPixmap final(w, h);
  QPainter painter(&final);
  final.fill(Qt::black);
  foreach (auto scr, scrs) {
    painter.drawPixmap(QPoint(p, 0), scr);
    p += scr.width();
  }
  return final;
}

Conspecific

Answer 2

For old Qt versions:

You can use primary screen's (desktop) virtual geometry and capture entire desktop without additional loops and calculations:

QRect desktopGeometry = qApp->primaryScreen()->virtualGeometry();
QPixmap desktopPixmap = qApp->primaryScreen()->grabWindow(qApp->desktop()->winId(), desktopGeometry.x(), desktopGeometry.y(), desktopGeometry.width(), desktopGeometry.height());

See also: QDesktopWidget

---

For new Qt versions:

QApplication::desktop() and QDesktopWidget are marked as obsolete, so for new projects it's recommended to use approach with screen enumerating.

Yeomanly

Answer 3

Answer provided by lrleon:

Based on this answer, I came up with this:

QPixmap grabScreens()
{
  QList<QScreen*> screens = QGuiApplication::screens();
  QList<QPixmap> scrs;
  int w = 0, h = 0, p = 0;

  foreach (auto scr, screens)
    {
      QRect g = scr->geometry();
      QPixmap pix = scr->grabWindow(0, g.x(), g.y(), g.width(), g.height());
      w += pix.width();
      h = max(h, pix.height());
      scrs.append(pix);
    }

  QPixmap final(w, h);
  QPainter painter(&final);
  final.fill(Qt::black);
  foreach (auto scr, scrs)
    {
      painter.drawPixmap(QPoint(p, 0), scr);
      p += scr.width();
    }

  return final;
}

Pleven