Capture QML drawing buffer, without displaying - qt

Question

I need to grab each QML (QtQuick 2) drawing frame and sent it over the network.
At the moment I have used method listed below, but this method has two big disadvantage
1) Due to Qt5 documentation grabWindow() function has performance issues
2) It can't work with hidden QML window
Is it possible to get OpenGL render buffer right after QQuickWindow::afterRendering ?
Using FBOs ? Shared opengl context ?
class Grab: public QObject
{
public:
Grab( QQuickWindow * wnd ) : wnd_(wnd) {}
public slots:
void Grabme()
{
QImage image = wnd_->grabWindow();
}
private:
QQuickWindow *wnd_;
};
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QtQuick2ApplicationViewer viewer;
viewer.setMainQmlFile(QStringLiteral("qml/grab1/main.qml"));
viewer.showExpanded();
Grab grab( &viewer );
QObject::connect( &viewer, &QtQuick2ApplicationViewer::frameSwapped,
&grab, &Grab::Grabme, Qt::DirectConnection );
return app.exec();
}

Answer 1

Example bellow can grab any qml content to FBO and then sent it as Image via signal.
Only one problem of this approach is visibility, grab window must be visible for successful grabbing. If anybody knows how to prevent this you can help me and provide more advanced approach.
// main.cpp
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
GrabWindow grab;
grab.setResizeMode( QQuickView::SizeViewToRootObject );
grab.setSource( QUrl::fromLocalFile("qml/main.qml") );
grab.setFlags( Qt::Popup );
grab.show();
return app.exec();
}
// grabwindow.hpp
#pragma once
#include <QOpenGLFramebufferObject>
#include <QScopedPointer>
#include <QQuickView>
#include <QImage>
class GrabWindow: public QQuickView
{
Q_OBJECT
signals:
void changeImage( const QImage &image );
public:
GrabWindow( QWindow * parent = 0 );
private slots:
void afterRendering();
void beforeRendering();
private:
QScopedPointer<QOpenGLFramebufferObject> fbo_;
};
// grabwindow.cpp
#include "grabwindow.hpp"
#include <limits>
GrabWindow::GrabWindow( QWindow * parent ) :
QQuickView( parent )
{
setClearBeforeRendering( false );
setPosition( std::numeric_limits<unsigned short>::max(), std::numeric_limits<unsigned short>::max() );
connect( this, SIGNAL( afterRendering() ), SLOT( afterRendering() ), Qt::DirectConnection );
connect( this, SIGNAL( beforeRendering() ), SLOT( beforeRendering() ), Qt::DirectConnection );
}
void GrabWindow::afterRendering()
{
if( !fbo_.isNull() )
{
emit changeImage( fbo_->toImage() );
}
}
void GrabWindow::beforeRendering()
{
if (!fbo_)
{
fbo_.reset(new QOpenGLFramebufferObject( size(), QOpenGLFramebufferObject::NoAttachment) );
setRenderTarget(fbo_.data());
}
}

Answer 2

I managed to find a trick to make grabWindow() work when the Window is "not visible". The trick is to set the window's visibility: Window.Minimized and the flags: Qt.Tool. The window is not displayed to the user, but to the Qt's internals it appears to be visible and the grabWindow() method call works as expected. Remember to call that method only once the scene has been initialised.
The only problem with this solution (that I have come across) is that if the window's color property is set to transparent, the captured content has black background.

Answer 3

With later versions of Qt 5.X you can also use the software render backend.
The following renders any scene in the background without any visible window or OpenGL tricks:
// main.cpp
#include <QGuiApplication>
#include <QQmlEngine>
#include <QQmlComponent>
#include <QQuickItem>
#include <QQuickWindow>
#include <QQuickRenderControl>
int main(int argc, char *argv[])
{
const char *source = "qrc:/main.qml";
if (argc > 1) source = argv[1];
QQuickWindow::setSceneGraphBackend(QSGRendererInterface::Software);
QGuiApplication app{argc, argv};
QQuickRenderControl renderControl;
QQuickWindow window{&renderControl};
QQmlEngine engine;
QQmlComponent component{
&engine,
QUrl{QString::fromUtf8(source)}
};
QQuickItem *rootItem = qobject_cast<QQuickItem *>(component.create());
window.contentItem()->setSize(rootItem->size());
rootItem->setParentItem(window.contentItem());
window.resize(rootItem->size().width(), rootItem->size().height());
QImage image = renderControl.grab();
image.save("output.png");
return 0;
}