QQuickRenderControl Class

The QQuickRenderControl class provides a mechanism for rendering the Qt Quick scenegraph onto an offscreen render target in a fully application-controlled manner. More...

Header: #include <QQuickRenderControl>
qmake: QT += quick
Since: Qt 5.4
Inherits: QObject

This class was introduced in Qt 5.4.

Public Functions

QQuickRenderControl(QObject *parent = nullptr)
virtual ~QQuickRenderControl() override
QImage grab()
void initialize(QOpenGLContext *gl)
void invalidate()
void polishItems()
void prepareThread(QThread *targetThread)
void render()
virtual QWindow *renderWindow(QPoint *offset)
bool sync()


Static Public Members

QWindow *renderWindowFor(QQuickWindow *win, QPoint *offset = nullptr)

Detailed Description

QQuickWindow and QQuickView and their associated internal render loops render the Qt Quick scene onto a native window. In some cases, for example when integrating with 3rd party OpenGL renderers, it might be beneficial to get the scene into a texture that can then be used in arbitrary ways by the external rendering engine. QQuickRenderControl makes this possible in a hardware accelerated manner, unlike the performance-wise limited alternative of using QQuickWindow::grabWindow()

When using a QQuickRenderControl, the QQuickWindow does not have to be shown or even created at all. This means there will not be an underlying native window for it. Instead, the QQuickWindow instance is associated with the render control, using the overload of the QQuickWindow constructor, and an OpenGL framebuffer object by calling QQuickWindow::setRenderTarget().

Management of the context and framebuffer object is up to the application. The context that will be used by Qt Quick must be created before calling initialize(). The creation of the framebuffer object can be deferred, see below. Qt 5.4 introduces the ability for QOpenGLContext to adopt existing native contexts. Together with QQuickRenderControl this makes it possible to create a QOpenGLContext that shares with an external rendering engine's existing context. This new QOpenGLContext can then be used to render the Qt Quick scene into a texture that is accessible by the other engine's context too.

Loading and instantiation of the QML components happen by using a QQmlEngine. Once the root object is created, it will need to be parented to the QQuickWindow's contentItem().

Applications will usually have to connect to 4 important signals:

To send events, for example mouse or keyboard events, to the scene, use QCoreApplication::sendEvent() with the QQuickWindow instance as the receiver.

Note: In general QQuickRenderControl is supported in combination with all Qt Quick backends. However, some functionality, in particular grab(), may not be available in all cases.

Member Function Documentation

QQuickRenderControl::QQuickRenderControl(QObject *parent = nullptr)

Constructs a QQuickRenderControl object, with parent object parent.

[signal] void QQuickRenderControl::renderRequested()

This signal is emitted when the scene graph needs to be rendered. It is not necessary to call sync().

Note: Avoid triggering rendering directly when this signal is emitted. Instead, prefer deferring it by using a timer for example. This will lead to better performance.

[signal] void QQuickRenderControl::sceneChanged()

This signal is emitted when the scene graph is updated, meaning that polishItems() and sync() needs to be called. If sync() returns true, then render() needs to be called.

Note: Avoid triggering polishing, synchronization and rendering directly when this signal is emitted. Instead, prefer deferring it by using a timer for example. This will lead to better performance.

[override virtual] QQuickRenderControl::~QQuickRenderControl()

Destroys the instance. Releases all scenegraph resources.

See also invalidate().

QImage QQuickRenderControl::grab()

Grabs the contents of the scene and returns it as an image.

Note: Requires the context to be current.

void QQuickRenderControl::initialize(QOpenGLContext *gl)

Initializes the scene graph resources. The context gl has to be the current OpenGL context or null if it is not relevant because a Qt Quick backend other than OpenGL is in use.

Note: Qt Quick does not take ownership of the context. It is up to the application to destroy it after a call to invalidate() or after the QQuickRenderControl instance is destroyed.

void QQuickRenderControl::invalidate()

Stop rendering and release resources. Requires a current context.

This is the equivalent of the cleanup operations that happen with a real QQuickWindow when the window becomes hidden.

This function is called from the destructor. Therefore there will typically be no need to call it directly. Pay attention however to the fact that this requires the context, that was passed to initialize(), to be the current one at the time of destroying the QQuickRenderControl instance.

Once invalidate() has been called, it is possible to reuse the QQuickRenderControl instance by calling initialize() again.

Note: This function does not take QQuickWindow::persistentSceneGraph() or QQuickWindow::persistentOpenGLContext() into account. This means that context-specific resources are always released.

void QQuickRenderControl::polishItems()

This function should be called as late as possible before sync(). In a threaded scenario, rendering can happen in parallel with this function.

void QQuickRenderControl::prepareThread(QThread *targetThread)

Prepares rendering the Qt Quick scene outside the GUI thread.

targetThread specifies the thread on which synchronization and rendering will happen. There is no need to call this function in a single threaded scenario.

void QQuickRenderControl::render()

Renders the scenegraph using the current context.

[virtual] QWindow *QQuickRenderControl::renderWindow(QPoint *offset)

Reimplemented in subclasses to return the real window this render control is rendering into.

If offset in non-null, it is set to the offset of the control inside the window.

Note: While not mandatory, reimplementing this function becomes essential for supporting multiple screens with different device pixel ratios and properly positioning popup windows opened from QML. Therefore providing it in subclasses is highly recommended.

[static] QWindow *QQuickRenderControl::renderWindowFor(QQuickWindow *win, QPoint *offset = nullptr)

Returns the real window that win is being rendered to, if any.

If offset in non-null, it is set to the offset of the rendering inside its window.

bool QQuickRenderControl::sync()

This function is used to synchronize the QML scene with the rendering scene graph.

If a dedicated render thread is used, the GUI thread should be blocked for the duration of this call.

Returns true if the synchronization changed the scene graph.

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