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>
CMake: find_package(Qt6 REQUIRED COMPONENTS Quick)
target_link_libraries(mytarget PRIVATE Qt6::Quick)
qmake: QT += quick
Inherits: QObject

Public Functions

QQuickRenderControl(QObject *parent = nullptr)
virtual ~QQuickRenderControl() override
(since 6.0) void beginFrame()
(since 6.6) QRhiCommandBuffer *commandBuffer() const
(since 6.0) void endFrame()
(since 6.0) bool initialize()
void invalidate()
void polishItems()
void prepareThread(QThread *targetThread)
void render()
virtual QWindow *renderWindow(QPoint *offset)
(since 6.6) QRhi *rhi() const
(since 6.0) int samples() const
(since 6.0) void setSamples(int sampleCount)
bool sync()
(since 6.0) QQuickWindow *window() const


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, Vulkan, Metal, or Direct 3D renderers, it can be useful to get the scene into a texture that can then be used in arbitrary ways by the external rendering engine. Such a mechanism is also essential when integrating with a VR framework. 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 must not be shown (it will not be visible on-screen) and there will not be an underlying native window for it. Instead, the QQuickWindow instance is associated with the render control object, using the overload of the QQuickWindow constructor, and a texture or image object specified via QQuickWindow::setRenderTarget(). The QQuickWindow object is still essential, because it represents the Qt Quick scene and provides the bulk of the scene management and event delivery mechanisms. It does not however act as a real on-screen window from the windowing system's perspective.

Management of the graphics devices, contexts, image and texture objects is up to the application. The device or context that will be used by Qt Quick must be created before calling initialize(). The creation of the texture 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. For Vulkan, Metal, and Direct 3D there are no Qt-provided wrappers for device objects, so existing ones can be passed as-is via QQuickWindow::setGraphicsDevice().

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.

For key events it may be also necessary to set the focus manually on the desired item. In practice this involves calling forceActiveFocus() on the desired item, for example the scene's root item, once it is associated with the scene (the QQuickWindow).

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

[explicit] QQuickRenderControl::QQuickRenderControl(QObject *parent = nullptr)

Constructs a QQuickRenderControl object, with parent object parent.

[override virtual noexcept] QQuickRenderControl::~QQuickRenderControl()

Destroys the instance. Releases all scenegraph resources.

See also invalidate().

[since 6.0] void QQuickRenderControl::beginFrame()

Specifies the start of a graphics frame. Calls to sync() or render() must be enclosed by calls to beginFrame() and endFrame().

Unlike the earlier OpenGL-only world of Qt 5, rendering with other graphics APIs requires more well-defined points of starting and ending a frame. When manually driving the rendering loop via QQuickRenderControl, it now falls to the user of QQuickRenderControl to specify these points.

A typical update step, including initialization of rendering into an existing texture, could look like the following. The example snippet assumes Direct3D 11 but the same concepts apply other graphics APIs as well.

if (!m_quickInitialized) {
    m_quickWindow->setGraphicsDevice(QQuickGraphicsDevice::fromDeviceAndContext(m_engine->device(), m_engine->context()));

    if (!m_renderControl->initialize())
        qWarning("Failed to initialize redirected Qt Quick rendering");

    m_quickWindow->setRenderTarget(QQuickRenderTarget::fromNativeTexture({ quint64(m_res.texture), 0 },
                                                                         QSize(QML_WIDTH, QML_HEIGHT),

    m_quickInitialized = true;


m_renderControl->endFrame(); // Qt Quick's rendering commands are submitted to the device context here

Note: This function does not need to be, and must not be, called when using the software adaptation of Qt Quick.

Note: Internally beginFrame() and endFrame() invoke beginOffscreenFrame() and endOffscreenFrame(), respectively. This implies that there must not be a frame (neither offscreen, nor swapchain-based) being recorded on the QRhi when this function is called.

This function was introduced in Qt 6.0.

See also endFrame(), initialize(), sync(), render(), QQuickGraphicsDevice, and QQuickRenderTarget.

[since 6.6] QRhiCommandBuffer *QQuickRenderControl::commandBuffer() const

Returns the current command buffer.

Once beginFrame() is called, a QRhiCommandBuffer is set up automatically. That is the command buffer Qt Quick scenegraph uses, but in some cases applications may also want to query it, for example to issue resource updates (for example, a texture readback).

The returned command buffer reference should only be used between beginFrame() and endFrame(). There are specific exceptions, for example calling lastCompletedGpuTime() on the command buffer right after endFrame(), but before the next beginFrame(), is valid.

Note: This function is not applicable and returns null when using the software adaptation of Qt Quick.

This function was introduced in Qt 6.6.

See also rhi(), beginFrame(), and endFrame().

[since 6.0] void QQuickRenderControl::endFrame()

Specifies the end of a graphics frame. Calls to sync() or render() must be enclosed by calls to beginFrame() and endFrame().

When this function is called, any graphics commands enqueued by the scenegraph are submitted to the context or command queue, whichever is applicable.

Note: This function does not need to be, and must not be, called when using the software adaptation of Qt Quick.

This function was introduced in Qt 6.0.

See also beginFrame(), initialize(), sync(), render(), QQuickGraphicsDevice, and QQuickRenderTarget.

[since 6.0] bool QQuickRenderControl::initialize()

Initializes the scene graph resources. When using a graphics API, such as Vulkan, Metal, OpenGL, or Direct3D, for Qt Quick rendering, QQuickRenderControl will set up an appropriate rendering engine when this function is called. This rendering infrastructure exists as long as the QQuickRenderControl exists.

To control what graphics API Qt Quick uses, call QQuickWindow::setGraphicsApi() with one of the QSGRendererInterface:GraphicsApi constants. That must be done before calling this function.

To prevent the scenegraph from creating its own device and context objects, specify an appropriate QQuickGraphicsDevice, wrapping existing graphics objects, by calling QQuickWindow::setGraphicsDevice().

To configure which device extensions to enable (for example, for Vulkan), call QQuickWindow::setGraphicsConfiguration() before this function.

Note: When using Vulkan, QQuickRenderControl does not create a QVulkanInstance automatically. Rather, it is the application's responsibility to create a suitable QVulkanInstance and associate it with the QQuickWindow. Before initializing the QVulkanInstance, it is strongly encouraged to query the list of Qt Quick's desired instance extensions by calling the static function QQuickGraphicsConfiguration::preferredInstanceExtensions() and to pass the returned list to QVulkanInstance::setExtensions().

Returns true on success, false otherwise.

Note: This function does not need to be, and must not be, called when using the software adaptation of Qt Quick.

With the default Qt Quick adaptation this function creates a new QRhi object, similarly to what would happen with an on-screen QQuickWindow when QQuickRenderControl was not used. To make this new QRhi object adopt some existing device or context resource (e.g. use an existing QOpenGLContext instead of creating a new one), use QQuickWindow::setGraphicsDevice() as mentioned above. When the application wants to make the Qt Quick rendering use an already existing QRhi object, that is possible as well via QQuickGraphicsDevice::fromRhi(). When such a QQuickGraphicsDevice, referencing an already existing QRhi, is set, there will be no new, dedicated QRhi object created in initialize().

This function was introduced in Qt 6.0.

See also QQuickRenderTarget, QQuickGraphicsDevice, and QQuickGraphicsConfiguration::preferredInstanceExtensions().

void QQuickRenderControl::invalidate()

Stop rendering and release resources.

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.

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::persistentGraphics() 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.

[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.

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

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

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

[since 6.6] QRhi *QQuickRenderControl::rhi() const

Returns the QRhi this QQuickRenderControl is associated with.

Note: The QRhi exists only when initialize() has successfully completed. Before that the return value is null.

Note: This function is not applicable and returns null when using the software adaptation of Qt Quick.

This function was introduced in Qt 6.6.

See also commandBuffer(), beginFrame(), and endFrame().

[since 6.0] int QQuickRenderControl::samples() const

Returns the current sample count. 1 or 0 means no multisampling.

This function was introduced in Qt 6.0.

See also setSamples().

[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.

[since 6.0] void QQuickRenderControl::setSamples(int sampleCount)

Sets the number of samples to use for multisampling. When sampleCount is 0 or 1, multisampling is disabled.

Note: This function is always used in combination with a multisample render target, which means sampleCount must match the sample count passed to QQuickRenderTarget::fromNativeTexture(), which in turn must match the sample count of the native texture.

This function was introduced in Qt 6.0.

See also samples(), initialize(), and QQuickRenderTarget.

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.

[since 6.0] QQuickWindow *QQuickRenderControl::window() const

Returns the QQuickWindow this QQuickRenderControl is associated with.

Note: A QQuickRenderControl gets associated with a QQuickWindow when constructing the QQuickWindow. The return value from this function is null before that point.

This function was introduced in Qt 6.0.

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