QQuickGraphicsConfiguration Class

The QQuickGraphicsConfiguration class is a container for low-level graphics settings that can affect how the underlying graphics API, such as Vulkan, is initialized by the Qt Quick scene graph. It can also control certain aspects of the scene graph renderer. More...

Header: #include <QQuickGraphicsConfiguration>
CMake: find_package(Qt6 COMPONENTS Quick REQUIRED)
target_link_libraries(mytarget PRIVATE Qt6::Quick)
qmake: QT += quick
Since: Qt 6.0

Public Functions

QQuickGraphicsConfiguration()
~QQuickGraphicsConfiguration()
QByteArrayList deviceExtensions() const
bool isDepthBufferEnabledFor2D() const
void setDepthBufferFor2D(bool enable)
void setDeviceExtensions(const QByteArrayList &extensions)

Detailed Description

When constructing and showing a QQuickWindow that uses Vulkan to render, a Vulkan instance (VkInstance), a physical device (VkPhysicalDevice), a device (VkDevice) and associated objects (queues, pools) are initialized through the Vulkan API. The same is mostly true when using QQuickRenderControl to redirect the rendering into a custom render target, such as a texture. While QVulkanInstance construction is under the application's control then, the initialization of other graphics objects happen the same way in QQuickRenderControl::initialize() as with an on-screen QQuickWindow.

For the majority of applications no additional configuration is needed because Qt Quick provides reasonable defaults for many low-level graphics settings, for example which device extensions to enable.

This will not alway be sufficient, however. In advanced use cases, when integrating direct Vulkan or other graphics API content, or when integrating with an external 3D or VR engine, such as, OpenXR, the application will want to specify its own set of settings when it comes to details, such as which device extensions to enable.

That is what this class enables. It allows specifying, for example, a list of device extensions that is then picked up by the scene graph when using Vulkan, or graphics APIs where the concept is applicable. Where some concepts are not applicable, the related settings are simply ignored.

Another class of settings are related to the scene graph's renderer. In some cases applications may want to control certain behavior,such as using the depth buffer when rendering 2D content. In Qt 5 such settings were either not controllable at all, or were managed through environment variables. In Qt 6, QQuickGraphicsConfiguration provides a new home for these settings, while keeping support for the legacy environment variables, where applicable.

Note: Setting a QQuickGraphicsConfiguration on a QQuickWindow must happen early enough, before the scene graph is initialized for the first time for that window. With on-screen windows this means the call must be done before invoking show() on the QQuickWindow or QQuickView. With QQuickRenderControl the configuration must be finalized before calling initialize().

See also QQuickWindow::setGraphicsConfiguration(), QQuickWindow, and QQuickRenderControl.

Member Function Documentation

QQuickGraphicsConfiguration::QQuickGraphicsConfiguration()

Constructs a default QQuickGraphicsConfiguration that does not specify any additional settings for the scene graph to take into account.

QQuickGraphicsConfiguration::~QQuickGraphicsConfiguration()

Destructor.

QByteArrayList QQuickGraphicsConfiguration::deviceExtensions() const

Returns the list of the requested additional device extensions.

See also setDeviceExtensions().

bool QQuickGraphicsConfiguration::isDepthBufferEnabledFor2D() const

Returns true if depth buffer usage is enabled for 2D content.

By default the value is true, unless the QSG_NO_DEPTH_BUFFER environment variable is set.

void QQuickGraphicsConfiguration::setDepthBufferFor2D(bool enable)

Sets the usage of depth buffer for 2D content to enable. When disabled, the Qt Quick scene graph never writes into the depth buffer.

By default the value is true, unless the QSG_NO_DEPTH_BUFFER environment variable is set.

The default value of true is the most optimal setting for the vast majority of scenes. Disabling depth buffer usage reduces the efficiency of the scene graph's batching.

There are cases however, when allowing the 2D content write to the depth buffer is not ideal. Consider a 3D scene as an "overlay" on top the 2D scene, rendered via Qt Quick 3D using a View3D with View3D::renderMode}{renderMode} set to Overlay. In this case, having the depth buffer filled by 2D content can cause unexpected results. This is because the way the 2D scene graph renderer generates and handles depth values is not necessarily compatible with how a 3D scene works. This may end up in depth value clashes, collisions, and unexpected depth test failures. Therefore, the robust approach here is to call this function with enable set to false, and disable depth buffer writes for the 2D content in the QQuickWindow.

Note: This flag is not fully identical to setting the QSG_NO_DEPTH_BUFFER environment variable. This flag does not control the depth-stencil buffers' presence. It is rather relevant for the rendering pipeline. To force not having depth/stencil attachments at all, set QSG_NO_DEPTH_BUFFER and QSG_NO_STENCIL_BUFFER. Be aware however that such a QQuickWindow, and any Item layers in it, may then become incompatible with items, such as View3D with certain operating modes, because 3D content requires a depth buffer. Calling this function is always safe, but can mean that resources, such as depth buffers, are created even though they are not actively used.

void QQuickGraphicsConfiguration::setDeviceExtensions(const QByteArrayList &extensions)

Sets the list of additional extensions to enable on the graphics device (such as, the VkDevice).

When rendering with a graphics API where the concept is not applicable, extensions will be ignored.

Note: The list specifies additional, extra extensions. Qt Quick always enables extensions that are required by the scene graph.

See also deviceExtensions().

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