Scene Graph Adaptations

Scene Graph Adaptations in Qt Quick

Originally, Qt Quick always relied on OpenGL (OpenGL ES 2.0 or OpenGL 2.0) to parse the scene graph and render the results to a render target

From Qt 5.8 onwards, Qt Quick also supports rendering in software, with OpenVG, and with Direct3D 12. This is realized by having additional scene graph adaptations, either in form of plugins (d3d12, openvg) or built-in to the Qt Quick library (software). The default adaptation continues to rely directly on OpenGL.

From Qt 5.14 onwards, the default adaptation gains the option of rendering via a graphics abstraction layer, the Qt Rendering Hardware Interface (RHI), provided by the QtGui module. When enabled, no direct OpenGL calls are made. Rather, the scene graph renders by using the APIs provided by the abstraction layer, which is then translated into OpenGL, Vulkan, Metal, or Direct 3D calls. Shader handling is also unified by writing shader code once, compiling to SPIR-V, and then translating to the language appropriate for the various graphics APIs.

Switch Between Adaptations in Your Application

Unlike software or d3d12, the RHI-based renderer is not an additional adaptation, and is always built-in. As of Qt 5.14 it can be enabled by setting the environment variable QSG_RHI to a non-zero value before starting the application, or via QQuickWindow::setSceneGraphBackend() in combination with QSGRendererInterface::GraphicsApi. When none of this is done, OpenGL is used directly like in previous versions.

Switching to a different adaptation can be achieved in two ways:

• Use an environment variable - Set the QT_QUICK_BACKEND or the legacy QMLSCENE_DEVICE environment variable before launching applications.
• Use a C++ API - Call QQuickWindow::setSceneGraphBackend() early on in the application's main() function.

The following backends are supported:

• Default - Request with the "" string or a QSGRendererInterface::GraphicsApi enum value different than the ones listed below.
• Software - Request with the "software" string or the QSGRendererInterface::Software enum value.
• Direct3D 12 - Request with the "d3d12" string or the QSGRendererInterface::Direct3D12 enum value.
• OpenVG - Request with the "openvg" string or the QSGRendererInterface::OpenVG enum value.

To find out which backend is in use, you can enable basic scene graph information logging via the QSG_INFO environment variable or the qt.scenegraph.general logging category. This results in some information being printed onto the debug output, during application startup.

Default Adaptation

When using OpenGL directly, the default adaptation is capable of providing the full Qt Quick 2 feature set. For more details, see Default Adaptation.

When using OpenGL, Vulkan, Metal, or Direct 3D via the RHI, the default adaptation is capable of providing most features, including the full batching renderer described in Default Adaptation, but some additional features may not be available as of Qt 5.14.

Software Adaptation

The Software adaptation is an alternative renderer for Qt Quick 2 that uses the raster paint engine to render the contents of the scene graph. For more details, see Software Adaptation.

Direct3D 12 (experimental)

The Direct3D 12 adaptation is an alternative renderer for Qt Quick 2 when running on Windows 10, both for Win32 and UWP applications. For more details, see Direct3D 12 Adaptation.

OpenVG

The OpenVG adaptation is an alternative renderer for Qt Quick 2 that renders the contents of the scene graph using OpenVG commands to provide hardware-accelerated 2D vector and raster graphics. For more details, see OpenVG Adaptation.