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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteShort answer: Qt 6 does not provide a setting that turns QML Canvas into a direct GPU framebuffer painter. Canvas draws to an in-memory image; the Qt Quick scene graph can then render the application through a GPU-backed graphics API. Configure and verify those as separate layers.
Understand what GPU acceleration does—and does not—change
In Qt 6, Canvas.Image is the supported Canvas render target. Qt describes it as “the only render target that is supported by all Qt Quick backends” in its Canvas QML Type documentation. The Canvas painting surface is an in-memory QImage; choosing a GPU-backed scene-graph renderer does not make Canvas paint directly into a GPU framebuffer.
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Canvas.FramebufferObject is not a Qt 6 alternative: Qt says this target is ignored as of Qt 6.0. The GPU-related configuration belongs to the Qt Quick scene graph, which uses the Rendering Hardware Interface (RHI) to translate rendering commands to graphics APIs such as OpenGL, Vulkan, Metal, or Direct3D. See Qt’s scene graph default renderer documentation.
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Use Canvas.Image, keep painting in onPaint, and mark the relevant area dirty when the content changes. For example:
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import QtQuick
Canvas {
id: canvas
width: 400
height: 300
renderTarget: Canvas.Image
renderStrategy: Canvas.Immediate
onPaint: {
const ctx = getContext("2d")
ctx.clearRect(0, 0, width, height)
ctx.fillStyle = "steelblue"
ctx.fillRect(20, 20, 120, 80)
}
function redraw() {
requestPaint()
}
}renderStrategy controls where Canvas painting commands run; it is not a switch for GPU acceleration. Qt documents these options in the Canvas QML Type reference:
Canvas.Immediateruns commands on the UI thread and is the default.Canvas.Threadeddefers commands to a private rendering thread.Canvas.Cooperativedefers commands to the application’s global render thread; it does not guarantee a thread separate from the GUI thread.
These strategies are subject to the graphics context. If the requested option is unsupported, the context may select a different strategy. Check the Canvas renderStrategy property at runtime rather than assuming the requested value took effect. The scene graph’s own render loop is a separate mechanism and does not change Canvas’s image-backed target.
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Request a Qt Quick graphics backend
Qt 6’s default Qt Quick adaptation uses RHI. In a supported deployment, QSG_RHI_BACKEND can request a backend such as vulkan, metal, opengl, d3d11, or d3d12. Qt may choose a backend based on platform and overrides, so treat this as a request to validate—not a guarantee that every target supports it.
- Run the application on the target operating system and graphics device without overriding the backend. Observe the default renderer first.
- If you need a particular documented RHI backend, set
QSG_RHI_BACKENDin the process environment before launching the application, using the backend name appropriate to that Qt version and platform. - Check the startup output and test on the actual target device. A successfully selected scene-graph backend still does not turn Canvas into a direct GPU painter.
Windows diagnostics
For Qt 6 on Windows, the documented default is Direct3D 11. Set QSG_INFO=1 before launching to inspect graphics-device information; consult Qt’s Windows graphics acceleration documentation for platform details. QSG_RHI_PREFER_SOFTWARE_RENDERER=1 requests preference for a software renderer, so it is useful when diagnosing fallback behavior—not for enabling GPU acceleration.
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Keep scene-graph and Canvas threading separate
The Qt Quick scene graph can use a basic or threaded render loop. Qt documents QSG_RENDER_LOOP for forcing a render loop and the qt.scenegraph.general logging category for inspecting scene-graph behavior in its Qt Quick Scene Graph documentation. These controls concern the scene graph; they do not alter Canvas’s Canvas.Image render target or replace its renderStrategy.
Choose Canvas based on update pattern, not the backend name
Canvas offers convenient HTML5-like 2D drawing in QML, but Qt notes that its JavaScript and Context2D approach can be more expensive and less performant in some cases. Qt warns against large Canvas surfaces, frequent updates, and animation in general: with accelerated graphics APIs, an update can cause a texture upload. Selecting a GPU-backed scene-graph API does not remove that cost.
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For frequent drawing changes, consider comparing Canvas with a C++ QQuickPaintedItem using QPainter, or another Qt Quick rendering approach. The relevant trade-offs are:
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- Surface and update rate: Canvas may suit small or infrequently changing drawings better than large, frequently refreshed or animated surfaces.
- Implementation: Canvas uses QML JavaScript and Context2D;
QQuickPaintedItemwith QPainter moves drawing implementation into C++. - Threading: Canvas strategy choices and the scene graph render loop are distinct; neither should be mistaken for a direct Canvas GPU target.
- Platform support: Backend availability and Canvas strategy support depend on the Qt version, graphics context, platform, and driver.
- Measured behavior: Profile the actual workload on target hardware. Qt’s cited documentation gives qualitative cautions, not a universal frame-rate gain or performance percentage.
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