There is no documented universal speed winner. Qt Canvas Painter is built for GPU-oriented 2D rendering through QRhi, while QQuickPaintedItem brings the QPainter API into Qt Quick and normally renders through an intermediate image in Qt 6. Choose based on your Qt release and graphics backend, rendering workload, existing code, resizing behavior, and image-quality needs—not a generic speed ranking.
How the two APIs render
Qt Canvas Painter: a GPU-oriented painting API
Qt describes QCanvasPainter as a painting API optimized for hardware-accelerated GPU painting. It is designed to render through QRhi and has no CPU backend. For Qt Quick, the integration uses QCanvasPainterItem and QCanvasPainterItemRenderer.
The API is an imperative 2D drawing interface broadly modeled on HTML Canvas 2D Context, with Qt-specific additions and some omissions. Its documented capabilities include paths, gradients, shadows, grid patterns, custom shader brushes, text wrapping, color effects, and tinted images. The Qt Quick gallery example illustrates the item-and-renderer integration; it is an example, not a performance test.
Release status matters: the Qt 6.11.1 module documentation labels Canvas Painter a Technology Preview and says it is outside Qt’s compatibility promises for that release. That label should not be generalized to every Qt version: Qt also publishes 6.12 documentation, so check the documentation for the exact release you plan to use.
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QQuickPaintedItem: QPainter through an intermediate surface
QQuickPaintedItem lets you use QPainter with the QML Scene Graph. In Qt 6, its normal render target is a QImage: Qt paints into the image, then uploads it as a texture for the scene graph. That upload can be slow for large items. The image-based path supports high-quality antialiasing and fast resizing.
Qt also cautions that QQuickPaintedItem uses an indirect 2D surface and a two-step rendering operation. Its general Qt Quick guidance says direct use of the scene graph API is significantly faster, but that is architectural guidance—not a measured Canvas Painter versus QQuickPaintedItem comparison. See the QQuickItem documentation.
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When to choose Canvas Painter or QQuickPaintedItem
| Consideration | Canvas Painter | QQuickPaintedItem |
|---|---|---|
| Rendering model | GPU-oriented through QRhi; no CPU backend. | QPainter into a QImage by default in Qt 6, followed by texture upload. |
| Best fit | A new or suitable GPU-oriented imperative 2D drawing implementation, provided the target Qt release and backend fit. | Existing QPainter code, QPainter-specific capabilities, or a workload that benefits from the image path’s antialiasing and resizing characteristics. |
| Potential cost or constraint | Check release maturity, compatibility expectations, backend support, and whether required features are available. | Large image-backed items can incur costly uploads; frequent resizing can make the framebuffer-object option a poor fit. |
| Performance evidence | Qt documentation gives no universal head-to-head benchmark. | Qt documentation gives no universal head-to-head benchmark. |
Canvas Painter is worth evaluating when you are starting fresh, want an API designed for GPU rendering, and can accept the release and backend constraints. QQuickPaintedItem is often the more practical choice when reusing substantial QPainter code would avoid migration work, or when its image-backed quality and resize behavior suit the visual workload.
What the QQuickPaintedItem framebuffer option changes
The framebuffer-object render target can alter QQuickPaintedItem’s trade-offs, but it is not a general-purpose switch for every Qt version or graphics API. According to the Qt 6.11.1 documentation:
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- Hardware-accelerated framebuffer-object painting is available from Qt 6.9 only when the rendering API is OpenGL.
- For Qt 6.0 through 6.8, the framebuffer-object setting is ignored for all rendering APIs.
- In Qt 6.9 and later, it is ignored on non-OpenGL APIs.
- Qt says it usually improves performance at the expense of antialiasing quality.
- Resizing the framebuffer object is costly, so this mode is a poor default for an item that changes size frequently.
These details are documented on the Qt 6.11.1 QQuickPaintedItem page; verify the corresponding documentation for the Qt version you ship.
How to compare performance for your application
Qt’s published guidance explains the rendering paths and their trade-offs, but provides no controlled, universal Canvas Painter versus QQuickPaintedItem benchmark. A useful comparison therefore needs to use your actual content and deployment conditions.
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- Match the environment. Test the same Qt release, target device, and graphics backend you intend to ship. Record these with the results.
- Use representative content. Include the paths, text, effects, gradients, or images your application really draws rather than a minimal demo.
- Vary the workload that can change the result. Compare realistic item dimensions and resolution, how often content changes, how much of the item updates, and how often it resizes.
- Check output quality as well as speed. Inspect antialiasing and image quality under the settings you would actually use. If evaluating the framebuffer-object path, include its quality trade-off.
- Keep implementation constraints in the test. Account for required API features, code reuse, release stability expectations, and whether the chosen backend is supported.
- Measure on target hardware. Compare the same scene and interactions on the devices and graphics backends that matter to users. Report the setup and workload with any performance numbers; do not treat one result as a universal ratio.
Threading and module details to check
QQuickPaintedItem’s paint() function is called by the scene graph and runs on the renderer thread rather than the main GUI thread. Qt warns against creating QObjects, emitting signals, or starting timers from that function because of thread-affinity concerns. Design the painting implementation with that execution context in mind.
Canvas Painter is a separate Qt module. The Qt 6.11.1 module page names the CMake component CanvasPainter and target Qt6::CanvasPainter; it lists commercial licensing and GPLv3. Confirm the current module documentation and your project’s licensing requirements for the release you intend to use.
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