Qt Quick Scene Graph

The Scene Graph in Qt Quick

Qt Quick 2 makes use of a dedicated scene graph that is then traversed and rendered via a graphics API such as OpenGL ES, OpenGL, Vulkan, Metal, or Direct 3D. Using a scene graph for graphics rather than the traditional imperative painting systems (QPainter and similar), means the scene to be rendered can be retained between frames and the complete set of primitives to render is known before rendering starts. This opens up for a number of optimizations, such as batch rendering to minimize state changes and discarding obscured primitives.

For example, say a user-interface contains a list of ten items where each item has a background color, an icon and a text. Using the traditional drawing techniques, this would result in 30 draw calls and a similar amount of state changes. A scene graph, on the other hand, could reorganize the primitives to render such that all backgrounds are drawn in one call, then all icons, then all the text, reducing the total amount of draw calls to only 3. Batching and state change reduction like this can greatly improve performance on some hardware.

The scene graph is closely tied to Qt Quick 2.0 and can not be used stand-alone. The scene graph is managed and rendered by the QQuickWindow class and custom Item types can add their graphical primitives into the scene graph through a call to QQuickItem::updatePaintNode().

The scene graph is a graphical representation of the Item scene, an independent structure that contains enough information to render all the items. Once it has been set up, it can be manipulated and rendered independently of the state of the items. On many platforms, the scene graph will even be rendered on a dedicated render thread while the GUI thread is preparing the next frame's state.

Qt Quick Scene Graph Structure

The scene graph is composed of a number of predefined node types, each serving a dedicated purpose. Although we refer to it as a scene graph, a more precise definition is node tree. The tree is built from QQuickItem types in the QML scene and internally the scene is then processed by a renderer which draws the scene. The nodes themselves do not contain any active drawing code nor virtual paint() function.

Even though the node tree is mostly built internally by the existing Qt Quick QML types, it is possible for users to also add complete subtrees with their own content, including subtrees that represent 3D models.

Nodes

The most important node for users is the QSGGeometryNode. It is used to define custom graphics by defining its geometry and material. The geometry is defined using QSGGeometry and describes the shape or mesh of the graphical primitive. It can be a line, a rectangle, a polygon, many disconnected rectangles, or complex 3D mesh. The material defines how the pixels in this shape are filled.

A node can have any number of children and geometry nodes will be rendered so they appear in child-order with parents behind their children.

The available nodes are:

Custom nodes are added to the scene graph by subclassing QQuickItem::updatePaintNode() and setting the QQuickItem::ItemHasContents flag.

For more details, see the Scene Graph - Custom Geometry.

Preprocessing

Nodes have a virtual QSGNode::preprocess() function, which will be called before the scene graph is rendered. Node subclasses can set the flag QSGNode::UsePreprocess and override the QSGNode::preprocess() function to do final preparation of their node. For example, dividing a bezier curve into the correct level of detail for the current scale factor or updating a section of a texture.

Node Ownership

Ownership of the nodes is either done explicitly by the creator or by the scene graph by setting the flag QSGNode::OwnedByParent. Assigning ownership to the scene graph is often preferable as it simplifies cleanup when the scene graph lives outside the GUI thread.

Materials

The material describes how the interior of a geometry in a QSGGeometryNode is filled. It encapsulates graphics shaders for the vertex and fragment stages of the graphics pipeline and provides ample flexibility in what can be achieved, though most of the Qt Quick items themselves only use very basic materials, such as solid color and texture fills.

For users who just want to apply custom shading to a QML Item type, it is possible to do this directly in QML using the ShaderEffect type.

Below is a complete list of material classes:

Convenience Nodes

The scene graph API is low-level and focuses on performance rather than convenience. Writing custom geometries and materials from scratch, even the most basic ones, requires a non-trivial amount of code. For this reason, the API includes a few convenience classes to make the most common custom nodes readily available.

• QSGSimpleRectNode - a QSGGeometryNode subclass which defines a rectangular geometry with a solid color material.
• QSGSimpleTextureNode - a QSGGeometryNode subclass which defines a rectangular geometry with a texture material.

Scene Graph and Rendering

The rendering of the scene graph happens internally in the QQuickWindow class, and there is no public API to access it. There are, however, a few places in the rendering pipeline where the user can attach application code. This can be used to add custom scene graph content or to insert arbitrary rendering commands by directly calling the graphics API (OpenGL, Vulkan, Metal, etc.) that is in use by the scene graph. The integration points are defined by the render loop.

For detailed description of how the scene graph renderer works, see Qt Quick Scene Graph Default Renderer.

There are two render loop variants available: basic, and threaded. basic is single-threaded, while threaded performs scene graph rendering on a dedicated thread. Qt attempts to choose a suitable loop based on the platform and possibly the graphics drivers in use. When this is not satisfactory, or for testing purposes, the environment variable QSG_RENDER_LOOP can be used to force the usage of a given loop. To verify which render loop is in use, enable the qt.scenegraph.general logging category.

Threaded Render Loop ('threaded')

On many configurations, the scene graph rendering will happen on a dedicated render thread. This is done to increase parallelism of multi-core processors and make better use of stall times such as waiting for a blocking swap buffer call. This offers significant performance improvements, but imposes certain restrictions on where and when interaction with the scene graph can happen.

The following is a simple outline of how a frame gets rendered with the threaded render loop and OpenGL. The steps are the same with other graphics APIs as well, apart from the OpenGL context specifics.

1. A change occurs in the QML scene, causing QQuickItem::update() to be called. This can be the result of for instance an animation or user input. An event is posted to the render thread to initiate a new frame.
2. The render thread prepares to draw a new frame and initiates a block on the GUI thread.
3. While the render thread is preparing the new frame, the GUI thread calls QQuickItem::updatePolish() to do final touch-up of items before they are rendered.
4. GUI thread is blocked.
5. The QQuickWindow::beforeSynchronizing() signal is emitted. Applications can make direct connections (using Qt::DirectConnection) to this signal to do any preparation required before calls to QQuickItem::updatePaintNode().
6. Synchronization of the QML state into the scene graph. This is done by calling the QQuickItem::updatePaintNode() function on all items that have changed since the previous frame. This is the only time the QML items and the nodes in the scene graph interact.
7. GUI thread block is released.
8. The scene graph is rendered:
   1. The QQuickWindow::beforeRendering() signal is emitted. Applications can make direct connections (using Qt::DirectConnection) to this signal to use custom graphics API calls which will then stack visually beneath the QML scene.
   2. Items that have specified QSGNode::UsePreprocess, will have their QSGNode::preprocess() function invoked.
   3. The renderer processes the nodes.
   4. The renderer generates states and records draw calls for the graphics API in use.
   5. The QQuickWindow::afterRendering() signal is emitted. Applications can make direct connections (using Qt::DirectConnection) to this signal to issue custom graphics API calls which will then stack visually over the QML scene.
   6. The frame is now ready. The buffers are swapped (OpenGL), or a present command is recorded and the command buffers are submitted to a graphics queue (Vulkan, Metal). QQuickWindow::frameSwapped() is emitted.
9. While the render thread is rendering, the GUI is free to advance animations, process events, etc.

The threaded renderer is currently used by default on Windows with Direct3D 11 and with OpenGL when using opengl32.dll, Linux excluding Mesa llvmpipe, macOS with Metal, mobile platforms, and Embedded Linux with EGLFS, and with Vulkan regardless of the platform. All this may change in future releases. It is always possible to force use of the threaded renderer by setting QSG_RENDER_LOOP=threaded in the environment.

Non-threaded Render Loop ('basic')

The non-threaded render loop is currently used by default on Windows with OpenGL when not using the system's standard opengl32.dll, macOS with OpenGL, and Linux with some drivers. For the latter this is mostly a precautionary measure, as not all combinations of OpenGL drivers and windowing systems have been tested.

On macOS and OpenGL, the threaded render loop is not supported when building with XCode 10 (10.14 SDK) or later, since this opts in to layer-backed views on macOS 10.14. You can build with Xcode 9 (10.13 SDK) to opt out of layer-backing, in which case the threaded render loop is available and used by default. There is no such restriction with Metal.

Even when using the non-threaded render loop, you should write your code as if you are using the threaded renderer, as failing to do so will make the code non-portable.

The following is a simplified illustration of the frame rendering sequence in the non-threaded renderer.

Driving Animations

What does Advance Animations refer to in the above diagrams?

By default, a Qt Quick animation (such, as a NumberAnimation) is driven by the default animation driver. This relies on basic system timers, such as QObject::startTimer(). The timer typically runs with an interval of 16 milliseconds. While this will never be fully accurate and also depends on the accuracy of timers in the underlying platform, it has the benefit of being independent of the rendering. It provides uniform results regardless of the display refresh rate and if synchronization to the display's vertical sync is active or not. This is how animations work with the basic render loop.

In order to provide more accurate results with less stutter on-screen, independent of the render loop design (be it single threaded or multiple threads) a render loop may decide to install its own custom animation driver, and take the operation of advancing it into its own hands, without relying on timers.

This is what the threaded render loop implements. In fact, it installs not one, but two animation drivers: one on the gui thread (to drive regular animations, such as NumberAnimation), and one on the render thread (to drive render thread animations, i.e. the Animator types, such as OpacityAnimator or XAnimator). Both of these are advanced during the preparation of a frame, i.e. animations are now synchronized with rendering. This makes sense due to presentation being throttled to the display's vertical sync by the underlying graphics stack.

Therefore, in the diagram for the threaded render loop above, there is an explicit Advance animations step on both threads. For the render thread, this is trivial: as the thread is being throttled to vsync, advancing animations (for Animator types) in each frame as if 16.67 milliseconds had elapsed gives more accurate results than relying on a system timer. (when throttled to the vsync timing, which is 1000/60 milliseconds with a 60 Hz refresh rate, it is fair to assume that it has been approximately that long since the same operation was done for the previous frame)

The same approach works for animations on the gui (main) thread too: due to the essential synchronization of data between the gui and render threads, the gui thread is effectively throttled to the same rate as the render thread, while still having the benefit of having less work to do, leaving more headroom for the application logic since much of the rendering preparations are now offloaded to the render thread.

While the above examples used 60 frames per second, Qt Quick is prepared for other refresh rates as well: the rate is queried from the QScreen and the platform. For example, with a 144 Hz screen the interval is 6.94 ms. At the same time this is exactly what can cause trouble if vsync-based throttling is not functioning as expected, because if what the render loop thinks is happening is not matching reality, incorrect animation pacing will occur.

In summary, the threaded render loop is expected to provide smoother animations with less stutter as long as the following conditions are met:

• There is exactly one window (as in QQuickWindow) on-screen.
• VSync-based throttling works as expected with the underyling graphics and display stack.

What if there is no or more than one window visible?

When there is no renderable window, for example because our QQuickWindow is minimized (Windows) or fully obscured (macOS), we cannot present frames, thus cannot rely on the thread "working" in lockstep with the screen refresh rate. In this case, the threaded render loop automatically switches over to a system timer based approach to drive animations, i.e. temporarily switching over to the mechanism the basic loop would use.

The same is true when there are more than one QQuickWindow instances on-screen. The model presented above for advancing animations on the gui thread, enabled by its synchronization with the render thread, is not satisfactory anymore, as there are now multiple sync points with multiple render threads. (one per window.) Here falling back to the system timer based approach becomes necessary as well, because how long and often the gui thread will block is now dependent on a number of factors, including the content in the windows (are they animating? how often are they updating?) and the graphics stack behavior (how exactly does it handle two or more threads presenting with wait-for-vsync?). As we cannot guarantee being throttled to the presentation rate of the window (which window would that be, to begin with?) in a stable, cross-platform manner, advancing animations cannot be based on the rendering.

This switch of animation handling mechanisms is transparent to the applications.

What if vsync-based throttling is disfunctional, globally disabled, or the application disabled it itself?

The threaded render loop relies on the graphics API implementation and/or the windowing system for throttling, for example, by requesting a swap interval of 1 in case of OpenGL (GLX, EGL, WGL), calling Present() with an interval of 1 for Direct 3D, or using the presentation mode FIFO with Vulkan.

Some graphics drivers allow users to override this setting and turn it off, ignoring Qt's request. An example of this would be a system wide control panel of the graphics driver that allows overriding the application's settings with regards to vsync. It can also happen that a graphics stack is unable to provide proper vsync-based throttling, which can be the case in some virtual machines (mainly due to using a software rasterization based implementation of OpenGL or Vulkan).

Without blocking in the swap/present operation (or some other graphics operation), such a render loop would advance animations too fast. This would be no issue with the basic render loop, because that always relies on system timers. With threaded, the behavior can vary based on the Qt version:

• If a system is known to be unable to provide vsync-based throttling, the only option before Qt 6.4 was to use the basic render loop, by manually setting QSG_RENDER_LOOP=basic in the environment before running the application.
• Starting with Qt 6.4, setting either the QSG_NO_VSYNC environment variable to a non-zero value, or the window's QSurfaceFormat::swapInterval() to 0 can both alleviate the problem as well: by explicitly requesting disabling vsync-based blocking, regardless of the request having any effect in practice, the threaded render loop can by extension recognize that relying on vsync to drive animations is futile, and it will fall back to using system timers, just as it would for more than one window.
• Even better, starting from Qt 6.4 the scenegraph also attempts to recognize using some simple heuristics that the frames are being presented "too fast", and automatically switch over to system timers if seen necessary. This means that in most cases there will be no need to do anything and applications will run animations as expected even when the default render loop is the threaded one. While this is transparent to applications, for troubleshooting and development purposes it is useful to know that this is logged with a "Window 0x7ffc8489c3d0 is determined to have broken vsync throttling ..." message printed when QSG_INFO or qt.scenegraph.general is enabled. This method has the downside of activating only after a small set of frames, given that it first needs to collect data to evaluate, meaning that when opening a QQuickWindow the application may still show overly fast animations for a short period of time. Additionally, it may not capture all possible vsync-broken situations.

Remember however, that by design none of this helps render thread animations (the Animator types). In the absence of vsync-based blocking, animators will advance incorrectly by default, faster than expected, even when the workarounds are activated for regular animations. If this becomes an issue, consider using the alternative animation driver by setting QSG_USE_SIMPLE_ANIMATION_DRIVER.

Custom control over rendering with QQuickRenderControl

When using QQuickRenderControl, the responsibility for driving the rendering loop is transferred to the application. In this case no built-in render loop is used. Instead, it is up to the application to invoke the polish, synchronize and rendering steps at the appropriate time. It is possible to implement either a threaded or non-threaded behavior similar to the ones shown above.

Additionally, applications may wish to implement and install their own QAnimationDriver in combination with QQuickRenderControl. This gives full control over driving Qt Quick animations, which can be particularly important for content that is not shown on screen, bearing no relation to the presentation rate simply because there is no presenting of the frame happening. This is optional, by default animations will advance based on the system timer.

Mixing Scene Graph and the native graphics API

The scene graph offers two methods for integrating application-provided graphics commands: by issuing OpenGL, Vulkan, Metal, etc. commands directly, and by creating a textured node in the scene graph.

By connecting to the QQuickWindow::beforeRendering() and QQuickWindow::afterRendering() signals, applications can make OpenGL calls directly into the same context as the scene graph is rendering to. With APIs like Vulkan or Metal, applications can query native objects, such as, the scene graph's command buffer, via QSGRendererInterface, and record commands to it as they see fit. As the signal names indicate, the user can then render content either under a Qt Quick scene or over it. The benefit of integrating in this manner is that no extra framebuffer nor memory is needed to perform the rendering, and a possibly expensive texturing step is eliminated. The downside is that Qt Quick decides when to call the signals and this is the only time the OpenGL application is allowed to draw.

The Scene Graph - OpenGL Under QML example gives an example on how to use these signals using OpenGL.

The Scene Graph - Direct3D 11 Under QML example gives an example on how to use these signals using Direct3D.

The Scene Graph - Metal Under QML example gives an example on how to use these signals using Metal.

The Scene Graph - Vulkan Under QML example gives an example on how to use these signals using Vulkan.

The other alternative, only available for OpenGL currently, is to create a QQuickFramebufferObject, render into it, and let it be displayed in the scene graph as a texture. The Scene Graph - Rendering FBOs example shows how this can be done.

Graphics APIs other than OpenGL can also follow this approach, even though QQuickFramebufferObject does not currently support them. Creating and rendering to a texture directly with the underlying API, followed by wrapping and using this resource in a Qt Quick scene in a custom QQuickItem, is demonstrated in the Scene Graph - Metal Texture Import example. That example uses Metal, the concepts however apply to all other graphics APIs as well.

Custom Items using QPainter

The QQuickItem provides a subclass, QQuickPaintedItem, which allows the users to render content using QPainter.

Logging Support

The scene graph has support for a number of logging categories. These can be useful in tracking down both performance issues and bugs in addition to being helpful to Qt contributors.

• qt.scenegraph.time.texture - logs the time spent doing texture uploads
• qt.scenegraph.time.compilation - logs the time spent doing shader compilation
• qt.scenegraph.time.renderer - logs the time spent in the various steps of the renderer
• qt.scenegraph.time.renderloop - logs the time spent in the various steps of the render loop. With the threaded render loop this gives an insight into the time elapsed between the various frame preparation steps both on the GUI and the render thread. It can therefore also be a useful troubleshooting tool, for example, to confirm how vsync-based throttling and other low-level Qt enablers, such as QWindow::requestUpdate(), affect the rendering and presentation pipeline.
• qt.scenegraph.time.glyph - logs the time spent preparing distance field glyphs
• qt.scenegraph.general - logs general information about various parts of the scene graph and the graphics stack
• qt.scenegraph.renderloop - creates a detailed log of the various stages involved in rendering. This log mode is primarily useful for developers working on Qt.

The legacy QSG_INFO environment variable is also available. Setting it to a non-zero value enables the qt.scenegraph.general category.

Scene Graph Backend

In addition to the public API, the scene graph has an adaptation layer which opens up the implementation to do hardware specific adaptations. This is an undocumented, internal and private plugin API, which lets hardware adaptation teams make the most of their hardware. It includes:

• Custom textures; specifically the implementation of QQuickWindow::createTextureFromImage and the internal representation of the texture used by Image and BorderImage types.
• Custom renderer; the adaptation layer lets the plugin decide how the scene graph is traversed and rendered, making it possible to optimize the rendering algorithm for a specific hardware or to make use of extensions which improve performance.
• Custom scene graph implementation of many of the default QML types, including its text and font rendering.
• Custom animation driver; allows the animation system to hook into the low-level display vertical refresh to get smooth rendering.
• Custom render loop; allows better control over how QML deals with multiple windows.