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QSGRenderNode Class

The QSGRenderNode class represents a set of custom rendering commands targeting the graphics API that is in use by the scenegraph. More...

Header: #include <QSGRenderNode>
CMake: find_package(Qt6 REQUIRED COMPONENTS Quick)
target_link_libraries(mytarget PRIVATE Qt6::Quick)
qmake: QT += quick
Since: Qt 5.8
Inherits: QSGNode

Public Types

enum RenderingFlag { BoundedRectRendering, DepthAwareRendering, OpaqueRendering }
flags RenderingFlags
enum StateFlag { DepthState, StencilState, ScissorState, ColorState, BlendState, …, RenderTargetState }
flags StateFlags

Public Functions

virtual ~QSGRenderNode() override
virtual QSGRenderNode::StateFlags changedStates() const
const QSGClipNode * clipList() const
virtual QSGRenderNode::RenderingFlags flags() const
qreal inheritedOpacity() const
const QMatrix4x4 * matrix() const
virtual void prepare()
virtual QRectF rect() const
virtual void releaseResources()
virtual void render(const QSGRenderNode::RenderState *state) = 0

Detailed Description

Member Type Documentation

enum QSGRenderNode::RenderingFlag
flags QSGRenderNode::RenderingFlags

Possible values for the bitmask returned from flags().

Constant Value Description
QSGRenderNode::BoundedRectRendering 0x01 Indicates that the implementation of render() does not render outside the area reported from rect() in item coordinates. Such node implementations can lead to more efficient rendering, depending on the scenegraph backend. For example, the software backend can continue to use the more optimal partial update path when all render nodes in the scene have this flag set.
QSGRenderNode::DepthAwareRendering 0x02 Indicates that the implementations of render() conforms to scenegraph expectations by only generating a Z value of 0 in scene coordinates which is then transformed by the matrices retrieved from RenderState::projectionMatrix() and matrix(), as described in the notes for render(). Such node implementations can lead to more efficient rendering, depending on the scenegraph backend. For example, the batching OpenGL renderer can continue to use a more optimal path when all render nodes in the scene have this flag set.
QSGRenderNode::OpaqueRendering 0x04 Indicates that the implementation of render() writes out opaque pixels for the entire area reported from rect(). By default the renderers must assume that render() can also output semi or fully transparent pixels. Setting this flag can improve performance in some cases.

The RenderingFlags type is a typedef for QFlags<RenderingFlag>. It stores an OR combination of RenderingFlag values.

See also render() and rect().

enum QSGRenderNode::StateFlag
flags QSGRenderNode::StateFlags

This enum is a bit mask identifying several states.

Constant Value Description
QSGRenderNode::DepthState 0x01 Depth
QSGRenderNode::StencilState 0x02 Stencil
QSGRenderNode::ScissorState 0x04 Scissor
QSGRenderNode::ColorState 0x08 Color
QSGRenderNode::BlendState 0x10 Blend
QSGRenderNode::CullState 0x20 Cull
QSGRenderNode::ViewportState 0x40 View poirt
QSGRenderNode::RenderTargetState 0x80 Render target

The StateFlags type is a typedef for QFlags<StateFlag>. It stores an OR combination of StateFlag values.

Member Function Documentation

[override virtual] QSGRenderNode::~QSGRenderNode()

Destructs the render node. Derived classes are expected to perform cleanup similar to releaseResources() in here.

When a low-level graphics API is in use, the scenegraph will make sure there is a CPU-side wait for the GPU to complete all work submitted to the scenegraph's graphics command queue before the scenegraph's nodes are deleted. Therefore there is no need to issue additional waits here, unless the render() implementation is using additional command queues.

See also releaseResources().

[virtual] QSGRenderNode::StateFlags QSGRenderNode::changedStates() const

When the underlying rendering API is OpenGL, this function should return a mask where each bit represents graphics states changed by the render() function:

With APIs other than OpenGL, the only relevant values are the ones that correspond to dynamic state changes recorded on the command list/buffer. For example, RSSetViewports, RSSetScissorRects, OMSetBlendState, OMSetDepthStencilState in case of D3D11, or vkCmdSetViewport, vkCmdSetScissor, vkCmdSetBlendConstants, vkCmdSetStencilRef in case of Vulkan, and only when such commands were added to the scenegraph's command list queried via the QSGRendererInterface::CommandList resource enum. States set in pipeline state objects do not need to be reported here. Similarly, draw call related settings (pipeline states, descriptor sets, vertex or index buffer bindings, root signature, descriptor heaps, etc.) are always set again by the scenegraph so render() can freely change them.

Note: RenderTargetState is no longer supported with APIs like Vulkan. This is by nature. render() is invoked while the Qt Quick scenegraph's main command buffer is recording a renderpass, so there is no possibility of changing the target and starting another renderpass (on that command buffer at least). Therefore returning a value with RenderTargetState set is not sensible.

The software backend exposes its QPainter and saves and restores before and after invoking render(). Therefore reporting any changed states from here is not necessary.

The function is called by the renderer so it can reset the states after rendering this node. This makes the implementation of render() simpler since it does not have to query and restore these states.

The default implementation returns 0, meaning no relevant state was changed in render().

Note: This function may be called before render().

const QSGClipNode *QSGRenderNode::clipList() const

Returns the current clip list.

[virtual] QSGRenderNode::RenderingFlags QSGRenderNode::flags() const

Returns flags describing the behavior of this render node.

The default implementation returns 0.

See also RenderingFlag and rect().

qreal QSGRenderNode::inheritedOpacity() const

Returns the current effective opacity.

const QMatrix4x4 *QSGRenderNode::matrix() const

Returns pointer to the current model-view matrix.

[virtual, since 6.0] void QSGRenderNode::prepare()

Called from the frame preparation phase. There is a call to this function before each invocation of render().

Unlike render(), this function is called before the scenegraph starts recording the render pass for the current frame on the underlying command buffer. This is useful when doing rendering with graphics APIs, such as Vulkan, where copy type of operations will need to be recorded before the render pass.

The default implementation is empty.

This function was introduced in Qt 6.0.

[virtual] QRectF QSGRenderNode::rect() const

Returns the bounding rectangle in item coordinates for the area render() touches. The value is only in use when flags() includes BoundedRectRendering, ignored otherwise.

Reporting the rectangle in combination with BoundedRectRendering is particularly important with the software backend because otherwise having a rendernode in the scene would trigger fullscreen updates, skipping all partial update optimizations.

For rendernodes covering the entire area of a corresponding QQuickItem the return value will be (0, 0, item->width(), item->height()).

Note: Nodes are also free to render outside the boundaries specified by the item's width and height, since the scenegraph nodes are not bounded by the QQuickItem geometry, as long as this is reported correctly from this function.

See also flags().

[virtual] void QSGRenderNode::releaseResources()

This function is called when all custom graphics resources allocated by this node have to be freed immediately. In case the node does not directly allocate graphics resources (buffers, textures, render targets, fences, etc.) through the graphics API that is in use, there is nothing to do here.

Failing to release all custom resources can lead to incorrect behavior in graphics device loss scenarios on some systems since subsequent reinitialization of the graphics system may fail.

Note: Some scenegraph backends may choose not to call this function. Therefore it is expected that QSGRenderNode implementations perform cleanup both in their destructor and in releaseResources().

Unlike with the destructor, it is expected that render() can reinitialize all resources it needs when called after a call to releaseResources().

With OpenGL, the scenegraph's OpenGL context will be current both when calling the destructor and this function.

[pure virtual] void QSGRenderNode::render(const QSGRenderNode::RenderState *state)

This function is called by the renderer and should paint this node with directly invoking commands in the graphics API (OpenGL, Direct3D, etc.) currently in use.

The effective opacity can be retrieved with inheritedOpacity().

The projection matrix is available through state, while the model-view matrix can be fetched with matrix(). The combined matrix is then the projection matrix times the model-view matrix. The correct stacking of the items in the scene is ensured by the projection matrix.

When using the provided matrices, the coordinate system for vertex data follows the usual QQuickItem conventions: top-left is (0, 0), bottom-right is the corresponding QQuickItem's width() and height() minus one. For example, assuming a two float (x-y) per vertex coordinate layout, a triangle covering half of the item can be specified as (width - 1, height - 1), (0, 0), (0, height - 1) using counter-clockwise direction.

Note: QSGRenderNode is provided as a means to implement custom 2D or 2.5D Qt Quick items. It is not intended for integrating true 3D content into the Qt Quick scene. That use case is better supported by QQuickFramebufferObject, QQuickWindow::beforeRendering(), or the equivalents of those for APIs other than OpenGL.

Note: QSGRenderNode can perform significantly better than texture-based approaches (such as, QQuickFramebufferObject), especially on systems where the fragment processing power is limited. This is because it avoids rendering to a texture and then drawing a textured quad. Rather, QSGRenderNode allows recording draw calls in line with the scenegraph's other commands, avoiding an additional render target and the potentially expensive texturing and blending.

Clip information is calculated before the function is called. Implementations wishing to take clipping into account can set up scissoring or stencil based on the information in state. The stencil buffer is filled with the necessary clip shapes, but it is up to the implementation to enable stencil testing.

Some scenegraph backends, software in particular, use no scissor or stencil. There the clip region is provided as an ordinary QRegion.

With the legacy, direct OpenGL based renderer, the following states are set on the render thread's context before this function is called:

  • glColorMask(true, true, true, true)
  • glDepthMask(false)
  • glDisable(GL_DEPTH_TEST)
  • glStencilFunc(GL_EQUAL, state.stencilValue, 0xff); glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP) depending on clip
  • glScissor(state.scissorRect.x(), state.scissorRect.y(), state.scissorRect.width(), state.scissorRect.height()) depending on clip
  • glEnable(GL_BLEND)
  • glDisable(GL_CULL_FACE)

States that are not listed above, but are covered by StateFlags, can have arbitrary values.

Note: There is no state set with other graphics APIs, considering that many of them do not have a concept of the traditional OpenGL state machine. Rather, it is up to the implementation to create pipeline state objects with the desired blending, scissor, and stencil tests enabled. Note that this also includes OpenGL via the RHI. New QSGRenderNode implementations are recommended to set all scissor, stencil and blend state explicitly (as shown in the above list), even if they are targeting OpenGL.

changedStates() should return which states this function changes. If a state is not covered by StateFlags, the state should be set to the default value according to the OpenGL specification. For other APIs, see the documentation for changedStates() for more information.

Note: Depth writes are disabled when this function is called (glDepthMask(false) with OpenGL). Enabling depth writes can lead to unexpected results, depending on the scenegraph backend in use and the content in the scene, so exercise caution with this.

For APIs other than OpenGL, it will likely be necessary to query certain API-specific resources (for example, the graphics device or the command list/buffer to add the commands to). This is done via QSGRendererInterface.

Assume nothing about the pipelines and dynamic states bound on the command list/buffer when this function is called.

With some graphics APIs it can be necessary to reimplement prepare() in addition, or alternatively connect to the QQuickWindow::beforeRendering() signal. These are called/emitted before recording the beginning of a renderpass on the command buffer (vkCmdBeginRenderPass with Vulkan, or starting to encode via MTLRenderCommandEncoder in case of Metal. Recording copy operations cannot be done inside render() with such APIs. Rather, do such operations either in prepare() or the slot connected to beforeRendering (with DirectConnection).

See also QSGRendererInterface and QQuickWindow::rendererInterface().

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