QSGRenderNode#

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

Inheritance diagram of PySide6.QtQuick.QSGRenderNode

Synopsis#

Functions#

Virtual functions#

Note

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Detailed Description#

class PySide6.QtQuick.QSGRenderNode#
PySide6.QtQuick.QSGRenderNode.StateFlag#

(inherits enum.Flag) This enum is a bit mask identifying several states.

Constant

Description

QSGRenderNode.DepthState

Depth

QSGRenderNode.StencilState

Stencil

QSGRenderNode.ScissorState

Scissor

QSGRenderNode.ColorState

Color

QSGRenderNode.BlendState

Blend

QSGRenderNode.CullState

Cull

QSGRenderNode.ViewportState

View poirt

QSGRenderNode.RenderTargetState

Render target
PySide6.QtQuick.QSGRenderNode.RenderingFlag#

(inherits enum.Flag) Possible values for the bitmask returned from flags() .

Constant

Description

QSGRenderNode.BoundedRectRendering

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

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 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

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.

See also

render() rect()

PySide6.QtQuick.QSGRenderNode.changedStates()#
Return type:

StateFlags

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

  • DepthState - depth write mask, depth test enabled, depth comparison function

  • StencilState - stencil write masks, stencil test enabled, stencil operations, stencil comparison functions

  • ScissorState - scissor enabled, scissor test enabled

  • ColorState - clear color, color write mask

  • BlendState - blend enabled, blend function

  • CullState - front face, cull face enabled

  • ViewportState - viewport

  • RenderTargetState - render target

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() .

PySide6.QtQuick.QSGRenderNode.clipList()#
Return type:

PySide6.QtQuick.QSGClipNode

Returns the current clip list.

PySide6.QtQuick.QSGRenderNode.inheritedOpacity()#
Return type:

float

Returns the current effective opacity.

PySide6.QtQuick.QSGRenderNode.matrix()#
Return type:

PySide6.QtGui.QMatrix4x4

Returns pointer to the current model-view matrix.

PySide6.QtQuick.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.

PySide6.QtQuick.QSGRenderNode.projectionMatrix()#
Return type:

PySide6.QtGui.QMatrix4x4

Returns pointer to the current projection matrix.

In render() this is the same matrix that is returned from projectionMatrix() . This getter exists so that prepare() also has a way to query the projection matrix.

When working with a modern graphics API, or Qt’s own graphics abstraction layer, it is more than likely that one will want to load *projectionMatrix() * *matrix() into a uniform buffer. That is however something that needs to be done in prepare() , so outside the recording of a render pass. That is why both matrices are queriable directly from the QSGRenderNode , both in prepare() and render() .

PySide6.QtQuick.QSGRenderNode.rect()#
Return type:

PySide6.QtCore.QRectF

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()

PySide6.QtQuick.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.

abstract PySide6.QtQuick.QSGRenderNode.render(state)#
Parameters:

state – PySide6.QtQuick.QSGRenderNode.RenderState

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 , 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)

  • glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA)

  • 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 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).