Shape QML Type

Renders a path More...

Import Statement: import QtQuick.Shapes 1.0
Since: Qt 5.10


Inherited By:

EllipseShape, LineShape, and MaskShape


Detailed Description

Renders a path either by generating geometry via QPainterPath and manual triangulation or by using a GPU vendor extension like GL_NV_path_rendering.

This approach is different from rendering shapes via QQuickPaintedItem or the 2D Canvas because the path never gets rasterized in software. Therefore Shape is suitable for creating shapes spreading over larger areas of the screen, avoiding the performance penalty for texture uploads or framebuffer blits. In addition, the declarative API allows manipulating, binding to, and even animating the path element properties like starting and ending position, the control points, etc.

The types for specifying path elements are shared between PathView and Shape. However, not all Shape implementations support all path element types, while some may not make sense for PathView. Shape's currently supported subset is: PathMove, PathLine, PathQuad, PathCubic, PathArc, PathSvg.

See Path for a detailed overview of the supported path elements.

Shape {
    width: 200
    height: 150
    anchors.centerIn: parent
    ShapePath {
        strokeWidth: 4
        strokeColor: "red"
        fillGradient: LinearGradient {
            x1: 20; y1: 20
            x2: 180; y2: 130
            GradientStop { position: 0; color: "blue" }
            GradientStop { position: 0.2; color: "green" }
            GradientStop { position: 0.4; color: "red" }
            GradientStop { position: 0.6; color: "yellow" }
            GradientStop { position: 1; color: "cyan" }
        strokeStyle: ShapePath.DashLine
        dashPattern: [ 1, 4 ]
        startX: 20; startY: 20
        PathLine { x: 180; y: 130 }
        PathLine { x: 20; y: 130 }
        PathLine { x: 20; y: 20 }

Like Item, Shape also allows any visual or non-visual objects to be declared as children. ShapePath objects are handled specially. This is useful since it allows adding visual items, like Rectangle or Image, and non-visual objects, like Timer directly as children of Shape.

The following list summarizes the available Shape rendering approaches:

  • When running with the default, OpenGL backend of Qt Quick, both the generic, triangulation-based and the NVIDIA-specific GL_NV_path_rendering methods are available. The choice is made at runtime, depending on the graphics driver's capabilities. When this is not desired, applications can force using the generic method by setting the Shape.vendorExtensionsEnabled property to false.
  • The software backend is fully supported. The path is rendered via QPainter::strokePath() and QPainter::fillPath() in this case.
  • The Direct 3D 12 backend is not currently supported.
  • The OpenVG backend is not currently supported.

When using Shape, it is important to be aware of potential performance implications:

When the application is running with the generic, triangulation-based Shape implementation, the geometry generation happens entirely on the CPU. This is potentially expensive. Changing the set of path elements, changing the properties of these elements, or changing certain properties of the Shape itself all lead to retriangulation of the affected paths on every change. Therefore, applying animation to such properties can affect performance on less powerful systems.

However, the data-driven, declarative nature of the Shape API often means better cacheability for the underlying CPU and GPU resources. A property change in one ShapePath will only lead to reprocessing the affected ShapePath, leaving other parts of the Shape unchanged. Therefore, a frequently changing property can still result in a lower overall system load than with imperative painting approaches (for example, QPainter).

If animating properties other than stroke and fill colors is a must, it is recommended to target systems providing GL_NV_path_rendering where the cost of property changes is smaller.

At the same time, attention must be paid to the number of Shape elements in the scene, in particular when using this special accelerated approach for GL_NV_path_rendering. The way such a Shape item is represented in the scene graph is different from an ordinary geometry-based item, and incurs a certain cost when it comes to OpenGL state changes.

As a general rule, scenes should avoid using separate Shape items when it is not absolutely necessary. Prefer using one Shape item with multiple ShapePath elements over multiple Shape items. Scenes that cannot avoid using a large number of individual Shape items should consider setting Shape.vendorExtensionsEnabled to false.

See also Qt Quick Examples - Shapes, Path, PathMove, PathLine, PathQuad, PathCubic, PathArc, and PathSvg.

Property Documentation

asynchronous : bool

When Shape.rendererType is Shape.GeometryRenderer, the input path is triangulated on the CPU during the polishing phase of the Shape. This is potentially expensive. To offload this work to separate worker threads, set this property to true.

When enabled, making a Shape visible will not wait for the content to become available. Instead, the gui/main thread is not blocked and the results of the path rendering are shown only when all the asynchronous work has been finished.

The default value is false.

[default] data : list<Object>

This property holds the ShapePath objects that define the contents of the Shape. It can also contain any other type of objects, since Shape, like Item, allows adding any visual or non-visual objects as children.

rendererType : enumeration

This property determines which path rendering backend is active.

  • Shape.UnknownRenderer - The renderer is unknown.
  • Shape.GeometryRenderer - The generic, driver independent solution for OpenGL. Uses the same CPU-based triangulation approach as QPainter's OpenGL 2 paint engine. This is the default on non-NVIDIA hardware when the default, OpenGL Qt Quick scenegraph backend is in use.
  • Shape.NvprRenderer - Path items are rendered by performing OpenGL calls using the GL_NV_path_rendering extension. This is the default on NVIDIA hardware when the default, OpenGL Qt Quick scenegraph backend is in use.
  • Shape.SoftwareRenderer - Pure QPainter drawing using the raster paint engine. This is the default, and only, option when the Qt Quick scenegraph is running with the software backend.

status : enumeration

This property determines the status of the Shape and is relevant when Shape.asynchronous is set to true.

  • Shape.Null - Not yet initialized.
  • Shape.Ready - The Shape has finished processing.
  • Shape.Processing - The path is being processed.

vendorExtensionsEnabled : bool

This property controls the usage of non-standard OpenGL extensions like GL_NV_path_rendering. To disable Shape.NvprRenderer and force a uniform behavior regardless of the graphics card and drivers, set this property to false.

The default value is true.

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