Canvas3D QML Type

Canvas that provides a 3D rendering context. More...

Import Statement: import QtCanvas3D 1.1
Since: QtCanvas3D 1.0




Detailed Description

The Canvas3D is a QML element that, when placed in your Qt Quick 2 scene, allows you to get a 3D rendering context and call 3D rendering API calls through that context object. Use of the rendering API requires knowledge of OpenGL-like rendering APIs.

There are two functions that are called by the Canvas3D implementation:

  • initializeGL is emitted before the first frame is rendered, and usually during that you get the 3D context and initialize resources to be used later on during the rendering cycle.
  • paintGL is emitted for each frame to be rendered, and usually during that you submit 3D rendering calls to draw whatever 3D content you want to be displayed.

See also Context3D.

Property Documentation

context : Context3D

This property can be used to access the context created with getContext() method.

See also getContext().

devicePixelRatio : float

Specifies the ratio between logical pixels (used by the Qt Quick) and actual physical on-screen pixels (used by the 3D rendering).

fps : int

This property specifies the current number of frames rendered per second. The value is recalculated every 500 ms, as long as any rendering is done.

Note: This property only gets updated after a Canvas3D frame is rendered, so if no frames are being drawn, this property value won't change. It is also based on the number of Canvas3D frames actually rendered since the value was last updated, so it may not accurately reflect the actual rendering performance when If Canvas3D.renderOnDemand property is true.

See also frameTimeMs.

pixelSize : size

Specifies the size of the render target surface in physical on-screen pixels used by the 3D rendering.

renderOnDemand : bool

If the value is false, the render loop runs constantly and Canvas3D.paintGL() signal is emitted once per frame. If the value is true, Canvas3D.paintGL() is only emitted when Canvas3D content needs to be re-rendered because a geometry change or some other event affecting the Canvas3D content occurred. The application can also request a render using Canvas3D.requestRender() method.

renderTarget : RenderTarget

Specifies how the rendering should be done.

  • Canvas3D.RenderTargetOffscreenBuffer indicates rendering is done into an offscreen buffer and the finished texture is used for the Canvas3D item. This is the default target.
  • Canvas3D.RenderTargetBackground indicates the rendering is done to the background of the Qt Quick scene, in response to QQuickWindow::beforeRendering() signal.
  • Canvas3D.RenderTargetForeground indicates the rendering is done to the foreground of the Qt Quick scene, in response to QQuickWindow::afterRendering() signal.

Canvas3D.RenderTargetBackground and Canvas3D.RenderTargetForeground targets render directly to the same framebuffer the rest of the Qt Quick scene uses. This will improve performance on platforms that are fill-rate limited, but using these targets imposes several limitations on the usage of Canvas3D:

  • Synchronous Context3D commands are not supported outside Canvas3D.initializeGL() signal handler when rendering directly to Qt Quick scene framebuffer, as they cause portions of the command queue to be executed outside the normal frame render sequence, which interferes with the frame clearing logic. Using them will usually result in Canvas3D content not rendering properly. A synchronous command is any Context3D command that requires waiting for Context3D command queue to finish executing before it returns, such as Context3D.getError(), Context3D.finish(), or Context3D.readPixels(). When in doubt, see the individual command documentation to see if that command is synchronous. If your application requires synchronous commands outside Canvas3D.initializeGL() signal handler, you should use Canvas3D.RenderTargetOffscreenBuffer render target.
  • Only Canvas3D items that fill the entire window are supported. Note that you can still control the actual rendering area by using an appropriate viewport.
  • The default framebuffer is automatically cleared by Canvas3D every time before the Qt Quick scene renders a frame, even if there are no Context3D commands queued for that frame. This requires Canvas3D to store the commands used to draw the previous frame in case the window is updated by some other component than Canvas3D and use those commands to render the Canvas3D content for frames that do not have fresh content. Only commands issued inside Canvas3D.paintGL() signal handler are stored this way. You need to make sure that the content of your Canvas3D.paintGL() signal handler is implemented so that it is safe to execute its commands repeatedly. Mainly this means making sure you don't use any synchronous commands or commands that create new persistent OpenGL resources there.
  • Issuing Context3D commands outside Canvas3D.paintGL() and Canvas3D.initializeGL() signal handlers can in some cases cause unwanted flickering of Canvas3D content, particularly if on-demand rendering is used. It is recommended to avoid issuing any Context3D commands outside these two signal handlers.
  • When drawing to the foreground, you should never issue a Context3D.clear(Context3D.GL_COLOR_BUFFER_BIT) command targeting the default framebuffer, as that will clear all other Qt Quick items from the scene. Clearing depth and stencil buffers is allowed.
  • Antialiasing is only supported if the surface format of the window supports multisampling. You may need to specify the surface format of the window explicitly in your main.cpp.
  • You lose the ability to control the z-order of the Canvas3D item itself, as it is always drawn either behind or in front of all other Qt Quick items.
  • The context attributes given as Canvas3D.getContext() parameters are ignored and the corresponding values of the Qt Quick context are used.
  • Drawing to the background or the foreground doesn't work when Qt Quick is using OpenGL core profile, as Canvas3D requires either OpenGL 2.x compatibility or OpenGL ES2.

This property can only be modified before the Canvas3D item has been rendered for the first time.

Signal Documentation

void contextLost()

Emitted when OpenGL context is lost. This happens whenever the parent window of the Canvas3D is destroyed (or otherwise loses its context), or Canvas3D is moved to a different window. Removing Canvas3D from a window and adding it back to the same window doesn't cause context loss, as long as the window itself stays alive.

When context is lost, all objects created by Context3D are invalidated.

See also contextRestored.

void contextRestored()

Emitted when OpenGL context is restored after a loss of context occurred. The Context3D attached to the canvas needs to be reinitialized, so initializeGL is also emitted after this signal.

See also contextLost.

void initializeGL()

Emitted once when Canvas3D is ready and OpenGL state initialization can be done by the client.

void paintGL()

Emitted each time a new frame should be drawn to Canvas3D. Driven by the Qt Quick scenegraph loop.

Method Documentation

int frameSetupTimeMs()

This method returns the number of milliseconds Canvas3D took to process the PaintGL signal for the previous frame. Before any frames have been rendered this method returns 0. This time doesn't include time spent on actual OpenGL rendering of the frame, nor the time the scene graph takes to present the frame to the screen. This value is updated after PaintGL signal handler returns.

This QML method was introduced in QtCanvas3D 1.1.

See also fps and frameTimeMs.

int frameTimeMs()

This method returns the number of milliseconds the renderer took to process the OpenGL portion of the rendering for the previous frame. Before any frames have been rendered this method returns 0. This time is measured from the point OpenGL commands are transferred to render thread to the time glFinish() returns, so it doesn't include the time spent parsing JavaScript, nor the time the scene graph takes to present the frame to the screen. This value is updated for the previous frame when the next frame OpenGL command transfer is done.

See also fps and frameSetupTimeMs.

Context3D getContext(string type, Canvas3DContextAttributes options)

Returns the 3D rendering context that allows 3D rendering calls to be made. The type parameter is ignored for now, but a string is expected to be given. If Canvas3D.renderTarget property value is either Canvas3D.RenderTargetBackground or Canvas3D.RenderTargetForeground, the options parameter is also ignored, the context attributes of the Qt Quick context are used, and the Canvas3DContextAttributes.preserveDrawingBuffer property is forced to false. The options parameter is only parsed when the first call to getContext() is made and is ignored in subsequent calls if given. If the first call is made without giving the options parameter, then the context and render target is initialized with default configuration.

See also Canvas3DContextAttributes, Context3D, and renderTarget.

Context3D getContext(string type)

Returns the 3D rendering context that allows 3D rendering calls to be made. The type parameter is ignored for now, but a string is expected to be given.

void requestRender()

Queues a new frame for rendering when Canvas3D.renderOnDemand property is true. Does nothing when Canvas3D.renderOnDemand property is false.

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