Provides a 2D canvas item enabling drawing via JavaScript 更多...
| import 语句： | import QtQuick 2.7 |
| Since: | Qt 5.0 |
| 继承： | Item |
| 继承者： | TraceCanvas |
The Canvas item allows drawing of straight and curved lines, simple and complex shapes, graphs, and referenced graphic images. It can also add text, colors, shadows, gradients, and patterns, and do low level pixel operations. The Canvas output may be saved as an image file or serialized to a URL.
Rendering to the Canvas is done using a Context2D object, usually as a result of the paint 信号。
To define a drawing area in the Canvas item set the
width
and
height
properties. For example, the following code creates a Canvas item which has a drawing area with a height of 100 pixels and width of 200 pixels:
import QtQuick 2.0 Canvas { id: mycanvas width: 100 height: 200 onPaint: { var ctx = getContext("2d"); ctx.fillStyle = Qt.rgba(1, 0, 0, 1); ctx.fillRect(0, 0, width, height); } }
Currently the Canvas item only supports the two-dimensional rendering context.
The Canvas item supports two render targets:
Canvas.Image
and
Canvas.FramebufferObject
.
The
Canvas.Image
render target is a
QImage
object. This render target supports background thread rendering, allowing complex or long running painting to be executed without blocking the UI. This is the only render target that is supported by all Qt Quick backends.
The Canvas.FramebufferObject render target utilizes OpenGL hardware acceleration rather than rendering into system memory, which in many cases results in faster rendering. Canvas.FramebufferObject relies on the OpenGL extensions
GL_EXT_framebuffer_multisample
and
GL_EXT_framebuffer_blit
for antialiasing. It will also use more graphics memory when rendering strategy is anything other than Canvas.Cooperative. Framebuffer objects may not be available with Qt Quick backends other than OpenGL.
The default render target is Canvas.Image and the default renderStrategy is Canvas.Immediate.
All HTML5 2D context pixel operations are supported. In order to ensure improved pixel reading/writing performance the
Canvas.Image
render target should be chosen. The
Canvas.FramebufferObject
render target requires the pixel data to be exchanged between the system memory and the graphic card, which is significantly more expensive. Rendering may also be synchronized with the V-sync signal (to avoid
screen tearing
) which will further impact pixel operations with
Canvas.FrambufferObject
render target.
Although the Canvas item provides an HTML5-like API, HTML5 canvas applications need to be modified to run in the Canvas item:
onPaint
handler and trigger painting by calling the
markDirty()
or
requestPaint()
方法。
onImageLoaded
handler.
Starting Qt 5.4, the Canvas is a 纹理提供程序 and can be used directly in ShaderEffects and other classes that consume texture providers.
注意： In general large canvases, frequent updates, and animation should be avoided with the Canvas.Image render target. This is because with accelerated graphics APIs each update will lead to a texture upload. Also, if possible, prefer QQuickPaintedItem and implement drawing in C++ via QPainter instead of the more expensive and likely less performing JavaScript and Context2D approach.
另请参阅 Context2D and QQuickPaintedItem .
Indicates when Canvas is able to provide a drawing context to operate on.
Holds the logical canvas size that the context paints on.
By default, the canvas size is the same size as the current canvas item size.
By setting the canvasSize, tileSize and canvasWindow, the Canvas item can act as a large virtual canvas with many separately rendered tile rectangles. Only those tiles within the current canvas window are painted by the Canvas render engine.
另请参阅 tileSize and canvasWindow .
Holds the active drawing context.
If the canvas is ready and there has been a successful call to getContext() 或 contextType property has been set with a supported context type, this property will contain the current drawing context, otherwise null.
The type of drawing context to use.
This property is set to the name of the active context type.
If set explicitly the canvas will attempt to create a context of the named type after becoming available.
The type name is the same as used in the getContext() call, for the 2d canvas the value will be "2d".
另请参阅 getContext() and available .
Holds the current canvas rendering strategy.
This hint is supplied along with renderTarget to the graphics context to determine the method of rendering. A renderStrategy, renderTarget or a combination may not be supported by a graphics context, in which case the context will choose appropriate options and Canvas will signal the change to the properties.
Configuration or runtime tests may cause the QML Scene Graph to render in the GUI thread. Selecting
Canvas.Cooperative
, does not guarantee rendering will occur on a thread separate from the GUI thread.
默认值为
Canvas.Immediate
.
另请参阅 renderTarget .
Holds the current canvas render target.
This hint is supplied along with renderStrategy to the graphics context to determine the method of rendering. A renderStrategy , renderTarget or a combination may not be supported by a graphics context, in which case the context will choose appropriate options and Canvas will signal the change to the properties.
The default render target is
Canvas.Image
.
This signal is emitted when an image has been loaded.
相应处理程序是
onImageLoaded
.
另请参阅 loadImage() .
|
paint ( rect region ) |
此信号被发射当 region needs to be rendered. If a context is active it can be referenced from the context property.
This signal can be triggered by markdirty(), requestPaint() or by changing the current canvas window.
相应处理程序是
onPaint
.
This signal is emitted after all context painting commands are executed and the Canvas has been rendered.
相应处理程序是
onPainted
.
This function will cancel the animation callback referenced by handle .
Returns a drawing context, or
null
if no context is available.
The contextId parameter names the required context. The Canvas item will return a context that implements the required drawing mode. After the first call to getContext, any subsequent call to getContext with the same contextId will return the same context object.
If the context type is not supported or the canvas has previously been requested to provide a different and incompatible context type,
null
将被返回。
Canvas only supports a 2d context.
|
isImageError ( url image ) |
返回 true 若 image failed to load.
另请参阅 loadImage() .
|
isImageLoaded ( url image ) |
返回 true 若 image is successfully loaded and ready to use.
另请参阅 loadImage() .
|
isImageLoading ( url image ) |
返回 true 若 image is currently loading.
另请参阅 loadImage() .
|
loadImage ( url image ) |
Loads the given
image
asynchronously.
When the image is ready, imageLoaded will be emitted. The loaded image can be unloaded by the unloadImage() 方法。
Note: Only loaded images can be painted on the Canvas item.
另请参阅 unloadImage , imageLoaded , isImageLoaded() , Context2D::createImageData() ，和 Context2D::drawImage() .
|
markDirty ( rect area ) |
Mark the given
area
as dirty, so that when this area is visible the canvas renderer will redraw it. This will trigger the
paint
信号。
另请参阅 paint and requestPaint() .
This function schedules callback to be invoked before composing the Qt Quick scene.
Request the entire visible region be re-drawn.
另请参阅 markDirty() .
Save the current canvas content into an image file filename . The saved image format is automatically decided by the filename 的后缀。
Note: calling this method will force painting the whole canvas, not just the current canvas visible window.
另请参阅 canvasWindow , canvasSize ，和 toDataURL() .
Returns a data URL for the image in the canvas.
默认 mimeType is "image/png".
另请参阅 save() .
|
unloadImage ( url image ) |
Unloads the
image
.
Once an image is unloaded it cannot be painted by the canvas context unless it is loaded again.
另请参阅 loadImage() , imageLoaded , isImageLoaded() , Context2D::createImageData() ，和 Context2D::drawImage .