在此页

• 句法
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

HTMLCanvasElement.getContext() method returns a drawing context on the canvas, or null if the context identifier is not supported, or the canvas has already been set to a different context mode.

Later calls to this method on the same canvas element, with the same contextType argument, will always return the same drawing context instance as was returned the first time the method was invoked. It is not possible to get a different drawing context object on a given canvas element.

句法

var ctx = canvas.getContext(contextType);
var ctx = canvas.getContext(contextType, contextAttributes);
					

参数

contextType

是 DOMString containing the context identifier defining the drawing context associated to the canvas. Possible values are:

• "2d" , leading to the creation of a CanvasRenderingContext2D object representing a two-dimensional rendering context.
• "webgl" (或 "experimental-webgl" ) which will create a WebGLRenderingContext object representing a three-dimensional rendering context. This context is only available on browsers that implement WebGL version 1 (OpenGL ES 2.0).
• "webgl2" which will create a WebGL2RenderingContext object representing a three-dimensional rendering context. This context is only available on browsers that implement WebGL version 2 (OpenGL ES 3.0).
• "bitmaprenderer" which will create an ImageBitmapRenderingContext which only provides functionality to replace the content of the canvas with a given ImageBitmap .

注意 : The identifier "experimental-webgl" is used in new implementations of WebGL. These implementations have either not reached test suite conformance, or the graphics drivers on the platform are not yet stable. The Khronos Group certifies WebGL implementations under certain conformance rules .

contextAttributes

You can use several context attributes when creating your rendering context, for example:

const gl = canvas.getContext('webgl', {
  antialias: false,
  depth: false
});
							

2d context attributes:

• alpha : Boolean that indicates if the canvas contains an alpha channel. If set to false , the browser now knows that the backdrop is always opaque, which can speed up drawing of transparent content and images.
• desynchronized : Boolean that hints the user agent to reduce the latency by desynchronizing the canvas paint cycle from the event loop
• (Gecko only) willReadFrequently : Boolean that indicates whether or not a lot of read-back operations are planned. This will force the use of a software (instead of hardware accelerated) 2D canvas and can save memory when calling getImageData() frequently. This option is only available, if the flag gfx.canvas.willReadFrequently.enable 被设为 true (which, by default, is only the case for B2G/Firefox OS).
• (Blink only) storage : String that indicates which storage is used ("persistent" by default).

WebGL context attributes:

• alpha : Boolean that indicates if the canvas contains an alpha buffer.
• desynchronized : Boolean that hints the user agent to reduce the latency by desynchronizing the canvas paint cycle from the event loop
• antialias : Boolean that indicates whether or not to perform anti-aliasing.
• depth : Boolean that indicates that the drawing buffer has a depth buffer of at least 16 bits.
• failIfMajorPerformanceCaveat : Boolean that indicates if a context will be created if the system performance is low or if no hardware GPU is available.
• powerPreference : A hint to the user agent indicating what configuration of GPU is suitable for the WebGL context. Possible values are:
  • "default" : Let the user agent decide which GPU configuration is most suitable. This is the default value.
  • "high-performance" : Prioritizes rendering performance over power consumption.
  • "low-power" : Prioritizes power saving over rendering performance.
• premultipliedAlpha : Boolean that indicates that the page compositor will assume the drawing buffer contains colors with pre-multiplied alpha.
• preserveDrawingBuffer : If the value is true the buffers will not be cleared and will preserve their values until cleared or overwritten by the author.
• stencil : Boolean that indicates that the drawing buffer has a stencil buffer of at least 8 bits.

返回值

A RenderingContext which is either a

• CanvasRenderingContext2D for "2d" ,
• WebGLRenderingContext for "webgl" and "experimental-webgl" ,
• WebGL2RenderingContext for "webgl2" or
• ImageBitmapRenderingContext for "bitmaprenderer" .

若 contextType doesn't match a possible drawing context, or differs from the first contextType requested, null 被返回。

范例

Given this <canvas> 元素：

<canvas id="canvas" width="300" height="300"></canvas>
					

You can get a 2d context of the canvas with the following code:

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
console.log(ctx); // CanvasRenderingContext2D { ... }
					

Now you have the 2D rendering context for a canvas and you can draw within it.

规范

规范	状态	注释
HTML 实时标准 The definition of 'HTMLCanvasElement.getContext' in that specification.	实时标准	无变化从最新快照起， HTML5
HTML 5.1 The definition of 'HTMLCanvasElement.getContext' in that specification.	推荐
HTML5 The definition of 'HTMLCanvasElement.getContext' in that specification.	推荐	Snapshot of the HTML 实时标准 containing the initial definition.

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
getContext	Chrome 1	Edge 12	Firefox 1.5	IE 9	Opera 9	Safari 2	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 10.1	Safari iOS 1	Samsung Internet Android 1.0
bitmaprenderer context	Chrome 56	Edge ≤79	Firefox 46	IE No	Opera 43	Safari No	WebView Android No	Chrome Android 56	Firefox Android 46	Opera Android 43	Safari iOS No	Samsung Internet Android 6.0
desynchronized 属性	Chrome 81 81 ChromeOS and Windows 75 ChromeOS only 71 Alternate Name Alternate Name Uses the non-standard name: lowLatency	Edge No	Firefox No	IE No	Opera No	Safari No	WebView Android No	Chrome Android No	Firefox Android No	Opera Android No	Safari iOS No	Samsung Internet Android No
failIfMajorPerformanceCaveat attribute	Chrome Yes	Edge 12	Firefox 41	IE Yes	Opera Yes	Safari ?	WebView Android No	Chrome Android No	Firefox Android 41	Opera Android ?	Safari iOS ?	Samsung Internet Android No
powerPreference 属性	Chrome No	Edge No	Firefox 63	IE No	Opera No	Safari No	WebView Android No	Chrome Android No	Firefox Android 63	Opera Android No	Safari iOS No	Samsung Internet Android No
WebGL context	Chrome 33 33 9 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Edge 79 79 12 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Firefox 24 24 3.6 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	IE 11 Alternate Name 11 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Opera 15 15 9 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Safari 5.1 Alternate Name 5.1 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	WebView Android 37 37 37 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Chrome Android 33 33 18 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Firefox Android Yes Alternate Name Yes Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Opera Android 14 14 10.1 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl	Safari iOS ?	Samsung Internet Android 2.0 2.0 1.0 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl
WebGL2 context	Chrome 56	Edge 79	Firefox 51 51 25 Alternate Name Alternate Name Uses the non-standard name: experimental-webgl2	IE No	Opera No	Safari No	WebView Android 56	Chrome Android No	Firefox Android No	Opera Android No	Safari iOS No	Samsung Internet Android No
2d alpha context attribute	Chrome 32	Edge ≤79	Firefox 30	IE No	Opera Yes	Safari No	WebView Android No	Chrome Android No	Firefox Android 30	Opera Android No	Safari iOS No	Samsung Internet Android No

图例

完整支持

完整支持

不支持

不支持

兼容性未知 ?

兼容性未知

见实现注意事项。

使用非标名称。

另请参阅

• The interface defining it, HTMLCanvasElement .
• OffscreenCanvas.getContext()
• Available rendering contexts: CanvasRenderingContext2D , WebGLRenderingContext and WebGL2RenderingContext and ImageBitmapRenderingContext .

元数据

• 最后修改： Aug 31, 2020

相关话题

1. 画布 API
2. HTMLCanvasElement
3. 特性

   1. height
   2. mozOpaque
   3. width
4. 方法

   1. captureStream()
   2. getContext()
   3. mozFetchAsStream()
   4. mozGetAsFile()
   5. toBlob()
   6. toDataURL()
   7. transferControlToOffscreen()
5. 事件

   1. webglcontextlost
6. 继承：

   1. HTMLElement
   2. 元素
   3. 节点
   4. EventTarget
7. Related pages for Canvas API

   1. Canvas
   2. CanvasGradient
   3. CanvasImageSource
   4. CanvasPattern
   5. CanvasRenderingContext2D
   6. ImageBitmap
   7. ImageBitmapFactories
   8. ImageBitmapRenderingContext
   9. ImageData
   10. OffscreenCanvas
   11. Path2D
   12. RenderingContext
   13. TextMetrics