HTMLCanvasElement.getContext()

Метод HTMLCanvasElement.getContext() возвращает контекст рисования на холсте, или null, если идентификатор контекста не определен.

Синтаксис

canvas.getContext(contextType, contextAttributes);

Параметры

contextType

Is a 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" (or "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" (or "experimental-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 a ImageBitmapRenderingContext which only provides functionality to replace the content of the canvas with a given ImageBitmap.

Примечание: Идентификаторы "experimental-webgl" или "experimental-webgl2" используются в новых реализациях WebGL. Эти реализации не достигли испытательного набора на соответствие или ситуация с графическими драйверами на платформе еще не стабильна The Khronos Group certifies WebGL implementations under certain conformance rules.

contextAttributes

Вы можете использовать несколько атрибутов контекста при создании контекста рендеринга, например:

canvas.getContext("webgl", 
                 { antialias: false,
                   depth: false });

2d атрибуты контекста:

• 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 then.
• (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 is set to 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.
• depth: Boolean that indicates that the drawing buffer has a depth buffer of at least 16 bits.
• stencil: Boolean that indicates that the drawing buffer has a stencil buffer of at least 8 bits.
• antialias: Boolean that indicates whether or not to perform anti-aliasing.
• 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.

• failIfMajorPerformanceCaveat: Boolean that indicates if a context will be created if the system performance is low.

Возвращаемое значение

RenderingContext, который представляет собой

• CanvasRenderingContext2D для "2d",
• WebGLRenderingContext для "webgl" и "experimental-webgl",
• WebGL2RenderingContext для "webgl2" и "experimental-webgl2", или
• ImageBitmapRenderingContext для "bitmaprenderer".

Если contextType не соответствует возможному контексту рисунка, то возвращается null.

Примеры

Given this <canvas> element:

<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.

Спецификации

Совместимость браузеров

[1] Chrome 9 and Gecko 1.9.2 initially implemented this as experimental-webgl. Since Chrome 33 and Gecko 24 it is implemented as the standard webgl.

[2] Internet Explorer 11, WebKit 5.1 and Firefox Mobile implemented this as experimental-webgl.

[3] Opera 9 implemented this as experimental-webgl, behind a user preference, in version 15.0 the user preference got removed.

[4] Gecko 25 implements this as "experimental-webgl2" behind the user preference webgl.enable-prototype-webgl2. Starting with Gecko 42, the string "webgl2" is used behind the same preference.

Смотри также

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