From 33058f2b292b3a581333bdfb21b8f671898c5060 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:40:17 -0500 Subject: initial commit --- .../web/api/offscreencanvas/getcontext/index.html | 121 +++++++++++++++++++++ 1 file changed, 121 insertions(+) create mode 100644 files/ja/web/api/offscreencanvas/getcontext/index.html (limited to 'files/ja/web/api/offscreencanvas/getcontext') diff --git a/files/ja/web/api/offscreencanvas/getcontext/index.html b/files/ja/web/api/offscreencanvas/getcontext/index.html new file mode 100644 index 0000000000..ce60c9f633 --- /dev/null +++ b/files/ja/web/api/offscreencanvas/getcontext/index.html @@ -0,0 +1,121 @@ +--- +title: OffscreenCanvas.getContext() +slug: Web/API/OffscreenCanvas/getContext +translation_of: Web/API/OffscreenCanvas/getContext +--- +
{{APIRef("Canvas API")}} {{SeeCompatTable}}
+ +

The OffscreenCanvas.getContext() method returns a drawing context for an offscreen canvas, or {{jsxref("null")}} if the context identifier is not supported.

+ +
+

Note: This API is currently implemented for WebGL1 and WebGL2 contexts only. See {{bug(801176)}} for Canvas 2D API support from workers.

+
+ +

構文

+ + + +
offscreen.getContext(contextType, contextAttributes);
+
+ +

引数

+ +
+
contextType
+
Is a {{domxref("DOMString")}} containing the context identifier defining the drawing context associated to the canvas. Possible values are: +
    +
  • "2d" creates a {{domxref("CanvasRenderingContext2D")}} object representing a two-dimensional rendering context.
    • +
  • "webgl" creates a {{domxref("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" creates a {{domxref("WebGL2RenderingContext")}} object representing a three-dimensional rendering context. This context is only available on browsers that implement WebGL version 2 (OpenGL ES 3.0). {{experimental_inline}}
    • +
  • +

    "bitmaprenderer" creates a {{domxref("ImageBitmapRenderingContext")}} which only provides functionality to replace the content of the canvas with a given {{domxref("ImageBitmap")}}.

    +
    • +
+ +

Note: The identifiers "experimental-webgl" or "experimental-webgl2" are also used in implementations of WebGL. These implementations have not reached test suite conformance, or the graphic drivers situation on the platform is 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:

+ + 
offscreen.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 then.
    • +
  • {{non-standard_inline}} (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 {{domxref("CanvasRenderingContext2D.getImageData", "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).
    • +
  • {{non-standard_inline}} (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.

    +
    • +
+
+
+ +

　返値

+ +

A {{domxref("RenderingContext")}} which is either a

+ + + +

If the contextType doesn't match a possible drawing context, null is returned.

+ +

例

+ + + +
var offscreen = new OffscreenCanvas(256, 256);
+var gl = offscreen.getContext("webgl");
+
+gl; // WebGLRenderingContext
+gl.canvas; // OffscreenCanvas
+ +

仕様書

+ + + + + + + + + + + + + + +
SpecificationStatusComment
{{SpecName('HTML WHATWG', "#dom-offscreencanvas-getcontext", "OffscreenCanvas.getContext()")}}{{Spec2('HTML WHATWG')}}
+ +

ブラウザの互換性

+ +
+ + +

{{Compat("api.OffscreenCanvas.getContext")}}

+
+ +

See also

+ + -- cgit v1.2.3-54-g00ecf