---
title: OffscreenCanvas.getContext()
slug: Web/API/OffscreenCanvas/getContext
translation_of: Web/API/OffscreenCanvas/getContext
---
<div>{{APIRef("Canvas API")}} {{SeeCompatTable}}</div>

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

<div class="note">
<p><strong>Note</strong>: This API is currently implemented for <a href="/en-US/docs/Web/API/WebGLRenderingContext">WebGL1</a> and <a href="/en-US/docs/Web/API/WebGL2RenderingContext">WebGL2</a> contexts only. See {{bug(801176)}} for <a href="/en-US/docs/Web/API/Canvas_API">Canvas 2D API</a> support from workers.</p>
</div>

<p><font face="x-locale-heading-primary, zillaslab, Palatino, Palatino Linotype, x-locale-heading-secondary, serif"><span style="font-size: 37.33327865600586px;"><strong>構文</strong></span></font></p>



<pre class="syntaxbox notranslate"><em>offscreen</em>.getContext(<em>contextType</em>, <em>contextAttributes</em>);
</pre>

<h3 id="引数">引数</h3>

<dl>
 <dt><code>contextType</code></dt>
 <dd>Is a {{domxref("DOMString")}} containing the context identifier defining the drawing context associated to the canvas. Possible values are:
 <ul>
  <li><strong><code>"2d"</code></strong> creates a {{domxref("CanvasRenderingContext2D")}} object representing a two-dimensional rendering context.</li>
  <li><strong><code>"webgl"</code></strong> creates a {{domxref("WebGLRenderingContext")}} object representing a three-dimensional rendering context. This context is only available on browsers that implement <a href="https://developer.mozilla.org/en-US/docs/Web/WebGL">WebGL</a> version 1 (OpenGL ES 2.0).</li>
  <li><strong><code>"webgl2"</code></strong> creates a {{domxref("WebGL2RenderingContext")}} object representing a three-dimensional rendering context. This context is only available on browsers that implement <a href="https://developer.mozilla.org/en-US/docs/Web/WebGL">WebGL</a> version 2 (OpenGL ES 3.0). {{experimental_inline}}</li>
  <li>
   <p><strong><code>"bitmaprenderer"</code></strong> creates a {{domxref("ImageBitmapRenderingContext")}} which only provides functionality to replace the content of the canvas with a given {{domxref("ImageBitmap")}}.</p>
  </li>
 </ul>

 <p>Note: The identifiers <strong><code>"experimental-webgl"</code></strong> or <strong><code>"experimental-webgl2"</code></strong> 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 <a href="https://www.khronos.org/">Khronos Group</a> certifies WebGL implementations under certain <a href="https://www.khronos.org/registry/webgl/sdk/tests/CONFORMANCE_RULES.txt">conformance rules</a>.</p>
 </dd>
 <dt><code>contextAttributes</code></dt>
 <dd>
 <p>You can use several context attributes when creating your rendering context, for example:</p>

 <pre class="brush: js notranslate">offscreen.getContext("webgl",
                 { antialias: false,
                   depth: false });</pre>
 2d context attributes:

 <ul>
  <li><strong><code>alpha</code></strong>: Boolean that indicates if the canvas contains an alpha channel. If set to <code>false</code>, the browser now knows that the backdrop is always opaque, which can speed up drawing of transparent content and images then.</li>
  <li>{{non-standard_inline}} (Gecko only) <strong><code>willReadFrequently</code></strong>: 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 <code>gfx.canvas.willReadFrequently.enable</code> is set to <code>true</code> (which, by default, is only the case for B2G/Firefox OS).</li>
  <li>{{non-standard_inline}} (Blink only) <strong><code>storage</code></strong>: String that indicates which storage is used ("persistent" by default).</li>
 </ul>
 WebGL context attributes:

 <ul>
  <li><strong><code>alpha</code></strong>: Boolean that indicates if the canvas contains an alpha buffer.</li>
  <li><strong><code>depth</code></strong>: Boolean that indicates that the drawing buffer has a depth buffer of at least 16 bits.</li>
  <li><strong><code>stencil</code></strong>: Boolean that indicates that the drawing buffer has a stencil buffer of at least 8 bits.</li>
  <li><strong><code>antialias</code></strong>: Boolean that indicates whether or not to perform anti-aliasing.</li>
  <li><strong><code>premultipliedAlpha</code></strong>: Boolean that indicates that the page compositor will assume the drawing buffer contains colors with pre-multiplied alpha.</li>
  <li><strong><code>preserveDrawingBuffer</code></strong>: If the value is true the buffers will not be cleared and will preserve their values until cleared or overwritten by the author.</li>
  <li>
   <p><code><strong>failIfMajorPerformanceCaveat</strong></code>: Boolean that indicates if a context will be created if the system performance is low.</p>
  </li>
 </ul>
 </dd>
</dl>

<h3 id="返値">　返値</h3>

<p>A {{domxref("RenderingContext")}} which is either a</p>

<ul>
 <li>{{domxref("CanvasRenderingContext2D")}} for <code>"2d"</code>,</li>
 <li>{{domxref("WebGLRenderingContext")}} for <code>"webgl"</code> and <code>"experimental-webgl"</code>,</li>
 <li>{{domxref("WebGL2RenderingContext")}} for <code>"webgl2"</code> and <code>"experimental-webgl2"</code> {{experimental_inline}}, or</li>
 <li>{{domxref("ImageBitmapRenderingContext")}} for <code>"bitmaprenderer"</code>.</li>
</ul>

<p>If the <code>contextType</code> doesn't match a possible drawing context, <code>null</code> is returned.</p>

<h2 id="例"><font face="x-locale-heading-primary, zillaslab, Palatino, Palatino Linotype, x-locale-heading-secondary, serif"><span style="font-size: 37.33327865600586px;"><strong>例</strong></span></font></h2>



<pre class="brush: js notranslate">var offscreen = new OffscreenCanvas(256, 256);
var gl = offscreen.getContext("webgl");

gl; // WebGLRenderingContext
gl.canvas; // OffscreenCanvas</pre>

<h2 id="仕様書"><font face="x-locale-heading-primary, zillaslab, Palatino, Palatino Linotype, x-locale-heading-secondary, serif"><span style="font-size: 37.33327865600586px;"><strong>仕様書</strong></span></font></h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('HTML WHATWG', "#dom-offscreencanvas-getcontext", "OffscreenCanvas.getContext()")}}</td>
   <td>{{Spec2('HTML WHATWG')}}</td>
   <td></td>
  </tr>
 </tbody>
</table>

<h2 id="ブラウザの互換性"><font face="x-locale-heading-primary, zillaslab, Palatino, Palatino Linotype, x-locale-heading-secondary, serif"><span style="font-size: 37.33327865600586px;"><strong>ブラウザの互換性</strong></span></font></h2>

<div>


<p>{{Compat("api.OffscreenCanvas.getContext")}}</p>
</div>

<h2 id="See_also">See also</h2>

<ul>
 <li>The interface defining this method: {{domxref("OffscreenCanvas")}}</li>
 <li>{{domxref("HTMLCanvasElement.getContext()")}}</li>
 <li>Available rendering contexts: {{domxref("CanvasRenderingContext2D")}}, {{domxref("WebGLRenderingContext")}}, {{domxref("WebGL2RenderingContext")}}, and {{domxref("ImageBitmapRenderingContext")}}</li>
</ul>