---
title: CanvasRenderingContext2D.arc()
slug: Web/API/CanvasRenderingContext2D/arc
translation_of: Web/API/CanvasRenderingContext2D/arc
---
<div>{{APIRef}}</div>

<p><code><strong>CanvasRenderingContext2D</strong></code><strong><code>.arc()</code></strong> 是 Canvas 2D API 绘制圆弧路径的方法。 圆弧路径的圆心在 <em>(x, y)</em> 位置，半径为<em> r</em> ，根据<em>anticlockwise</em> （默认为顺时针）指定的方向从 <em>startAngle</em> 开始绘制，到 <em>endAngle</em> 结束。</p>

<h2 id="语法">语法</h2>

<pre class="syntaxbox">void <var><em>ctx</em>.arc(x, y, radius, startAngle, endAngle, anticlockwise);</var>
</pre>

<h3 id="Parameters">Parameters</h3>

<dl>
 <dt><code>x</code></dt>
 <dd>圆弧中心（圆心）的 x 轴坐标。</dd>
 <dt><code>y</code></dt>
 <dd>圆弧中心（圆心）的 y 轴坐标。</dd>
 <dt><code>radius</code></dt>
 <dd>圆弧的半径。</dd>
 <dt><code>startAngle</code></dt>
 <dd>圆弧的起始点， x轴方向开始计算，单位以弧度表示。</dd>
 <dt><code>endAngle</code></dt>
 <dd>圆弧的终点， 单位以弧度表示。</dd>
 <dt><code>anticlockwise</code> {{optional_inline}}</dt>
 <dd>可选的{{jsxref("Boolean")}}值 ，如果为 <code>true</code>，逆时针绘制圆弧，反之，顺时针绘制。 </dd>
</dl>

<h2 id="示例">示例</h2>

<h3 id="Using_the_arc_method" name="Using_the_arc_method">使用 <code>arc</code> 方法</h3>

<p>这是一段绘制圆的简单的代码片段。</p>

<h4 id="HTML">HTML</h4>

<pre class="brush: html">&lt;canvas id="canvas"&gt;&lt;/canvas&gt;
</pre>

<h4 id="JavaScript">JavaScript</h4>

<pre class="brush: js">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

ctx.beginPath();
ctx.arc(75, 75, 50, 0, 2 * Math.PI);
ctx.stroke();
</pre>

<p>修改下面的代码并在线查看 canvas 的变化：</p>

<div class="hidden">
<h6 id="Playable_code" name="Playable_code">Playable code</h6>

<pre class="brush: html">&lt;canvas id="canvas" width="400" height="200" class="playable-canvas"&gt;&lt;/canvas&gt;
&lt;div class="playable-buttons"&gt;
  &lt;input id="edit" type="button" value="Edit" /&gt;
  &lt;input id="reset" type="button" value="Reset" /&gt;
&lt;/div&gt;
&lt;textarea id="code" class="playable-code"&gt;
ctx.beginPath();
ctx.arc(50, 50, 50, 0, 2 * Math.PI, false);
ctx.stroke();&lt;/textarea&gt;
</pre>

<pre class="brush: js">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;

function drawCanvas() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  eval(textarea.value);
}

reset.addEventListener("click", function() {
  textarea.value = code;
  drawCanvas();
});

edit.addEventListener("click", function() {
  textarea.focus();
})

textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);
</pre>
</div>

<p>{{ EmbedLiveSample('Playable_code', 700, 360) }}</p>

<h3 id="Different_shapes_demonstrated" name="Different_shapes_demonstrated">不同的形状演示</h3>

<p>在此例中，使用 arc() 尽可能地绘制不同的形状。</p>

<div class="hidden">
<h6 id="HTML_2">HTML</h6>

<pre class="brush: html">&lt;canvas id="canvas" width="150" height="200"&gt;&lt;/canvas&gt;
</pre>

<h6 id="JavaScript_2">JavaScript</h6>
</div>

<pre class="brush: js">var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');

// Draw shapes
for (i=0;i&lt;4;i++){
  for(j=0;j&lt;3;j++){
    ctx.beginPath();
    var x          = 25+j*50;               // x coordinate
    var y          = 25+i*50;               // y coordinate
    var radius     = 20;                    // Arc radius
    var startAngle = 0;                     // Starting point on circle
    var endAngle   = Math.PI+(Math.PI*j)/2; // End point on circle
    var clockwise  = i%2==0 ? false : true; // clockwise or anticlockwise

    ctx.arc(x,y,radius,startAngle,endAngle, clockwise);

    if (i&gt;1){
      ctx.fill();
    } else {
      ctx.stroke();
    }
  }
}</pre>

<p>{{ EmbedLiveSample('Different_shapes_demonstrated', 160, 210, "https://mdn.mozillademos.org/files/204/Canvas_arc.png") }}</p>

<h2 id="规范描述">规范描述</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', "scripting.html#dom-context-2d-arc", "CanvasRenderingContext2D.arc")}}</td>
   <td>{{Spec2('HTML WHATWG')}}</td>
   <td> </td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容性">浏览器兼容性</h2>

<p>{{ CompatibilityTable() }}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

<h3 id="Gecko-specific_注解">Gecko-specific 注解</h3>

<p>从 Gecko 2.0 {{geckoRelease("2.0")}}开始：</p>

<ul>
 <li><code>anticlockwise</code> 参数是可选的，</li>
 <li>描述了 radius 为负数会抛出 {{domxref("DOMError", "IndexSizeError")}} 错误（“索引或长度为负数，或者超过允许的数值”）。</li>
</ul>

<h2 id="参见">参见</h2>

<ul>
 <li>接口定义， {{domxref("CanvasRenderingContext2D")}}</li>
</ul>