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+---
+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 style="display: none;">
+<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 style="display: none;">
+<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>