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

<p><code><strong>CanvasRenderingContext2D</strong></code><strong><code>.lineCap</code></strong> 是 Canvas 2D API 指定如何绘制每一条线段末端的属性。有3个可能的值，分别是：<code>butt</code>, <code>round</code> and <code>square</code>。默认值是 <code>butt。</code></p>

<p>参见 <a href="/en-US/docs/Web/API/Canvas_API/Tutorial">Canvas Tutorial</a> 中的 <a href="/en-US/docs/Web/API/Canvas_API/Tutorial/Applying_styles_and_colors">Applying styles and color</a> 章节。</p>

<h2 id="语法">语法</h2>

<pre class="syntaxbox"><var><em>ctx</em>.lineCap = "butt";
<var><em>ctx</em>.lineCap = "round";
<var><em>ctx</em>.lineCap = "square";</var></var></var></pre>

<h3 id="选项">选项</h3>

<dl>
 <dt><code>butt</code></dt>
 <dd>线段末端以方形结束。</dd>
 <dt><code>round</code></dt>
 <dd>线段末端以圆形结束。</dd>
 <dt><code>square</code></dt>
 <dd>线段末端以方形结束，但是增加了一个宽度和线段相同，高度是线段厚度一半的矩形区域。</dd>
</dl>

<h2 id="示例">示例</h2>

<h3 id="Using_the_lineCap_property" name="Using_the_lineCap_property">使用 <code>lineCap</code> 属性</h3>

<p>这是一段简单的代码片段，使用 <code>lineCap</code> 属性绘制以圆形结尾的线段。</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; highlight[7]">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

ctx.beginPath();
ctx.moveTo(0,0);
ctx.lineWidth = 15;
ctx.lineCap = "round";
ctx.lineTo(100, 100);
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.moveTo(0,0);
ctx.lineWidth = 15;
ctx.lineCap = "round";
ctx.lineTo(100, 100);
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="A_lineCap_example" name="A_lineCap_example"><code>lineCap</code> 例子</h3>

<p>在这个例子中绘制了3条线段， 每条线段都设置了不同的 <code style="font-style: normal;">lineCap</code> 属性值。通过2条导航线能够精确地看到3条已绘制线段之间的不同。 每条线段的顶端和末端都能在导航线上准确的反映出来。</p>

<p>左侧的线段使用了默认值 <code>butt</code> ， 和导航线是完全平齐的。 第二条线段使用了 <code>round</code> 选项，在线段末端增加了一个半径为线短宽度一半的半圆。右侧的线段使用了 <code>square</code> 选项，增加了一个宽度和线段相同，高度是线段厚度一半的矩形区域。</p>

<pre class="brush: js">var ctx = document.getElementById('canvas').getContext('2d');
var lineCap = ['butt','round','square'];

// Draw guides
ctx.strokeStyle = '#09f';
ctx.beginPath();
ctx.moveTo(10,10);
ctx.lineTo(140,10);
ctx.moveTo(10,140);
ctx.lineTo(140,140);
ctx.stroke();

// Draw lines
ctx.strokeStyle = 'black';
for (var i = 0; i &lt; lineCap.length; i++) {
  ctx.lineWidth = 15;
  ctx.lineCap = lineCap[i];
  ctx.beginPath();
  ctx.moveTo(25+i*50,10);
  ctx.lineTo(25+i*50,140);
  ctx.stroke();
}
</pre>

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

<p>{{EmbedLiveSample("A_lineCap_example", "180", "180", "https://mdn.mozillademos.org/files/236/Canvas_linecap.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-linecap", "CanvasRenderingContext2D.lineCap")}}</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="WebKitBlink-specific_注解">WebKit/Blink-specific 注解</h3>

<ul>
 <li>在基于WebKit- 和 Blink- 的浏览器中，除了此属性外，还实现了一个不标准的并且不赞成使用的方法 <code>ctx.setLineCap()</code> 。</li>
</ul>

<h2 id="参见">参见</h2>

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