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

<p><code><strong>CanvasRenderingContext2D</strong></code><strong><code>.globalAlpha</code></strong> 是 Canvas 2D API 用来描述在canvas上绘图之前，设置图形和图片透明度的属性。 数值的范围从 0.0 （完全透明）到1.0 （完全不透明）。</p>

<p>在 <a href="/en-US/docs/Web/API/Canvas_API/Tutorial">Canvas Tutorial</a> 中参见 <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>ctx.globalAlpha = value;</var>
</pre>

<h3 id="选项">选项</h3>

<dl>
 <dt><code>value</code></dt>
 <dd>数字在 0.0  （完全透明）和 1.0 （完全不透明）之间。 默认值是 1.0。 如果数值不在范围内，包括{{jsxref("Infinity")}} 和{{jsxref("NaN")}} ，无法赋值，并且 <code>globalAlpha</code> 会保持原有的数值。</dd>
</dl>

<h2 id="示例">示例</h2>

<h3 id="Using_the_globalAlpha_property" name="Using_the_globalAlpha_property">使用 <code>globalAlpha</code> 属性</h3>

<p>这是一段使用 <code>globalAlpha</code> 属性的简单代码片段，绘制了2个半透明的矩形。</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[4]">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

ctx.globalAlpha = 0.5;

ctx.fillStyle = "blue";
ctx.fillRect(10, 10, 100, 100);

ctx.fillStyle = "red";
ctx.fillRect(50, 50, 100, 100);
</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" style="height:120px;"&gt;
ctx.globalAlpha = 0.5;

ctx.fillStyle = "blue";
ctx.fillRect(10, 10, 100, 100);

ctx.fillStyle = "red";
ctx.fillRect(50, 50, 100, 100);&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, 380) }}</p>

<h3 id="A_globalAlpha_example" name="A_globalAlpha_example"><code>globalAlpha</code> 例子</h3>

<p>此例中， 绘制了4个不同背景色的正方形。 在他们上面，绘制半透明的圆形。 将那个点绘制的所有图形的 <code>globalAlpha</code> 属性值都设置为0.2。通过 for 循环绘制半径逐渐增大的圆形。 最终形成的结果是放射性渐变。通过不停地叠加圆形， 使得先前绘制的圆形的透明度越来越暗。通过增加循环数量绘制更多的圆形，图片中心的背景将会变成完全不透明。</p>

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

// draw background
ctx.fillStyle = '#FD0';
ctx.fillRect(0,0,75,75);
ctx.fillStyle = '#6C0';
ctx.fillRect(75,0,75,75);
ctx.fillStyle = '#09F';
ctx.fillRect(0,75,75,75);
ctx.fillStyle = '#F30';
ctx.fillRect(75,75,75,75);
ctx.fillStyle = '#FFF';

// set transparency value
ctx.globalAlpha = 0.2;

// Draw semi transparent circles
for (i=0;i&lt;7;i++){
  ctx.beginPath();
  ctx.arc(75,75,10+10*i,0,Math.PI*2,true);
  ctx.fill();
}
</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_globalAlpha_example", "180", "180", "https://mdn.mozillademos.org/files/232/Canvas_globalalpha.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-globalalpha", "CanvasRenderingContext2D.globalAlpha")}}</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>

<ul>
 <li>从 Gecko 5.0开始， 对 <code>globalAlpha</code> 指定无效的值不再抛出 <code>SYNTAX_ERR</code> 异常； 它们已经被默默地忽略了。</li>
</ul>

<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.setAlpha()</code> 。</li>
</ul>

<h2 id="参见">参见</h2>

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