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diff --git a/files/zh-cn/web/api/canvasrenderingcontext2d/globalalpha/index.html b/files/zh-cn/web/api/canvasrenderingcontext2d/globalalpha/index.html new file mode 100644 index 0000000000..a12923f33f --- /dev/null +++ b/files/zh-cn/web/api/canvasrenderingcontext2d/globalalpha/index.html @@ -0,0 +1,217 @@ +--- +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"><canvas id="canvas"></canvas> +</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"><canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas> +<div class="playable-buttons"> + <input id="edit" type="button" value="Edit" /> + <input id="reset" type="button" value="Reset" /> +</div> +<textarea id="code" class="playable-code" style="height:120px;"> +ctx.globalAlpha = 0.5; + +ctx.fillStyle = "blue"; +ctx.fillRect(10, 10, 100, 100); + +ctx.fillStyle = "red"; +ctx.fillRect(50, 50, 100, 100);</textarea> +</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<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"><canvas id="canvas" width="150" height="150"></canvas></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> |