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---
title: CanvasRenderingContext2D.clearRect()
slug: Web/API/CanvasRenderingContext2D/clearRect
translation_of: Web/API/CanvasRenderingContext2D/clearRect
---
<div>{{APIRef}}</div>
<p>Canvas 2D API 的 <code><strong>CanvasRenderingContext2D</strong></code><strong><code>.clearRect()</code></strong> 方法可以設定指定矩形（經由坐標 <em>(x, y)</em> 及大小 <em>(width, height)</em>）範圍內的所有像素為透明，清除所有先前繪製的內容。</p>
<h2 id="語法">語法</h2>
<pre class="syntaxbox">void <var><em>ctx</em>.clearRect(x, y, width, height);</var>
</pre>
<h3 id="參數">參數</h3>
<dl>
<dt><code>x</code></dt>
<dd>The x axis of the coordinate for the rectangle starting point.</dd>
<dt><code>y</code></dt>
<dd>The y axis of the coordinate for the rectangle starting point.</dd>
<dt><code>width</code></dt>
<dd>The rectangle's width.</dd>
<dt><code>height</code></dt>
<dd>The rectangle's height.</dd>
</dl>
<h2 id="Usage_notes">Usage notes</h2>
<p>A common problem with <code>clearRect</code> is that it may appear it does not work when not <a href="/en-US/docs/Web/API/Canvas_API/Tutorial/Drawing_shapes#Drawing_paths">using paths properly</a>. Don't forget to call {{domxref("CanvasRenderingContext2D.beginPath", "beginPath()")}} before starting to draw the new frame after calling <code>clearRect</code>.</p>
<h2 id="範例">範例</h2>
<h3 id="Using_the_clearRect_method">Using the <code>clearRect</code> method</h3>
<p>This is just a simple code snippet which uses the <code>clearRect</code> method.</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:[11]">var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(20, 20);
ctx.lineTo(200, 20);
ctx.lineTo(120, 120);
ctx.closePath(); // draws last line of the triangle
ctx.stroke();
ctx.clearRect(10, 10, 100, 100);
// clear the whole canvas
// ctx.clearRect(0, 0, canvas.width, canvas.height);
</pre>
<p>Edit the code below and see your changes update live in the canvas:</p>
<div class="hidden">
<h6 id="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:140px;">
ctx.beginPath();
ctx.moveTo(20,20);
ctx.lineTo(200,20);
ctx.lineTo(120,120);
ctx.closePath(); // draws last line of the triangle
ctx.stroke();
ctx.clearRect(10, 10, 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, 400) }}</p>
<h2 id="規範">規範</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-clearrect", "CanvasRenderingContext2D.clearRect")}}</td>
<td>{{Spec2('HTML WHATWG')}}</td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="瀏覽器相容性">瀏覽器相容性</h2>
{{Compat("api.CanvasRenderingContext2D.clearRect")}}
<h2 id="參見">參見</h2>
<ul>
<li>The interface defining it, {{domxref("CanvasRenderingContext2D")}}</li>
<li>{{domxref("CanvasRenderingContext2D.fillRect()")}}</li>
<li>{{domxref("CanvasRenderingContext2D.strokeRect()")}}</li>
</ul>
|
