【HTML5】Canvas基础知识讲解

迈不过友情╰ 2022-08-06 12:24 241阅读 0赞

<table style="word-wrap:break-word; empty-cells:show; border-collapse:collapse; table-layout:fixed; width:698px; color:rgb(0,0,0); font-family:微软雅黑,Tahoma,Helvetica,SimSun,sans-serif; font-size:14px; line-height:25.2000007629395px"> 
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   <td style="word-wrap:break-word"><a href="http://www.html5cn.org/" style="word-wrap:break-word; color:rgb(68,68,68)!important; text-decoration:none!important; border-bottom-width:1px; border-bottom-style:solid; border-bottom-color:blue" rel="nofollow">HTML5</a>规范引进了很多新特性，其中最令人期待的之一就是Canvas元素。HTML5 Canvas提供了通过JavaScript绘制图形的方法，此方法使用简单但功能强大。每一个Canvas元素都有一个“上下文(context)” (想象成绘图板上的一页)，在其中可以绘制任意图形。浏览器支持多个Canvas上下文，并通过不同的API提供图形绘制功能。<br style="word-wrap:break-word"> <br style="word-wrap:break-word"> 大部分的浏览器都支持2D Canvas上下文——包括Opera，Firefox，Konqueror和Safari。而且某些版本的Opera还支持3D Canvas，Firefox也可以通过插件形式支持 3D Canvas。<br style="word-wrap:break-word"> <br style="word-wrap:break-word"> 本文介绍主要介绍2D Canvas基础以及如何使用基本Canvas函数，如线条、形状、图像和文字等。为了理解此文章，你最好了解JavaScript基础知识。<br style="word-wrap:break-word"> 下面开始我们的讲解：<br style="word-wrap:break-word"> <br style="word-wrap:break-word"> <span style="word-wrap:break-word">canvas基础</span><br style="word-wrap:break-word"> <br style="word-wrap:break-word"> 创建Canvas的方法很简单，只需要在HTML页面中添加&lt;canvas&gt;元素就可以了：<br style="word-wrap:break-word"> <br style="word-wrap:break-word"> 
    <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
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      <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">&lt;canvas id="myCanvas" width="300" height="300"&gt;<br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">这是一个Canvas标签案例<br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">&lt;/canvas&gt;</li> 
      </ol> 
     </div> 
     <span style="word-wrap:break-word">复制代码</span> 
    </div> <br style="word-wrap:break-word"> 为了能在JavaScript中引用Canvas，最好给它设置ID ；也需要给Canvas设定高度和宽度。<br style="word-wrap:break-word"> <br style="word-wrap:break-word"> 要在Canvas画布中绘制图形需要使用JavaScript。首先通过getElementById函数找到Canvas元素，然后初始化上下文，之后可以使用上下文API绘制各种图形。下面的脚本可以在Canvas中绘制一个矩形：<br style="word-wrap:break-word"> <br style="word-wrap:break-word"> 
    <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
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      <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">var elem = document.getElementById('myCanvas');<br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">if (elem &amp;&amp; elem.getContext) { <br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;var context = elem.getContext('2d');<br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;if (context) { <br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp; &nbsp; context.fillRect(0, 0, 150, 100);<br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;}<br style="word-wrap:break-word"> </li> 
       <li style="word-wrap:break-word; margin:0px; padding:0px">}</li> 
      </ol> 
     </div> 
     <span style="word-wrap:break-word">复制代码</span> 
    </div> <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。<br style="word-wrap:break-word"> <br style="word-wrap:break-word"> <span style="word-wrap:break-word">2D context API</span> 
    <div style="word-wrap:break-word"> 
     <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>介绍了如何创建Canvas后，让我们来看看2D Canvas API，看看能用这些函数做些什么。 
     <br style="word-wrap:break-word"> 
     <br style="word-wrap:break-word"> 在上面的例子中，我们展示了绘制矩形是多么简单，而通过fillStyle和strokeStyle属性可以更轻松的设置矩形的填充和线条。颜色值使用方法和CSS一样：十六进制数、rgb()、rgba() 和hsla()。 
     <br style="word-wrap:break-word"> 
     <br style="word-wrap:break-word"> 通过fillRect可以绘制带填充的矩形；使用strokeRect可以绘制只有边框没有填充的矩形。如果想清除部分Canvas可以使用clearRect。上述三个方法的参数相同：x, y, width, height。前两个参数设定 (x,y) 坐标，后两个参数设置矩形的高度和宽度。 
     <br style="word-wrap:break-word"> 下面将是一个综合实例的JavaScript脚本： 
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     <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
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       <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
        <li style="word-wrap:break-word; margin:0px; padding:0px">context.fillStyle&nbsp; &nbsp;= '#00f';<br style="word-wrap:break-word"> </li> 
        <li style="word-wrap:break-word; margin:0px; padding:0px">context.strokeStyle = '#f00';<br style="word-wrap:break-word"> </li> 
        <li style="word-wrap:break-word; margin:0px; padding:0px">context.lineWidth&nbsp; &nbsp;= 4;<br style="word-wrap:break-word"> </li> 
        <li style="word-wrap:break-word; margin:0px; padding:0px">context.fillRect&nbsp;&nbsp;(0,&nbsp; &nbsp;0, 150, 50);<br style="word-wrap:break-word"> </li> 
        <li style="word-wrap:break-word; margin:0px; padding:0px">context.strokeRect(0,&nbsp;&nbsp;60, 150, 50);<br style="word-wrap:break-word"> </li> 
        <li style="word-wrap:break-word; margin:0px; padding:0px">context.clearRect (30, 25,&nbsp;&nbsp;90, 60);<br style="word-wrap:break-word"> </li> 
        <li style="word-wrap:break-word; margin:0px; padding:0px">context.strokeRect(30, 25,&nbsp;&nbsp;90, 60);</li> 
       </ol> 
      </div> 
      <span style="word-wrap:break-word">复制代码</span> 
     </div> 
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     <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。 
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     <span style="word-wrap:break-word">路径</span> 
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      <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>通过Canvas路径（path）可以绘制任意形状。可以先绘制轮廓，然后绘制边框和填充。创建自定义形状很简单，使用beginPath()开始绘制，然后使用直线、曲线和其他图形绘制你的图形。绘制完毕后调用fill和stroke即可添加填充或者设置边框。调用 closePath()结束自定义图形绘制。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 下面是一个绘制三角的案例的JavaScript脚本： 
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      <br style="word-wrap:break-word"> 
      <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
       <div style="word-wrap:break-word"> 
        <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.fillStyle&nbsp; &nbsp;= '#00f';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.strokeStyle = '#f00';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.lineWidth&nbsp; &nbsp;= 4;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.beginPath();<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.moveTo(10, 10);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.lineTo(100, 10);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.lineTo(10, 100);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.lineTo(10, 10);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.fill();<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.stroke();<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.closePath();</li> 
        </ol> 
       </div> 
       <span style="word-wrap:break-word">复制代码</span> 
      </div> 
      <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 
      <span style="word-wrap:break-word">插入图像</span> 
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      <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>drawImage方法允许在Canvas中插入其他图像(img和Canvas元素) 。在Opera中可以在Canvas中绘制SVG图形。此方法比较复杂，可以有3个、5个或9个参数： 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 3个参数：最基本的drawImage使用方法。一个参数指定图像位置，另两个参数设置图像在Canvas中的位置。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 5个参数：中级的drawImage使用方法，包括上面所述3个参数，加两个参数指明插入图像宽度和高度 (如果你想改变图像大小)。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 9个参数：最复杂drawImage杂使用方法，包含上述5个参数外，另外4个参数设置源图像中的位置和高度宽度。这些参数允许你在显示图像前动态裁剪源图像。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 下面是上述三个使用方法的例子： 
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      <br style="word-wrap:break-word"> 
      <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
       <div style="word-wrap:break-word"> 
        <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.drawImage(img_elem, dx, dy);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.drawImage(img_elem, dx, dy, dw, dh);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.drawImage(img_elem, sx, sy, sw, sh, dx, dy, dw, dh);</li> 
        </ol> 
       </div> 
       <span style="word-wrap:break-word">复制代码</span> 
      </div> 
      <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 
      <span style="word-wrap:break-word">像素级操作</span> 
     </div> 
     <div style="word-wrap:break-word"> 
      <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>2D Context API提供了三个方法用于像素级操作：createImageData，getImageData和putImageData。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> ImageData对象保存了图像像素值。每个对象有三个属性: width,height和data。data属性类型为CanvasPixelArray，用于储存width*height*4个像素值。每一个像素有RGB值和透明度alpha值(其值为0至255，包括alpha在内。)。像素的顺序从左至右，从上到下，按行存储。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 来看一个例子： 
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      <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
       <div style="word-wrap:break-word"> 
        <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">var imgd = context.createImageData(50,50);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">var pix = imgd.data;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">for (var i = 0; n = pix.length, i &lt; n; i += 4) { <br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;pix[i&nbsp;&nbsp;] = 255;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;pix[i+3] = 127;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">}<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.putImageData(imgd, 0,0);</li> 
        </ol> 
       </div> 
       <span style="word-wrap:break-word">复制代码</span> 
      </div> 
      <br style="word-wrap:break-word"> 注意: 不是所有浏览器都实现了createImageData。在支持的浏览器中，需要通过getImageData方法获取ImageData对象。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 通过ImageData可以完成很多功能。如可以实现图像滤镜，或可以实现数学可视化 (如分形和其他特效)。来看一个实例： 
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      <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
       <div style="word-wrap:break-word"> 
        <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">var imgd = context.getImageData(x, y width, height);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">var pix = imgd.data;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">for (var i = 0, n = pix.length; i &lt; n; i += 4) { <br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;pix[i&nbsp;&nbsp;] = 255 - pix[i&nbsp;&nbsp;];<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;pix[i+1] = 255 - pix[i+1];<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">&nbsp;&nbsp;pix[i+2] = 255 - pix[i+2];<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">}<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.putImageData(imgd,x, y);</li> 
        </ol> 
       </div> 
       <span style="word-wrap:break-word">复制代码</span> 
      </div> 
      <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。 
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      <span style="word-wrap:break-word">文字</span> 
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      <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>虽然最近的WebKit版本和Firefox 3.1 nightly build才开始支持Text API，为了保证文章完整性我决定仍在这里介绍文字API。 
      <br style="word-wrap:break-word"> context对象可以设置以下text属性： 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> font：文字字体，同CSS font-family属性； 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> textAlign：文字水平对齐方式。可取属性值: start,end,left,right,center。默认值：start； 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> textBaseline：文字竖直对齐方式。可取属性值：top,hanging,middle,alphabetic,ideographic,bottom。默认值：alphabetic。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 有两个方法可以绘制文字：fillText和strokeText。第一个绘制带fillStyle填充的文字，后者绘制只有strokeStyle边框的文字。两者的参数相同：要绘制的文字和文字的位置(x,y)坐标。还有一个可选选项——最大宽度。如果需要的话，浏览器会缩减文字以让它适应指定宽度。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 文字对齐属性影响文字与设置的(x,y)坐标的相对位置。 
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      <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
       <div style="word-wrap:break-word"> 
        <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">ontext.fillStyle&nbsp; &nbsp; = '#00f';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.font&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;= 'italic 30px sans-serif';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.textBaseline = 'top';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.fillText&nbsp;&nbsp;('Hello world!', 0, 0);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.font&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;= 'bold 30px sans-serif';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.strokeText('Hello world!', 0, 50);</li> 
        </ol> 
       </div> 
       <span style="word-wrap:break-word">复制代码</span> 
      </div> 
      <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。 
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      <span style="word-wrap:break-word">阴影</span> 
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     <div style="word-wrap:break-word"> 
      <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>目前只有Konqueror和Firefox 3.1 nightly build支持Shadows API。API的属性为：shadowColor：阴影颜色。其值和CSS颜色值一致。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> shadowBlur：设置阴影模糊程度。此值越大，阴影越模糊。其效果和Photoshop的高斯模糊滤镜相同。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> shadowOffsetX和shadowOffsetY：阴影的x和y偏移量，单位是像素。 
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      <br style="word-wrap:break-word"> 
      <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
       <div style="word-wrap:break-word"> 
        <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.shadowOffsetX = 5;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.shadowOffsetY = 5;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.shadowBlur&nbsp; &nbsp; = 4;<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.shadowColor&nbsp; &nbsp;= 'rgba(255, 0, 0, 0.5)';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.fillStyle&nbsp; &nbsp;&nbsp;&nbsp;= '#00f';<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">context.fillRect(20, 20, 150, 100);</li> 
        </ol> 
       </div> 
       <span style="word-wrap:break-word">复制代码</span> 
      </div> 
      <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 
      <span style="word-wrap:break-word">颜色渐变</span> 
     </div> 
     <div style="word-wrap:break-word"> 
      <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>除了CSS颜色， fillStyle和strokeStyle属性可以设置为CanvasGradient对象。——通过CanvasGradient可以为线条和填充使用颜色渐变。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 欲创建CanvasGradient对象，可以使用两个方法：createLinearGradient和createRadialGradient。前者创建线性颜色渐变，后者创建圆形颜色渐变。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 创建颜色渐变对象后，可以使用对象的addColorStop方法添加颜色中间值。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 下面的代码演示了颜色渐变使用方法： 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 
      <div style="word-wrap:break-word; overflow:hidden; margin:10px 0px; padding:5px 10px; color:rgb(102,102,102); background:rgb(247,247,247)"> 
       <div style="word-wrap:break-word"> 
        <ol style="word-wrap:break-word; margin:1em 2em; padding:0px"> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">var gradient1 = context.createLinearGradient(sx,sy,dx,dy);<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">gradient1.addColorStop(0,&nbsp; &nbsp;'#f00');<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">gradient1.addColorStop(0.5, '#ff0');<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">gradient1.addColorStop(1,&nbsp; &nbsp;'#00f');<br style="word-wrap:break-word"> </li> 
         <li style="word-wrap:break-word; margin:0px; padding:0px">var gradient2 = context.createRadialGradient(sx,sy,sr,dx,dy, dr);</li> 
        </ol> 
       </div> 
       <span style="word-wrap:break-word">复制代码</span> 
      </div> 
      <br style="word-wrap:break-word"> 可以把上面代码放置在文档head部分中，或者放在外部JavaScript文件中。 
      <br style="word-wrap:break-word"> 
      <br style="word-wrap:break-word"> 
      <span style="word-wrap:break-word">小结</span> 
     </div> 
     <div style="word-wrap:break-word"> 
      <span style="word-wrap:break-word"><br style="word-wrap:break-word"> </span>Canvas是HTML5最让人期待的特性之一，目前已获得大部分Web浏览器支持Canvas可以帮助创建游戏、增强图形用户界面。2D context API提供大量图形绘制功能——我希望通过本文你了解了Canvas使用，并且你有兴趣了解更多！ 
      <br style="word-wrap:break-word"> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>