什么是SVG
SVG是一种图像文件格式，它的英文全称为Scalable Vector Graphics，意思为可缩放的矢量图形。它是基于XML（Extensible Markup Language），由World Wide Web Consortium（W3C）联盟进行开发的。严格来说应该是一种开放标准的矢量图形语言，可让你设计激动人心的、高分辨率的Web图形页面。用户可以直接用代码来描绘图像，可以用任何文字处理工具打开SVG图像，通过改变部分代码来使图像具有交互功能，并可以随时插入到HTML中通过浏览器来观看。
SVG从1999年由一组加入W3C的公司启动开发，与之前1998年提交给W3C的标准（PGML、VML）构成竞争。SVG从这两个格式吸取了经验。
SVG允许3种图形对象类型：矢量图形、栅格图像以及文本。图形对象——包括PNG、JPEG这些栅格图像——能够被编组、设计、转换及集成进先前的渲染对象中。文本可以在任何适用于应用程序的XML名字空间之内，从而提高SVG图形的搜索能力和无障碍性。SVG提供的功能集涵盖了嵌套转换、裁剪路径、Alpha通道、滤镜效果、模板对象以及可扩展性。
SVG严格遵从XML语法，并用文本格式的描述性语言来描述图像内容，因此是一种和图像分辨率无关的矢量图形格式。
SVG格式具有以下优点：
图像文件可读，易于修改和编辑（理论上如此，但实际上却是因为各种不同的SVG档编辑器而可能存储成不易解读的SVG文件）
与现有技术可以互动融合。例如，SVG技术本身的动态部分（包括时序控制和动画）就是基于SMIL标准。另外，SVG文件还可嵌入JavaScript（严格地说，应该是ECMAScript）脚本来控制SVG对象
SVG图形格式可以方便的创建文字索引，从而实现基于内容的图像搜索
SVG图形格式支持多种滤镜和特殊效果，在不改变图像内容的前提下可以实现位图格式中类似文字阴影的效果。
SVG图形格式可以用来动态生成图形。例如，可用SVG动态生成具有交互功能的地图，嵌入网页中，并显示给终端用户。
SVG格式具有以下缺点：
如何和已经占有重要市场份额的矢量图形格式Adobe Animate竞争的问题。事实上，Adobe Animate在Flash CC 2014版及以后的版本就都支持直接导出SVG文件了。
SVG的本地运行环境下的厂家支持程度。
由于原始的SVG档是遵从XML语法，导致数据采用未压缩的方式存放，因此相较于其他的矢量图形格式，同样的文件内容会比其他的文件格式稍大。Adobe因此使用gzip压缩开发出压缩的SVG档格式，附档名为 .svgz， 但此种文件格式除了Adobe旗下的软件以外，未被广泛支持使用。
旧版的SVG Viewer无法正确显示出使用新版SVG格式的矢量图形。
SVG与canvas的区别
SVG - 实际开发中，多用SVG
不依赖分辨率
支持事件处理器
不适合游戏
大型渲染区域的程序
Canvas
依赖分辨率
不支持事件绑定
最适合网页游戏
可以用图片格式保存图像
不管是SVG还是Canvas，在一个页面中都可以同时定义多个
示例效果图
示例源代码
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
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