html5使用canvas绘制图片并下载

一、简介
HTML5 <canvas> 元素用于图形的绘制，通过脚本 (通常是JavaScript)来完成.
<canvas> 标签只是图形容器，您必须使用脚本来绘制图形。
你可以通过多种方法使用 canvas 绘制路径,盒、圆、字符以及添加图像。
二、如何构建画布、绘制图片、通过a标签进行下载
1.绘制画布
var canvas = document.createElement('canvas');
canvas.height = 1334;
canvas.width =750;
canvas.style.width = 750 + "px";
canvas.style.height = 1334 + "px";
2.创建 context 对象
var ctx = canvas.getContext('2d');
3.生成一张图片_img1
var img = new Image();
img.crossOrigin = 'Anonymous';
img.src=("#downloadPoster1").attr('href', canvas.toDataURL("image/png")); //将画布转成图片路径赋值给a标签的href
var activityName = ('#downloadPoster1').attr("download", activityName);//给a标签添加download属性，属性值为下载的图片的名称
};
5.点击a标签下载绘制好的图片

6.图片跨域问题处理方法
a.创建图片的时候 img.crossOrigin = 'Anonymous';
b.在请求图片的时候添加请求头

三、整体代码

绘制图片

function canvasPosterImg(){
("#qrCodeImg").attr("style","width: 100%;height: auto;display: block");
("#qrCodePoster img").attr('src'));
//解决跨域，将跨域图片路径转为base64格式
var img = new Image();
var canvas = document.createElement('canvas');
canvas.height = 1334;
canvas.width =750;
canvas.style.width = 750 + "px";
canvas.style.height = 1334 + "px";
var ctx = canvas.getContext('2d');
img.crossOrigin = 'Anonymous';
img.src=("#downloadPoster1").attr('href', canvas.toDataURL("image/png"));
var activityName = ('#downloadPoster1').attr("download", activityName);
};

}

四、下载图片（使用压缩包的方式下载图片）
a.引入对应的工具包

<script src="/javascript/activity/js_zip/FileSaver.js"></script>
<script src="/javascript/activity/js_zip/jszip.min.js"></script>
/**

• 批量下载海报
  */
  function brachDownLoadPoster(activitySharePageInfoList){
  //获取渠道二维码的列表
  //activitySharePageInfoList 为需要下载的图片列表的详情信息
  if(activitySharePageInfoList == null || activitySharePageInfoList.length < 1){
  .each(activitySharePageInfoList, function (i, shareInfo) {
  var shareInfoId = shareInfo.id ;
  var shareName = shareInfo.name;
  var img = new Image();
  var canvas = document.createElement('canvas');
  canvas.height = 1334;
  canvas.width =750;
  canvas.style.width = 750 + "px";
  canvas.style.height = 1334 + "px";
  var ctx = canvas.getContext('2d');
  img.crossOrigin = 'Anonymous';
  img.src=('#qrCode'+shareInfoId+' img')[0];
  ctx.drawImage(img2, QRleft1, QRtop1,QRwidth1,QRwidth1);
  var url = canvas.toDataURL("image/png")
  // 构造a标签并模拟点击
  var imgData = url.split('base64,')[1];
  zip.file(shareName+"图片.png", imgData , { base64:true });
  if(zipNo==size ){
  //下载压缩包
  var activityName = ("#ma-Updata").modal('hide');
  zipNo = 0;
  });
  }
  };
  })
  }