<el-dialog
  title="查看图片"
  :visible.sync="dialogJPG"
  append-to-body>
  <canvas id="mycanvas" width="940" height="570"></canvas>
</el-dialog>

#mycanvas {
  border: 1px solid rgb(199, 198, 198);
}

// imageUrl为后台提供图片地址
doDraw(imageUrl){
  // 获取canvas
  var canvas = document.getElementById("mycanvas")
  // 由于弹窗，确保已获取到
  var a = setInterval(() =>{
    // 重复获取
    canvas = document.getElementById("mycanvas")
    if(!canvas){
     return false
    } else {
      clearInterval(a)
      // 可以理解为一个画笔，可画路径、矩形、文字、图像
      var context = canvas.getContext('2d')
      var img = new Image()
      img.src = imageUrl
      // 加载图片
      img.onload = function(){
        if(img.complete){
          // 根据图像重新设定了canvas的长宽
          canvas.setAttribute("width",img.width)
          canvas.setAttribute("height",img.height)
          // 绘制图片
          context.drawImage(img,0,0,img.width,img.height)
        }
      }
    }
  },1)
},

context.strokeStyle = "red"
context.lineWidth = 3;
context.strokeRect(x, y, width, height)

context.moveTo(x1,y1) 
context.lineTo(x2,y2)
context.strokeStyle = "red"
context.lineWidth = 3;
context.stroke()

context.font = "26px Arial bolder"
context.fillStyle = 'red'
context.fillText(text,x,y)

// 图片地址和图片名称
downIamge (imgsrc, name) { 
  let image = new Image()
  image.setAttribute('crossOrigin', 'anonymous')
  image.onload = function () {
    let canvas = document.createElement('canvas')
    canvas.width = image.width
    canvas.height = image.height
    let context = canvas.getContext('2d')
    context.drawImage(image, 0, 0, image.width, image.height)
    let url = canvas.toDataURL('image/jpg') 
    let a = document.createElement('a')
    let event = new MouseEvent('click')
    a.download = name
    a.href = url
    a.dispatchEvent(event)
  }
  image.src = imgsrc
},