加水印

 addWaterMarker(str, domstr) {
    let can = document.createElement('canvas')
    let doms = document.querySelectorAll(domstr)
    can.width = 150
    can.height = 90
    can.style.display = 'none'
    let cans = can.getContext('2d')
    cans.rotate(-20 * Math.PI / 180)
    cans.font = '16px Microsoft JhengHei'
    cans.fillStyle = 'rgba(17, 17, 17, 0.15)'
    cans.textAlign = 'left'
    cans.textBaseline = 'Middle'
    cans.fillText('用户ID：' + str, 0, 90)
    doms.forEach((item) => {
      item.appendChild(can)
      item.style.backgroundImage = `url(${can.toDataURL('image/png')})`
    })
  }