甩锅技能：让前端他们自己生成海报

Html2canvas(document.querySelector("#poster")).then(canvas => {
 // base64 编码的图片
  const imageBase64Url = canvas.toDataURL("image/png", 1.0);
 // 转换成 Blob 类型
  const imageBlob = self.dataURItoBlob(self.posterUrl);
 // 添加 canvas 到 body
  document.body.appendChild(canvas)
});

dataURItoBlob(base64Data) {
  var byteString;
  if (base64Data.split(',')[0].indexOf('base64') >= 0){
      byteString = atob(base64Data.split(',')[1]);
  }else{
      byteString = unescape(base64Data.split(',')[1]);
  }
  var mimeString = base64Data.split(',')[0].split(':')[1].split(';')[0];
  var ia = new Uint8Array(byteString.length);
  for (var i = 0; i < byteString.length; i++) {
      ia[i] = byteString.charCodeAt(i);
  }
  return new Blob([ia], {type: mimeString});
}