如何将指定区域导出为图片？

$("#saveImage").click(function () {
  if (isPrinting) {
    return;
  }
  spread.bind(GC.Spread.Sheets.Events.BeforePrint, function (s, e) {
    var sheet = spread.getSheet(1);
    console.log(sheet.printInfo());
    var iframe = e.iframe;
    var images = iframe.contentWindow.document.getElementsByTagName("img");
    for (var i = 0; i < images.length; i++) {
      var img = images[i];
      console.log(img);
      var canvas = document.createElement("canvas");
      canvas.height = img.naturalHeight;
      canvas.width = img.naturalWidth;
      var ctx = canvas.getContext("2d");
      ctx.fillStyle = "#FFF";
      ctx.fillRect(0, 0, canvas.width, canvas.height);
      ctx.drawImage(img, 0, 0); //, img.width, img.height);
      canvas.toBlob(
        function (blob) {
          saveAs(blob, "print.jpeg");
        },
        "image/jpeg",
        1
      );
    }
    e.cancel = true;
    setTimeout(function () {
      isPrinting = false;
      spread.unbind(GC.Spread.Sheets.Events.BeforePrint);
    }, 10);
  });
  isPrinting = true;
  spread.print();
});

$("#printPreview").click(function () {
  var json = spread.toJSON();
  var jsonStr = JSON.stringify(json);
  var newSpread = new GC.Spread.Sheets.Workbook();
  newSpread.fromJSON(JSON.parse(jsonStr));
  newSpread.bind(GC.Spread.Sheets.Events.BeforePrint, function (s, e) {
    var iframe = e.iframe;
    $("#previewFrame").html(iframe.contentWindow.document.body.innerHTML); //把得到的img存到list //list.push(iframe.contentWindow.document.getElementsByTagName("img")[0].src)
    e.cancel = true;
    setTimeout(function () {
      isPrinting = false;
      newSpread.unbind(GC.Spread.Sheets.Events.BeforePrint);
    }, 100);
  });
  $("#previewFrame").html("加载中！");
  $("#previewContent").show();
  isPrinting = true; //设置sheet的打印规则
  printSetting();
  setTimeout(function () {
    newSpread.print();
  }, 100);
});