canvas drawImage的图片，上传服务器格式问题解决方案.

rotateImg () {
  if (this.curImageUrl) {
    this.rotateNum++;
 let degree = 0;
 let rotateInput = this.rotateNum * 90;
 degree += parseInt(rotateInput);
 degree %= 360;
 let img = new Image();
 img.setAttribute("crossOrigin",'Anonymous');
 img.src = this.curImageUrl;
 img.onload = () => {
      let $w = img.width, $h = img.height, $c = document.getElementById("uploadImageCanvas");
 let ctx = $c.getContext("2d");
 if (degree === 90) {
        $c.width = $h;
 $c.height = $w;
 ctx.clearRect(0, 0, $h, $w);
 ctx.rotate(degree / 180 * Math.PI);
 ctx.translate(0, -$h);
 ctx.drawImage(img, 0, 0);
 } else if (degree === 270) {
        $c.width = $h;
 $c.height = $w;
 ctx.clearRect(0, 0, $h, $w);
 ctx.rotate(degree / 180 * Math.PI);
 ctx.translate(-$w, 0);
 ctx.drawImage(img, 0, 0);
 } else {
        $c.width = $w;
 $c.height = $h;
 ctx.clearRect(0, 0, $w, $h);
 ctx.translate($w / 2, $h / 2);
 ctx.rotate(degree / 180 * Math.PI);
 ctx.translate(-$w / 2, -$h / 2);
 ctx.drawImage(img, $w / 2 - $w / 2, $h / 2 - $h / 2);
 }
      $c.toBlob((blob) => {
        let fd = new FormData();
 let fileBlob = new File([blob], new Date().getTime() + '.png')
        fd.append("file", fileBlob);
 handUpload(fd).then(res => {
          console.log(res)
        })
      });
 }
  } else {
    this.rotateNum = 0;
 }
}

实例是，操作canvas上图片旋转的。