vue图片压缩

	// 处理图片
    imgPreview(file) {
      let self = this;
      // 看支持不支持FileReader
      if (!file || !window.FileReader) return;
      if (/^image/.test(file.type)) {
        // 创建一个reader
        let reader = new FileReader();
        // 将图片2将转成 base64 格式
        reader.readAsDataURL(file);
        // 读取成功后的回调
        reader.onloadend = function () {
          let result = this.result;
          let img = new Image();
          img.src = result;
          //判断图片是否大于500K,是就直接上传，反之压缩图片
          if (this.result.length <= 500 * 1024) {
            // 上传图片
            self.postImg(this.result);
          } else {
            img.onload = function () {
              let data = self.compress(img);
              // 上传图片
              self.postImg(data);
            };
          }
        };
      }
    },
    // 压缩图片
    compress(img) {
      let canvas = document.createElement("canvas");
      let ctx = canvas.getContext("2d");
      //瓦片canvas
      let tCanvas = document.createElement("canvas");
      let tctx = tCanvas.getContext("2d");
      // let initSize = img.src.length;
      let width = img.width;
      let height = img.height;
      //如果图片大于四百万像素，计算压缩比并将大小压至400万以下
      let ratio;
      if ((ratio = (width * height) / 4000000) > 1) {
        // console.log("大于400万像素");
        ratio = Math.sqrt(ratio);
        width /= ratio;
        height /= ratio;
      } else {
        ratio = 1;
      }
      canvas.width = width;
      canvas.height = height;
      //    铺底色
      ctx.fillStyle = "#fff";
      ctx.fillRect(0, 0, canvas.width, canvas.height);
      //如果图片像素大于100万则使用瓦片绘制
      let count;
      if ((count = (width * height) / 1000000) > 1) {
        // console.log("超过100W像素");
        count = ~~(Math.sqrt(count) + 1); //计算要分成多少块瓦片
        //      计算每块瓦片的宽和高
        let nw = ~~(width / count);
        let nh = ~~(height / count);
        tCanvas.width = nw;
        tCanvas.height = nh;
        for (let i = 0; i < count; i++) {
          for (let j = 0; j < count; j++) {
            tctx.drawImage(
              img,
              i * nw * ratio,
              j * nh * ratio,
              nw * ratio,
              nh * ratio,
              0,
              0,
              nw,
              nh
            );
            ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
          }
        }
      } else {
        ctx.drawImage(img, 0, 0, width, height);
      }

      //进行最小压缩
      let ndata = canvas.toDataURL("image/jpeg", 0.2);
      tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;
      // console.log(ndata);
      return ndata;
    },```