导出后正常的图片就变形了

 var canvas = res[0].node;
        var ctx = canvas.getContext('2d');
        var width = this.data.xRight - this.data.xLeft;
        var height = this.data.yBottom - this.data.yTop;
        wx.canvasToTempFilePath({
          x: this.data.xLeft + this.data.maxLeft,
          y: this.data.yTop + this.data.maxTop,
          width: width,
          height: height,
          destWidth: width * 4,
          destHeight: height * 4,
          canvas: canvas,
          success: ((res => { 
            var obj = {
              'url': res.tempFilePath
            };
            if (isSave) {
              this.triggerEvent('run', obj);
              this.setData({
                show: false
              })
            } else {
              wx.previewImage({
                url: res.tempFilePath,
                urls: [res.tempFilePath],
              })
            }
            wx.hideLoading()
          }))
        })