wx.getImageInfo({
          src: photo.tempFilePaths[0],
          success: function (res) {
            var ctx = wx.createCanvasContext('photo_canvas');
            var ratio = 2;
            var canvasWidth = res.width;
            var canvasHeight = res.height;
            // 保证宽高均在200以内
            while (canvasWidth > 300 || canvasHeight > 300) {
              //比例取整
              canvasWidth = Math.trunc(res.width / ratio)
              canvasHeight = Math.trunc(res.height / ratio)
              ratio++;
            }
            that.setData({
              canvasWidth: canvasWidth,
              canvasHeight: canvasHeight
            })//设置canvas尺寸
            ctx.drawImage(photo.tempFilePaths[0], 0, 0, canvasWidth, canvasHeight)
            ctx.draw()
            //下载canvas图片
            setTimeout(function () {
              wx.canvasToTempFilePath({
                canvasId: 'photo_canvas',
                success: function (res) {
                  console.log(res.tempFilePath)
                  filePath = res.tempFilePath;
                  that.setData({
                    invoice: filePath
                  });

                },
                fail: function (error) {
                  console.log(error)
                }
              })
            }, 100)
          },
          fail: function (error) {
            console.log(error)
          }
        })