为什么我保存的图片是白色的，只有白色

wx.canvasToTempFilePath({

          x: 0,

          y: 0,

          width: 600,

          height: 800,

          destWidth: 600,

          destHeight: 800,

          fileType: ‘jpg’,

          canvasId: ‘myCanvas’,

          success: function (res) {

            console.log(res.tempFilePath);

          }

        })