app使用canvas绘制，导出jpg格式图片全黑

app使用canvas绘制，导出jpg格式图片全黑。

canvasToPath(width , height) {  
      const quality = getQuality(this.imgSize)  
      uni.canvasToTempFilePath(  
        {  
          canvasId: this.getCurrentUseCanvasInfo.canvasId,  
          quality: quality,  
          width:width ,  
          height:height,  
          destWidth: width,  
          destHeight: height,  
          fileType:'jpg',  
          success: (res)=> {  

            const tempFilePath = res.tempFilePath;  
            this.uploadImg(tempFilePath)  
          },  
          fail: (err) => {  

            console.log(err);  
          },  
        },  
      );  
    },