HarmonyOS 怎么通过createPixelMap将本地的资源文件png转换为PixelMap，用于消息推送设置smallicon？

async getPixelMapByImage():Promise<image.PixelMap>{ 
    const context : Context = getContext(this); 
    // 获取resourceManager资源管理器 
    const resourceMgr : resourceManager.ResourceManager = context.resourceManager; 
    let rawFileDescriptor = await resourceMgr.getRawFd('food.png') 
    const imageSource : image.ImageSource = image.createImageSource(rawFileDescriptor); 
    let decodingOptions : image.DecodingOptions = { 
      editable: true, 
      desiredPixelFormat: 3, 
    } 
    // 创建pixelMap并进行简单的旋转和缩放 
    const pixelMap:image.PixelMap = await imageSource.createPixelMap(decodingOptions) 
    this.pixelMap =  pixelMap 
    this.imageInfo = await pixelMap.getImageInfo() 
    return pixelMap; 
  }