public async Task WriteToFileAsync(StorageFolder folder,SoftwareBitmap sb,string fileName)
        {
           
            if (sb != null)
            {
                // save image file to cache
                StorageFile file = await folder.CreateFileAsync(fileName, CreationCollisionOption.OpenIfExists);
                using (IRandomAccessStream stream = await file.OpenAsync(FileAccessMode.ReadWrite))
                {
                    BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, stream);
                    encoder.SetSoftwareBitmap(sb);
                    await encoder.FlushAsync();
                }
            }
        }