C# 中兴图书馆可以'；t解码彩色照片[WP8]

我想解码画廊照片中的二维码。如果我选择了一张只包含二维码的照片，它会工作，但是如果它是一张包含二维码的普通照片，结果是空的。我做错了什么

                BitmapImage bitmap = new BitmapImage();
            bitmap.SetSource(e.ChosenPhoto);
            WriteableBitmap wb = new WriteableBitmap(bitmap);

            BarcodeReader reader = new BarcodeReader {AutoRotate = true, TryHarder = true};
            Result result = reader.Decode(wb);

ps.我试图用这种方式解码亮度源：

            var writeableBmp = new WriteableBitmap(1, 1);
            writeableBmp.SetSource(e.ChosenPhoto);

            writeableBmp = new WriteableBitmap(writeableBmp.PixelWidth, writeableBmp.PixelHeight);
            e.ChosenPhoto.Position = 0;
            writeableBmp.SetSource(e.ChosenPhoto);
            LuminanceSource ls;
            byte[] buffer = new byte[writeableBmp.PixelHeight * writeableBmp.PixelWidth];
            using (MemoryStream ms = new MemoryStream())
            {
                int read;
                while ((read = e.ChosenPhoto.Read(buffer, 0, buffer.Length)) > 0)
                {
                    ms.Write(buffer, 0, read);
                }

                ls = new RGBLuminanceSource(ms.ToArray(), writeableBmp.PixelWidth, writeableBmp.PixelHeight,
                    RGBLuminanceSource.BitmapFormat.Gray8);

            }

但如果我这样做了，它甚至不能读取以前工作过的二维码