Android imread返回空矩阵，而ifstream可以打开并读取相同的文件

我正在尝试从本机代码部分读取图像。 Java part I用于从资源中提取文件：

String unpackFileFromResources(int resID, String subdir, String fname)
    {
        String full_path = "";
        try
        {
            // load cascade file from application resources
            InputStream is = getResources().openRawResource(resID);

            File cascadeDir = new File(Environment.getExternalStorageDirectory(), subdir);
            if (!cascadeDir.exists())
            {
                cascadeDir.mkdirs();
            }
            mCascadeFile = new File(cascadeDir, fname);
            FileOutputStream os = new FileOutputStream(mCascadeFile);
            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = is.read(buffer)) != -1)
            {
                os.write(buffer, 0, bytesRead);
            }
            is.close();
            os.close();
            full_path = mCascadeFile.getAbsolutePath();
        } catch (IOException e)
        {
            e.printStackTrace();
            Log.e(TAG, "Failed to load cascade. Exception thrown: " + e);
        }
        return full_path;
    }

OpenCV加载程序回调：

private BaseLoaderCallback mLoaderCallback = new BaseLoaderCallback(this)
    {
        @Override
        public void onManagerConnected(int status)
        {
            switch (status)
            {
            case LoaderCallbackInterface.SUCCESS:
            {
                Log.i(TAG, "OpenCV loaded successfully");

                // Load native library after(!) OpenCV initialization
                System.loadLibrary("mixed_sample");
                String path = unpackFileFromResources(R.raw.lena, "cascade", "lena.jpg");
                ReadFile(path);
            }
                break;
            default:
            {
                super.onManagerConnected(status);
            }
                break;
            }
        }
    };

和本地部分：

#define LOG_TAG "MyLog"
#define LOGD(...) ((void)__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__))

JNIEXPORT void JNICALL Java_com_smorodov_opencv_MainActivity_ReadFile(JNIEnv* jenv, jobject, jstring jFileName)
    {
    const char* jnamestr = jenv->GetStringUTFChars(jFileName, NULL);
    string stdFileName(jnamestr);

     // This works perfect
     ifstream fl(jnamestr);
     fl.seekg(0, ios::end );
     size_t len = fl.tellg();
     char *ret = new char[len];
     fl.seekg(0, ios::beg);
     fl.read(ret, len);
     fl.close();
     delete[] ret;

     // This returns empty matrix
     Mat m=imread(stdFileName,0);

    LOGD("%s",stdFileName.c_str());
    LOGD("fl.read %d bytes",len);
    LOGD("m.empty() = %d",m.empty());
    }

当我运行此代码时，我会得到以下输出：

/存储/仿真/0/cascade/lena.jpg 05-08 16:01:57.721：

D/MyLog（27433）：fl.read 91814字节05-08 16:01:57.721：

D/MyLog（27433）：m.empty（）=1

---

问题是：是否可以从本机代码中读取image/Mat/cascade。

imdecode来自ifstream的字节，看看它是否损坏？谢谢您的回复。它成功地解码了。但我需要使工作标准化opencv的文件IO方法。