OpenCV -- ffmpeg 视频输入输出VideoCapture和VideoWriter的使用

opencv依赖了ffmpeg，所以可以轻松对avi视频文件进行操作。

打开视频文件或摄像头视频需要使用Opencv中的VideoCapture类，保存视频或摄像头视频到本地磁盘，需要使用Opencv中的VideoWriter类。

代码：

bool isInit= false;
VideoCapture *inputVideo=NULL;
VideoWriter *outputVideo=NULL;

JNIEXPORT void JNICALL
Java_org_opencv_samples_tutorial2_Tutorial2Activity_FindFeatures(JNIEnv *, jobject, jlong addrGray,
                                                                 jlong addrRgba) {
    Mat &mGr = *(Mat *) addrGray;
    Mat &mRgb = *(Mat *) addrRgba;
    vector<KeyPoint> v;

    looperAddNum++;
    if (looperAddNum > 15) {
        looperAddNum = 0;
        if (looperIndexNum < 100)
            looperIndexNum += 1;
    }



    // code start-----------------------------------------------------------------------------

    if (!isInit){
        LOGI("index start");
        const string source      = "/data/data/org.opencv.samples.tutorial2/cache/Megamind.avi";           // the source file name
        inputVideo=new VideoCapture(source.c_str());
        if (!inputVideo->isOpened()){
            LOGI("open video error");
            return;
        }
        Mat lsFrame;
        *inputVideo>> lsFrame;
        if (lsFrame.empty())
            return;
        string::size_type pAt = source.find_last_of('.');                  // Find extension point
        const string NAME = source.substr(0, pAt) + "R" + ".avi";   // Form the new name with container
        LOGI("index looper %d %d", lsFrame.cols, lsFrame.rows);
        outputVideo=new VideoWriter();
        outputVideo->open(NAME, VideoWriter::fourcc('M', 'J', 'P', 'G'), 25, Size(lsFrame.cols, lsFrame.rows), true);
        if (!outputVideo->isOpened())
        {
            LOGI("index open video error %s", NAME.c_str());
            return;
        }
        isInit= true;
    }




    if (isInit){
        Mat videoFrame;
        *inputVideo >> videoFrame;
        if (videoFrame.empty())
            return;
        // show video
        Mat dstW = mRgb(Rect(0,0, videoFrame.cols, videoFrame.rows));
        cvtColor(videoFrame, dstW, COLOR_RGB2BGRA);

        int channel = 2; // Select the channel to save
        Mat res;
        vector<Mat> spl;
        split(videoFrame, spl);                // process - extract only the correct channel
        for (int i =0; i < 3; ++i)
            if (i != channel)
                spl[i] = Mat::zeros(Size(videoFrame.cols, videoFrame.rows), spl[0].type());
        merge(spl, res);
        *outputVideo << res;
    }


    // code end-----------------------------------------------------------------------------
}

代码说明：
1、使用VideoCapture读内容文件获取视频信息。
2、初始化输出视频VideoWriter。
3、开始读取mat 写入 VideoWriter。
4、同时将其中两个通道数据设置为0（黑色）。

编解码器标识说明：
CV_FOURCC(‘P’, ‘I’, ‘M’, ‘1’) = MPEG-1 codec
CV_FOURCC(‘M’, ‘J’, ‘P’, ‘G’) = motion-jpeg codec
CV_FOURCC(‘M’, ‘P’, ‘4’, ‘2’) = MPEG-4.2 codec
CV_FOURCC(‘D’, ‘I’, ‘V’, ‘3’) = MPEG-4.3 codec
CV_FOURCC(‘D’, ‘I’, ‘V’, ‘X’) = MPEG-4 codec
CV_FOURCC(‘U’, ‘2’, ‘6’, ‘3’) = H263 codec
CV_FOURCC(‘I’, ‘2’, ‘6’, ‘3’) = H263I codec
CV_FOURCC(‘F’, ‘L’, ‘V’, ‘1’) = FLV1 codec