[opencv introduction to mastery 9] opencv video capture, image and video conversion

One 、 The relationship between pictures and videos

The picture is still , Put many pictures with gradual changes , Organized by time , That is to say, video .
The video is linked , Pause every moment in the video , Then the picture is displayed .

1. Video frame
   Every instant picture in the video pauses, and the picture is the video frame

2. Video frame rate
   Frames per second (Frames per Second, Jane ：FPS）, This word is often used in film and television production and video games . Because of the special physiological structure of human eyes , If the frame rate of the picture is higher than 16 When , It would be considered coherent , This phenomenon is called visual staying . That's why film is shot one by one , Then quickly play .
   For the game , First person shooters pay more attention to FPS High and low , If FPS<30 Words , The game will appear incoherent .30fps Is acceptable , But improve performance to 60fps It can obviously enhance the sense of interaction and realism , But generally speaking, more than 75fps Generally, it is not easy to notice that there is a significant increase in fluency . If the frame rate exceeds the screen refresh rate, it will only waste graphics processing power , Because the monitor can't update so fast , In this way, the frame rate exceeding the refresh rate is wasted .

Two 、 Video and picture conversion

1. Video capture

There are several steps for video capture ：
Video on
Read video frames
choice

	// Open the video  
    VideoCapture captrue = VideoCapture("test.mp4");
    // Open the failure  
    if (!captrue.isOpened())
    {
    
        return 1;
    }

    int rate = captrue.get(cv::CAP_PROP_FPS);
    int frameCount = captrue.get(cv::CAP_PROP_FRAME_COUNT);
    int frameWidth = captrue.get(cv::CAP_PROP_FRAME_WIDTH);
    int framehHeight = captrue.get(cv::CAP_PROP_FRAME_HEIGHT);
    int format = captrue.get(cv::CAP_PROP_FORMAT);
    int video_duration = frameCount / rate;

    cout << "---------------------------------------------" << endl;
    cout << " Pixel width of video frame :" << frameWidth << endl;
    cout << " Pixel height of video frame :" << framehHeight << endl;
    cout << " Frame rate of recorded video ( frame / second ):" << rate << endl;
    cout << " Total frames of video files :" << frameCount << endl;
    cout << " The format of the image :" << format << endl;
    cout << " Video duration :" << video_duration << endl;
    cout << "---------------------------------------------" << endl;


    namedWindow("Video", WINDOW_NORMAL);
    resizeWindow("Video", 480, 270);

    // Video write object  
    cv::VideoWriter write;
    // Write the video file name  
    std::string outFlie = "videoTest.mp4";
    // Open video file , Prepare to write 
    //write.open(outFlie, -1, rate, Size(frameWidth, framehHeight), true);
    write.open(outFlie, CV_FOURCC('D', 'I', 'V', 'X'), rate, Size(frameWidth, framehHeight), true);
    
    /* from 6700 Frame start reading */
    bool stop = false;
    cv::Mat frame;
    captrue.set(CV_CAP_PROP_POS_FRAMES, 6700);      // Jump to the 6700 frame 
    
    // Loop read video frames 
    while (!stop)
    {
    
        // Read frame  
        if (!captrue.read(frame))
            break;
        cv::imshow("Video", frame);
        // write file  
        write.write(frame);
        if (cv::waitKey(10) > 0)
        {
    
            stop = true;
        }
    }
    // Release object  
    captrue.release();
    write.release();

2. Video read pictures

	VideoCapture captrue = VideoCapture(filename);
    if (!captrue.isOpened()) {
    
        return 1;
    }

    int picNum = 0;
    char picName[32] = "";
    cv::Mat frame;
    while (1)
    {
    
        if (!captrue.read(frame))
            break;
        picNum++;
        cout << "picNum:" << picNum << endl;
        memset(picName, 0, sizeof(picName));
        sprintf(picName, "pic_%d.jpg", picNum);
        imwrite(picName, frame);
    }

3. Picture synthesis video

	int idx = 0;
    char picName[64];

    for (idx = 1; idx < 100; idx++)
    {
    
        sprintf(picName, "pic%d_out.jpg", idx);
        cout << "read picture:" << picName << endl;
        Mat image = imread(string(picName));
        if (image.empty())
        {
    
            cout << "read picture failed!" << endl;
            continue;
        }

        static VideoWriter saveVideo("output2.avi", CV_FOURCC('M', 'J', 'P', 'G'), 25, Size(image.cols, image.rows));
        saveVideo << image;

        waitKey(50);
    }