OpenCV (4) Image and Video

1. Read and write a image:

Read a image and convert it to a gray-scale image. Save the gray-scale image.

#include<iostream>
#include<opencv2/opencv.hpp>

using namespace std;
using namespace cv;

int main() {
    Mat gray_img = imread("data/image/mini.jpg", 
                     IMREAD_GRAYSCALE);
    imshow("mini", gray_img);
    waitKey(0);
    destroyAllWindows();
    
    // save the image.
    imwrite("data/image/gray_mini.jpg", gray_img);
    
    return 0;    
}

2. Open a video:

The video is composed of a series of graphics, each one is called frame.

#include<iostream>
#include<opencv2/opencv.hpp>

using namespace std;
using namespace cv;

int main() {
    // init an object of VideoCapture.
    VideoCapture cap("data/video/ggo.mp4");
    
    // make sure that the video is opened successfully.
    if (!cap.isOpened()) {
        cout << "Capture open fail !!" << endl;
    }

    Mat frame;
    // read the video frame by frame.
    // if the frame is not a null frame, display the frame.
    while(cap.read(frame)) {
        imshow("frame", frame);

        // break the while loop if user press esc.
        char key = (char) waitKey(30);
        if (key == 27) {
            break;
        }
    }

    destroyAllWindows();
    return 0;    
}

3. cv::VideoCapture::operator>>():

cv::VideoCapture::operator>>() is similar to std::istream::operator>>().

Mat frame;
cap >> frame;
while(!frame.empty()) {
    imshow("frame", frame);
    char key = (char) waitKey(30);
    if (key == 27) {
        break;
    }
    cap >> frame;
}

4. Use VideoCapture to get the information of the video:

int main() {
    VideoCapture cap("data/video/prince.mp4");
    if (!cap.isOpened()) {
        cout << "Capture open fail !!" << endl;
    }

    cout << "width: " << cap.get(CAP_PROP_FRAME_WIDTH) << endl;
    cout << "height: " << cap.get(CAP_PROP_FRAME_HEIGHT) << endl;
    cout << "FPS: " << cap.get(CAP_PROP_FPS) << endl;
    
    return 0;    
}

Reference

[1] Adrian Kaehler, Gary Bradski, Learning OpenCV 3 , O'Reilly Media (2017)