Inter-laying a video sequence to another video in OpenCV

Question

How can I add a small video sequence to another video using OpenCV?

To elaborate, let's say I have a video playing which is to be interactive where let's say the user viewing the video gestures something and a short sequence plays at the bottom or at the corner of the existing video.

Answer 1

For each frame, you need to copy an image with the content you need inside the video frame. The steps are:

1. Define the size of the overlay frame
2. Define where to show the overlay frame

3. For each frame

   1. Fill the overlay frame with some content
   2. Copy the overlay frame in the defined position in the original frame.

This small snippet will show a random noise overlay window on bottom right of the camera feed:

#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;


int main()
{
    // Video capture frame
    Mat3b frame;
    // Overlay frame
    Mat3b overlayFrame(100, 200);

    // Init VideoCapture
    VideoCapture cap(0);

    // check if we succeeded
    if (!cap.isOpened()) {
        cerr << "ERROR! Unable to open camera\n";
        return -1;
    }

    // Get video size
    int w = cap.get(CAP_PROP_FRAME_WIDTH);
    int h = cap.get(CAP_PROP_FRAME_HEIGHT);

    // Define where the show the overlay frame 
    Rect roi(w - overlayFrame.cols, h - overlayFrame.rows, overlayFrame.cols, overlayFrame.rows);

    //--- GRAB AND WRITE LOOP
    cout << "Start grabbing" << endl
        << "Press any key to terminate" << endl;
    for (;;)
    {
        // wait for a new frame from camera and store it into 'frame'
        cap.read(frame);

        // Fill overlayFrame with something meaningful (here random noise)
        randu(overlayFrame, Scalar(0, 0, 0), Scalar(256, 256, 256));

        // Overlay
        overlayFrame.copyTo(frame(roi));

        // check if we succeeded
        if (frame.empty()) {
            cerr << "ERROR! blank frame grabbed\n";
            break;
        }
        // show live and wait for a key with timeout long enough to show images
        imshow("Live", frame);
        if (waitKey(5) >= 0)
            break;
    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}