Install GStreamer 1.18 on Raspberry Pi 4.

						
						#include <opencv2/opencv.hpp>
						
						std::string gstreamer_pipeline(int device, int capture_width, int capture_height, int framerate, int display_width, int display_height) {
						    return
						            " v4l2src device=/dev/video"+ std::to_string(device) + " !"
						            " video/x-raw,"
						            " width=(int)" + std::to_string(capture_width) + ","
						            " height=(int)" + std::to_string(capture_height) + ","
						            " framerate=(fraction)" + std::to_string(framerate) +"/1 !"
						            " videoconvert ! videoscale !"
						            " video/x-raw,"
						            " width=(int)" + std::to_string(display_width) + ","
						            " height=(int)" + std::to_string(display_height) + " ! appsink";
						}
						////////////
						int main()
						{
						    //pipeline parameters
						    int capture_width = 1280 ;
						    int capture_height = 720 ;
						    int display_width = 640 ;
						    int display_height = 360 ;
						    int framerate = 30 ;
						
						    std::string pipeline = gstreamer_pipeline(0,capture_width, capture_height,
						                                              display_width, display_height, framerate);
						    
						    cv::VideoCapture cap(pipeline, cv::CAP_GSTREAMER);
						    if(!cap.isOpened()) {
						        std::cout << "Failed to open camera." << std::endl;
						        return (-1);
						    }
						
						
						    
						

						
						#include <opencv2/opencv.hpp>
						#include "RTSPcam.h"
						
						using namespace std;
						
						int main()
						{
						    cv::Mat frame;
						    RTSPcam cam;
						
						    cv::namedWindow("Camera", cv::WINDOW_AUTOSIZE);
						
						//    cam.Open(0);                                 //RaspiCam
						//    cam.Open(1);                                 //Webcam
						    cam.Open("rtsp://192.168.178.129:8554/test/"); //RTSP you can dump anything OpenCV eats.
						//    cam.Open("udpsrc port=5200 ! application/x-rtp, media=video, clock-rate=90000, payload=96 ! rtpjpegdepay ! jpegdec ! videoconvert ! appsink", cv::CAP_GSTREAMER);  //UDP  
						//    cam.Open("tcpclientsrc host=192.168.178.129 port=5000 ! jpegdec ! videoconvert ! appsink", cv::CAP_GSTREAMER); //TCP
						//    cam.Open("James.mp4");                       //movie    
						//    cam.Open("/home/pi/Pictures/plant.jpg");     //picture
						//    cam.Open("/home/pi/Pictures/garden");        //folder
						
						    while(true)
						    {
						        if(!cam.GetLatestFrame(frame)){
						            cout << "Capture read error" << endl;
						            break;
						        }
						        ///place here your time consuming algorithms
						
						        cv::imshow("Camera",frame);
						        char esc = cv::waitKey(2);
						        if(esc == 27) break;
						    }
						    cv::destroyAllWindows() ;
						    return 0;
						}