[OpenCV] line segment detector example

#include "lsd.h"

#include <opencv2/videoio.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/core.hpp>
#include <opencv2/imgproc.hpp>

#include <string>
#include <iostream>
#include <thread>
#include <chrono>

template<typename T>
void initWebcam(cv::VideoCapture& videoCapture, T cameraNumber)
{
    // Get access to the default camera.
    try {   // Surround the OpenCV call by a try/catch block so we can give a useful error message!
        videoCapture.open(cameraNumber);
    }
    catch (cv::Exception& e) {
        std::cerr << "ERROR: Could not access the camera!" << std::endl;
        exit(1);
    }
    if (!videoCapture.isOpened()) {
        std::cerr << "ERROR: Could not access the camera!" << std::endl;
        exit(1);
    }
    std::cout << "Loaded camera " << cameraNumber << "." << std::endl;
}

int main() {
    cv::Mat frame;
    cv::VideoCapture cap;
    initWebcam(cap, "up1.mp4");
    std::this_thread::sleep_for(std::chrono::seconds(1));

    cap >> frame;
    while (!frame.empty()) {
        cv::Mat tmp, src_gray;
        frame.copyTo(src_gray);
        cv::cvtColor(frame, tmp, cv::COLOR_RGB2GRAY);
        tmp.convertTo(src_gray, CV_64FC1);
        int cols = src_gray.cols;
        int rows = src_gray.rows;

        image_double image = new_image_double(cols, rows);
        image->data = src_gray.ptr<double>(0);
        ntuple_list ntl = lsd(image);

        cv::Mat lsd = cv::Mat::zeros(rows, cols, CV_8UC1);
        cv::Point pt1, pt2;
        for (int j = 0; j != ntl->size; ++j) {
            pt1.x = ntl->values[0 + j * ntl->dim];
            pt1.y = ntl->values[1 + j * ntl->dim];
            pt2.x = ntl->values[2 + j * ntl->dim];
            pt2.y = ntl->values[3 + j * ntl->dim];
            cv::line(lsd, pt1, pt2, cv::Scalar(255), 1);
        }

        free_ntuple_list(ntl);
        cv::imshow("Result", frame);
        cv::namedWindow("lsd", cv::WINDOW_AUTOSIZE);
        cv::imshow("lsd", lsd);
        if (cv::waitKey(30) >= 0)
            break;

        cap >> frame;
    }

    return 0;
}