camcap.cpp

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

int main()
{
    const char * const window = "Capture (Press ESC to exit)";
    const double scale = 0.75;
    const int delay = 32;   // [msec]

    cv::VideoCapture cap(0);
    if (!cap.isOpened()) {
        std::cerr << "fail to open cv::VideoCapture" << std::endl;
        return 2; 
    }

    const double width = cap.get(cv::CAP_PROP_FRAME_WIDTH) * scale;
    const double height = cap.get(cv::CAP_PROP_FRAME_HEIGHT) * scale;
    cap.set(cv::CAP_PROP_FRAME_WIDTH, width);
    cap.set(cv::CAP_PROP_FRAME_HEIGHT, height);
    std::cout << "frame=" << width << 'x' << height << std::endl;

    cv::namedWindow(window);
    cv::Mat frame;
    do {
        cap >> frame;
        cv::imshow(window, frame);
    } while (cv::waitKey(delay) != '\x1b');

    return 0;
}

$ brew install science/opencv3
...
$ clang++ -std=c++14 -W -Wall -O2 -o camcap camcap.cpp -I/usr/local/opt/opencv3/include -L/usr/local/opt/opencv3/lib -lopencv_core -lopencv_highgui -lopencv_videoio
$ ./camcap