0. 图像读取与显示

示例1，来自learning OpenCV 3的配套源码
https://github.com/oreillymedia/Learning-OpenCV-3_examples/blob/master/example_02-01.cpp

//Example 2-1. A simple OpenCV program that loads an image from disk and displays it
//on the screen
#include <opencv2/opencv.hpp>void help(char** argv ) {std::cout << "\n"<< "A simple OpenCV program that loads and displays an image from disk\n"<< argv[0] <<" <path/filename>\n"<< "For example:\n"<< argv[0] << " ../fruits.jpg\n"<< std::endl;
}int main( int argc, char** argv ) {if (argc != 2) {help(argv);return 0;}cv::Mat img = cv::imread( argv[1], -1 );if( img.empty() ) return -1;cv::namedWindow( "Example 2-1", cv::WINDOW_AUTOSIZE );cv::imshow( "Example 2-1", img );cv::waitKey( 0 );cv::destroyWindow( "Example 2-1" );return 0;
}

1. 视频文件读取与操作

1.1 示例代码

示例1：
https://github.com/opencv/opencv/blob/4.x/samples/cpp/videocapture_starter.cpp

示例2:
来自cv::VideoCapture Class Reference的Detailed Description

如果觉得上面那个写的比较复杂，不直观，可以看看简单的：

#include <iostream>
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/videoio.hpp>using namespace cv;
int main(int argc, char **argv)
{Mat frame;// 初始化一个VIDEOCAPTURE对象capVideoCapture cap;// 使用默认的API打开默认的相机// cap.open(0);// 或者更先进的用法，选择任意一个API后端int deviceID = 0;        // 0 = open default cameraint apiID = cv::CAP_ANY; // 0 = autodetect default API// 使用选择的API打开选择的相机cap.open(deviceID, apiID);// 判断是否正确打开视频流if (!cap.isOpened()){std::cerr << "ERROR! Unable to open camera\n";return -1;}// 开始读取视频std::cout << "Start grabbing" << std::endl<< "Press any key to terminate" << std::endl;for (;;){// 等待从视频读入新帧并把它存储到'frame'中cap.read(frame);// 检查是否读取成功if (frame.empty()){std::cerr << "ERROR! blank frame grabbed\n";break;}// 实时显示，每5ms显示一帧图像直到有任意按键imshow("Live", frame);if (waitKey(5) >= 0)break;}// 相机对象会在VideoCapture析构函数中自动释放return 0;
}

示例3:
来自learning OpenCV 3的配套源码
https://github.com/oreillymedia/Learning-OpenCV-3_examples/blob/master/example_02-03.cpp

int main(int argc, char **argv)
{cv::namedWindow("Example2");cv::VideoCapture cap;// VideoCapture可以打开和关闭很多ffmpeg支持的视频文件cap.open(std::string(argv[1]));cv::Mat frame;if (cap.isOpened()){for (;;){cap >> frame;// Stream operator to read the next video frameif (frame.empty())break;cv::imshow