C++ 编译并运行一个最简单的openCV 程序（纯命令行）

前言

1. 在进入这篇文章之前应该学会：C++ hello world编译过程
2. 本文代码来自：Lab 1 - OpenCV 的安装和使用

代码

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

int main(int argc, char* argv[]) {
    Mat image = imread("opencv-logo.png"); // 载入名为 "opencv-logo.png" 的图片
    namedWindow("hello"); // 创建一个标题为 "hello" 的窗口
    imshow("hello", image); // 在窗口 "hello" 中显示图片
    waitKey(0); // 等待用户按下键盘
    destroyWindow("hello"); // 销毁窗口 "hello"
    return 0;
}

macOS安装 openCV

• brew install opencv, 安装在/usr/local/Cellar/opencv 目录下
• 同时建议安装 brew install pkg-config
• 参考 https://www.geeksforgeeks.org/how-to-install-opencv-for-c-on-macos/

错误与解决方法

1. g++ main.cpp -o main.out -I /usr/local/Cellar/opencv/4.5.4_4/include/opencv4
   出现错误 error: no template named 'initializer_list' in namespace 'std' error: "OpenCV 4.x+ requires enabled C++11 support"
   解决方法：g++ -std=c++11 main.cpp -o main.out -I /usr/local/Cellar/opencv/4.5.4_4/include/opencv4
2. 以上命令出现错误：

   Undefined symbols for architecture x86_64:
     "cv::namedWindow(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int)", referenced from:
   	  _main in main-1d11bd.o
     "cv::destroyWindow(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
   	  _main in main-1d11bd.o
     "cv::Mat::~Mat()", referenced from:
   	  _main in main-1d11bd.o
     "cv::imread(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int)", referenced from:
   	  _main in main-1d11bd.o
     "cv::imshow(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, cv::_InputArray const&)", referenced from:
   	  _main in main-1d11bd.o
     "cv::waitKey(int)", referenced from:
   	  _main in main-1d11bd.o
   ld: symbol(s) not found for architecture x86_64
   clang: error: linker command failed with exit code 1 (use -v to see invocation)
   

   解决方法：
   g++ -std=c++11 main.cpp -o main.out -I /usr/local/Cellar/opencv/4.5.4_4/include/opencv4 -L /usr/local/Cellar/opencv/4.5.4_4/lib -lopencv_highgui -lopencv_imgcodecs -lopencv_core
   分析：
   • 出错是因为只用-I给出了头文件的包括路径，但没有告诉链接器应该链接的库文件的位置(-L)和名称(-l，-labc指的是链接libabc.a)。
   • 如果只用-I给出包括路径，可以正确生成机器代码（即完成编译四步的前三步）（使用-c选项）: g++ -std=c++11 -c main.cpp -I /usr/local/Cellar/opencv/4.5.4_4/include/opencv4 -L /usr/local/Cellar/opencv/4.5.4_4/lib
   • 直接找到链接器缺失的库，比如此处，库opencv_highgui 包含了cv::namedWindow, cv::destroyWindow, cv::imshow, cv::waitKey， 库opencv_imgcodecs包含了 cv::imread， 库opencv_core 包含了cv::Mat::~Mat。

pkg-config

一个个找对应的库很麻烦，一般直接告诉编译器 openCV 的所有库的名称和位置，这可以用pkg-config 实现。

• g++ $(pkg-config --cflags --libs opencv) test.cpp -o Test
• pkg-config --cflags --libs opencv4 的输出：

-I/usr/local/opt/opencv/include/opencv4 -L/usr/local/opt/opencv/lib -lopencv_gapi -lopencv_stitching -lopencv_alphamat -lopencv_aruco -lopencv_barcode -lopencv_bgsegm -lopencv_bioinspired -lopencv_ccalib -lopencv_dnn_objdetect -lopencv_dnn_superres -lopencv_dpm -lopencv_face -lopencv_freetype -lopencv_fuzzy -lopencv_hfs -lopencv_img_hash -lopencv_intensity_transform -lopencv_line_descriptor -lopencv_mcc -lopencv_quality -lopencv_rapid -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_sfm -lopencv_stereo -lopencv_structured_light -lopencv_phase_unwrapping -lopencv_superres -lopencv_optflow -lopencv_surface_matching -lopencv_tracking -lopencv_highgui -lopencv_datasets -lopencv_text -lopencv_plot -lopencv_videostab -lopencv_videoio -lopencv_viz -lopencv_wechat_qrcode -lopencv_xfeatures2d -lopencv_shape -lopencv_ml -lopencv_ximgproc -lopencv_video -lopencv_xobjdetect -lopencv_objdetect -lopencv_calib3d -lopencv_imgcodecs -lopencv_features2d -lopencv_dnn -lopencv_flann -lopencv_xphoto -lopencv_photo -lopencv_imgproc -lopencv_core

• 参考： https://stackoverflow.com/a/24985811