如何解决OpenCV C++代码中cv::face::LBPHFaceRecognizer的链接错误？

问题背景 #

编写了一个用于处理人脸图像目录的OpenCV C++训练器代码，运行环境为OpenCV 4.2.0 + OpenCV contrib 4.2.0，使用最新版Visual Studio。

代码实现 #

#include <iostream> 
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
#include <filesystem>
#include <opencv2/opencv.hpp>
#include <opencv2/face.hpp>
#include <opencv2/face/facerec.hpp>
using namespace cv;
using namespace cv::face;
using namespace std;
namespace fs = std::filesystem;

void assurePathExists(const string& path) {
    fs::create_directories(fs::path(path).parent_path());
}

int main() {
    Ptr<LBPHFaceRecognizer> recognizer = LBPHFaceRecognizer::create();
    CascadeClassifier detector;
    detector.load("C:/Users/781932/Downloads/haarcascade_frontalface_default.xml");

    string datasetPath = "C:/Users/781932/Documents/STEMVisageVision";

    vector<Mat> faceSamples;
    vector<int> ids;

    vector<string> imagePaths;
    for (const auto& entry : fs::directory_iterator(datasetPath)) {
        if (entry.is_regular_file()) {
            imagePaths.push_back(entry.path().string());
        }
    }

    for (const auto& imagePath : imagePaths) {
        Mat img = imread(imagePath, IMREAD_GRAYSCALE);
        int id = stoi(fs::path(imagePath).stem().string().substr(1));

        vector<Rect> faces;
        detector.detectMultiScale(img, faces);

        for (const auto& face : faces) {
            faceSamples.push_back(img(face));
            ids.push_back(id);
        }
    }

    recognizer->train(faceSamples, ids);

    string trainerPath = "trainer/trainer.yml";
    assurePathExists(trainerPath);
    recognizer->write(trainerPath);

    return 0;
}

编译错误 #

清理并重建解决方案后，代码无语法错误，但触发链接错误：

Build started...
1>------ Build started: Project: 5_30_Trainer, Configuration: Debug x64 ------
1>5_30_Trainer.obj : error LNK2019: **unresolved external symbol "public: static struct cv::Ptr<class cv::face::LBPHFaceRecognizer> __cdecl cv::face::LBPHFaceRecognizer::create(int,int,int,int,double)" (?create@LBPHFaceRecognizer@face@cv@@SA?AU?$Ptr@VLBPHFaceRecognizer@face@cv@@@3@HHHHN@Z) referenced in function main**
1>C:\Users\781932\source\repos\5_30_Trainer\x64\Debug\5_30_Trainer.exe : fatal error LNK1120: 1 unresolved externals
1>Done building project "5_30_Trainer.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
========== Build started at 9:21 AM and took 02.605 seconds ==========

问题原因及解决方向 #

该错误属于未解析外部符号，说明链接器无法找到LBPHFaceRecognizer::create的实现，可从以下几点排查：

• 未添加contrib face模块依赖库：OpenCV 4.2.0的contrib face模块，Debug版本库文件为opencv_face420d.lib，Release版本为opencv_face420.lib，需在项目属性的链接器->输入->附加依赖项中添加对应编译模式的库文件。
• 编译模式/架构不匹配：确保项目的编译模式（Debug/Release）、平台架构（x64/x86）与本地OpenCV及contrib库的编译参数完全一致，比如Debug模式不能链接Release版本的库。
• contrib模块未正确编译：若自行编译OpenCV与contrib，需确认face模块编译成功并生成了对应库文件；若使用预编译包，需下载包含contrib组件的完整版本。
• 包含/库目录配置不全：检查项目属性的VC++目录->包含目录是否包含OpenCV contrib的include文件夹，库目录是否包含contrib库文件所在路径。

内容的提问来源于stack exchange，提问作者Francis RE