f(i, j) = \left{\begin{array}{cc} 0, & (\text { gray }< = 100) \ 255, & (\text { gray }>100) \end{array}\right.

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

#define DEFAULT_CARD_WIDTH 640
#define DEFAULT_CARD_HEIGHT 400

#define FIX_IDCARD_SIZE Size(DEFAULT_CARD_WIDTH, DEFAULT_CARD_HEIGHT)

#define FIX_TEMPLATE_SIZE Size(153, 28)

using namespace std;
using namespace cv;

int main() {
    std::cout << "Hello, World!" << std::endl;
    Mat src = imread("src.png");
    imshow("src", src);

    //處理身份證
    Mat src_img = src;
    //1、無損壓縮 640*400 （通用卡片類的處理方式）
    resize(src_img, src_img, FIX_IDCARD_SIZE);
    imshow("dst", src_img);
    
    Mat dst_img;
    //2、灰度化
    Mat dst;
    cvtColor(src_img, dst, COLOR_BGR2GRAY);
    imshow("gray", dst);
    
    //3、二值化（降噪）
    threshold(dst, dst, 100, 255, THRESH_BINARY);
    imshow("threshold", dst);
    
    // 4.1 腐蝕、膨脹
    Mat erodeElement = getStructuringElement(MORPH_RECT, Size(20, 10));
    erode(dst, dst, erodeElement);
    imshow("erode", dst);

    //4、輪廓檢測，把所有的連續(xù)的閉包用矩形包起來
    /*
     * 一個(gè)矩形用兩個(gè)點(diǎn)表示，contours就包含了很多矩形
     */
    vector<vector<Point>> contours;
    vector<Rect> rects;
    
    findContours(dst, contours, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0));
    
    for (size_t i = 0; i < contours.size(); i++)
    {
        // 基于兩點(diǎn)構(gòu)建矩形
        Rect rect = boundingRect(contours.at(i));
        // 繪制矩形
        rectangle(dst, rect, Scalar(0, 0, 255));
        imshow("contours", dst);
        // 對符合條件的圖片進(jìn)行篩選，寬高比大于1：9的
        if(rect.width > rect.height*9)
        {
            cout << "找到了" << endl;
            rects.push_back(rect);
            rectangle(dst, rect, Scalar(0, 0, 255));
    
            // 還需要再次矯正
            //dst_img = src_img(rect);
        }
    }
    
    // imshow("dst_Img", dst_img);
    
    // 如果只找到了一個(gè)矩形，說明這個(gè)就是，如果多個(gè)就找出縱坐標(biāo)最低的矩形
    if(rects.size() == 1)
    {
        Rect rect = rects.at(0);
        dst_img = src_img(rect);
    }else
    {
        int lowPoint = 0;
        Rect finalRect;
    
        for (size_t i = 0; i < rects.size(); ++i)
        {
            Rect rect = rects.at(i);
            Point p = rect.tl();
            if(rect.tl().y > lowPoint)
            {
                lowPoint = rect.tl().y;
                finalRect = rect;
            }
        }
    
        rectangle(dst, finalRect, Scalar(255, 255, 0));
        dst_img = src_img(finalRect);
    }
    
    
    imshow("dst_Img", dst_img);
    waitKey();
    return 0;
}

cmake_minimum_required (VERSION 3.8)

project(opencv_idcard)

set(CMAKE_CXX_STANDARD 11)

add_executable (opencv_idcard "opencv_idcard.cpp" )

set(OpenCV_DIR "D:/develop/opencv-4.5.4/opencv-4.5.4-build")

find_package(OpenCV REQUIRED)

include_directories(${OpenCV_INCLUDE_DIRS})

target_link_libraries(opencv_idcard ${OpenCV_LIBS})