读写xml和yml文件 opencv mat vector

有时候我们处理完图像后需要保存一下数据到文件上，以供下一步的处理。

一个比较广泛的需求场景就是：我们对一幅图像进行特征提取之后，需要把特征点信息保存到文件上，以供后面的机器学习分类操作。那么如果遇到这样的场景，我们有什么好方法，搭建这类的小型数据库文件？

更好的办法是使用xml和yml，因为他们更具有可读性，简直就是为保存数据结构而生的好方法！OpenCV提供了很好用的读写xml/yml的类，我们只要掌握其读写要领，很容易就可以实现这个小型数据库。

1. XML、YAML文件的打开和关闭

---

1.1 文件打开

FileStorage的文件操作模式一共分为四种：READ，WRITE，APPEND，MEMORY。

string fileName  = "E:\\test.yml"; // YAML
string fileName2 = "E:\\test.xml"; // XML
// write file
FileStorage fs(fileName , FileStorage::WRITE);
// read file
FileStorage fs2(fileName , FileStorage::READ);
// or use: cv::FileStorage::open
fs2.open(fileName , FileStorage::READ);

1.2 文件关闭

FileStorage文件关闭比较简单：fs.release();

2.文件读写

---

FileStorage文件读与写的方法与C++语言中的文件流对象的使用很像，对>>和<<进行了重载，分别用于文件读取和写入。很棒的是，FileStorage支持一些常用格式的直接读写，例如字符、字符串、数字、cv::Mat等。

2.1 文本和数字的输入和输出

写入文件使用 << 运算符，例如：

fs << "frameCount" << 5;  // 字符和数字
Mat_<double> cameraMat = Mat_<double>::zeros(3, 3); 
fs << "Camera Intrinsic Matrix" << cameraMat; // cv::Mat

读取文件，使用 >> 运算符，例如

int itNr;  
fs["iterationNr"] >> itNr;  
itNr = (int) fs["iterationNr"];

2.2. OpenCV数据结构的输入和输出，和基本的C++形式相同

Mat R = Mat_<uchar >::eye (3, 3),  
T = Mat_<double>::zeros(3, 1);  
fs << "R" << R; // Write cv::Mat  
fs << "T" << T;  
fs["R"] >> R; // Read cv::Mat  
fs["T"] >> T;  

2.3 vector（arrays） 和 maps的输入和输出

vector要注意在第一个元素前加上“[”，在最后一个元素前加上”]”。例如：

fs << "strings" << "["; // text - string sequence  
fs << "image1.jpg" << "Awesomeness" << "baboon.jpg";  
fs << "]"; // close sequence  

对于map结构的操作使用的符号是”{“和”}”，例如：

fs << "Mapping"; // text - mapping  
fs << "{" << "One" << 1;  
fs << "Two" << 2 << "}";  

读取这些结构的时候，会用到FileNode和FileNodeIterator数据结构。对FileStorage类的[]操作符会返回FileNode数据类型，对于一连串的node，可以使用FileNodeIterator结构，例如：

FileNode n = fs["strings"]; // Read string sequence - Get node  
if (n.type() != FileNode::SEQ)  
{  
cerr << "strings is not a sequence! FAIL" << endl;  
return 1;  
}  
FileNodeIterator it = n.begin(), it_end = n.end(); // Go through the node  
for (; it != it_end; ++it)  
cout << (string)*it << endl;  

3.code ex

---

#include<opencv2\opencv.hpp>   
#include<opencv2\highgui\highgui.hpp>

using namespace std;
using namespace cv;

typedef struct
{
    int x;
    int y;
    string s;
}test_t;


int main(int argc, char** argv)
{
    FileStorage fs("test.xml", FileStorage::WRITE); //填入写操作

    //测试数据
    int a1 = 2;
    char a2 = -1;
    string str = "hello sysu!";
    int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };
    test_t t = { 3,4,"hi sysu" };
    map<string, int> m;
    m["kobe"] = 100;
    m["james"] = 99;
    m["curry"] = 98;

    //写入文件操作,先写标注在写数据
    fs << "int_data" << a1;
    fs << "char_data" << a2;
    fs << "string_data" << str;

    //写入数组
    fs <<"array_data"<< "["; //数组开始
    for (int i = 0; i < 10; i++)
    {
        fs << arr[i];
    }
    fs << "]"; //数组结束

    //写入结构体
    fs << "struct_data" << "{"; //结构体开始
    fs << "x" << t.x;
    fs << "y" << t.y;
    fs << "s" << t.s;
    fs << "}";  //结构结束


    //map的写入 
    fs << "map_data" << "{";  //map的开始写入
    map<string, int>::iterator it = m.begin();
    for (; it != m.end(); it++)
    {
        fs << it->first << it->second;
    }
    fs << "}";  //map写入结束


    return 0;
}

// file write
#include "opencv2/opencv.hpp"
#include <time.h>

using namespace cv;
using namespace std;

int main(int, char** argv)
{
    FileStorage fs("test.yml", FileStorage::WRITE);

    fs << "frameCount" << 5;
    time_t rawtime; time(&rawtime);
    fs << "calibrationDate" << asctime(localtime(&rawtime));
    Mat cameraMatrix = (Mat_<double>(3,3) << 1000, 0, 320, 0, 1000, 240, 0, 0, 1);
    Mat distCoeffs = (Mat_<double>(5,1) << 0.1, 0.01, -0.001, 0, 0);
    fs << "cameraMatrix" << cameraMatrix << "distCoeffs" << distCoeffs;
    fs << "features" << "[";
    for( int i = 0; i < 3; i++ )
    {
        int x = rand() % 640;
        int y = rand() % 480;
        uchar lbp = rand() % 256;

        fs << "{:" << "x" << x << "y" << y << "lbp" << "[:";
        for( int j = 0; j < 8; j++ )
            fs << ((lbp >> j) & 1);
        fs << "]" << "}";
    }
    fs << "]";
    fs.release();
    return 0;
}

// file read
FileStorage fs2("test.yml", FileStorage::READ);

// first method: use (type) operator on FileNode.
int frameCount = (int)fs2["frameCount"];

std::string date;
// second method: use FileNode::operator >>
fs2["calibrationDate"] >> date;

Mat cameraMatrix2, distCoeffs2;
fs2["cameraMatrix"] >> cameraMatrix2;
fs2["distCoeffs"] >> distCoeffs2;

cout << "frameCount: " << frameCount << endl
     << "calibration date: " << date << endl
     << "camera matrix: " << cameraMatrix2 << endl
     << "distortion coeffs: " << distCoeffs2 << endl;

FileNode features = fs2["features"];
FileNodeIterator it = features.begin(), it_end = features.end();
int idx = 0;
std::vector<uchar> lbpval;

// iterate through a sequence using FileNodeIterator
for( ; it != it_end; ++it, idx++ )
{
    cout << "feature #" << idx << ": ";
    cout << "x=" << (int)(*it)["x"] << ", y=" << (int)(*it)["y"] << ", lbp: (";
    // you can also easily read numerical arrays using FileNode >> std::vector operator.
    (*it)["lbp"] >> lbpval;
    for( int i = 0; i < (int)lbpval.size(); i++ )
        cout << " " << (int)lbpval[i];
    cout << ")" << endl;
}
fs.release();