OpenCV中XML文件和YAML文件的读写

OpenCV中XML文件和YAML文件的读写

代码如下：

#include <opencv2/core/core.hpp>
#include <iostream>
#include <string>

using namespace cv;
using namespace std;

static void help(char** av)
{
    cout << endl
        << av[0] << " shows the usage of the OpenCV serialization functionality."         << endl
        << "usage: "                                                                      << endl
        <<  av[0] << " outputfile.yml.gz"                                                 << endl
        << "The output file may be either XML (xml) or YAML (yml/yaml). You can even compress it by "
        << "specifying this in its extension like xml.gz yaml.gz etc... "                  << endl
        << "With FileStorage you can serialize objects in OpenCV by using the << and >> operators" << endl
        << "For example: - create a class and have it serialized"                         << endl
        << "             - use it to read and write matrices."                            << endl;
}

class MyData
{
public:
    MyData() : A(0), X(0), id()
    {}
    explicit MyData(int) : A(97), X(CV_PI), id("mydata1234") // explicit to avoid implicit conversion
    {}
    void write(FileStorage& fs) const                        //Write serialization for this class
    {
        fs << "{" << "A" << A << "X" << X << "id" << id << "}";
    }
    void read(const FileNode& node)                          //Read serialization for this class
    {
        A = (int)node["A"];
        X = (double)node["X"];
        id = (string)node["id"];
    }
public:   // Data Members
    int A;
    double X;
    string id;
};

//These write and read functions must be defined for the serialization in FileStorage to work
static void write(FileStorage& fs, const std::string&, const MyData& x)
{
    x.write(fs);
}
static void read(const FileNode& node, MyData& x, const MyData& default_value = MyData()){
    if(node.empty())
        x = default_value;
    else
        x.read(node);
}

// This function will print our custom class to the console
static ostream& operator<<(ostream& out, const MyData& m)
{
    out << "{ id = " << m.id << ", ";
    out << "X = " << m.X << ", ";
    out << "A = " << m.A << "}";
    return out;
}

int main(int ac, char** av)
{
    if (ac != 2)
    {
        help(av);
        return 1;
    }

    string filename = av[1];
    { //write
        Mat R = Mat_<uchar>::eye(3, 3),
            T = Mat_<double>::zeros(3, 1);
        MyData m(1);

        FileStorage fs(filename, FileStorage::WRITE);

        fs << "iterationNr" << 100;
        fs << "strings" << "[";                              // text - string sequence
        fs << "image1.jpg" << "Awesomeness" << "../data/baboon.jpg";
        fs << "]";                                           // close sequence

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

        fs << "R" << R;                                      // cv::Mat
        fs << "T" << T;

        fs << "MyData" << m;                                // your own data structures

        fs.release();                                       // explicit close
        cout << "Write Done." << endl;
    }

    {//read
        cout << endl << "Reading: " << endl;
        FileStorage fs;
        fs.open(filename, FileStorage::READ);

        int itNr;
        //fs["iterationNr"] >> itNr;
        itNr = (int) fs["iterationNr"];
        cout << itNr;
        if (!fs.isOpened())
        {
            cerr << "Failed to open " << filename << endl;
            help(av);
            return 1;
        }

        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;


        n = fs["Mapping"];                                // Read mappings from a sequence
        cout << "Two  " << (int)(n["Two"]) << "; ";
        cout << "One  " << (int)(n["One"]) << endl << endl;


        MyData m;
        Mat R, T;

        fs["R"] >> R;                                      // Read cv::Mat
        fs["T"] >> T;
        fs["MyData"] >> m;                                 // Read your own structure_

        cout << endl
            << "R = " << R << endl;
        cout << "T = " << T << endl << endl;
        cout << "MyData = " << endl << m << endl << endl;

        //Show default behavior for non existing nodes
        cout << "Attempt to read NonExisting (should initialize the data structure with its default).";
        fs["NonExisting"] >> m;
        cout << endl << "NonExisting = " << endl << m << endl;
    }

    cout << endl
        << "Tip: Open up " << filename << " with a text editor to see the serialized data." << endl;

    return 0;
}

Explanation

这里只讨论XML 和 YAML 文件的读取，这是两种不同的可能会序列化的数据结构: mappings (类似STL的map) 和 element sequence (类似STL的vector). 两种结构的区别为 map 的每一个元素都有一个具有唯一性的名字，通过具有唯一性的名字（键）可以访问对应的元素值.对于序列，你需要通过他们来查询一个特定的项目.

１　XML/YAML 文件的代开和关闭

OpenCV中XML/YAML数据结构类型为cv::FileStorage，要打开硬盘上的文件，可以基于构造函数或者open()函数

string filename = "I.xml";
FileStorage fs(filename, FileStorage::WRITE);
//...
fs.open(filename, FileStorage::READ);

函数中的第二个参数为：　WRITE, READ 或 APPEND，表示不同的操作类型（写，读或追加）。文件的扩展名表明输出文件的格式，当时用扩展名*.xml.gz*.时文件会被压缩。

当cv::FileStorage对象销毁时，文件会被自动关闭，可以通过显式调用关闭文件。

fs.release(); // explicit close

２　文本和数字的输入输出 数据结构采用和ＳＴＬ标准库一样的输出操作符<< ，输出任何类型的数据结构首先需要指定文件名.

fs << "iterationNr" << 100;

读入是一个简单的通过 [] 操作符实现的地址和映射操作 或者 通过 >> 操作符读 :

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

３　 OpenCV 数据结构的读入和输出.  C++风格的方法:

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;

４　vectors (arrays) 和 associative maps的输入输出

向前面说的，我们可以输出maps 和 sequences (array, vector) 。首先我们输出变量的名字，然后指定类型为 sequence 或者 map.

对于 sequence在第一个元素前输出 "[" 字符 并且在最后一个元素后输出 "]" 字符:

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

对于 maps the drill是同样的，但是使用 "{" 和 "}" 分割字符:

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

读取操作使用 cv::FileNode 和  cv::FileNodeIterator 数据结构.    cv::FileStorage 的操作符[]返回的是 cv::FileNode 数据类型. If the node is sequential we can use the cv::FileNodeIterator to iterate through the items:

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;

对于 maps 使用[] 操作符访问给定的条目（或者 >> 操作符):

n = fs["Mapping"];                                // Read mappings from a sequence
cout << "Two  " << (int)(n["Two"]) << "; ";
cout << "One  " << (int)(n["One"]) << endl << endl;

５　 读写自己的数据结构 　假设有如下结构：

class MyData
{
public:
      MyData() : A(0), X(0), id() {}
public:   // Data Members
   int A;
   double X;
   string id;
};

通过 在自定义类的内部和外部添加读写函数，借助OpenCV I/O XML/YAML 接口序列化是可能的 (就像 OpenCV的数据结构一样 ) 。

类内部的部分:

void write(FileStorage& fs) const                        //Write serialization for this class
{
  fs << "{" << "A" << A << "X" << X << "id" << id << "}";
}
void read(const FileNode& node)                          //Read serialization for this class
{
  A = (int)node["A"];
  X = (double)node["X"];
  id = (string)node["id"];
}

类外部的部分：

void write(FileStorage& fs, const std::string&, const MyData& x)
{
  x.write(fs);
}

void read(const FileNode& node, MyData& x, const MyData& default_value = MyData())
{
  if(node.empty())
    x = default_value;
  else
    x.read(node);
}

接下来将会定义读取不存在的节点将会发生的情况. 这种情况下返回默认的初始化值, 然而，一个更详细的解决办法是对于实例返回一个负值作为object ID.

一旦添加了使用 >>作为写操作符，使用 <<为读操作符的４个函数:

MyData m(1);
fs << "MyData" << m;                                // your own data structures
fs["MyData"] >> m;    

或者执行一个不存在读:

fs["NonExisting"] >> m;   // Do not add a fs << "NonExisting" << m command for this to work
cout << endl << "NonExisting = " << endl << m << endl;

结果

只打印出定义的数字。在控制台屏幕上看到：

Write Done.
Reading:
100image1.jpg
Awesomeness
baboon.jpg
Two  2; One  1
R = [1, 0, 0;
  0, 1, 0;
  0, 0, 1]
T = [0; 0; 0]
MyData =
{ id = mydata1234, X = 3.14159, A = 97}
Attempt to read NonExisting (should initialize the data structure with its default).
NonExisting =
{ id = , X = 0, A = 0}
Tip: Open up output.xml with a text editor to see the serialized data.

输出的XML文件

<?xml version="1.0"?>
<opencv_storage>
<iterationNr>100</iterationNr>
<strings>
  image1.jpg Awesomeness baboon.jpg</strings>
<Mapping>
  <One>1</One>
  <Two>2</Two></Mapping>
<R type_id="opencv-matrix">
  <rows>3</rows>
  <cols>3</cols>
  <dt>u</dt>
  <data>
    1 0 0 0 1 0 0 0 1</data></R>
<T type_id="opencv-matrix">
  <rows>3</rows>
  <cols>1</cols>
  <dt>d</dt>
  <data>
    0. 0. 0.</data></T>
<MyData>
  <A>97</A>
  <X>3.1415926535897931e+000</X>
  <id>mydata1234</id></MyData>
</opencv_storage>

输出的 YAML 文件:

%YAML:1.0
iterationNr: 100
strings:
   - "image1.jpg"
   - Awesomeness
   - "baboon.jpg"
Mapping:
   One: 1
   Two: 2
R: !!opencv-matrix
   rows: 3
   cols: 3
   dt: u
   data: [ 1, 0, 0, 0, 1, 0, 0, 0, 1 ]
T: !!opencv-matrix
   rows: 3
   cols: 1
   dt: d
   data: [ 0., 0., 0. ]
MyData:
   A: 97
   X: 3.1415926535897931e+000
   id: mydata1234