Opencv对XML和YAML文件实现I/O操作

1. XML、YAML文件的打开和关闭

XML\YAML文件在OpenCV中的数据结构为FileStorage，打开操作例如：

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

文件关闭操作会在FileStorage结构销毁时自动进行，但也可调用如下函数实现

fs.release();

2.文本和数字的输入和输出

写入文件使用 << 运算符，例如：

fs << "iterationNr" << 100;

读取文件，使用 >> 运算符，例如

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

3. 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;

4. 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;

5. 读写自己的数据结构

这部分比较复杂，参考最后的实例中的MyData结构自己领悟吧

最后，我这里上一个实例，供大家参考。

源文件里填入如下代码：

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

using namespace cv;
using namespace std;

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
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);
}

// This function will print our custom class to the console
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" << "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;
}

编译后，在命令行进入到文件目录，执行test test.xml，运行结果如下，生成一个test . xml文件，内容如下：

<?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>