OpenCV学习笔记（六）——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>