在OpenCV的学习中，经常会用到文件的输入输出，特别是XML和YAML格式的输出文件，如果大家有做过人脸识别就可以体会到，用主分量分析法得到的特征脸、平均脸等等数据都会被保存成为XML格式，方便下次使用时调用，OpenCV2版本比OpenCV1版本省去了很多函数，大家通过OpenCV中文网就可以查到，而且OpenCV2版本的输入输出使用与STL相同的 <</>> 输入/输出操作符，实现起来非常简单，在学习的过程中要特别注意，输入输出是map数据结构还是sequence数据结构，这两种结构上的操作会有所不同，现在我们就看看范例吧。
1、代码实现
#include "stdafx.h"
#include <opencv2/core/core.hpp>
#include <iostream>
#include <string>
using namespace cv;
using namespace std;
static void help()
{
cout << endl
<<" shows the usage of the OpenCV serialization functionality." << endl
<< "usage: " << endl
<< " 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)
{
string filename = "F:\\OpenCV2.4.8\\file_input_output\\file_input_output\\input_output.xml";
{ //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();
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;
char ch=NULL;
while (ch!='c')
{
cin>>ch;
}
return 0;
}
2、运行结果
图1、运行结果
图2、文件格式
3、总结
范例很简单，大家看看就可以明白了，这里强调的两点是：
①、输入输出是sequence还是map
sequence：输入操作：在第一个元素前输出“[”字符，并在最后一个元素后输出“]”字符。如：
fs << "strings" << "["; // 文本 - 字符串序列 fs << "image1.jpg" << "Awesomeness" << "baboon.jpg"; fs << "]"; // 序列结束输出操作： 可使用 FileNode 和 FileNodeIterator 数据结构。 FileStorage 的[] 操作符将返回一个 FileNode 数据类型。如果这个节点是序列化的，我们可以使用 FileNodeIterator 来迭代遍历所有元素。如:
FileNode n = fs["strings"]; // 读取字符串序列 - 获取节点 if (n.type() != FileNode::SEQ) { cerr << "strings is not a sequence! FAIL" << endl; return 1; } FileNodeIterator it = n.begin(),it_end = n.end(); // 遍历节点 for (; it != it_end; ++it) cout << (string)*it << endl;map: 输入操作： 采用”{“和”}“作为分隔符。如：
fs << "Mapping"; // 文本 - mapping
fs << "{" << "One" << 1;
fs << "Two" << 2 << "}"; 输出操作：
可以用 [] 操作符访问指定的元素（或者 >> 操作符）。如：
n = fs["Mapping"]; // 从序列中读取map cout << "Two " << (int)(n["Two"]) << "; "; cout << "One " << (int)(n["One"]) << endl << endl;
②、读写自定义类型
读写自定义类型时，需要自己写内部和外部的读写函数，学完后就可以使用OpenCV I/O XML/YAML接口对其进行序列化（就像对OpenCV数据结构进行序列化一样）。
4、用到的类和函数
FileStorage：
当OpenCV想打开或保存文件时，可以使用FileStorage的构造函数，或者用open（）函数
string filename = "I.xml"; FileStorage fs(filename,FileStorage::WRITE); \\... fs.open(filename,FileStorage::READ);无论以哪种方式绑定，函数中的第二个参数都以常量形式指定你要对文件进行操作的类型，包括：WRITE,READ 或 APPEND。文件扩展名决定了你将采用的输出格式。如果你指定扩展名如 .xml.gz ，输出甚至可以是压缩文件。
当FileStorage对象被销毁时，文件将自动关闭。当然你也可以显示调用release函数:
fs.release();
FileNode：
FileStorage 的[] 操作符将返回一个 FileNode 数据类型，它可以是一个矩阵、数据、头结点等等，包含了所有的文件内容，而文件节点的类型可以通过FileNode::type()方法获得。
