今天学习一下Opencv中一个用处很广泛的功能，xml/yml 格式文件的输入和输出，这在特征，算法参数等数据类型的保存和载入中肯定需要用到，掌握opencv中文件的输入输出类，会使这一个过程十分简单愉快。当然文件的输入输出功能用处很广，有待大家去挖掘。对应的例程是 (TUTORIAL) file_input_output
先来看下程序的运行结果，这里我们设置的xml输出路径为 f:\image_set\test.xml
可以看出，例程先将数据写入到文件中，然后再从文件中读出数据，并打印出来。下面具体看下是怎么操作的。
{ //write
Mat R = Mat_<uchar>::eye(3, 3),
T = Mat_<double>::zeros(3, 1);
MyData m(1);
FileStorage fs(filename, FileStorage::WRITE); // 构造一个 FileStorage对象，只提供写操作
fs << "iterationNr" << 100;
fs << "strings" << "["; // text - string sequence // 用 [ ] 作为字符串的输入开始和结束标记
fs << "image1.jpg" << "Awesomeness" << "baboon.jpg";
fs << "]"; // close sequence
fs << "Mapping"; // text - mapping // 用 { }作为map 索引的输入开始和结束标记
fs << "{" << "One" << 1;
fs << "Two" << 2 << "}";
fs << "R" << R; // cv::Mat // 输出Mat，“R”作为Mat的名称
fs << "T" << T;
fs << "MyData" << m; // your own data structures // 输出自定义类型 , 需要自己编写函数重载
fs.release(); // explicit close // 写完后，要释放文件资源
cout << "Write Done." << endl;
}
//! the full constructor that opens file storage for reading or writing
CV_WRAP FileStorage(const string& source, int flags, const string& encoding=string());
1. 文件路径，类型为string
2. flag参数，用来决定对象的存储模式，包括一下几种模式
//! file storage mode
enum
{
READ=0, //! read mode
WRITE=1, //! write mode
APPEND=2, //! append mode
MEMORY=4,
FORMAT_MASK=(7<<3),
FORMAT_AUTO=0,
FORMAT_XML=(1<<3),
FORMAT_YAML=(2<<3)
};
<?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>
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;
}template<typename _Tp> static inline FileStorage& operator << (FileStorage& fs, const _Tp& value)
{
if( !fs.isOpened() )
return fs;
if( fs.state == FileStorage::NAME_EXPECTED + FileStorage::INSIDE_MAP )
CV_Error( CV_StsError, "No element name has been given" );
write( fs, fs.elname, value );
if( fs.state & FileStorage::INSIDE_MAP )
fs.state = FileStorage::NAME_EXPECTED + FileStorage::INSIDE_MAP;
return fs;
}
这是一个模板函数，通过一个重载函数write实现不同类型的写操作，而这个write函数只是提供一个接口，具体的文件写入操作，在各种类型中有分别的实现。同理，FileStorage 的read也是利用类似的技巧实现的。这样做的目的也是为了提供代码的重载性和扩展性，这样FileStorage就可以为任意的自定义类型提供接口，实现在各自的类型中实现，但用户新添加一种类型时，不需要改动FileStorage中的任何代码，这是个很高明的技巧。
下面看下FileStorage 的read操作
{//read
cout << endl << "Reading: " << endl;
FileStorage fs;
fs.open(filename, FileStorage::READ); 构造一个FileStorage，只提供读操作
int itNr;
//fs["iterationNr"] >> itNr;
itNr = (int) fs["iterationNr"]; //利用[ ]查找iterationNr的值
cout << itNr;
if (!fs.isOpened())
{
cerr << "Failed to open " << filename << endl;
help(av);
return 1;
}
FileNode n = fs["strings"]; // Read string sequence - Get node // 寻找名为 strings的节点
if (n.type() != FileNode::SEQ) // 判断节点的值类型 FileNode::SEQ代表字符串类型
{
cerr << "strings is not a sequence! FAIL" << endl;
return 1;
}
FileNodeIterator it = n.begin(), it_end = n.end(); // Go through the node // 遍历节点，输出所有字符串，迭代的风格
for (; it != it_end; ++it)
cout << (string)*it << endl;
n = fs["Mapping"]; // Read mappings from a sequence
cout << "Two " << (int)(n["Two"]) << "; "; // 查找"Two"的值
cout << "One " << (int)(n["One"]) << endl << endl;
MyData m;
Mat R, T;
fs["R"] >> R; // Read cv::Mat // 查找到“R”的Mat，并读入到 R中
fs["T"] >> T;
fs["MyData"] >> m; // Read your own structure_ 自定义类型的读取
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;
}
同样的，自定义类型的读入操作需要自己定义。