OpenCV文件输入和输出使用XML和YAML文件

目标

您会找到以下问题的答案：

• 如何使用YAML或XML文件打印和读取文本和OpenCV文本条目？
• OpenCV数据结构如何做同样的操作？
• 如何为您的数据结构做这个？
• OpenCV数据结构的使用，如cv :: FileStorage，cv :: FileNode或cv :: FileNodeIterator。

源代码

您可以从这里下载，也可以在samples/cpp/tutorial_code/core/file_input_output/file_input_output.cppOpenCV源代码库中找到它。

以下是如何实现目标列表中枚举的所有内容的示例代码。

#include <opencv2/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;
}

说明

这里我们仅谈论XML和YAML文件输入。您的输出（及其相应的输入）文件可能只有这些扩展中的一个和结构来自此。它们是可以序列化的两种数据结构：映射（如STL映射）和元素序列（如STL向量）。这些之间的区别是，在地图中，每个元素都有一个唯一的名称，通过您可以访问它。对于序列，您需要通过它们查询特定项目。

• XML / YAML文件打开和关闭。在将任何内容写入此类文件之前，您需要将其打开并结束关闭。OpenCV中的XML / YAML数据结构是cv :: FileStorage。要指定文件在硬盘驱动器上绑定的结构，您可以使用其构造函数或open（）函数：

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

您使用第二个参数中的任何一个是一个常数，指定您可以在其上执行的操作类型：WRITE，READ或APPEND。文件名中指定的扩展名也可以确定要使用的输出格式。如果指定扩展名，例如* .xml.gz *，输出可能会被压缩。

当cv :: FileStorage对象被销毁时，文件会自动关闭。但是，您可以使用发布功能显式地调用此功能：

fs.release();                                       // explicit close

• 文本和数字的输入和输出。数据结构使用与STL库相同的<<输出运算符。为了输出任何类型的数据结构，我们首先需要指定其名称。我们只需打印出这个名称即可。对于基本类型，您可以使用值的打印符号：

fs << “iterationNr” << 100;

读入是一个简单的寻址（通过[]操作符）和转换操作或通过>>操作符读取：

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

• OpenCV数据结构的输入/输出。那么这些行为就像基本的C ++类型一样：

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;

• 向量（数组）和关联图的输入/输出。如前所述，我们可以输出地图和序列（数组，向量）。再次，我们首先打印变量的名称，然后我们必须指定我们的输出是序列还是地图。
  

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

对于地图，钻头是一样的，现在我们使用“{”和“}”分隔符：

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

要读取这些，我们使用cv :: FileNode和cv :: FileNodeIterator数据结构。cv :: FileStorage类的[]运算符返回一个cv :: FileNode数据类型。如果节点是顺序的，我们可以使用cv :: FileNodeIterator遍历项目：

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;

对于地图，您可以再次使用[]运算符来访问给定项目（或>>运算符）：

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

• 读写自己的数据结构。假设你有一个数据结构，如：

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

通过OpenCV I / O XML / YAML接口（就像OpenCV数据结构一样），可以通过在类中添加一个读取和写入函数来对其进行序列化。对于内部：

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"];
}

那么你需要在类之外添加以下函数定义：

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);
}

在这里您可以看到，在阅读部分中，我们定义了如果用户尝试读取不存在的节点会发生什么。在这种情况下，我们只返回默认的初始化值，但是更详细的解决方案是返回一个对象ID的减号值。

一旦添加了这四个函数，就可以使用>>操作符进行写操作，而<<操作符用于读取：

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

或尝试阅读一个不存在的阅读：

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

结果

主要是我们打印出定义的数字。您可以在控制台的屏幕上看到：

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.

不过，输出xml文件中可能会看到的更有趣：

<?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>

或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