[opencv] opencv from introduction to mastery -- detailed explanation of input and output XML and yaml files

1、XML and YAML File info

XML A document is a kind of Meta markup language , That is, developers can define their own tags according to their own needs . It is a semantic and structural language , Describes the... Of the document Semantics and structure .

YAML Data centric , Is a high readability , The format used to express a sequence of data ..yml and .yaml by YAML The suffix of the file .

YAML Try to compare XML A more agile way to accomplish XML The task accomplished .

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2、FileStorage Class operation file

The following procedure is generally used to write or read data to XML or YAML In file

（1） Instantiate a FileStorage Class object , use Default with parameters The constructor for completes initialization , Or use FileStorage::open() Member function assisted initialization

（2） Use the flow operator << Document in progress Write operation , perhaps >> Document in progress Read operation

（3） Use FileStorage::release() Function destructs FileStorage Class object , Close the file at the same time

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Right below 2 The three steps in

【 First step 】XML、YAML Opening of files

（1） Prepare file write operation

FileStorage yes OpenCV in XML and YAML Of documents Storage class , Encapsulates all relevant information . yes OpenCV From file Reading data or Write data to file A class that must be used when .

Constructor is FileStorage::FIleStorage, There are two overloads ：

FIleStorage::FileStorage()
FIleStorage::FileStorage(const string& source, int flags, const string& encoding=string())

For the first constructor without parameters , You can use member functions FileStorage::open Write data ：

FileStorage fs;
fs.open("abc.xml", FileStorage::WRITE);

For the second constructor with parameters , Write operation ：

fs.open("abc.xml", FileStorage::WRITE);

（2） Prepare file read operation

use FileStorage::READ Read the identifier ：

The first way ：

fs.open("abc.xml", FileStorage::READ);

The second way ：

FileStorage fs;
fs.open("abc.xml", FileStorage::READ);

The above operation is based on XML File as an example , about YAML The same applies to documents .

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【 The second step 】 Read and write files

（1） Input and output of text and numbers

Well defined FileStorage After class object , write file have access to “<<” Operator , Such as

fs<<“iterationNr”<<100

Read the file , Use “>>” Operator , Such as ：

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

（2）OpenCV Input and output of data structure

OpenCV The input and output of data structure and basic C++ In the same form , Such as ：

// Initialization of manual data structure 
Mat R = Mat_<uchar>::eye(3, 3);
Mat T = Mat_<double>::zeros(3, 1);

// towards Mat Middle write data 
fs << "R" << R;
fs << "T" << T;

// from Mat Read data from 
fs["R"] >> R;
fs["T"] >> T;

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【 The third step 】vector（arrays） and maps Input and output of

about vector structure Input and output of , Note that you need to add... Before an element “[”, Add... Before the last element “]”, Such as ：

fs << "string" << "[";  // Start reading in string Text sequence 
fs << "imagel.jpg" << "Awesomeness" << "baboon.jpg";
fs << "]";  // Close the sequence 

about maps structure The operation of , The symbol used is “{” and “}”, Such as ：

fs << "mapping";  // Start reading in mapping Text 
fs << "{" << "One" << 1;
fs << "Two" << 2 << "}";

When reading these structures , use FileNode and FileNodeIterator data structure .FileStorage Class “[”、“]” The operator returns FileNode data type ; For a series of node, have access to FileNoelIterator structure , Such as ：

FileNode n = fs["strings"];   //  Read the string sequence to get the node 
if(n.type() != FileNode::SEQ)
{
    cerr << " An error occurred ！ The string is not a sequence ！" << endl;
    return 1;
}

FileNodeIterator it = n.begin(), it_end = n.end();   // Traversal node 
for(; it != it_end; ++it)
    cout << (string)*it << endl;

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【 Fourth parts 】 File close

File closing will FileStorage Class is automatically closed when it is destroyed , However, it is also possible to show that it is called Destructor FileStorage::release() Realization .

Destructor FileStorage::release() Will deconstruct FileStorage Class object , Close the file at the same time .

Call the process ：

fs.release();

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The sample program

【1】 The sample program ：XML and YAML Writing files

//---------------------------------【 The header file 、 The namespace contains some 】-------------------------------
//		 describe ： Contains the header file and namespace used by the program 
//------------------------------------------------------------------------------------------------
#include "opencv2/opencv.hpp"  
#include <time.h>  
using namespace cv;


//-----------------------------------【main( ) function 】--------------------------------------------
//	 describe ： The entry function of the console application , Our program starts here 
//-----------------------------------------------------------------------------------------------
int main()
{
    // initialization 
    FileStorage fs("test.yaml", FileStorage::WRITE);

    // Start file writing 
    fs << "frameCount" << 5;
    time_t rawtime; time(&rawtime);
    fs << "calibrationDate" << asctime(localtime(&rawtime));  // Write the current system time 
    Mat cameraMatrix = (Mat_<double>(3, 3) << 1000, 0, 320, 0, 1000, 240, 0, 0, 1);
    Mat distCoeffs = (Mat_<double>(5, 1) << 0.1, 0.01, -0.001, 0, 0);
    fs << "cameraMatrix" << cameraMatrix << "distCoeffs" << distCoeffs;
    fs << "features" << "[";
    for (int i = 0; i < 3; i++)
    {
        int x = rand() % 640;
        int y = rand() % 480;
        uchar lbp = rand() % 256;

        fs << "{:" << "x" << x << "y" << y << "lbp" << "[:";
        for (int j = 0; j < 8; j++)
            fs << ((lbp >> j) & 1);
        fs << "]" << "}";
    }
    fs << "]";
    fs.release();

    printf("\n The file is read and written , Please view the generated files in the project directory ~");
    getchar();

    return 0;
}

The running result is ：

Generated files

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【2】 The sample program ：XML and YAML File reading

//---------------------------------【 The header file 、 The namespace contains some 】-------------------------------
//		 describe ： Contains the header file and namespace used by the program 
//------------------------------------------------------------------------------------------------       
#include "opencv2/opencv.hpp"  
#include <time.h>  
# include <string>
using namespace cv;
using namespace std;


int main()
{
    // initialization 
    FileStorage fs2("test.yaml", FileStorage::READ);

    //  The first method , Yes FileNode operation 
    int frameCount = (int)fs2["frameCount"];

    string date;
    //  The second method , Use FileNode Operator > > 
    fs2["calibrationDate"] >> date;

    Mat cameraMatrix2, distCoeffs2;
    fs2["cameraMatrix"] >> cameraMatrix2;
    fs2["distCoeffs"] >> distCoeffs2;

    cout << "frameCount: " << frameCount << endl
        << "calibration date: " << date << endl
        << "camera matrix: " << cameraMatrix2 << endl
        << "distortion coeffs: " << distCoeffs2 << endl;

    FileNode features = fs2["features"];
    FileNodeIterator it = features.begin(), it_end = features.end();
    int idx = 0;
    vector<uchar> lbpval;

    // Use FileNodeIterator Ergodic sequence 
    for (; it != it_end; ++it, idx++)
    {
        cout << "feature #" << idx << ": ";
        cout << "x=" << (int)(*it)["x"] << ", y=" << (int)(*it)["y"] << ", lbp: (";
        //  We can also use filenode > > std::vector Operators can easily read numeric arrays 
        (*it)["lbp"] >> lbpval;
        for (int i = 0; i < (int)lbpval.size(); i++)
            cout << " " << (int)lbpval[i];
        cout << ")" << endl;
    }
    fs2.release();

    // Program end , Output some help text 
    printf("\n File read complete , Please enter any key to end the program ~");
    getchar();

    return 0;
}

The running result is ：