OpenCV持久化（一）

在OpenCV中，采用FileStorage类进行数据持久化，可以采用XML或YAML格式存储数据。

将数据写入XML或YAML文件，可采用以下步骤：

1、创建FileStorage对象。可以调用构造函数 FileStorage::FileStorage()，并传入文件名参数；或者调用默认构造函数，然后调用 FileStorage::open()。

2、利用重载输出操作符<<，在operations.hpp中，输出操作符<<为模板函数：

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;
}

3、调用FileStorage::release()，关闭文件，并释放所有内存。

FileStorage在core.hpp定义如下：

class CV_EXPORTS_W FileStorage
{
public:
    //! 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)
    };
    enum
    {
        UNDEFINED=0,
        VALUE_EXPECTED=1,
        NAME_EXPECTED=2,
        INSIDE_MAP=4
    };
    //! the default constructor
    CV_WRAP FileStorage();
    //! the full constructor that opens file storage for reading or writing
    CV_WRAP FileStorage(const string& source, int flags, const string& encoding=string());
    //! the constructor that takes pointer to the C FileStorage structure
    FileStorage(CvFileStorage* fs);
    //! the destructor. calls release()
    virtual ~FileStorage();

    //! opens file storage for reading or writing. The previous storage is closed with release()
    CV_WRAP virtual bool open(const string& filename, int flags, const string& encoding=string());
    //! returns true if the object is associated with currently opened file.
    CV_WRAP virtual bool isOpened() const;
    //! closes the file and releases all the memory buffers
    CV_WRAP virtual void release();
    //! closes the file, releases all the memory buffers and returns the text string
    CV_WRAP string releaseAndGetString();

    //! returns the first element of the top-level mapping
    CV_WRAP FileNode getFirstTopLevelNode() const;
    //! returns the top-level mapping. YAML supports multiple streams
    CV_WRAP FileNode root(int streamidx=0) const;
    //! returns the specified element of the top-level mapping
    FileNode operator[](const string& nodename) const;
    //! returns the specified element of the top-level mapping
    CV_WRAP FileNode operator[](const char* nodename) const;

    //! returns pointer to the underlying C FileStorage structure
    CvFileStorage* operator *() { return fs; }
    //! returns pointer to the underlying C FileStorage structure
    const CvFileStorage* operator *() const { return fs; }
    //! writes one or more numbers of the specified format to the currently written structure
    void writeRaw( const string& fmt, const uchar* vec, size_t len );
    //! writes the registered C structure (CvMat, CvMatND, CvSeq). See cvWrite()
    void writeObj( const string& name, const void* obj );

    //! returns the normalized object name for the specified file name
    static string getDefaultObjectName(const string& filename);

    Ptr<CvFileStorage> fs; //!< the underlying C FileStorage structure
    string elname; //!< the currently written element
    vector<char> structs; //!< the stack of written structures
    int state; //!< the writer state
};

从XML或YAML文件中读取数据，可采用以下步骤：

1、利用构造函数 FileStorage::FileStorage()或方法 FileStorage::open()打开文件，这个过程包含解析文件，并将文件节点和数据存入内存中。

2、读取节点数据。利用下标操作符 FileStorage::operator [](), FileNode::operator []()或 FileNodeIterator。

3、调用 FileStorage::release() 关闭文件。

OpenCV利用 FileNode 存储XML或YAML文件中的每个节点，并用于读写。FileNode在 core.hpp 文件中定义如下：

class CV_EXPORTS_W_SIMPLE FileNode
{
public:
    //! type of the file storage node
    enum
    {
        NONE=0, //!< empty node
        INT=1, //!< an integer
        REAL=2, //!< floating-point number
        FLOAT=REAL, //!< synonym or REAL
        STR=3, //!< text string in UTF-8 encoding
        STRING=STR, //!< synonym for STR
        REF=4, //!< integer of size size_t. Typically used for storing complex dynamic structures where some elements reference the others
        SEQ=5, //!< sequence
        MAP=6, //!< mapping
        TYPE_MASK=7,
        FLOW=8, //!< compact representation of a sequence or mapping. Used only by YAML writer
        USER=16, //!< a registered object (e.g. a matrix)
        EMPTY=32, //!< empty structure (sequence or mapping)
        NAMED=64 //!< the node has a name (i.e. it is element of a mapping)
    };
    //! the default constructor
    CV_WRAP FileNode();
    //! the full constructor wrapping CvFileNode structure.
    FileNode(const CvFileStorage* fs, const CvFileNode* node);
    //! the copy constructor
    FileNode(const FileNode& node);
    //! returns element of a mapping node
    FileNode operator[](const string& nodename) const;
    //! returns element of a mapping node
    CV_WRAP FileNode operator[](const char* nodename) const;
    //! returns element of a sequence node
    CV_WRAP FileNode operator[](int i) const;
    //! returns type of the node
    CV_WRAP int type() const;

    //! returns true if the node is empty
    CV_WRAP bool empty() const;
    //! returns true if the node is a "none" object
    CV_WRAP bool isNone() const;
    //! returns true if the node is a sequence
    CV_WRAP bool isSeq() const;
    //! returns true if the node is a mapping
    CV_WRAP bool isMap() const;
    //! returns true if the node is an integer
    CV_WRAP bool isInt() const;
    //! returns true if the node is a floating-point number
    CV_WRAP bool isReal() const;
    //! returns true if the node is a text string
    CV_WRAP bool isString() const;
    //! returns true if the node has a name
    CV_WRAP bool isNamed() const;
    //! returns the node name or an empty string if the node is nameless
    CV_WRAP string name() const;
    //! returns the number of elements in the node, if it is a sequence or mapping, or 1 otherwise.
    CV_WRAP size_t size() const;
    //! returns the node content as an integer. If the node stores floating-point number, it is rounded.
    operator int() const;
    //! returns the node content as float
    operator float() const;
    //! returns the node content as double
    operator double() const;
    //! returns the node content as text string
    operator string() const;

    //! returns pointer to the underlying file node
    CvFileNode* operator *();
    //! returns pointer to the underlying file node
    const CvFileNode* operator* () const;

    //! returns iterator pointing to the first node element
    FileNodeIterator begin() const;
    //! returns iterator pointing to the element following the last node element
    FileNodeIterator end() const;

    //! reads node elements to the buffer with the specified format
    void readRaw( const string& fmt, uchar* vec, size_t len ) const;
    //! reads the registered object and returns pointer to it
    void* readObj() const;

    // do not use wrapper pointer classes for better efficiency
    const CvFileStorage* fs;
    const CvFileNode* node;
};

写XML、YAML文件时主要注意的几点：

1、XML和YAML可以嵌套的两种集合类型：映射（mappings）、序列（sequences）。映射集合类似STL中的std::map和Python中的字典，序列集合类似STL中std::vector和 Python的序列；

2、当写入映射结构的数据时，节点的值（value）紧跟着节点的键(key)；当写入序列结构的数据时，一个一个写入即可；

3、写入映射结构数据时，格式如下：fs << "{" << element_key << element_value << "}"

4、写入序列结构数据时，格式如下：fs << "[" << element_value << …… << "]"

5、写入映射、序列嵌套的数据时，以"{:"代替"{"，"[:" 代替 "["。

示例代码如下：

#include <QtCore/QCoreApplication>
#include <opencv2/core/core.hpp>
#include <time.h>
#include <iostream>

void writeYAML()
{
    cv::FileStorage fs("../file/test.yml", cv::FileStorage::WRITE);

    fs << "frameCount" << 5;
    time_t rawtime;
    time(&rawtime);

    fs << "calibrationDate" << asctime(localtime(&rawtime));

    cv::Mat cameraMatrix = (cv::Mat_<double>(3,3) << 1000,0,320,0,1000,240,0,0,1);
    cv::Mat distCoeffs = (cv::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;
        unsigned char lbp = rand() % 256;

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

void readYAML()
{
    cv::FileStorage fs2("../file/test.yml",cv::FileStorage::READ);

    int frameCount = (int)fs2["frameCount"];
    std::string date;
    fs2["calibrationDate"] >> date;

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

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

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

    for(; it != it_end; ++it, idx++)
    {
        std::cout << "features #" << idx << ": ";
        std::cout << "x=" << (int)(*it)["x"] << ", y=" <<
                     (int)(*it)["y"] << ", lbp: (";

        (*it)["lbp"] >> lbpval;
        for(int i = 0; i < (int)lbpval.size(); i++)
            std::cout << " " << (int)lbpval[i];
        std::cout << ")" << std::endl;
    }
    fs2.release();
}

int main(int argc, char *argv[])
{
//    QCoreApplication a(argc, argv);
//    return a.exec();

//    writeYAML();
    readYAML();

    return 0;
}

写入的YAML文件如下：

读取YAML文件输出如下：