reading and writing xml files

我们可以将varIoUs OpenCV 的 data strucures 以及C++ 中的primitive data structures 写入到XML 格式或者 YAML 格式的文件中去，也可以从这些文件中读取我们存储进去的Open data structures 或者 primitive data structures。 XML 文件就是可扩展标记语言。用来标记数据，定义数据类型。

使用如下的procedures 进行write something to xml or yaml， xml 和yaml 格式的文件的后缀名字是'.xml' 和 '.yml'/ '.yaml'。

（1）创建 FileStorage 的 object，并open it for writing。可以使用FileStorage::FileStorage() 的constructor进去文件名字。也可以先使用default constructor,然后

调用 FileStorage::open()。

(2)使用 << 写入到文件中， >> 读取文件中的数据。

(3) 使用 FileStorage::release() close the file。 FileStorage 的destructor also closes the file。

为了解释下面的程序，我们需要介绍如下的几个程序：

头文件<ctime>

//defined in header file <ctime>
// typedef /*unspecified*/ time_t
// arithmetic type capable of representing times
// almost always an integral value holding the number of seconds since 00:00,Jan 1,1970 UTC
time_t

函数 std:: time也定义在<ctime>下面：

函数的prototype:

std::time_t time( std::time_t *time );

description:

Returns the current calendar time encoded as a std::time_t object.

如果time 指针不是NULL,即不是 time(nullptr)，而是time(seconds)形式，那么返回的值也存储在seconds中指向的位置。

<ctime>中localtime() 函数：

作用是converts time since epoch to calendar time expressed as local time。

std::tm* localtime( const std::time_t *time );

其中tm 是一个time的structure:

struct tm {
   int tm_sec;         /* seconds,range 0 to 59          */
   int tm_min;         /* minutes,range 0 to 59           */
   int tm_hour;        /* hours,range 0 to 23             */
   int tm_mday;        /* day of the month,range 1 to 31  */
   int tm_mon;         /* month,range 0 to 11             */
   int tm_year;        /* The number of years since 1900   */
   int tm_wday;        /* day of the week,range 0 to 6    */
   int tm_yday;        /* day in the year,range 0 to 365  */
   int tm_isdst;       /* daylight saving time             */	
};

<ctime>中asctime() 函数：

prototype:

char* asctime( const std::tm* time_ptr );

如下作用：

Converts given calendar time std::tm to a textual representation. The resulting string has the following format: 

Www Mmm dd hh:mm:ss yyyy


Www - the day of the week (one of Mon,Tue,Wed,Thu,Fri,Sat,Sun). 

Mmm - the month (one of Jan,Feb,Mar,Apr,May,Jun,Jul,Aug,Sep,Oct,Nov,Dec). 

dd - the day of the month 

hh - hours 

mm - minutes 

ss - seconds 

yyyy - years

测试程序如下：

#include <ctime>
#include <iostream>
 
int main()
{
    std::time_t result = std::time(nullptr);
    std::cout << std::asctime(std::localtime(&result))
              << result << " seconds since the Epoch\n";
}

Output: 

Wed Sep 21 10:27:52 2011
1316615272 seconds since the Epoch

<cstdlib>下面定义的rand函数，用于generate a psudon random number between 0 and RAND_MAX（是一个常数，在 <cstdlib>下面被定义为

#define<span style="white-space:pre">	</span>RAND_MAX<span style="white-space:pre">	</span>0x7FFF

）注意产生的随机数的范围为[0,RAND_MAX].

产生规定范围的随机数的方法如下：

v1 = rand() % 100;         // v1 in the range 0 to 99
v2 = rand() % 100 + 1;     // v2 in the range 1 to 100
v3 = rand() % 30 + 1985;   // v3 in the range 1985-2014

另外，还有函数srand() 函数，需要喂进去一种子。例如：

srand (time(NULL));

最后关于C++的 bit shift operator:

分为左移和右移,左移是在数字的二进制表示后面移位补0，右移是在左边移位补0（右移的时候，如果是有符号的数，需要extends the top bit）：

The shift operators bitwise shift the value on their left by the number of bits on their right:-
<< shifts left and adds zeros at the right end.
>> shifts right and adds either 0s,if value is an unsigned type,or extends the top bit (to preserve the sign) if its a signed type.

所以有：

2 << 4 为32,

-8 >> 3 是 -1。

opencv 相关程序如下：

#include <opencv2/opencv.hpp>
#include <ctime>

using namespace cv;

int main() {
    FileStorage fs("test.yml",FileStorage::WRITE);

    fs << "frameCount" << 5;
    time_t rawtime;
    time(&rawtime);
    fs << "calibrate" << asctime(localtime(&rawtime));
    Mat cameraMatrix = (Mat_<double>(3,3) << 1000,320,1000,240,1);
    Mat distCoeffs = (Mat_<double>(5,1) << 0.1,0.01,-0.01,0);
    fs << "cameraMatrix" << cameraMatrix << "distCoeffs" << distCoeffs;

    fs << "features" << "[";
    for(int i = 0; i < 3; i++) {
        int x = rand() % 640; // [0,639]
        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();
    return 0;
}

运行后test.yml 文件如下：

%YAML:1.0
frameCount: 5
calibrate: "Tue Jan 06 13:57:03 2015\n"
cameraMatrix: !!opencv-matrix
   rows: 3
   cols: 3
   dt: d
   data: [ 1000.,0.,320.,1000.,240.,1. ]
distCoeffs: !!opencv-matrix
   rows: 5
   cols: 1
   dt: d
   data: [ 1.0000000000000001e-001,1.0000000000000000e-002,-1.0000000000000000e-002,0. ]
features:
   - { x:41,y:227,lbp:[ 0,1,1 ] }
   - { x:260,y:449,0 ] }
   - { x:598,y:78,0 ] }

现在将上述的程序的行：

FileStorage fs("test.yml",FileStorage::WRITE);

改为.xml 的文件后缀其余不变：

FileStorage fs("test.xml",FileStorage::WRITE);

再次编译运行程序，打开test.xml 文件，内容如下：

<?xml version="1.0"?>
<opencv_storage>
<frameCount>5</frameCount>
<calibrate>"Tue Jan 06 14:04:03 2015
"</calibrate>
<cameraMatrix type_id="opencv-matrix">
  <rows>3</rows>
  <cols>3</cols>
  <dt>d</dt>
  <data>
    1000. 0. 320. 0. 1000. 240. 0. 0. 1.</data></cameraMatrix>
<distCoeffs type_id="opencv-matrix">
  <rows>5</rows>
  <cols>1</cols>
  <dt>d</dt>
  <data>
    1.0000000000000001e-001 1.0000000000000000e-002
    -1.0000000000000000e-002 0. 0.</data></distCoeffs>
<features>
  <_><x>41</x>
    <y>227</y>
    <lbp>
      0 1 1 1 1 1 0 1</lbp></_>
  <_><x>260</x>
    <y>449</y>
    <lbp>
      0 0 1 1 0 1 1 0</lbp></_>
  <_><x>598</x>
    <y>78</y>
    <lbp>
      0 1 0 0 1 0 1 0</lbp></_></features>
</opencv_storage>

reading and writing xml files

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