一.MNIST手写字体文件说明
MNIST手写字体数据库下载地址http://yann.lecun.com/exdb/mnist/ 。
MNIST手写字体的数据库说明在下载网站的下面也有，为了便于写程序，数据库文件说明如下：
从上面的数据库说明可以看出来，MNIST手写字体数据库包含了是个文件，每个文件都是单纯的普通文件格式，因此，可以采用C++的文件流将其打开，每一个文件除了几个字节的文件头之外，就是剩下的要数据部分了。因此，可以先将文件的文件头读进来，然后利用magic number进行验证，验证所读的文件是否为MNIST文件。
由于MNIST存储的格式是大端存储的，和大部分Intel处理器的存储方式不同，所以，直接将文件头的前面四个直接存储为int类型或者long类型的话，是无法获得正确的数值的，还需要进行从大端模式到小端模式的转换，而且，在不同的处理器上面，int类型和long类型存储的位数是不一样的，所以，用int类型或者long类型来读取文件头的前面四个字节不具有可移植性。
在C++标准中，char类型的长度被定义为一个字节，这个在不同的处理器上面是不变的，因此，可以采用char类型的数组来存储文件头的部分，同时，使用char类型的数组来进行大端模式到小端模式的转换也是很容易的。
大端模式和小端模式的定义的区别如下：
大端模式：高位字节放在内存低地址处，低位字节放在内存高地址处；
小端模式：低位字节放在内存低地址处，高位字节放在内存高地址处；Intel处理器一般为小端模式。
下面是定义图片文件的文件头和标签的文件头
struct MNISTImageFileHeader
{
unsigned char MagicNumber[4];
unsigned char NumberOfImages[4];
unsigned char NumberOfRows[4];
unsigned char NumberOfColums[4];
};
struct MNISTLabelFileHeader
{
unsigned char MagicNumber[4];
unsigned char NumberOfLabels[4];
};
由于大端模式是把高位字节放在内存的低位处，所以，char类型的数组的低字节表示的就是原来的数的高位，所以，在将char类型的数组转化为整数的时候，只要将数组的第一个元素左移24位，第二个元素左移16位，第三个元素左移8位，然后将这些元素相加起来就可以了，下面是用迭代的方法实现的代码。
int ConvertCharArrayToInt(unsigned char* array, int LengthOfArray)
{
if (LengthOfArray < 0)
{
return -1;
}
int result = static_cast<signed int>(array[0]);
for (int i = 1; i < LengthOfArray; i++)
{
result = (result << 8) + array[i];
}
return result;
}
在有了上面的将大端模式存储的char类型数组转化为整数之后，我们就可以将MNIST的文件按照普通的文件格式读进来了，在读取了文件头之后，就可以用普通的读文件的方法将数据部分放在一个char数组中，然后就可以将其放在OpenCV的Mat对象中了。下面是全部读取MNIST文件的代码。
#ifndef MNIST_H
#define MNIST_H
#include <iostream>
#include <fstream>
#include <opencv2/opencv.hpp>
struct MNISTImageFileHeader
{
unsigned char MagicNumber[4];
unsigned char NumberOfImages[4];
unsigned char NumberOfRows[4];
unsigned char NumberOfColums[4];
};
struct MNISTLabelFileHeader
{
unsigned char MagicNumber[4];
unsigned char NumberOfLabels[4];
};
const int MAGICNUMBEROFIMAGE = 2051;
const int MAGICNUMBEROFLABEL = 2049;
int ConvertCharArrayToInt(unsigned char* array, int LengthOfArray);
bool IsImageDataFile(unsigned char* MagicNumber, int LengthOfArray);
bool IsLabelDataFile(unsigned char* MagicNumber, int LengthOfArray);
cv::Mat ReadData(std::fstream& DataFile, int NumberOfData, int DataSizeInBytes);
cv::Mat ReadImageData(std::fstream& ImageDataFile, int NumberOfImages);
cv::Mat ReadLabelData(std::fstream& LabelDataFile, int NumberOfLabel);
cv::Mat ReadImages(std::string& FileName);
cv::Mat ReadLabels(std::string& FileName);
#endif // MNIST_H
/**
* @file ReadData.cpp The file contains the functions used to read image data
* and label data from the origin mnist file
* @author sheng
* @version 1.0.0
* @date 2014-04-09
*
* @function
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-09 1.0.0 build the module
*/
#include <MNIST.h>
/**
* @brief IsImageDataFile Check the input MagicNumber is equal to
* MAGICNUMBEROFIMAGE
* @param MagicNumber The array of the magicnumber to be checked
* @param LengthOfArray The length of the array
* @return true, if the magcinumber is mathed;
* false, otherwise.
*
* @author sheng
* @version 1.0.0
* @date 2014-04-08
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-08 1.0.0 build the function
*/
bool IsImageDataFile(unsigned char* MagicNumber, int LengthOfArray)
{
int MagicNumberOfImage = ConvertCharArrayToInt(MagicNumber, LengthOfArray);
if (MagicNumberOfImage == MAGICNUMBEROFIMAGE)
{
return true;
}
return false;
}
/**
* @brief IsImageDataFile Check the input MagicNumber is equal to
* MAGICNUMBEROFLABEL
* @param MagicNumber The array of the magicnumber to be checked
* @param LengthOfArray The length of the array
* @return true, if the magcinumber is mathed;
* false, otherwise.
*
* @author sheng
* @version 1.0.0
* @date 2014-04-08
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-08 1.0.0 build the function
*/
bool IsLabelDataFile(unsigned char *MagicNumber, int LengthOfArray)
{
int MagicNumberOfLabel = ConvertCharArrayToInt(MagicNumber, LengthOfArray);
if (MagicNumberOfLabel == MAGICNUMBEROFLABEL)
{
return true;
}
return false;
}
/**
* @brief ReadData Read the data in a opened file
* @param DataFile The file which the data is read from.
* @param NumberOfData The number of the data
* @param DataSizeInBytes The size fo the every data
* @return The Mat which rows is a data,
* Return a empty Mat if the file is not opened or the some flag was
* seted when reading the data.
*
* @author sheng
* @version 1.0.0
* @date 2014-04-08
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-08 1.0.0 build the function
*/
cv::Mat ReadData(std::fstream& DataFile, int NumberOfData, int DataSizeInBytes)
{
cv::Mat DataMat;
// read the data if the file is opened.
if (DataFile.is_open())
{
int AllDataSizeInBytes = DataSizeInBytes * NumberOfData;
unsigned char* TmpData = new unsigned char[AllDataSizeInBytes];
DataFile.read((char *)TmpData, AllDataSizeInBytes);
// // If the state is good, convert the array to a mat.
// if (!DataFile.fail())
// {
// DataMat = cv::Mat(NumberOfData, DataSizeInBytes, CV_8UC1,
// TmpData).clone();
// }
DataMat = cv::Mat(NumberOfData, DataSizeInBytes, CV_8UC1,
TmpData).clone();
delete [] TmpData;
DataFile.close();
}
return DataMat;
}
/**
* @brief ReadImageData Read the Image data from the MNIST file.
* @param ImageDataFile The file which contains the Images.
* @param NumberOfImages The number of the images.
* @return The mat contains the image and each row of the mat is a image.
* Return empty mat is the file is closed or the data is not matching
* the number.
*
* @author sheng
* @version 1.0.0
* @date 2014-04-08
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-08 1.0.0 build the function
*/
cv::Mat ReadImageData(std::fstream& ImageDataFile, int NumberOfImages)
{
int ImageSizeInBytes = 28 * 28;
return ReadData(ImageDataFile, NumberOfImages, ImageSizeInBytes);
}
/**
* @brief ReadLabelData Read the label data from the MNIST file.
* @param LabelDataFile The file contained the labels.
* @param NumberOfLabel The number of the labels.
* @return The mat contains the labels and each row of the mat is a label.
* Return empty mat is the file is closed or the data is not matching
* the number.
*
* @author sheng
* @version 1.0.0
* @date 2014-04-08
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-08 1.0.0 build the function
*/
cv::Mat ReadLabelData(std::fstream& LabelDataFile, int NumberOfLabel)
{
int LabelSizeInBytes = 1;
return ReadData(LabelDataFile, NumberOfLabel, LabelSizeInBytes);
}
/**
* @brief ReadImages Read the Training images.
* @param FileName The name of the file.
* @return The mat contains the image and each row of the mat is a image.
* Return empty mat is the file is closed or the data is not matched.
*
* @author sheng
* @version 1.0.0
* @date 2014-04-08
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-08 1.0.0 build the function
*/
cv::Mat ReadImages(std::string& FileName)
{
std::fstream File(FileName.c_str(), std::ios_base::in | std::ios_base::binary);
if (!File.is_open())
{
return cv::Mat();
}
MNISTImageFileHeader FileHeader;
File.read((char *)(&FileHeader), sizeof(FileHeader));
if (!IsImageDataFile(FileHeader.MagicNumber, 4))
{
return cv::Mat();
}
int NumberOfImage = ConvertCharArrayToInt(FileHeader.NumberOfImages, 4);
return ReadImageData(File, NumberOfImage);
}
/**
* @brief ReadLabels Read the label from the MNIST file.
* @param FileName The name of the file.
* @return The mat contains the image and each row of the mat is a image.
* Return empty mat is the file is closed or the data is not matched.
*
* @author sheng
* @version 1.0.0
* @date 2014-04-08
*
* @histroy <author> <date> <version> <description>
* sheng 2014-04-08 1.0.0 build the function
*/
cv::Mat ReadLabels(std::string& FileName)
{
std::fstream File(FileName.c_str(), std::ios_base::in | std::ios_base::binary);
if (!File.is_open())
{
return cv::Mat();
}
MNISTLabelFileHeader FileHeader;
File.read((char *)(&FileHeader), sizeof(FileHeader));
if (!IsLabelDataFile(FileHeader.MagicNumber, 4))
{
return cv::Mat();
}
int NumberOfImage = ConvertCharArrayToInt(FileHeader.NumberOfLabels, 4);
return ReadLabelData(File, NumberOfImage);
}