当前位置：首页 > article >正文

OpenCV学习笔记（二）：读取mnist数据集

article 2025/9/11 11:26:00

1.mnist简介

数据集主页链接：

MNIST handwritten digit database, Yann LeCun, Corinna Cortes and Chris Burgeshttp://yann.lecun.com/exdb/mnist/有四个文件：

train-images-idx3-ubyte: 训练集图像 60000张 尺寸28x28灰度图
train-labels-idx1-ubyte: 训练集标签 60000个 [0,9]
t10k-images-idx3-ubyte:  测试集图像 10000张 尺寸28x28灰度图
t10k-labels-idx1-ubyte:  测试集标签 10000个 [0,9]

对于图像文件而言，内部数据格式如下：

magic number：magic数
image number：图像数
image width ：图像宽度
image height：图像高度
image data1 ：图像数据1...
image dataN ：图像数据N

对于标签文件而言，内部数据格式如下：

magic number：magic数
label number：标签数
label1      ：标签1...
labelN      ：标签N

2.mnist数据解析

实际就是按照文件内部排布顺序，依次来读取对应内容，具体代码如下：

#include "opencv2/opencv.hpp"
#include <iostream>#ifndef uchar
#	define uchar unsigned char
#endif int ReadNumber(FILE* fp, int len) {uchar* buffer = new uchar[len];fread(buffer, len, 1, fp);int result = 0;for (int i = 0; i < len; ++i) {int temp = buffer[i];result += temp << ((len - 1 - i) * 8);}delete[] buffer;return result;
}void ReadImage(FILE* fp, int len, uchar* buffer) {fread(buffer, len, 1, fp);
}int main() {const char* trainImgFile = "./train-images.idx3-ubyte";const char* trainLabelFile = "./train-labels.idx1-ubyte";FILE* imgIn = fopen(trainImgFile, "rb");if (imgIn == NULL) {std::cout << "open: " << trainImgFile << "failed." << std::endl;return -1;}fseek(imgIn, 0, 0);int imgMagic  = ReadNumber(imgIn, 4);int imgNum    = ReadNumber(imgIn, 4);int imgWidth  = ReadNumber(imgIn, 4);int imgHeight = ReadNumber(imgIn, 4);std::cout << "magic: " << imgMagic << " imgNum: " << imgNum << " imgWidth: " <<imgWidth << " imgHeight: " << imgHeight << std::endl;if (imgMagic != 2051) {std::cout << "error image magic number: " << imgMagic << std::endl;fclose(imgIn);return -1;}int imgSize = imgWidth * imgHeight;uchar* buffer = new uchar[imgSize];for (int i = 0; i < imgNum; ++i) {ReadImage(imgIn, imgSize, buffer);cv::Mat img = cv::Mat(imgHeight, imgWidth, CV_8UC1, buffer);cv::imwrite("train/image/" + std::to_string(i) + ".jpg", img);}delete[] buffer;fclose(imgIn);FILE* labelIn = fopen(trainLabelFile, "rb");int labelMagic = ReadNumber(labelIn, 4);int labelNum = ReadNumber(labelIn, 4);if (labelMagic != 2049) {std::cout << "error label magic number: " << labelMagic << std::endl;fclose(labelIn);return -1;}for (int i = 0; i < labelNum; ++i) {int label = ReadNumber(labelIn, 1);std::cout << "i: " << i << " label: " << label << std::endl;}fclose(labelIn);return 0;
}

这里说一下解析Number为什么这样写，中间为什么要引入一个int类型的temp，主要是uchar做移位操作会被截断，所以这里先赋值给一个int然后再进行移位操作。