2019-12-15

OpenCV FileStorage 用法與 YAML 檔案讀取寫入範例

本篇介紹如何使用 OpenCV 的 FileStorage 來做 YAML 檔案格式的寫入與讀取，在 OpenCV 許多應用都需要使用資料的儲存於讀取，例如相機的3D校正需要儲存校正後的結果矩陣，以利下次使用；在機器學習的應用上需要將訓練學習得到的參數保存等。OpenCV 提供了 XML/YAML 檔案格式作儲存與讀取，以下將介紹如何使用 FileStorage 來讀取與寫入 YAML。

FileStorage 使用介紹

FileStorage 的使用流程如下：

初始化一個 FileStorage 的變數並且開檔
使用 << 進行資料的寫入，>> 進行資料讀取，類似 C++ 中的檔案操作
使用 FileStorage::release() 函數釋放記憶體與關檔

使用 FileStorage 寫入資料

以下範例示範開檔、寫入int數值、寫入矩陣、寫入系統時間，最後關檔。

opencv-FileStorage-write.cpp

// g++ opencv-FileStorage-write.cpp `pkg-config --cflags --libs opencv`
#include <time.h>
#include <iostream>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

bool writeYaml() {
    std::string filename = "output.yaml";
    cv::FileStorage fs(filename, FileStorage::WRITE);
    if (!fs.isOpened()) {
        cout << "failed to open file " << filename << endl;
        return false;
    }

    // write int
    int imageWidth= 5;
    int imageHeight= 10;
    fs << "imageWidth" << imageWidth;
    fs << "imageHeight" << imageHeight;

    // write a Mat
    cv::Mat m1= Mat::eye(3, 3, CV_8U);
    cv::Mat m2= Mat::ones(3, 3, CV_8U);
    cv::Mat resultMat= (m1+1).mul(m1+2);
    fs << "resultMat" << resultMat;

    // write multi-variables 
    cv::Mat cameraMatrix = (Mat_<double>(3,3) << 700, 0, 320, 0, 700, 240, 0, 0, 1);
    cv::Mat distCoeffs = (Mat_<double>(5,1) << 0.1, 0.01, -0.001, 0, 0);
    fs << "cameraMatrix" << cameraMatrix << "distCoeffs" << distCoeffs;
 
    // write local time
    time_t rawtime;
    time(&rawtime); // #include <time.h>
    fs << "calibrationDate" << asctime(localtime(&rawtime));

    fs.release();
    return true;
}

int main(int argc, const char *argv[])
{
    writeYaml();
    return 0;
}

使用 FileStorage 讀取資料

opencv-FileStorage-read.cpp

// g++ opencv-FileStorage-read.cpp `pkg-config --cflags --libs opencv`
#include <iostream>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

bool readYaml() {
    std::string filename = "output.yaml";
    cv::FileStorage fs(filename, FileStorage::READ);
    if (!fs.isOpened()) {
        cout << "failed to open file " << filename << endl;
        return false;
    }

    int width;
    int height;
    fs["imageWidth"] >> width;
    fs["imageHeight"] >> height;
    cout << "width = " << width << endl;
    cout << "height = " <<height << endl;
 
    // read Mat
    cv::Mat resultMatRead;
    fs["resultMat"] >> resultMatRead;
    cout << "resultMat = \n" << resultMatRead << endl;
    
    cv::Mat cameraMatrix, distCoeffs;
    fs["cameraMatrix"] >> cameraMatrix;
    fs["distCoeffs"] >> distCoeffs;
    cout << "cameraMatrix = \n" << cameraMatrix << endl;
    cout << "distCoeffs = \n" << distCoeffs << endl;
 
    // read string
    string timeRead;
    fs["calibrationDate"] >> timeRead;
    cout << "calibrationDate = " << timeRead << endl;
 
    fs.release();
    return true;
}

int main(int argc, const char *argv[])
{
    readYaml();
    return 0;
}

輸出

width = 5
height = 10
resultMat = 
[  6,   2,   2;
   2,   6,   2;
   2,   2,   6]
cameraMatrix = 
[700, 0, 320;
 0, 700, 240;
 0, 0, 1]
distCoeffs = 
[0.1;
 0.01;
 -0.001;
 0;
 0]
calibrationDate = Wed Dec 18 23:04:36 2019

參考
[1] OpenCV: cv::FileStorage Class Reference
https://docs.opencv.org/3.4.8/da/d56/classcv_1_1FileStorage.html
[2] OpenCV —數據持久化: FileStorage類的數據存取操作與示例 - iracer的博客
https://blog.csdn.net/iracer/article/details/51339377
[3] 使用OpenCV的FileStorage模塊持久化存取數據(Python)
http://zhaoxuhui.top/blog/2018/06/29/PythonOpenCVFileStorage.html