Question

I am trying to load a matrix from .yaml file but opencv gives me the following error :

OpenCV Error: Parsing Error (myFile.yaml(1): valid xml should start with ‘)
OpenCV Error: Parsing Error (myFile.yaml(1): Tag should start with ‘<‘> in unknown function)

this is my Write into Storage file, whcih works fine :

cv::FileStorage fs("myFile.yaml", cv::FileStorage::APPEND);   
while(counter<_imgPtrVector.size()){    
    unsigned char* _pointer=(unsigned char*)_imgPtrVector.at(counter);
    cv::Mat _matrixImage(cv::Size( width,height), CV_8UC1,_pointer , cv::Mat::AUTO_STEP);       
    fs <<"Matrix"<<_matrixImage;
    counter++;
}

but when I want to load the data from the same file I got those errors; this is the code for Reading from storage file :

cv::FileStorage f("myFile.yaml", cv::FileStorage::READ );
cv::Mat mat(cv::Size( width,height), CV_8UC1);  
if(f.isOpened()){
    cv::FileNode n = f["Matrix"];
    if (n.type() != cv::FileNode::SEQ){
     std::cout << "error!";
    }
    f["Matrix"] >>  mat; 
}

Answer 1

The problem might be that you are always appending to an existing file. So you might have to change your code to:

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

This will recreate the file each time your program runs.

The OpenCV docs have an example on how to write with XML/YAML Persistence that is pretty clear:

#include "opencv2/opencv.hpp"
#include <time.h>

using namespace cv;

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

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

And there is another example that shows how to read.