OpenCV  3.4.0-rc
Open Source Computer Vision
Classes
XML/YAML Persistence

Classes

class  cv::FileNode
 File Storage Node class. More...
 
class  cv::FileNodeIterator
 used to iterate through sequences and mappings. More...
 
class  cv::FileStorage
 XML/YAML/JSON file storage class that encapsulates all the information necessary for writing or reading data to/from a file. More...
 

Detailed Description

XML/YAML/JSON file storages.

Writing to a file storage.

You can store and then restore various OpenCV data structures to/from XML (http://www.w3c.org/XML), YAML (http://www.yaml.org) or JSON (http://www.json.org/) formats. Also, it is possible to store and load arbitrarily complex data structures, which include OpenCV data structures, as well as primitive data types (integer and floating-point numbers and text strings) as their elements.

Use the following procedure to write something to XML, YAML or JSON:

  1. Create new FileStorage and open it for writing. It can be done with a single call to FileStorage::FileStorage constructor that takes a filename, or you can use the default constructor and then call FileStorage::open. Format of the file (XML, YAML or JSON) is determined from the filename extension (".xml", ".yml"/".yaml" and ".json", respectively)
  2. Write all the data you want using the streaming operator <<, just like in the case of STL streams.
  3. Close the file using FileStorage::release. FileStorage destructor also closes the file.

Here is an example:

#include "opencv2/opencv.hpp"
#include <time.h>
using namespace cv;
int main(int, char** argv)
{
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;
}

The sample above stores to YML an integer, a text string (calibration date), 2 matrices, and a custom structure "feature", which includes feature coordinates and LBP (local binary pattern) value. Here is output of the sample:

%YAML:1.0
frameCount: 5
calibrationDate: "Fri Jun 17 14:09:29 2011\n"
cameraMatrix: !!opencv-matrix
rows: 3
cols: 3
dt: d
data: [ 1000., 0., 320., 0., 1000., 240., 0., 0., 1. ]
distCoeffs: !!opencv-matrix
rows: 5
cols: 1
dt: d
data: [ 1.0000000000000001e-01, 1.0000000000000000e-02,
-1.0000000000000000e-03, 0., 0. ]
features:
- { x:167, y:49, lbp:[ 1, 0, 0, 1, 1, 0, 1, 1 ] }
- { x:298, y:130, lbp:[ 0, 0, 0, 1, 0, 0, 1, 1 ] }
- { x:344, y:158, lbp:[ 1, 1, 0, 0, 0, 0, 1, 0 ] }

As an exercise, you can replace ".yml" with ".xml" or ".json" in the sample above and see, how the corresponding XML file will look like.

Several things can be noted by looking at the sample code and the output:

Reading data from a file storage.

To read the previously written XML, YAML or JSON file, do the following:

  1. Open the file storage using FileStorage::FileStorage constructor or FileStorage::open method. In the current implementation the whole file is parsed and the whole representation of file storage is built in memory as a hierarchy of file nodes (see FileNode)
  2. Read the data you are interested in. Use FileStorage::operator [], FileNode::operator [] and/or FileNodeIterator.
  3. Close the storage using FileStorage::release.

Here is how to read the file created by the code sample above:

FileStorage fs2("test.yml", FileStorage::READ);
// first method: use (type) operator on FileNode.
int frameCount = (int)fs2["frameCount"];
String date;
// second method: use FileNode::operator >>
fs2["calibrationDate"] >> date;
Mat cameraMatrix2, distCoeffs2;
fs2["cameraMatrix"] >> cameraMatrix2;
fs2["distCoeffs"] >> distCoeffs2;
cout << "frameCount: " << frameCount << endl
<< "calibration date: " << date << endl
<< "camera matrix: " << cameraMatrix2 << endl
<< "distortion coeffs: " << distCoeffs2 << endl;
FileNode features = fs2["features"];
FileNodeIterator it = features.begin(), it_end = features.end();
int idx = 0;
std::vector<uchar> lbpval;
// iterate through a sequence using FileNodeIterator
for( ; it != it_end; ++it, idx++ )
{
cout << "feature #" << idx << ": ";
cout << "x=" << (int)(*it)["x"] << ", y=" << (int)(*it)["y"] << ", lbp: (";
// you can also easily read numerical arrays using FileNode >> std::vector operator.
(*it)["lbp"] >> lbpval;
for( int i = 0; i < (int)lbpval.size(); i++ )
cout << " " << (int)lbpval[i];
cout << ")" << endl;
}
fs2.release();

Format specification

([count]{u|c|w|s|i|f|d})... where the characters correspond to fundamental C++ types:

count is the optional counter of values of a given type. For example, 2if means that each array element is a structure of 2 integers, followed by a single-precision floating-point number. The equivalent notations of the above specification are iif, 2i1f and so forth. Other examples: u means that the array consists of bytes, and 2d means the array consists of pairs of doubles.

See also
filestorage.cpp