How to save a vector of keypoints using openCV - c

Question

I was wondering if it was possible to save out a vector of cv::KeyPoints using the CvFileStorage class or the cv::FileStorage class. Also is it the same process to read it back in?
Thanks.

Answer 1

I am not sure about what you really expect :
The code I provide you is simply an example, to show how the file storage works in the OpenCV C++ bindings. It assumes here that you write in the XML file all the Keypoints separately, with their name being their position in the vector they were stored in.
It assumes aswell that when you read them back, you know the number of them you want to read, if not, the code is a little bit more complex. You'll find a way (if for instance you read the filestorage and test what it gives you, if it doesn't give you anything, then it means there is no more point to read) -it's just an idea, you have to find a solution, maybe this piece of code will be enough for you.
I should precise that i use ostringstream to put the integer in the string and by the way change the place where it will be written in the *.yml file.
//TO WRITE
vector<Keypoint> myKpVec;
FileStorage fs(filename,FileStorage::WRITE);
ostringstream oss;
for(size_t i;i<myKpVec.size();++i) {
oss << i;
fs << oss.str() << myKpVec[i];
}
fs.release();
//TO READ
vector<Keypoint> myKpVec;
FileStorage fs(filename,FileStorage::READ);
ostringstream oss;
Keypoint aKeypoint;
for(size_t i;i<myKpVec.size();<++i) {
oss << i;
fs[oss.str()] >> aKeypoint;
myKpVec.push_back(aKeypoint);
}
fs.release();
Julien,

Answer 2

char* key;
FileStorage f;
vector<Keypoint> keypoints;
//writing
write(f, key, keypoints);
//reading
read(f[key], keypoints);

Answer 3

int main() {
String filename = "data.xml";
FileStorage fs(filename,FileStorage::WRITE);
Vector<Mat> vecMat;
Mat A(3,3,CV_32F, Scalar(5));
Mat B(3,3,CV_32F, Scalar(6));
Mat C(3,3,CV_32F, Scalar(7));
vecMat.push_back(A);
vecMat.push_back(B);
vecMat.push_back(C);
//ostringstream oss;
for(int i = 0;i<vecMat.size();i++) {
stringstream ss;
ss << i;
string str = "x" + ss.str();
fs << str << vecMat[i];
}
fs.release();
vector<Mat> matVecRead;
FileStorage fr(filename,FileStorage::READ);
Mat aMat;
int countlabel = 0;
while(1) {
stringstream ss;
ss << countlabel;
string str = "x" + ss.str();
cout << str << endl;
fr[str] >> aMat;
if (fr[str].isNone() == 1) {
break;
}
matVecRead.push_back(aMat.clone());
countlabel ++;
}
fr.release();
for( unsigned j = 0; j < matVecRead.size(); j++){
cout << matVecRead[j] << endl;
}
}
Put a letter eg 'a' infront of the numbering as the OPENCV XML Format specify the xml KEY must start with a letter.
This is a code to save Vector<Mat> for visual studio 2010, i think it will works for Vector<KeyPoints>