How to write Keypoints into document of txt

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I would recommend using YML rather than txt, because with YML it will be much easier to access data afterwards.

To write in yml file your keypoints and descriptors use this:

 fileString = "keypoints.yml";
 cv::FileStorage fs(fileString, cv::FileStorage::WRITE);

 write(fs,"keyPointsSURF", keypointsSURF);              
 write(fs,"descriptorsSURF", descriptorsSURF);  
fs.release()

It's very simnple to access the data:

    fs.open("keypoints.yml", FileStorage::READ);
        if(fs.isOpened())
        {
            read(fs["descriptorsSURF"], descriptorsSURF); 
            read(fs["keyPointsSURF"], keypointsSURF);   
    }   
fs.release();

The "read" command can be swapped with this too, I personally prefer the "read" method:

fs["descriptorsSURF"] >> descriptorsSURF;
fs["keyPointsSURF"] >> keypointsSURF;