Keypoints and Descriptors in SURF

edit

I'm new to openCV i m currently working on stitching algorithm.

In step1 the key points are detected using SURF detector, 1)which key points are detected by SURF? 2) the argument uses "Hessian",assigned a value 400 ("taken from openCV tutorial"). What is the role of this argument? 3) In the next step, the key points from step1 are used to computer "descriptors"? what are these descriptors?

Thanks for your help.!

//-- Step 1: Detect the keypoints using SURF Detector int minHessian = 400;

SurfFeatureDetector detector(minHessian);

std::vector<KeyPoint> keypoints_1, keypoints_2;

detector.detect(image1, keypoints_1);
detector.detect(image2, keypoints_2);

//-- Step 2: Calculate descriptors 
SurfDescriptorExtractor extractor;

Mat descriptors_1, descriptors_2;

extractor.compute(image1, keypoints_1, descriptors_1);
extractor.compute(image2, keypoints_2, descriptors_2);