Generate local features For each keypoint by using SIFT

Question

I have an image and i want to locate key points by using SIFT detector and group them, then i want to generate local features for each key point by using SIFT, would you please help me how I can do it ? Please give me any suggestions I really appreciate your help

Answer 1

I'm not sure that I understand what you mean, but if you extract SIFT features from an image, you automatically get the feature descriptor which is used to compare features to each other. Of course you also get the feature location, size, direction and hessian value with it.

While you can group those features by there position in the image, but there is currently no way that I'm aware of to compare those groups, since they may be locally related, but can have wildly different feature descriptors.

Also I would suggest SURF. It is faster and not patent encumbered.

Have a look at the examples from OpenCV if you want specific instructions on how to retrieve and compare descriptors.

Answer 2

If you are using opencv here are the commands to do it, else if you are using the matlab see the link MATCHING_using surf

---

USING OPENCV::

// you can change the parameters for your requirement

    double hessianThreshold=200; 
    int octaves=3;
    int octaveLayers=4;
    bool upright=false;
    vector<KeyPoint>keypoints;

//The detector detects the keypoints in an image here image is RGBIMAGE of Mat type

SurfFeatureDetector detector( hessianThreshold, octaves, octaveLayers, upright );
detector.detect(RGB_IMAGE, keypoints);

//The extractor computesthe local features around the keypoints

SurfDescriptorExtractor extractor;
Mat descriptors;
extractor.compute( last_ref, keypoints, descriptors);

// all the key points local features are stored in rows one after another in descriptors matrix...

Hope it is useful:)