opencv FLANN with ORB descriptors?

Question

I am trying to use FLANN with ORB descriptors, but opencv crashes with this simple code:

vector<vector<KeyPoint> > dbKeypoints;
vector<Mat> dbDescriptors;
vector<Mat> objects;   

/*
  load Descriptors from images (with OrbDescriptorExtractor())
*/

FlannBasedMatcher matcher;

matcher.add(dbDescriptors); 
matcher.train() //> Crash!

If I use SurfDescriptorExtractor() it works well.

How can I solve this?

OpenCV says:

OpenCV Error: Unsupported format or combination of formats (type=0
) in unknown function, file D:\Value\Personal\Parthenope\OpenCV\modules\flann\sr
c\miniflann.cpp, line 299

score 34 · Answer 1 · answered Aug 03 '12 at 15:20

34

Flann needs the descriptors to be of type CV_32F so you need to convert them! find_object/example/main.cpp:

if(dbDescriptors.type()!=CV_32F) {
    dbDescriptors.convertTo(dbDescriptors, CV_32F);
}

may work ;-)

answered Aug 03 '12 at 15:20

Hans Sperker

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If somebody reaches this question but uses the OpenCV for Java, it might be CvType.CV_32F instead of CV_32F. This is due the structure that the OpenCV project decided to do the migration of code from C/C++. – xarlymg89 Mar 19 '13 at 17:31
1

is FLANNMatcher will be faster than BFMatcher if I convert descriptors? – happy_marmoset Dec 09 '13 at 18:19
I have opened another [question](http://stackoverflow.com/questions/43830849/opencv-use-flann-with-orb-descriptors-to-match-features) to ask about an error obtained with this supposed fix. – Santiago Gil May 07 '17 at 16:15

score 7 · Accepted Answer · answered Jul 27 '12 at 21:24

7

It's a bug. It will be fixed soon.

http://answers.opencv.org/question/503/how-to-use-the-lshindexparams/

answered Jul 27 '12 at 21:24

dynamic

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score 7 · Answer 3 · answered Apr 20 '15 at 19:49

7

When using ORB you should construct your matcher like so:

FlannBasedMatcher matcher(new cv::flann::LshIndexParams(5, 24, 2));

I've also seen this constructor suggested:

FlannBasedMatcher matcher(new flann::LshIndexParams(20,10,2));

answered Apr 20 '15 at 19:49

Rick Smith

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score 5 · Answer 4 · answered Oct 24 '18 at 13:22

Binary-string descriptors - ORB, BRIEF, BRISK, FREAK, AKAZE etc.

Floating-point descriptors - SIFT, SURF, GLOH etc.

Feature matching of binary descriptors can be efficiently done by comparing their Hamming distance as opposed to Euclidean distance used for floating-point descriptors.

For comparing binary descriptors in OpenCV, use FLANN + LSH index or Brute Force + Hamming distance.

http://answers.opencv.org/question/59996/flann-error-in-opencv-3/

By default FlannBasedMatcher works as KDTreeIndex with L2 norm. This is the reason why it works well with SIFT/SURF descriptors and throws an exception for ORB descriptor.

Binary features and Locality Sensitive Hashing (LSH)

Performance comparison between binary and floating-point descriptors

opencv FLANN with ORB descriptors?

4 Answers4

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