Common Interfaces of Descriptor Extractors

Extractors of keypoint descriptors in OpenCV have wrappers with a common interface that enables you to easily switch between different algorithms solving the same problem. This section is devoted to computing descriptors represented as vectors in a multidimensional space. All objects that implement the vector descriptor extractors inherit the DescriptorExtractor interface.

Note

• An example explaining keypoint extraction can be found at opencv_source_code/samples/cpp/descriptor_extractor_matcher.cpp
• An example on descriptor evaluation can be found at opencv_source_code/samples/cpp/detector_descriptor_evaluation.cpp

DescriptorExtractor

class DescriptorExtractor : public Algorithm

Abstract base class for computing descriptors for image keypoints.

class CV_EXPORTS DescriptorExtractor
{
public:
    virtual ~DescriptorExtractor();

    void compute( InputArray image, vector<KeyPoint>& keypoints,
                  OutputArray descriptors ) const;
    void compute( InputArrayOfArrays images, vector<vector<KeyPoint> >& keypoints,
                  OutputArrayOfArrays descriptors ) const;

    virtual void read( const FileNode& );
    virtual void write( FileStorage& ) const;

    virtual int descriptorSize() const = 0;
    virtual int descriptorType() const = 0;
    virtual int defaultNorm() const = 0;

    static Ptr<DescriptorExtractor> create( const String& descriptorExtractorType );

protected:
    ...
};

In this interface, a keypoint descriptor can be represented as a dense, fixed-dimension vector of a basic type. Most descriptors follow this pattern as it simplifies computing distances between descriptors. Therefore, a collection of descriptors is represented as Mat , where each row is a keypoint descriptor.

DescriptorExtractor::compute

Computes the descriptors for a set of keypoints detected in an image (first variant) or image set (second variant).

C++: void DescriptorExtractor::compute(InputArray image, vector<KeyPoint>& keypoints, OutputArray descriptors) const

C++: void DescriptorExtractor::compute(InputArrayOfArrays images, vector<vector<KeyPoint>>& keypoints, OutputArrayOfArrays descriptors) const

Python: cv2.DescriptorExtractor_create.compute(image, keypoints[, descriptors]) → keypoints, descriptors

Parameters:

• image – Image.
• images – Image set.
• keypoints – Input collection of keypoints. Keypoints for which a descriptor cannot be computed are removed. Sometimes new keypoints can be added, for example: SIFT duplicates keypoint with several dominant orientations (for each orientation).
• descriptors – Computed descriptors. In the second variant of the method descriptors[i] are descriptors computed for a keypoints[i]. Row j is the keypoints (or keypoints[i]) is the descriptor for keypoint j-th keypoint.

DescriptorExtractor::create

Creates a descriptor extractor by name.

C++: Ptr<DescriptorExtractor> DescriptorExtractor::create(const String& descriptorExtractorType)

Python: cv2.DescriptorExtractor_create(descriptorExtractorType) → retval

Parameters:descriptorExtractorType – Descriptor extractor type.

The current implementation supports the following types of a descriptor extractor:

• "BRISK" – BRISK
• "ORB" – ORB