Common Interfaces of Descriptor Extractors

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

DescriptorExtractor

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DescriptorExtractor

Abstract base class for computing descriptors for image keypoints.

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

    void compute( const Mat& image, vector<KeyPoint>& keypoints,
                  Mat& descriptors ) const;
    void compute( const vector<Mat>& images, vector<vector<KeyPoint> >& keypoints,
                  vector<Mat>& descriptors ) const;

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

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

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

protected:
    ...
};

In this interface we assume a keypoint descriptor can be represented as a dense, fixed-dimensional vector of some basic type. Most descriptors used in practice follow this pattern, as it makes it very easy to compute distances between descriptors. Therefore we represent a collection of descriptors as a Mat() , where each row is one keypoint descriptor.

cv::DescriptorExtractor::compute

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void DescriptorExtractor::compute(const Mat& image, vector<KeyPoint>& keypoints, Mat& descriptors) const

Compute the descriptors for a set of keypoints detected in an image (first variant)

or image set (second variant).

param image:The image. param keypoints:  The keypoints. Keypoints for which a descriptor cannot be computed are removed. param descriptors:  The descriptors. Row i is the descriptor for keypoint i.

void DescriptorExtractor::compute(const vector<Mat>& images, vector<vector<KeyPoint> >& keypoints, vector<Mat>& descriptors) const

• images The image set.

• keypoints Input keypoints collection. keypoints[i] is keypoints

  detected in images[i]. Keypoints for which a descriptor can not be computed are removed.

• descriptors Descriptor collection. descriptors[i] are descriptors computed for

  a set keypoints[i].

cv::DescriptorExtractor::read

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void DescriptorExtractor::read(const FileNode& fn)

Read descriptor extractor object from file node.

Parameters:

• fn – File node from which detector will be read.

cv::DescriptorExtractor::write

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void DescriptorExtractor::write(FileStorage& fs) const

Write descriptor extractor object to file storage.

Parameters:

• fs – File storage in which detector will be written.

cv::DescriptorExtractor::create

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DescriptorExtractor()

..

:param :

SiftFeatureDetector() ```` SurfFeatureDetector() ```` BriefFeatureDetector() ```` OpponentColorDescriptorExtractor() ````

SiftDescriptorExtractor

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SiftDescriptorExtractor

Wrapping class for descriptors computing using SIFT() class.

class SiftDescriptorExtractor : public DescriptorExtractor
{
public:
    SiftDescriptorExtractor(
        const SIFT::DescriptorParams& descriptorParams=SIFT::DescriptorParams(),
        const SIFT::CommonParams& commonParams=SIFT::CommonParams() );
    SiftDescriptorExtractor( double magnification, bool isNormalize=true,
        bool recalculateAngles=true, int nOctaves=SIFT::CommonParams::DEFAULT_NOCTAVES,
        int nOctaveLayers=SIFT::CommonParams::DEFAULT_NOCTAVE_LAYERS,
        int firstOctave=SIFT::CommonParams::DEFAULT_FIRST_OCTAVE,
        int angleMode=SIFT::CommonParams::FIRST_ANGLE );

    virtual void read (const FileNode &fn);
    virtual void write (FileStorage &fs) const;
    virtual int descriptorSize() const;
    virtual int descriptorType() const;
protected:
    ...
}

SurfDescriptorExtractor

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SurfDescriptorExtractor

Wrapping class for descriptors computing using SURF() class.

class SurfDescriptorExtractor : public DescriptorExtractor
{
public:
    SurfDescriptorExtractor( int nOctaves=4,
                             int nOctaveLayers=2, bool extended=false );

    virtual void read (const FileNode &fn);
    virtual void write (FileStorage &fs) const;
    virtual int descriptorSize() const;
    virtual int descriptorType() const;
protected:
    ...
}

CalonderDescriptorExtractor

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CalonderDescriptorExtractor

Wrapping class for descriptors computing using RTreeClassifier() class.

template<typename T>
class CalonderDescriptorExtractor : public DescriptorExtractor
{
public:
    CalonderDescriptorExtractor( const string& classifierFile );

    virtual void read( const FileNode &fn );
    virtual void write( FileStorage &fs ) const;
    virtual int descriptorSize() const;
    virtual int descriptorType() const;
protected:
    ...
}

OpponentColorDescriptorExtractor

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OpponentColorDescriptorExtractor

Adapts a descriptor extractor to compute descripors in Opponent Color Space (refer to van de Sande et al., CGIV 2008 “Color Descriptors for Object Category Recognition”). Input RGB image is transformed in Opponent Color Space. Then unadapted descriptor extractor (set in constructor) computes descriptors on each of the three channel and concatenate them into a single color descriptor.

class OpponentColorDescriptorExtractor : public DescriptorExtractor
{
public:
    OpponentColorDescriptorExtractor( const Ptr<DescriptorExtractor>& dextractor );

    virtual void read( const FileNode& );
    virtual void write( FileStorage& ) const;
    virtual int descriptorSize() const;
    virtual int descriptorType() const;
protected:
    ...
};

BriefDescriptorExtractor

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BriefDescriptorExtractor

Class for computing BRIEF descriptors described in paper of Calonder M., Lepetit V., Strecha C., Fua P.: ‘’BRIEF: Binary Robust Independent Elementary Features.’’ 11th European Conference on Computer Vision (ECCV), Heraklion, Crete. LNCS Springer, September 2010.

class BriefDescriptorExtractor : public DescriptorExtractor
{
public:
    static const int PATCH_SIZE = 48;
    static const int KERNEL_SIZE = 9;

    // bytes is a length of descriptor in bytes. It can be equal 16, 32 or 64 bytes.
    BriefDescriptorExtractor( int bytes = 32 );

    virtual void read( const FileNode& );
    virtual void write( FileStorage& ) const;
    virtual int descriptorSize() const;
    virtual int descriptorType() const;
protected:
    ...
};