common_interfaces_of_generic_descriptor_matchers.rst



Common Interfaces of Generic Descriptor Matchers
Matchers 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 matching descriptors
that cannot be represented as vectors in a multidimensional space. GenericDescriptorMatcher is a more generic interface for descriptors. It does not make any assumptions about descriptor representation.
Every descriptor with the
:ocv:class:`DescriptorExtractor` interface has a wrapper with the GenericDescriptorMatcher interface (see
:ocv:class:`VectorDescriptorMatcher` ).
There are descriptors such as the One-way descriptor and Ferns that have the GenericDescriptorMatcher interface implemented but do not support DescriptorExtractor.

GenericDescriptorMatcher
Abstract interface for extracting and matching a keypoint descriptor. There are also :ocv:class:`DescriptorExtractor` and :ocv:class:`DescriptorMatcher` for these purposes but their interfaces are intended for descriptors represented as vectors in a multidimensional space. GenericDescriptorMatcher is a more generic interface for descriptors. DescriptorMatcher and GenericDescriptorMatcher have two groups of match methods: for matching keypoints of an image with another image or with an image set.
class GenericDescriptorMatcher
{
public:
    GenericDescriptorMatcher();
    virtual ~GenericDescriptorMatcher();

    virtual void add( const vector<Mat>& images,
                      vector<vector<KeyPoint> >& keypoints );

    const vector<Mat>& getTrainImages() const;
    const vector<vector<KeyPoint> >& getTrainKeypoints() const;
    virtual void clear();

    virtual void train() = 0;

    virtual bool isMaskSupported() = 0;

    void classify( const Mat& queryImage,
                   vector<KeyPoint>& queryKeypoints,
                   const Mat& trainImage,
                   vector<KeyPoint>& trainKeypoints ) const;
    void classify( const Mat& queryImage,
                   vector<KeyPoint>& queryKeypoints );

    /*
     * Group of methods to match keypoints from an image pair.
     */
    void match( const Mat& queryImage, vector<KeyPoint>& queryKeypoints,
                const Mat& trainImage, vector<KeyPoint>& trainKeypoints,
                vector<DMatch>& matches, const Mat& mask=Mat() ) const;
    void knnMatch( const Mat& queryImage, vector<KeyPoint>& queryKeypoints,
                   const Mat& trainImage, vector<KeyPoint>& trainKeypoints,
                   vector<vector<DMatch> >& matches, int k,
                   const Mat& mask=Mat(), bool compactResult=false ) const;
    void radiusMatch( const Mat& queryImage, vector<KeyPoint>& queryKeypoints,
                      const Mat& trainImage, vector<KeyPoint>& trainKeypoints,
                      vector<vector<DMatch> >& matches, float maxDistance,
                      const Mat& mask=Mat(), bool compactResult=false ) const;
    /*
     * Group of methods to match keypoints from one image to an image set.
     */
    void match( const Mat& queryImage, vector<KeyPoint>& queryKeypoints,
                vector<DMatch>& matches, const vector<Mat>& masks=vector<Mat>() );
    void knnMatch( const Mat& queryImage, vector<KeyPoint>& queryKeypoints,
                   vector<vector<DMatch> >& matches, int k,
                   const vector<Mat>& masks=vector<Mat>(), bool compactResult=false );
    void radiusMatch( const Mat& queryImage, vector<KeyPoint>& queryKeypoints,
                      vector<vector<DMatch> >& matches, float maxDistance,
                      const vector<Mat>& masks=vector<Mat>(), bool compactResult=false );

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

    virtual Ptr<GenericDescriptorMatcher> clone( bool emptyTrainData=false ) const = 0;

protected:
    ...
};


GenericDescriptorMatcher::add
Adds images and their keypoints to the training collection stored in the class instance.

GenericDescriptorMatcher::getTrainImages
Returns a train image collection.

GenericDescriptorMatcher::getTrainKeypoints
Returns a train keypoints collection.

GenericDescriptorMatcher::clear
Clears a train collection (images and keypoints).

GenericDescriptorMatcher::train
Trains descriptor matcher
Prepares descriptor matcher, for example, creates a tree-based structure, to extract descriptors or to optimize descriptors matching.

GenericDescriptorMatcher::isMaskSupported
Returns true if a generic descriptor matcher supports masking permissible matches.

GenericDescriptorMatcher::classify
Classifies keypoints from a query set.
The method classifies each keypoint from a query set. The first variant of the method takes a train image and its keypoints as an input argument. The second variant uses the internally stored training collection that can be built using the GenericDescriptorMatcher::add method.
The methods do the following:

Call the GenericDescriptorMatcher::match method to find correspondence between the query set and the training set.
Set the class_id field of each keypoint from the query set to class_id of the corresponding keypoint from the training set.


GenericDescriptorMatcher::match
Finds the best match in the training set for each keypoint from the query set.
The methods find the best match for each query keypoint. In the first variant of the method, a train image and its keypoints are the input arguments. In the second variant, query keypoints are matched to the internally stored training collection that can be built using the GenericDescriptorMatcher::add method.     Optional mask (or masks) can be passed to specify which query and training descriptors can be matched. Namely, queryKeypoints[i] can be matched with trainKeypoints[j] only if mask.at<uchar>(i,j) is non-zero.

GenericDescriptorMatcher::knnMatch
Finds the k best matches for each query keypoint.
The methods are extended variants of GenericDescriptorMatch::match. The parameters are similar, and the  the semantics is similar to DescriptorMatcher::knnMatch. But this class does not require explicitly computed keypoint descriptors.

GenericDescriptorMatcher::radiusMatch
For each query keypoint, finds the training keypoints not farther than the specified distance.
The methods are similar to DescriptorMatcher::radius. But this class does not require explicitly computed keypoint descriptors.

GenericDescriptorMatcher::read
Reads a matcher object from a file node.

GenericDescriptorMatcher::write
Writes a match object to a file storage.

GenericDescriptorMatcher::clone
Clones the matcher.

OneWayDescriptorMatcher
Wrapping class for computing, matching, and classifying descriptors using the
:ocv:class:`OneWayDescriptorBase` class.
class OneWayDescriptorMatcher : public GenericDescriptorMatcher
{
public:
    class Params
    {
    public:
        static const int POSE_COUNT = 500;
        static const int PATCH_WIDTH = 24;
        static const int PATCH_HEIGHT = 24;
        static float GET_MIN_SCALE() { return 0.7f; }
        static float GET_MAX_SCALE() { return 1.5f; }
        static float GET_STEP_SCALE() { return 1.2f; }

        Params( int poseCount = POSE_COUNT,
                Size patchSize = Size(PATCH_WIDTH, PATCH_HEIGHT),
                string pcaFilename = string(),
                string trainPath = string(), string trainImagesList = string(),
                float minScale = GET_MIN_SCALE(), float maxScale = GET_MAX_SCALE(),
                float stepScale = GET_STEP_SCALE() );

        int poseCount;
        Size patchSize;
        string pcaFilename;
        string trainPath;
        string trainImagesList;

        float minScale, maxScale, stepScale;
    };

    OneWayDescriptorMatcher( const Params& params=Params() );
    virtual ~OneWayDescriptorMatcher();

    void initialize( const Params& params, const Ptr<OneWayDescriptorBase>& base=Ptr<OneWayDescriptorBase>() );

    // Clears keypoints stored in collection and OneWayDescriptorBase
    virtual void clear();

    virtual void train();

    virtual bool isMaskSupported();

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

    virtual Ptr<GenericDescriptorMatcher> clone( bool emptyTrainData=false ) const;
protected:
    ...
};


FernDescriptorMatcher
Wrapping class for computing, matching, and classifying descriptors using the
:ocv:class:`FernClassifier` class.
class FernDescriptorMatcher : public GenericDescriptorMatcher
{
public:
    class Params
    {
    public:
        Params( int nclasses=0,
                int patchSize=FernClassifier::PATCH_SIZE,
                int signatureSize=FernClassifier::DEFAULT_SIGNATURE_SIZE,
                int nstructs=FernClassifier::DEFAULT_STRUCTS,
                int structSize=FernClassifier::DEFAULT_STRUCT_SIZE,
                int nviews=FernClassifier::DEFAULT_VIEWS,
                int compressionMethod=FernClassifier::COMPRESSION_NONE,
                const PatchGenerator& patchGenerator=PatchGenerator() );

        Params( const string& filename );

        int nclasses;
        int patchSize;
        int signatureSize;
        int nstructs;
        int structSize;
        int nviews;
        int compressionMethod;
        PatchGenerator patchGenerator;

        string filename;
    };

    FernDescriptorMatcher( const Params& params=Params() );
    virtual ~FernDescriptorMatcher();

    virtual void clear();

    virtual void train();

    virtual bool isMaskSupported();

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

    virtual Ptr<GenericDescriptorMatcher> clone( bool emptyTrainData=false ) const;

protected:
        ...
};


VectorDescriptorMatcher
Class used for matching descriptors that can be described as vectors in a finite-dimensional space.
class CV_EXPORTS VectorDescriptorMatcher : public GenericDescriptorMatcher
{
public:
    VectorDescriptorMatcher( const Ptr<DescriptorExtractor>& extractor, const Ptr<DescriptorMatcher>& matcher );
    virtual ~VectorDescriptorMatcher();

    virtual void add( const vector<Mat>& imgCollection,
                      vector<vector<KeyPoint> >& pointCollection );
    virtual void clear();
    virtual void train();
    virtual bool isMaskSupported();

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

    virtual Ptr<GenericDescriptorMatcher> clone( bool emptyTrainData=false ) const;

protected:
    ...
};

Example:
VectorDescriptorMatcher matcher( new SurfDescriptorExtractor,
                                 new BruteForceMatcher<L2<float> > );