Common Interfaces of Feature Detectors

Feature detectors in OpenCV have wrappers with a common interface that enables you to easily switch between different algorithms solving the same problem. All objects that implement keypoint detectors inherit the FeatureDetector interface.


class KeyPoint

Data structure for salient point detectors.

Point2f pt

coordinates of the keypoint

float size

diameter of the meaningful keypoint neighborhood

float angle

computed orientation of the keypoint (-1 if not applicable)

float response

the response by which the most strong keypoints have been selected. Can be used for further sorting or subsampling

int octave

octave (pyramid layer) from which the keypoint has been extracted

int class_id

object id that can be used to clustered keypoints by an object they belong to


The keypoint constructors

C++: KeyPoint::KeyPoint()
C++: KeyPoint::KeyPoint(Point2f _pt, float _size, float _angle=-1, float _response=0, int _octave=0, int _class_id=-1)
C++: KeyPoint::KeyPoint(float x, float y, float _size, float _angle=-1, float _response=0, int _octave=0, int _class_id=-1)
Python: cv2.KeyPoint(x, y, _size[, _angle[, _response[, _octave[, _class_id]]]]) → <KeyPoint object>
  • x – x-coordinate of the keypoint
  • y – y-coordinate of the keypoint
  • _pt – x & y coordinates of the keypoint
  • _size – keypoint diameter
  • _angle – keypoint orientation
  • _response – keypoint detector response on the keypoint (that is, strength of the keypoint)
  • _octave – pyramid octave in which the keypoint has been detected
  • _class_id – object id


class FeatureDetector

Abstract base class for 2D image feature detectors.

class CV_EXPORTS FeatureDetector
    virtual ~FeatureDetector();

    void detect( const Mat& image, vector<KeyPoint>& keypoints,
                 const Mat& mask=Mat() ) const;

    void detect( const vector<Mat>& images,
                 vector<vector<KeyPoint> >& keypoints,
                 const vector<Mat>& masks=vector<Mat>() ) const;

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

    static Ptr<FeatureDetector> create( const string& detectorType );



Detects keypoints in an image (first variant) or image set (second variant).

C++: void FeatureDetector::detect(const Mat& image, vector<KeyPoint>& keypoints, const Mat& mask=Mat() ) const
C++: void FeatureDetector::detect(const vector<Mat>& images, vector<vector<KeyPoint>>& keypoints, const vector<Mat>& masks=vector<Mat>() ) const
  • image – Image.
  • images – Image set.
  • keypoints – The detected keypoints. In the second variant of the method keypoints[i] is a set of keypoints detected in images[i] .
  • mask – Mask specifying where to look for keypoints (optional). It must be a 8-bit integer matrix with non-zero values in the region of interest.
  • masks – Masks for each input image specifying where to look for keypoints (optional). masks[i] is a mask for images[i].


Reads a feature detector object from a file node.

C++: void FeatureDetector::read(const FileNode& fn)
  • fn – File node from which the detector is read.


Writes a feature detector object to a file storage.

C++: void FeatureDetector::write(FileStorage& fs) const
  • fs – File storage where the detector is written.


Creates a feature detector by its name.

C++: Ptr<FeatureDetector> FeatureDetector::create(const string& detectorType)
  • detectorType – Feature detector type.

The following detector types are supported:

Also a combined format is supported: feature detector adapter name ( "Grid"GridAdaptedFeatureDetector, "Pyramid"PyramidAdaptedFeatureDetector ) + feature detector name (see above), for example: "GridFAST", "PyramidSTAR" .


class FastFeatureDetector

Wrapping class for feature detection using the FAST() method.

class FastFeatureDetector : public FeatureDetector
    FastFeatureDetector( int threshold=1, bool nonmaxSuppression=true );
    virtual void read( const FileNode& fn );
    virtual void write( FileStorage& fs ) const;


class GoodFeaturesToTrackDetector

Wrapping class for feature detection using the goodFeaturesToTrack() function.

class GoodFeaturesToTrackDetector : public FeatureDetector
    class Params
        Params( int maxCorners=1000, double qualityLevel=0.01,
                double minDistance=1., int blockSize=3,
                bool useHarrisDetector=false, double k=0.04 );
        void read( const FileNode& fn );
        void write( FileStorage& fs ) const;

        int maxCorners;
        double qualityLevel;
        double minDistance;
        int blockSize;
        bool useHarrisDetector;
        double k;

    GoodFeaturesToTrackDetector( const GoodFeaturesToTrackDetector::Params& params=
                                            GoodFeaturesToTrackDetector::Params() );
    GoodFeaturesToTrackDetector( int maxCorners, double qualityLevel,
                                 double minDistance, int blockSize=3,
                                 bool useHarrisDetector=false, double k=0.04 );
    virtual void read( const FileNode& fn );
    virtual void write( FileStorage& fs ) const;


class MserFeatureDetector

Wrapping class for feature detection using the MSER class.

class MserFeatureDetector : public FeatureDetector
    MserFeatureDetector( CvMSERParams params=cvMSERParams() );
    MserFeatureDetector( int delta, int minArea, int maxArea,
                         double maxVariation, double minDiversity,
                         int maxEvolution, double areaThreshold,
                         double minMargin, int edgeBlurSize );
    virtual void read( const FileNode& fn );
    virtual void write( FileStorage& fs ) const;


class StarFeatureDetector

Wrapping class for feature detection using the StarDetector class.

class StarFeatureDetector : public FeatureDetector
    StarFeatureDetector( int maxSize=16, int responseThreshold=30,
                         int lineThresholdProjected = 10,
                         int lineThresholdBinarized=8, int suppressNonmaxSize=5 );
    virtual void read( const FileNode& fn );
    virtual void write( FileStorage& fs ) const;


class DenseFeatureDetector

Class for generation of image features which are distributed densely and regularly over the image.

    class DenseFeatureDetector : public FeatureDetector
            DenseFeatureDetector( float initFeatureScale=1.f, int featureScaleLevels=1,
                          float featureScaleMul=0.1f,
                          int initXyStep=6, int initImgBound=0,
                          bool varyXyStepWithScale=true,
                          bool varyImgBoundWithScale=false );

The detector generates several levels (in the amount of featureScaleLevels) of features. Features of each level are located in the nodes of a regular grid over the image (excluding the image boundary of given size). The level parameters (a feature scale, a node size, a size of boundary) are multiplied by featureScaleMul with level index growing depending on input flags, viz.:

  • Feature scale is multiplied always.
  • The grid node size is multiplied if varyXyStepWithScale is true.
  • Size of image boundary is multiplied if varyImgBoundWithScale is true.


class SimpleBlobDetector

Class for extracting blobs from an image.

class SimpleBlobDetector : public FeatureDetector
struct Params
    float thresholdStep;
    float minThreshold;
    float maxThreshold;
    size_t minRepeatability;
    float minDistBetweenBlobs;

    bool filterByColor;
    uchar blobColor;

    bool filterByArea;
    float minArea, maxArea;

    bool filterByCircularity;
    float minCircularity, maxCircularity;

    bool filterByInertia;
    float minInertiaRatio, maxInertiaRatio;

    bool filterByConvexity;
    float minConvexity, maxConvexity;

SimpleBlobDetector(const SimpleBlobDetector::Params &parameters = SimpleBlobDetector::Params());


The class implements a simple algorithm for extracting blobs from an image:

  1. Convert the source image to binary images by applying thresholding with several thresholds from minThreshold (inclusive) to maxThreshold (exclusive) with distance thresholdStep between neighboring thresholds.
  2. Extract connected components from every binary image by findContours() and calculate their centers.
  3. Group centers from several binary images by their coordinates. Close centers form one group that corresponds to one blob, which is controlled by the minDistBetweenBlobs parameter.
  4. From the groups, estimate final centers of blobs and their radiuses and return as locations and sizes of keypoints.

This class performs several filtrations of returned blobs. You should set filterBy* to true/false to turn on/off corresponding filtration. Available filtrations:

  • By color. This filter compares the intensity of a binary image at the center of a blob to blobColor. If they differ, the blob is filtered out. Use blobColor = 0 to extract dark blobs and blobColor = 255 to extract light blobs.
  • By area. Extracted blobs have an area between minArea (inclusive) and maxArea (exclusive).
  • By circularity. Extracted blobs have circularity (\frac{4*\pi*Area}{perimeter * perimeter}) between minCircularity (inclusive) and maxCircularity (exclusive).
  • By ratio of the minimum inertia to maximum inertia. Extracted blobs have this ratio between minInertiaRatio (inclusive) and maxInertiaRatio (exclusive).
  • By convexity. Extracted blobs have convexity (area / area of blob convex hull) between minConvexity (inclusive) and maxConvexity (exclusive).

Default values of parameters are tuned to extract dark circular blobs.


class GridAdaptedFeatureDetector

Class adapting a detector to partition the source image into a grid and detect points in each cell.

class GridAdaptedFeatureDetector : public FeatureDetector
     * detector            Detector that will be adapted.
     * maxTotalKeypoints   Maximum count of keypoints detected on the image.
     *                     Only the strongest keypoints will be kept.
     * gridRows            Grid row count.
     * gridCols            Grid column count.
    GridAdaptedFeatureDetector( const Ptr<FeatureDetector>& detector,
                                int maxTotalKeypoints, int gridRows=4,
                                int gridCols=4 );
    virtual void read( const FileNode& fn );
    virtual void write( FileStorage& fs ) const;


class PyramidAdaptedFeatureDetector

Class adapting a detector to detect points over multiple levels of a Gaussian pyramid. Consider using this class for detectors that are not inherently scaled.

class PyramidAdaptedFeatureDetector : public FeatureDetector
    PyramidAdaptedFeatureDetector( const Ptr<FeatureDetector>& detector,
                                   int levels=2 );
    virtual void read( const FileNode& fn );
    virtual void write( FileStorage& fs ) const;


class DynamicAdaptedFeatureDetector

Adaptively adjusting detector that iteratively detects features until the desired number is found.

class DynamicAdaptedFeatureDetector: public FeatureDetector
    DynamicAdaptedFeatureDetector( const Ptr<AdjusterAdapter>& adjuster,
        int min_features=400, int max_features=500, int max_iters=5 );

If the detector is persisted, it “remembers” the parameters used for the last detection. In this case, the detector may be used for consistent numbers of keypoints in a set of temporally related images, such as video streams or panorama series.

DynamicAdaptedFeatureDetector uses another detector, such as FAST or SURF, to do the dirty work, with the help of AdjusterAdapter . If the detected number of features is not large enough, AdjusterAdapter adjusts the detection parameters so that the next detection results in a bigger or smaller number of features. This is repeated until either the number of desired features are found or the parameters are maxed out.

Adapters can be easily implemented for any detector via the AdjusterAdapter interface.

Beware that this is not thread-safe since the adjustment of parameters requires modification of the feature detector class instance.

Example of creating DynamicAdaptedFeatureDetector :

//sample usage:
//will create a detector that attempts to find
//100 - 110 FAST Keypoints, and will at most run
//FAST feature detection 10 times until that
//number of keypoints are found
Ptr<FeatureDetector> detector(new DynamicAdaptedFeatureDetector (100, 110, 10,
                              new FastAdjuster(20,true)));


The constructor

C++: DynamicAdaptedFeatureDetector::DynamicAdaptedFeatureDetector(const Ptr<AdjusterAdapter>& adjuster, int min_features, int max_features, int max_iters)
  • adjusterAdjusterAdapter that detects features and adjusts parameters.
  • min_features – Minimum desired number of features.
  • max_features – Maximum desired number of features.
  • max_iters – Maximum number of times to try adjusting the feature detector parameters. For FastAdjuster , this number can be high, but with Star or Surf many iterations can be time-consuming. At each iteration the detector is rerun.


class AdjusterAdapter

Class providing an interface for adjusting parameters of a feature detector. This interface is used by DynamicAdaptedFeatureDetector . It is a wrapper for FeatureDetector that enables adjusting parameters after feature detection.

class AdjusterAdapter: public FeatureDetector
   virtual ~AdjusterAdapter() {}
   virtual void tooFew(int min, int n_detected) = 0;
   virtual void tooMany(int max, int n_detected) = 0;
   virtual bool good() const = 0;
   virtual Ptr<AdjusterAdapter> clone() const = 0;
   static Ptr<AdjusterAdapter> create( const string& detectorType );

See FastAdjuster, StarAdjuster, and SurfAdjuster for concrete implementations.


Adjusts the detector parameters to detect more features.

C++: void AdjusterAdapter::tooFew(int min, int n_detected)
  • min – Minimum desired number of features.
  • n_detected – Number of features detected during the latest run.


void FastAdjuster::tooFew(int min, int n_detected)


Adjusts the detector parameters to detect less features.

C++: void AdjusterAdapter::tooMany(int max, int n_detected)
  • max – Maximum desired number of features.
  • n_detected – Number of features detected during the latest run.


void FastAdjuster::tooMany(int min, int n_detected)


Returns false if the detector parameters cannot be adjusted any more.

C++: bool AdjusterAdapter::good() const


bool FastAdjuster::good() const
        return (thresh_ > 1) && (thresh_ < 200);


Creates an adjuster adapter by name

C++: Ptr<AdjusterAdapter> AdjusterAdapter::create(const string& detectorType)

Creates an adjuster adapter by name detectorType. The detector name is the same as in FeatureDetector::create(), but now supports "FAST", "STAR", and "SURF" only.


class FastAdjuster

AdjusterAdapter for FastFeatureDetector. This class decreases or increases the threshold value by 1.

class FastAdjuster FastAdjuster: public AdjusterAdapter
        FastAdjuster(int init_thresh = 20, bool nonmax = true);


class StarAdjuster

AdjusterAdapter for StarFeatureDetector. This class adjusts the responseThreshhold of StarFeatureDetector.

class StarAdjuster: public AdjusterAdapter
        StarAdjuster(double initial_thresh = 30.0);