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# Feature Detection And Description

## ocl::Canny

Finds edges in an image using the [Canny86] algorithm.

C++:`` void `ocl::``Canny`(const oclMat& image, oclMat& edges, double low_thresh, double high_thresh, int apperture_size=3, bool L2gradient=false)

C++:`` void `ocl::``Canny`(const oclMat& image, CannyBuf& buf, oclMat& edges, double low_thresh, double high_thresh, int apperture_size=3, bool L2gradient=false)

C++:`` void `ocl::``Canny`(const oclMat& dx, const oclMat& dy, oclMat& edges, double low_thresh, double high_thresh, bool L2gradient=false)

C++:`` void `ocl::``Canny`(const oclMat& dx, const oclMat& dy, CannyBuf& buf, oclMat& edges, double low_thresh, double high_thresh, bool L2gradient=false)

  Parameters:

  - image – Single-channel 8-bit input image.
  - dx – First derivative of image in the vertical direction. Support only `CV_32S` type.
  - dy – First derivative of image in the horizontal direction. Support only `CV_32S` type.
  - edges – Output edge map. It has the same size and type as `image` .
  - low_thresh – First threshold for the hysteresis procedure.
  - high_thresh – Second threshold for the hysteresis procedure.
  - apperture_size – Aperture size for the `Sobel()` operator.
  - L2gradient – Flag indicating whether a more accurate L_2 norm =\sqrt{(dI/dx)^2 + (dI/dy)^2} should be used to compute the image gradient magnitude ( `L2gradient=true` ), or a faster default L_1 norm =|dI/dx|+|dI/dy| is enough ( `L2gradient=false` ).
  - buf – Optional buffer to avoid extra memory allocations (for many calls with the same sizes).

See also

`Canny()`

## ocl::BruteForceMatcher_OCL_base

class `ocl::``BruteForceMatcher_OCL_base`

Brute-force descriptor matcher. For each descriptor in the first set, this matcher finds the closest descriptor in the second set by trying each one. This descriptor matcher supports masking permissible matches between descriptor sets.

    class BruteForceMatcher_OCL_base
    {
    public:
            enum DistType {L1Dist = 0, L2Dist, HammingDist};

        // Add descriptors to train descriptor collection.
        void add(const std::vector<oclMat>& descCollection);

        // Get train descriptors collection.
        const std::vector<oclMat>& getTrainDescriptors() const;

        // Clear train descriptors collection.
        void clear();

        // Return true if there are no train descriptors in collection.
        bool empty() const;

        // Return true if the matcher supports mask in match methods.
        bool isMaskSupported() const;

        void matchSingle(const oclMat& query, const oclMat& train,
            oclMat& trainIdx, oclMat& distance,
            const oclMat& mask = oclMat());

        static void matchDownload(const oclMat& trainIdx,
            const oclMat& distance, std::vector<DMatch>& matches);
        static void matchConvert(const Mat& trainIdx,
            const Mat& distance, std::vector<DMatch>& matches);

        void match(const oclMat& query, const oclMat& train,
            std::vector<DMatch>& matches, const oclMat& mask = oclMat());

        void makeGpuCollection(oclMat& trainCollection, oclMat& maskCollection,
            const vector<oclMat>& masks = std::vector<oclMat>());

        void matchCollection(const oclMat& query, const oclMat& trainCollection,
            oclMat& trainIdx, oclMat& imgIdx, oclMat& distance,
            const oclMat& maskCollection);

        static void matchDownload(const oclMat& trainIdx, oclMat& imgIdx,
            const oclMat& distance, std::vector<DMatch>& matches);
        static void matchConvert(const Mat& trainIdx, const Mat& imgIdx,
            const Mat& distance, std::vector<DMatch>& matches);

        void match(const oclMat& query, std::vector<DMatch>& matches,
            const std::vector<oclMat>& masks = std::vector<oclMat>());

        void knnMatchSingle(const oclMat& query, const oclMat& train,
            oclMat& trainIdx, oclMat& distance, oclMat& allDist, int k,
            const oclMat& mask = oclMat());

        static void knnMatchDownload(const oclMat& trainIdx, const oclMat& distance,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);
        static void knnMatchConvert(const Mat& trainIdx, const Mat& distance,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);

        void knnMatch(const oclMat& query, const oclMat& train,
            std::vector< std::vector<DMatch> >& matches, int k,
            const oclMat& mask = oclMat(), bool compactResult = false);

        void knnMatch2Collection(const oclMat& query, const oclMat& trainCollection,
            oclMat& trainIdx, oclMat& imgIdx, oclMat& distance,
            const oclMat& maskCollection = oclMat());

        static void knnMatch2Download(const oclMat& trainIdx, const oclMat& imgIdx, const oclMat& distance,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);
        static void knnMatch2Convert(const Mat& trainIdx, const Mat& imgIdx, const Mat& distance,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);

        void knnMatch(const oclMat& query, std::vector< std::vector<DMatch> >& matches, int k,
            const std::vector<oclMat>& masks = std::vector<oclMat>(),
            bool compactResult = false);

        void radiusMatchSingle(const oclMat& query, const oclMat& train,
            oclMat& trainIdx, oclMat& distance, oclMat& nMatches, float maxDistance,
            const oclMat& mask = oclMat());

        static void radiusMatchDownload(const oclMat& trainIdx, const oclMat& distance, const oclMat& nMatches,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);
        static void radiusMatchConvert(const Mat& trainIdx, const Mat& distance, const Mat& nMatches,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);

        void radiusMatch(const oclMat& query, const oclMat& train,
            std::vector< std::vector<DMatch> >& matches, float maxDistance,
            const oclMat& mask = oclMat(), bool compactResult = false);

        void radiusMatchCollection(const oclMat& query, oclMat& trainIdx, oclMat& imgIdx, oclMat& distance, oclMat& nMatches, float maxDistance,
            const std::vector<oclMat>& masks = std::vector<oclMat>());

        static void radiusMatchDownload(const oclMat& trainIdx, const oclMat& imgIdx, const oclMat& distance, const oclMat& nMatches,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);
        static void radiusMatchConvert(const Mat& trainIdx, const Mat& imgIdx, const Mat& distance, const Mat& nMatches,
            std::vector< std::vector<DMatch> >& matches, bool compactResult = false);

        void radiusMatch(const oclMat& query, std::vector< std::vector<DMatch> >& matches, float maxDistance,
            const std::vector<oclMat>& masks = std::vector<oclMat>(), bool compactResult = false);

                DistType distType;

    private:
        std::vector<oclMat> trainDescCollection;
    };

The class `BruteForceMatcher_OCL_base` has an interface similar to the class `DescriptorMatcher`. It has two groups of `match` methods: for matching descriptors of one image with another image or with an image set. Also, all functions have an alternative to save results either to the GPU memory or to the CPU memory. `BruteForceMatcher_OCL_base` supports only the `L1<float>`, `L2<float>`, and `Hamming` distance types.

See also

`DescriptorMatcher`, `BFMatcher`

## ocl::BruteForceMatcher_OCL_base::match

Finds the best match for each descriptor from a query set with train descriptors.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``match`(const oclMat& query, const oclMat& train, std::vector<DMatch>& matches, const oclMat& mask=oclMat())

C++:`` void `ocl::BruteForceMatcher_OCL_base::``matchSingle`(const oclMat& query, const oclMat& train, oclMat& trainIdx, oclMat& distance, const oclMat& mask=oclMat())

C++:`` void `ocl::BruteForceMatcher_OCL_base::``match`(const oclMat& query, std::vector<DMatch>& matches, const std::vector<oclMat>& masks=std::vector<oclMat>())

C++:`` void `ocl::BruteForceMatcher_OCL_base::``matchCollection`(const oclMat& query, const oclMat& trainCollection, oclMat& trainIdx, oclMat& imgIdx, oclMat& distance, const oclMat& masks=oclMat() )

See also

`DescriptorMatcher::match()`

## ocl::BruteForceMatcher_OCL_base::makeGpuCollection

Performs a GPU collection of train descriptors and masks in a suitable format for the `ocl::BruteForceMatcher_OCL_base::matchCollection()` function.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``makeGpuCollection`(oclMat& trainCollection, oclMat& maskCollection, const vector<oclMat>& masks=std::vector<oclMat>())

## ocl::BruteForceMatcher_OCL_base::matchDownload

Downloads matrices obtained via `ocl::BruteForceMatcher_OCL_base::matchSingle()` or `ocl::BruteForceMatcher_OCL_base::matchCollection()` to vector with `DMatch`.

C++:`` static void `ocl::BruteForceMatcher_OCL_base::``matchDownload`(const oclMat& trainIdx, const oclMat& distance, std::vector<DMatch>& matches)

C++:`` static void `ocl::BruteForceMatcher_OCL_base::``matchDownload`(const oclMat& trainIdx, const oclMat& imgIdx, const oclMat& distance, std::vector<DMatch>& matches)

## ocl::BruteForceMatcher_OCL_base::matchConvert

Converts matrices obtained via `ocl::BruteForceMatcher_OCL_base::matchSingle()` or `ocl::BruteForceMatcher_OCL_base::matchCollection()` to vector with `DMatch`.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``matchConvert`(const Mat& trainIdx, const Mat& distance, std::vector<DMatch>& matches)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``matchConvert`(const Mat& trainIdx, const Mat& imgIdx, const Mat& distance, std::vector<DMatch>& matches)

## ocl::BruteForceMatcher_OCL_base::knnMatch

Finds the `k` best matches for each descriptor from a query set with train descriptors.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatch`(const oclMat& query, const oclMat& train, std::vector<std::vector<DMatch>>& matches, int k, const oclMat& mask=oclMat(), bool compactResult=false)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatchSingle`(const oclMat& query, const oclMat& train, oclMat& trainIdx, oclMat& distance, oclMat& allDist, int k, const oclMat& mask=oclMat())

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatch`(const oclMat& query, std::vector<std::vector<DMatch>>& matches, int k, const std::vector<oclMat>& masks=std::vector<oclMat>(), bool compactResult=false )

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatch2Collection`(const oclMat& query, const oclMat& trainCollection, oclMat& trainIdx, oclMat& imgIdx, oclMat& distance, const oclMat& maskCollection=oclMat())

  Parameters:

  - query – Query set of descriptors.
  - train – Training set of descriptors. It is not be added to train descriptors collection stored in the class object.
  - k – Number of the best matches per each query descriptor (or less if it is not possible).
  - mask – Mask specifying permissible matches between the input query and train matrices of descriptors.
  - compactResult – If `compactResult` is `true` , the `matches` vector does not contain matches for fully masked-out query descriptors.

The function returns detected `k` (or less if not possible) matches in the increasing order by distance.

The third variant of the method stores the results in GPU memory.

See also

`DescriptorMatcher::knnMatch()`

## ocl::BruteForceMatcher_OCL_base::knnMatchDownload

Downloads matrices obtained via `ocl::BruteForceMatcher_OCL_base::knnMatchSingle()` or `ocl::BruteForceMatcher_OCL_base::knnMatch2Collection()` to vector with `DMatch`.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatchDownload`(const oclMat& trainIdx, const oclMat& distance, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatch2Download`(const oclMat& trainIdx, const oclMat& imgIdx, const oclMat& distance, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

If `compactResult` is `true` , the `matches` vector does not contain matches for fully masked-out query descriptors.

## ocl::BruteForceMatcher_OCL_base::knnMatchConvert

Converts matrices obtained via `ocl::BruteForceMatcher_OCL_base::knnMatchSingle()` or `ocl::BruteForceMatcher_OCL_base::knnMatch2Collection()` to CPU vector with `DMatch`.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatchConvert`(const Mat& trainIdx, const Mat& distance, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``knnMatch2Convert`(const Mat& trainIdx, const Mat& imgIdx, const Mat& distance, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

If `compactResult` is `true` , the `matches` vector does not contain matches for fully masked-out query descriptors.

## ocl::BruteForceMatcher_OCL_base::radiusMatch

For each query descriptor, finds the best matches with a distance less than a given threshold.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatch`(const oclMat& query, const oclMat& train, std::vector<std::vector<DMatch>>& matches, float maxDistance, const oclMat& mask=oclMat(), bool compactResult=false)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatchSingle`(const oclMat& query, const oclMat& train, oclMat& trainIdx, oclMat& distance, oclMat& nMatches, float maxDistance, const oclMat& mask=oclMat())

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatch`(const oclMat& query, std::vector<std::vector<DMatch>>& matches, float maxDistance, const std::vector<oclMat>& masks=std::vector<oclMat>(), bool compactResult=false)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatchCollection`(const oclMat& query, oclMat& trainIdx, oclMat& imgIdx, oclMat& distance, oclMat& nMatches, float maxDistance, const std::vector<oclMat>& masks=std::vector<oclMat>())

  Parameters:

  - query – Query set of descriptors.
  - train – Training set of descriptors. It is not added to train descriptors collection stored in the class object.
  - maxDistance – Distance threshold.
  - mask – Mask specifying permissible matches between the input query and train matrices of descriptors.
  - compactResult – If `compactResult` is `true` , the `matches` vector does not contain matches for fully masked-out query descriptors.

The function returns detected matches in the increasing order by distance.

The methods work only on devices with the compute capability >= 1.1.

The third variant of the method stores the results in GPU memory and does not store the points by the distance.

See also

`DescriptorMatcher::radiusMatch()`

## ocl::BruteForceMatcher_OCL_base::radiusMatchDownload

Downloads matrices obtained via `ocl::BruteForceMatcher_OCL_base::radiusMatchSingle()` or `ocl::BruteForceMatcher_OCL_base::radiusMatchCollection()` to vector with `DMatch`.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatchDownload`(const oclMat& trainIdx, const oclMat& distance, const oclMat& nMatches, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatchDownload`(const oclMat& trainIdx, const oclMat& imgIdx, const oclMat& distance, const oclMat& nMatches, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

If `compactResult` is `true` , the `matches` vector does not contain matches for fully masked-out query descriptors.

## ocl::BruteForceMatcher_OCL_base::radiusMatchConvert

Converts matrices obtained via `ocl::BruteForceMatcher_OCL_base::radiusMatchSingle()` or `ocl::BruteForceMatcher_OCL_base::radiusMatchCollection()` to vector with `DMatch`.

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatchConvert`(const Mat& trainIdx, const Mat& distance, const Mat& nMatches, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

C++:`` void `ocl::BruteForceMatcher_OCL_base::``radiusMatchConvert`(const Mat& trainIdx, const Mat& imgIdx, const Mat& distance, const Mat& nMatches, std::vector<std::vector<DMatch>>& matches, bool compactResult=false)

If `compactResult` is `true` , the `matches` vector does not contain matches for fully masked-out query descriptors.

## ocl::HOGDescriptor

struct `ocl::``HOGDescriptor`

The class implements Histogram of Oriented Gradients ([Dalal2005]) object detector.

    struct CV_EXPORTS HOGDescriptor
    {
        enum { DEFAULT_WIN_SIGMA = -1 };
        enum { DEFAULT_NLEVELS = 64 };
        enum { DESCR_FORMAT_ROW_BY_ROW, DESCR_FORMAT_COL_BY_COL };

        HOGDescriptor(Size win_size=Size(64, 128), Size block_size=Size(16, 16),
                      Size block_stride=Size(8, 8), Size cell_size=Size(8, 8),
                      int nbins=9, double win_sigma=DEFAULT_WIN_SIGMA,
                      double threshold_L2hys=0.2, bool gamma_correction=true,
                      int nlevels=DEFAULT_NLEVELS);

        size_t getDescriptorSize() const;
        size_t getBlockHistogramSize() const;

        void setSVMDetector(const vector<float>& detector);

        static vector<float> getDefaultPeopleDetector();
        static vector<float> getPeopleDetector48x96();
        static vector<float> getPeopleDetector64x128();

        void detect(const oclMat& img, vector<Point>& found_locations,
                    double hit_threshold=0, Size win_stride=Size(),
                    Size padding=Size());

        void detectMultiScale(const oclMat& img, vector<Rect>& found_locations,
                              double hit_threshold=0, Size win_stride=Size(),
                              Size padding=Size(), double scale0=1.05,
                              int group_threshold=2);

        void getDescriptors(const oclMat& img, Size win_stride,
                            oclMat& descriptors,
                            int descr_format=DESCR_FORMAT_COL_BY_COL);

        Size win_size;
        Size block_size;
        Size block_stride;
        Size cell_size;
        int nbins;
        double win_sigma;
        double threshold_L2hys;
        bool gamma_correction;
        int nlevels;

    private:
        // Hidden
    }

Interfaces of all methods are kept similar to the `CPU HOG` descriptor and detector analogues as much as possible.

## ocl::HOGDescriptor::HOGDescriptor

Creates the `HOG` descriptor and detector.

C++:`` `ocl::HOGDescriptor::``HOGDescriptor`(Size win_size=Size(64, 128), Size block_size=Size(16, 16), Size block_stride=Size(8, 8), Size cell_size=Size(8, 8), int nbins=9, double win_sigma=DEFAULT_WIN_SIGMA, double threshold_L2hys=0.2, bool gamma_correction=true, int nlevels=DEFAULT_NLEVELS)

  Parameters:

  - win_size – Detection window size. Align to block size and block stride.
  - block_size – Block size in pixels. Align to cell size. Only (16,16) is supported for now.
  - block_stride – Block stride. It must be a multiple of cell size.
  - cell_size – Cell size. Only (8, 8) is supported for now.
  - nbins – Number of bins. Only 9 bins per cell are supported for now.
  - win_sigma – Gaussian smoothing window parameter.
  - threshold_L2hys – L2-Hys normalization method shrinkage.
  - gamma_correction – Flag to specify whether the gamma correction preprocessing is required or not.
  - nlevels – Maximum number of detection window increases.

## ocl::HOGDescriptor::getDescriptorSize

Returns the number of coefficients required for the classification.

C++:`` size_t `ocl::HOGDescriptor::``getDescriptorSize`()`const`

## ocl::HOGDescriptor::getBlockHistogramSize

Returns the block histogram size.

C++:`` size_t `ocl::HOGDescriptor::``getBlockHistogramSize`()`const`

## ocl::HOGDescriptor::setSVMDetector

Sets coefficients for the linear SVM classifier.

C++:`` void `ocl::HOGDescriptor::``setSVMDetector`(const vector<float>& detector)

## ocl::HOGDescriptor::getDefaultPeopleDetector

Returns coefficients of the classifier trained for people detection (for default window size).

C++:`` static vector<float> `ocl::HOGDescriptor::``getDefaultPeopleDetector`()

## ocl::HOGDescriptor::getPeopleDetector48x96

Returns coefficients of the classifier trained for people detection (for 48x96 windows).

C++:`` static vector<float> `ocl::HOGDescriptor::``getPeopleDetector48x96`()

## ocl::HOGDescriptor::getPeopleDetector64x128

Returns coefficients of the classifier trained for people detection (for 64x128 windows).

C++:`` static vector<float> `ocl::HOGDescriptor::``getPeopleDetector64x128`()

## ocl::HOGDescriptor::detect

Performs object detection without a multi-scale window.

C++:`` void `ocl::HOGDescriptor::``detect`(const oclMat& img, vector<Point>& found_locations, double hit_threshold=0, Size win_stride=Size(), Size padding=Size())

  Parameters:

  - img – Source image. `CV_8UC1` and `CV_8UC4` types are supported for now.
  - found_locations – Left-top corner points of detected objects boundaries.
  - hit_threshold – Threshold for the distance between features and SVM classifying plane. Usually it is 0 and should be specfied in the detector coefficients (as the last free coefficient). But if the free coefficient is omitted (which is allowed), you can specify it manually here.
  - win_stride – Window stride. It must be a multiple of block stride.
  - padding – Mock parameter to keep the CPU interface compatibility. It must be (0,0).

## ocl::HOGDescriptor::detectMultiScale

Performs object detection with a multi-scale window.

C++:`` void `ocl::HOGDescriptor::``detectMultiScale`(const oclMat& img, vector<Rect>& found_locations, double hit_threshold=0, Size win_stride=Size(), Size padding=Size(), double scale0=1.05, int group_threshold=2)

  Parameters:

  - img – Source image. See `ocl::HOGDescriptor::detect()` for type limitations.
  - found_locations – Detected objects boundaries.
  - hit_threshold – Threshold for the distance between features and SVM classifying plane. See `ocl::HOGDescriptor::detect()` for details.
  - win_stride – Window stride. It must be a multiple of block stride.
  - padding – Mock parameter to keep the CPU interface compatibility. It must be (0,0).
  - scale0 – Coefficient of the detection window increase.
  - group_threshold – Coefficient to regulate the similarity threshold. When detected, some objects can be covered by many rectangles. 0 means not to perform grouping. See `groupRectangles()` .

## ocl::HOGDescriptor::getDescriptors

Returns block descriptors computed for the whole image.

C++:`` void `ocl::HOGDescriptor::``getDescriptors`(const oclMat& img, Size win_stride, oclMat& descriptors, int descr_format=DESCR_FORMAT_COL_BY_COL)

  Parameters:

  - img – Source image. See `ocl::HOGDescriptor::detect()` for type limitations.
  - win_stride – Window stride. It must be a multiple of block stride.
  - descriptors – 2D array of descriptors.
  - descr_format –

    Descriptor storage format:

    - DESCR_FORMAT_ROW_BY_ROW - Row-major order.
    - DESCR_FORMAT_COL_BY_COL - Column-major order.

The function is mainly used to learn the classifier.

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