OpenCV: cv::KeyPoint Class Reference

| OpenCV 3.4.2 Open Source Computer Vision

Public Member Functions | Static Public Member Functions | Public Attributes | List of all members
cv::KeyPoint Class Reference
Core functionality » Basic structures

Data structure for salient point detectors. More...

`#include "types.hpp"`

## Public Member Functions
| KeyPoint ()
| the default constructor More...
| KeyPoint (Point2f _pt, float _size, float _angle=-1, float _response=0, int _octave=0, int _class_id=-1)
| KeyPoint (float x, float y, float _size, float _angle=-1, float _response=0, int _octave=0, int _class_id=-1)
size_t | hash () const

## Static Public Member Functions
static void | convert (const std::vector< KeyPoint > &keypoints, std::vector< Point2f > &points2f, const std::vector< int > &keypointIndexes=std::vector< int >())
static void | convert (const std::vector< Point2f > &points2f, std::vector< KeyPoint > &keypoints, float size=1, float response=1, int octave=0, int class_id=-1)
static float | overlap (const KeyPoint &kp1, const KeyPoint &kp2)

## Public Attributes
float | angle
int | class_id
| object class (if the keypoints need to be clustered by an object they belong to) More...
int | octave
| octave (pyramid layer) from which the keypoint has been extracted More...
Point2f | pt
| coordinates of the keypoints More...
float | response
| the response by which the most strong keypoints have been selected. Can be used for the further sorting or subsampling More...
float | size
| diameter of the meaningful keypoint neighborhood More...

## Detailed Description

Data structure for salient point detectors.

The class instance stores a keypoint, i.e. a point feature found by one of many available keypoint detectors, such as Harris corner detector, FAST, StarDetector, SURF, SIFT etc.

The keypoint is characterized by the 2D position, scale (proportional to the diameter of the neighborhood that needs to be taken into account), orientation and some other parameters. The keypoint neighborhood is then analyzed by another algorithm that builds a descriptor (usually represented as a feature vector). The keypoints representing the same object in different images can then be matched using KDTree or another method.

## Constructor & Destructor Documentation

## ◆ KeyPoint() [1/3]

cv::KeyPoint::KeyPoint | ( |  | ) |

the default constructor

## ◆ KeyPoint() [2/3]

cv::KeyPoint::KeyPoint | ( | Point2f | _pt,
|  | float | _size,
|  | float | _angle = `-1`,
|  | float | _response = `0`,
|  | int | _octave = `0`,
|  | int | _class_id = `-1`
| ) |  |

Parameters

  _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

## ◆ KeyPoint() [3/3]

cv::KeyPoint::KeyPoint | ( | float | x,
|  | float | y,
|  | float | _size,
|  | float | _angle = `-1`,
|  | float | _response = `0`,
|  | int | _octave = `0`,
|  | int | _class_id = `-1`
| ) |  |

Parameters

  x | x-coordinate of the keypoint
  y | y-coordinate 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

## Member Function Documentation

## ◆ convert() [1/2]

static void cv::KeyPoint::convert | ( | const std::vector< KeyPoint > & | keypoints,
|  | std::vector< Point2f > & | points2f,
|  | const std::vector< int > & | keypointIndexes = `std::vector< int >()`
| ) |  |
| static

This method converts vector of keypoints to vector of points or the reverse, where each keypoint is assigned the same size and the same orientation.

Parameters

  keypoints | Keypoints obtained from any feature detection algorithm like SIFT/SURF/ORB
  points2f | Array of (x,y) coordinates of each keypoint
  keypointIndexes | Array of indexes of keypoints to be converted to points. (Acts like a mask to convert only specified keypoints)

## ◆ convert() [2/2]

static void cv::KeyPoint::convert | ( | const std::vector< Point2f > & | points2f,
|  | std::vector< KeyPoint > & | keypoints,
|  | float | size = `1`,
|  | float | response = `1`,
|  | int | octave = `0`,
|  | int | class_id = `-1`
| ) |  |
| static

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

  points2f | Array of (x,y) coordinates of each keypoint
  keypoints | Keypoints obtained from any feature detection algorithm like SIFT/SURF/ORB
  size | keypoint diameter
  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

## ◆ hash()

size_t cv::KeyPoint::hash | ( |  | ) | const

## ◆ overlap()

static float cv::KeyPoint::overlap | ( | const KeyPoint & | kp1,
|  | const KeyPoint & | kp2
| ) |  |
| static

This method computes overlap for pair of keypoints. Overlap is the ratio between area of keypoint regions' intersection and area of keypoint regions' union (considering keypoint region as circle). If they don't overlap, we get zero. If they coincide at same location with same size, we get 1.

Parameters

  kp1 | First keypoint
  kp2 | Second keypoint

## Member Data Documentation

## ◆ angle

float cv::KeyPoint::angle

computed orientation of the keypoint (-1 if not applicable); it's in [0,360) degrees and measured relative to image coordinate system, ie in clockwise.

## ◆ class_id

int cv::KeyPoint::class_id

object class (if the keypoints need to be clustered by an object they belong to)

## ◆ octave

int cv::KeyPoint::octave

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

## ◆ pt

Point2f cv::KeyPoint::pt

coordinates of the keypoints

## ◆ response

float cv::KeyPoint::response

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

## ◆ size

float cv::KeyPoint::size

diameter of the meaningful keypoint neighborhood

The documentation for this class was generated from the following file:

- core/include/opencv2/core/types.hpp