Table of Contents

Class ORB

Namespace
OpenCvSharp
Assembly
OpenCvSharp.dll

Class implementing the ORB (oriented BRIEF) keypoint detector and descriptor extractor.

described in @cite RRKB11 . The algorithm uses FAST in pyramids to detect stable keypoints, selects the strongest features using FAST or Harris response, finds their orientation using first-order moments and computes the descriptors using BRIEF (where the coordinates of random point pairs (or k-tuples) are rotated according to the measured orientation).

public class ORB : Feature2D, IDisposable, ICvPtrHolder
Inheritance
ORB
Implements
Inherited Members

Constructors

ORB(nint) 

protected ORB(nint p)

Parameters

p nint

Properties

EdgeThreshold 

public int EdgeThreshold { get; set; }

Property Value

int

FastThreshold 

public int FastThreshold { get; set; }

Property Value

int

FirstLevel 

public int FirstLevel { get; set; }

Property Value

int

MaxFeatures 

public int MaxFeatures { get; set; }

Property Value

int

NLevels 

public int NLevels { get; set; }

Property Value

int

PatchSize 

public int PatchSize { get; set; }

Property Value

int

ScaleFactor 

public double ScaleFactor { get; set; }

Property Value

double

ScoreType 

public ORBScoreType ScoreType { get; set; }

Property Value

ORBScoreType

WTA_K 

public int WTA_K { get; set; }

Property Value

int

Methods

Create(int, float, int, int, int, int, ORBScoreType, int, int)

The ORB constructor

public static ORB Create(int nFeatures = 500, float scaleFactor = 1.2, int nLevels = 8, int edgeThreshold = 31, int firstLevel = 0, int wtaK = 2, ORBScoreType scoreType = ORBScoreType.Harris, int patchSize = 31, int fastThreshold = 20)

Parameters

nFeatures int

The maximum number of features to retain.

scaleFactor float

Pyramid decimation ratio, greater than 1. scaleFactor==2 means the classical pyramid, where each next level has 4x less pixels than the previous, but such a big scale factor will degrade feature matching scores dramatically. On the other hand, too close to 1 scale factor will mean that to cover certain scale range you will need more pyramid levels and so the speed will suffer.

nLevels int

The number of pyramid levels. The smallest level will have linear size equal to input_image_linear_size/pow(scaleFactor, nlevels - firstLevel).

edgeThreshold int

This is size of the border where the features are not detected. It should roughly match the patchSize parameter.

firstLevel int

The level of pyramid to put source image to. Previous layers are filled with upscaled source image.

wtaK int

The number of points that produce each element of the oriented BRIEF descriptor. The default value 2 means the BRIEF where we take a random point pair and compare their brightnesses, so we get 0/1 response. Other possible values are 3 and 4. For example, 3 means that we take 3 random points (of course, those point coordinates are random, but they are generated from the pre-defined seed, so each element of BRIEF descriptor is computed deterministically from the pixel rectangle), find point of maximum brightness and output index of the winner (0, 1 or 2). Such output will occupy 2 bits, and therefore it will need a special variant of Hamming distance, denoted as NORM_HAMMING2 (2 bits per bin). When WTA_K=4, we take 4 random points to compute each bin (that will also occupy 2 bits with possible values 0, 1, 2 or 3).

scoreType ORBScoreType

The default HARRIS_SCORE means that Harris algorithm is used to rank features (the score is written to KeyPoint::score and is used to retain best nfeatures features); FAST_SCORE is alternative value of the parameter that produces slightly less stable keypoints, but it is a little faster to compute.

patchSize int

size of the patch used by the oriented BRIEF descriptor. Of course, on smaller pyramid layers the perceived image area covered by a feature will be larger.

fastThreshold int

the fast threshold

Returns

ORB

DisposeManaged()

Releases managed resources

protected override void DisposeManaged()
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