Class ORB

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).

Inheritance
System.Object
DisposableObject
DisposableCvObject
Algorithm
Feature2D
ORB
Implements
Inherited Members
Namespace: OpenCvSharp
Assembly: OpenCvSharp.dll
Syntax
public class ORB : Feature2D, ICvPtrHolder

Constructors

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ORB(IntPtr)

Declaration
protected ORB(IntPtr p)
Parameters
Type Name Description
IntPtr p

Properties

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EdgeThreshold

Declaration
public int EdgeThreshold { get; set; }
Property Value
Type Description
System.Int32
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FastThreshold

Declaration
public int FastThreshold { get; set; }
Property Value
Type Description
System.Int32
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FirstLevel

Declaration
public int FirstLevel { get; set; }
Property Value
Type Description
System.Int32
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MaxFeatures

Declaration
public int MaxFeatures { get; set; }
Property Value
Type Description
System.Int32
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NLevels

Declaration
public int NLevels { get; set; }
Property Value
Type Description
System.Int32
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PatchSize

Declaration
public int PatchSize { get; set; }
Property Value
Type Description
System.Int32
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ScaleFactor

Declaration
public double ScaleFactor { get; set; }
Property Value
Type Description
System.Double
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ScoreType

Declaration
public ORBScoreType ScoreType { get; set; }
Property Value
Type Description
ORBScoreType
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WTA_K

Declaration
public int WTA_K { get; set; }
Property Value
Type Description
System.Int32

Methods

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Create(Int32, Single, Int32, Int32, Int32, Int32, ORBScoreType, Int32, Int32)

The ORB constructor

Declaration
public static ORB Create(int nFeatures = 500, float scaleFactor = 1.2F, int nLevels = 8, int edgeThreshold = 31, int firstLevel = 0, int wtaK = 2, ORBScoreType scoreType = ORBScoreType.Harris, int patchSize = 31, int fastThreshold = 20)
Parameters
Type Name Description
System.Int32 nFeatures

The maximum number of features to retain.
System.Single scaleFactor

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.
System.Int32 nLevels

The number of pyramid levels. The smallest level will have linear size equal to input_image_linear_size/pow(scaleFactor, nlevels - firstLevel).
System.Int32 edgeThreshold

This is size of the border where the features are not detected. It should roughly match the patchSize parameter.
System.Int32 firstLevel

The level of pyramid to put source image to. Previous layers are filled with upscaled source image.
System.Int32 wtaK

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).
ORBScoreType scoreType

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.
System.Int32 patchSize

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.
System.Int32 fastThreshold

the fast threshold
Returns
Type Description
ORB
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DisposeManaged()

Releases managed resources

Declaration
protected override void DisposeManaged()
Overrides

Implements