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ORBPoints

Object for storing ORB keypoints

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Description

AnORBPointsobject stores the Oriented FAST and rotated BRIEF (ORB) keypoints in an image. You can specify the keypoints and store them as anORBPointsobject. You can also use thedetectORBFeaturesfunction to detect the ORB keypoints in an image. ThedetectORBFeaturesfunction stores the detected ORB keypoints as anORBPointsobject. UseObject Functionsto plot, select, and manipulate the detected ORB keypoints.

Creation

Syntax

points = ORBPoints
points = ORBPoints(location)
points = ORBPoints(location,Name,Value)

Description

points= ORBPointscreates anORBPointsobject with default property values.

example

points= ORBPoints(location)creates anORBPointsobject from a set of location coordinates specified bylocation. The location input sets theLocationproperty.

example

points= ORBPoints(location,Name,Value)sets properties of the object using one or more name-value pair arguments. Enclose each property name in quotes. For example,ORBPoints(location,'Count',15)creates anORBPointsobject withCountproperty set to 15.

Properties

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This property is read-only.

Location of keypoints, specified as anM-by-2 matrix. Each row is of the form [xy] and represents the location of a keypoint.Mis the number of keypoints. You cannot set this property as a name-value pair. Use thelocationinput argument.

This property is read-only.

Strength of keypoints, specified as one of these values:

  • A scalar — Detected keypoints have the same strength. In this case, the object assigns the sameMetricvalue to all keypoints.

  • AnM-element vector — Detected keypoints have different strength values. In this case, the object assigns differentMetricvalue to each keypoint.Mis the number of keypoints.

Example:ORBPoints(location,'Metric',0.5)

Data Types:single

This property is read-only.

Number of keypoints held by the object, specified as a nonnegative integer.

This property is read-only.

Scale factor, specified as one of these values:

  • A scalar — All keypoints are detected at the same level of decomposition. In this case, the object assigns the sameScalevalue to all keypoints.

  • AnM-element vector — The keypoints are detected at different levels of decomposition. In this case, the object assigns differentScalevalue to each keypoint.Mis the number of keypoints.

The scale factor specifies the level of decomposition at which a keypoint is detected.

Example:ORBPoints(位置,' Scale',1.2)

Data Types:single

This property is read-only.

Angle of keypoints in radians, specified as one of these values:

  • A scalar — Detected keypoints are of the same orientation. In this case, the object assigns the sameOrientationvalue to all keypoints.

  • AnM-element vector — Detected keypoints are of different orientation. In this case, the object assigns differentOrientationvalue to each keypoint.Mis the number of keypoints.

The angle made by a keypoint is defined with reference to the horizontal axis of the image. The coordinate of the keypoint is set as the origin of the axis.

Example:ORBPoints(location,'Orientation',0.7854)

Data Types:single|double|int8|int16|int32|int64|uint8|uint16|uint32|uint64

Object Functions

isempty Determine if points object is empty
length Number of stored points
plot Plot points
selectStrongest Select points with strongest metrics
size Return size of points object
selectUniform Select uniformly distributed subset of feature points

Examples

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Open Live Script

Read an image into the workspace.

I = imread('licensePlates.jpg');

Convert the image into a grayscale image.

I = im2gray(我);

Specify the location of keypoints in the image.

location = [400 398;...485 343;...274 323;...274 367;...241 313;...302 213];

Create anORBPointsobject and display its properties.

points = ORBPoints(location)
points = 6x1 ORBPoints array with properties: Location: [6x2 single] Metric: [6x1 single] Count: 6 Scale: [6x1 single] Orientation: [6x1 single]

Inspect theScaleandOrientationproperties of theORBPointsobject.

points.Scale
ans =6x1 single column vector1 1 1 1 1 1
points.Orientation
ans =6x1 single column vector0 0 0 0 0 0

Specify the scale value for each keypoint.

scale = [2.1 2.5 2.5 4 2.3 3.9];

Specify the angle values for the keypoints as 0.7854 radians.

angle = 0.7854;

Create anORBPointsobject with the keypoints and the updated scale and angle values. The output is anORBPointsobject containing keypoints with the modifiedScaleandOrientationproperties.

points = ORBPoints(location,'Scale',scale,'Orientation',angle);

Inspect the updatedScalevalues.

points.Scale
ans =6x1 single column vector2.1000 2.5000 2.5000 4.0000 2.3000 3.9000

Inspect the updatedOrientationvalues. Since theOrientationvalue is a scalar, the object assigns same value to all keypoints.

points.Orientation
ans =6x1 single column vector0.7854 0.7854 0.7854 0.7854 0.7854 0.7854

Display the image and plot the ORB keypoints on the image.

figure imshow(I) holdon情节(分)

Figure contains an axes object. The axes object contains 3 objects of type image, line.

Open Live Script

Read an image into the workspace.

I = imread('cameraman.tif');

Use thedetectORBFeaturesfunction to detect ORB keypoints in the image. The function returns the detected ORB keypoints as anORBPointsobject.

points = detectORBFeatures(I);

Use theselectUniformobject function to select 10 ORB keypoints. The output of theselectUniformobject function is anORBPointsobject.

newPoints = selectUniform(points,10,size(I))
newPoints = 10 x1 ORBPoints数组属性:Location: [10x2 single] Metric: [10x1 single] Count: 10 Scale: [10x1 single] Orientation: [10x1 single]

Display the location and scale of the selected keypoints on the image.

figure imshow(I) holdonplot(newPoints)

Figure contains an axes object. The axes object contains 3 objects of type image, line.

Display the (x, y) coordinates of the selected keypoints.

x = newPoints.Location(:,1); y = newPoints.Location(:,2); [x y]
ans =10x2 single matrix147 62 111 63 143 67 127 89 47 124 34 154 183 205 113 206 111 207 148 66

Display the orientation of the selected keypoints.

newPoints.Orientation
ans =10x1 single column vector5.4682 0.7888 5.3084 6.2443 2.8221 3.8440 6.1212 6.0344 4.8840 5.5535

Tips

AlthoughORBPointscan hold many points, it is a scalar object. Therefore,numel(ORBPoints)always returns 1. This value can differ fromlength(ORBPoints), which returns the true number of points held by the object.

References

[1] Rublee, E., V. Rabaud, K. Konolige, and G. Bradski. "ORB: An Efficient Alternative to SIFT or SURF." InProceedings of the 2011 International Conference on Computer Vision, pp. 2564–2571. Barcelona, Spain: IEEE, 2011.

Extended Capabilities

Version History

Introduced in R2019a

See Also

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