cornerPoints

Object for storing corner points

Description

This object stores information about feature points detected from a 2-D grayscale image.

Creation

Syntax

points = cornerPoints(location)

points = cornerPoints(location,Name=Value)

Description

points = cornerPoints(location) constructs a cornerPoints object from an M-by-2 array [x y] of location coordinates.

points = cornerPoints(location,Name=Value) specifies options using one or more name-value arguments in addition to the previous syntax. For example cornerPoints(location,Metric=12.0) sets strength of the detected feature to 12.0.

Input Arguments

location — Location coordinates

M-by-2 array

Location of points, specified as an M-by-2 array of [x y] coordinates.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: points = cornerPoints(location,Metric=12.0) specifies a radius of 12.0 (in pixels) of the cornerPoints sampling pattern used during feature extraction.

Metric — Strength of detected feature

0.0 (default) | numeric scalar

Strength of detected feature, specified as a numeric value. The algorithm uses a determinant of an approximated Hessian.

Properties

Location — Location of keypoints

[] (default) | M-by-2 matrix

This property is read-only.

Location of keypoints, specified as an M-by-2 matrix. Each row is of the form [x y] and represents the location of a keypoint. M is the number of keypoints. You cannot set this property, use the location input argument instead.

Count — Number of points

0 (default) | integer

Number of points held by the corner points object, specified as a numeric value.

Metric — Strength of detected feature

0.0 (default) | numeric scalar

Strength of detected feature, specified as a numeric value. The algorithm uses a determinant of an approximated Hessian.

Object Functions

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

Examples

Plot Strongest Features from Detected Feature Points

Read an image.

I = imread('cameraman.tif');

Detect feature points.

points = detectHarrisFeatures(I);

Display the ten strongest points.

strongest = selectStrongest(points,10);
imshow(I)
hold on
plot(strongest)

Display the (x,y) coordinates of the strongest points.

strongest.Location

ans = 10x2 single matrix

  112.4516  208.4412
  108.6510  228.1681
  136.6969  114.7962
  181.4160  205.9876
  135.5823  123.4529
  100.4951  174.3253
  146.7581   94.7393
  135.2899   92.6485
  129.8439  110.0350
  130.5716   91.0424

Create Corner Points Object and Display Points

Create a checkerboard image.

I = checkerboard(50,2,2);

Load the locations of corner points.

location = [51    51    51   100   100   100   151   151   151; ...
            50   100   150    50   101   150    50   100   150]';

Save the points in a cornerPoints object.

points = cornerPoints(location);

Display the points on the checkerboard.

imshow(I)
hold on
plot(points)

Tips

Although cornerPoints may hold many points, it is a scalar object. Therefore, numel(cornerPoints) always returns 1. This value may differ from length(cornerPoints), which returns the true number of points held by the object.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• To index locations with this object, use the syntax: points.Location(idx,:), for points object. See visionRecovertformCodeGeneration_kernel.m, which is used in the Introduction to Code Generation with Feature Matching and Registration example.

Version History

Introduced in R2012a