keyPointsToStruct

Convert OpenCV KeyPoints object to MATLAB structure

Since R2021b

Syntax

mlstruct = keyPointsToStruct(keypoints)

Description

mlstruct = keyPointsToStruct(keypoints) converts an OpenCV KeyPoints object to a MATLAB^® structure.

Examples

Detect Keypoints in Image Using OpenCV in MATLAB

This example shows how to use the prebuilt MATLAB interface for the OpenCV function cv::Fast in MATLAB to detect keypoints in an image. Additionally, use the keyPointsToStruct utility function to write the keypoints returned by the OpenCV cv::Fast function to a MATLAB structure.

Add the MATLAB interface to OpenCV package names to the import list.

import clib.opencv.*;
import vision.opencv.util.*;

Read an image into the MATLAB workspace.

img = imread("elephant.jpg");

Create MATLAB interface objects for the OpenCV MatND and InputArray classes to store the input image.

[inputMat,inputArray] = createMat(img);

Create a MATLAB interface object for the OpenCV KeyPoint vector by using the clibArray function.

keyPointsVec = clibArray("clib.opencv.cv.KeyPoint",0);

Specify the parameters for computing keypoints using the FAST detector.

threshold = 100;
nonmaxSuppression = true;

Compute keypoints in the image by calling the OpenCV function cv::FAST in MATLAB.

cv.FAST(inputArray,keyPointsVec,threshold,nonmaxSuppression);

Convert the KeyPoints object returned by the OpenCV function into a MATLAB structure.

mlstruct = keyPointsToStruct(keyPointsVec);

Inspect the fields in the output MATLAB structure.

mlstruct

mlstruct = struct with fields:
       Location: [48x2 double]
          Scale: [48x1 double]
         Metric: [48x1 double]
           Misc: [48x1 double]
    Orientation: [48x1 double]

Display the input image and plot the detected keypoints.

figure
imshow(img)
hold on
plot(mlstruct.Location(:,1),mlstruct.Location(:,2),"*r")
hold off

Input Arguments

keypoints — OpenCV KeyPoints class

MATLAB interface object

OpenCV KeyPoints class, specified as a MATLAB interface object. This interface object is a representation of the KeyPoints class returned by any of the OpenCV functions for keypoint detection.

Output Arguments

mlstruct — Keypoints detected using OpenCV function

structure

Keypoints detected using the OpenCV function, returned as a MATLAB structure with fields Location, Scale, Metric, Misc, and Orientation.

Fields	Description
Location	x and y-coordinates of the keypoints.
Scale	Diameter of the neighborhood region around the keypoints.
Metric	Strength of the keypoints.
Orientation	Orientation of the keypoints.

Version History

Introduced in R2021b