OpenCV goodFeaturesToTrack in Obj-C-C++ - c++

Question

I'm trying to iterate over the points i get from cv::goodFeaturesToTrack.
I retrieve my points using this code:
std::vector<std::vector<cv::Point2f> > corners;
cv::goodFeaturesToTrack(image,corners, 500, 0.01, 10);
My idea, which doesn't work:
for (size_t idx = 0; idx < corners.size(); idx++) {
cv::circle(image,corners.at(idx),radius,color,thickness);
}
Any ideas?

Answer 1

The detector goodFeaturesToTrack (indeed all feature detectors) populate a vector of features, while you are trying to pass it a vector of a vector of features. The remainder of your code looks fine but you should change the line
std::vector<std::vector<cv::Point2f>> corners;
to
std::vector<cv::Point2f> corners;
and hopefully all will be well.

Answer 2

Not sure what you mean by "doesn't work". But, you might try looking at the goodFeaturesToTrack_Demo.cpp to see if that usage will fix your problem.
Update with code:
/**
* #function goodFeaturesToTrack_Demo.cpp
* #brief Demo code for detecting corners using Shi-Tomasi method
* #author OpenCV team
*/
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace cv;
using namespace std;
/// Global variables
Mat src, src_gray;
int maxCorners = 23;
int maxTrackbar = 100;
RNG rng(12345);
const char* source_window = "Image";
/// Function header
void goodFeaturesToTrack_Demo( int, void* );
/**
* #function main
*/
int main( int, char** argv )
{
/// Load source image and convert it to gray
src = imread( argv[1], 1 );
cvtColor( src, src_gray, COLOR_BGR2GRAY );
/// Create Window
namedWindow( source_window, WINDOW_AUTOSIZE );
/// Create Trackbar to set the number of corners
createTrackbar( "Max corners:", source_window, &maxCorners, maxTrackbar, goodFeaturesToTrack_Demo );
imshow( source_window, src );
goodFeaturesToTrack_Demo( 0, 0 );
waitKey(0);
return(0);
}
/**
* #function goodFeaturesToTrack_Demo.cpp
* #brief Apply Shi-Tomasi corner detector
*/
void goodFeaturesToTrack_Demo( int, void* )
{
if( maxCorners < 1 ) { maxCorners = 1; }
/// Parameters for Shi-Tomasi algorithm
vector<Point2f> corners;
double qualityLevel = 0.01;
double minDistance = 10;
int blockSize = 3;
bool useHarrisDetector = false;
double k = 0.04;
/// Copy the source image
Mat copy;
copy = src.clone();
/// Apply corner detection
goodFeaturesToTrack( src_gray,
corners,
maxCorners,
qualityLevel,
minDistance,
Mat(),
blockSize,
useHarrisDetector,
k );
/// Draw corners detected
cout<<"** Number of corners detected: "<<corners.size()<<endl;
int r = 4;
for( size_t i = 0; i < corners.size(); i++ )
{ circle( copy, corners[i], r, Scalar(rng.uniform(0,255), rng.uniform(0,255), rng.uniform(0,255)), -1, 8, 0 ); }
/// Show what you got
namedWindow( source_window, WINDOW_AUTOSIZE );
imshow( source_window, copy );
}