Detecting corners location in subpixeles

Goal

In this tutorial you will learn how to:

• Use the OpenCV function cv::cornerSubPix to find more exact corner positions (more exact than integer pixels).

Theory

Code

This tutorial code's is shown lines below. You can also download it from here

#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>

using namespace cv;
using namespace std;

Mat src, src_gray;

int maxCorners = 10;
int maxTrackbar = 25;

RNG rng(12345);
char* source_window = "Image";

void goodFeaturesToTrack_Demo( int, void* );

/* @function main */
int main( int argc, char** argv )
{
  src = imread( argv[1], 1 );
  cvtColor( src, src_gray, COLOR_BGR2GRAY );

  namedWindow( source_window, WINDOW_AUTOSIZE );

  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; }

  vector<Point2f> corners;
  double qualityLevel = 0.01;
  double minDistance = 10;
  int blockSize = 3;
  bool useHarrisDetector = false;
  double k = 0.04;

  Mat copy;
  copy = src.clone();

  goodFeaturesToTrack( src_gray,
               corners,
               maxCorners,
               qualityLevel,
               minDistance,
               Mat(),
               blockSize,
               useHarrisDetector,
               k );


  cout<<"** Number of corners detected: "<<corners.size()<<endl;
  int r = 4;
  for( int 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 ); }

  namedWindow( source_window, WINDOW_AUTOSIZE );
  imshow( source_window, copy );

  Size winSize = Size( 5, 5 );
  Size zeroZone = Size( -1, -1 );
  TermCriteria criteria = TermCriteria( TermCriteria::EPS + TermCriteria::MAX_ITER, 40, 0.001 );

  cornerSubPix( src_gray, corners, winSize, zeroZone, criteria );

  for( int i = 0; i < corners.size(); i++ )
     { cout<<" -- Refined Corner ["<<i<<"]  ("<<corners[i].x<<","<<corners[i].y<<")"<<endl; }
}

Explanation

Result

Here is the result: