Accurate subpixel edge location in C (OPENCV) - opencv

Question

I want to find subpixel and I resarched this topic However I think that subpixels must be such as 152.6 , 49.3 ...
I find this document in opencv http://docs.opencv.org/2.4/modules/imgproc/doc/feature_detection.html?highlight=cornersubpix#cornersubpix
And I try this code
#include <iostream>
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/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 = 50;
RNG rng(11111);
char* source_window = "Image";
void goodFeaturesToTrack_Demo( int, void* );
int main( int argc, char** argv )
{
src = imread( "a.png", 1 );
cvtColor( src, src_gray, CV_BGR2GRAY );
namedWindow( source_window, CV_WINDOW_AUTOSIZE );
createTrackbar( "Max corners:", source_window, &maxCorners, maxTrackbar, goodFeaturesToTrack_Demo);
imshow( source_window, src );
goodFeaturesToTrack_Demo( 0, 0 );
waitKey(0);
return(0);
}
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, CV_WINDOW_AUTOSIZE );
imshow( source_window, copy );
Size winSize = Size( 10, 10 );
Size zeroZone = Size( -1, -1 );
TermCriteria criteria = TermCriteria( CV_TERMCRIT_EPS + CV_TERMCRIT_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; }
}
But I have this result:
this code ofind only corner's subpixel I want to find edge's subpixel