Opencv Dilation and Skeleton(c++) - c++

Question

I have this image :
And I applied dilation with this code :
int dilation_elem = 0;
int dilation_size = 0;
int const max_elem = 2;
int const max_kernel_size = 21;
void Dilation( int, void* );
int main( int argc, char** argv )
{
src = imread("a18.png");
if( !src.data )
{ return -1; }
namedWindow( "Dilation Demo", CV_WINDOW_AUTOSIZE );
cvMoveWindow( "Dilation Demo", src.cols, 0 );
createTrackbar( "Element:\n 0: Rect \n 1: Cross \n 2: Ellipse", "Dilation Demo",
&dilation_elem, max_elem,
Dilation );
createTrackbar( "Kernel size:\n 2n +1", "Dilation Demo",
&dilation_size, max_kernel_size,
Dilation );
//int dilation_size =7;
/// Default start
Dilation( 0, 0 );
waitKey(0);
return 0;
}
void Dilation( int, void* )
{
int dilation_type;
if( dilation_elem == 0 ){ dilation_type = MORPH_RECT; }
else if( dilation_elem == 1 ){ dilation_type = MORPH_CROSS; }
else if( dilation_elem == 2) { dilation_type = MORPH_ELLIPSE; }
Mat element = getStructuringElement( dilation_type,
Size( 2*dilation_size + 1, 2*dilation_size+1 ),
Point( dilation_size, dilation_size ) );
dilate( src, dilation_dst, element );
imshow( "Dilation Demo", dilation_dst );
imwrite("a18d.png",dilation_dst);
}
And after this step I get this consequent:
And the final step is skeleton :
#include <iostream>
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
int main()
{
cv::Mat img = cv::imread("a18d.png", 0);
cv::threshold(img, img, 127, 255, cv::THRESH_BINARY);
cv::Mat skel(img.size(), CV_8UC1, cv::Scalar(0));
cv::Mat temp;
cv::Mat eroded;
cv::Mat element = cv::getStructuringElement(cv::MORPH_CROSS, cv::Size(3,3));
bool done;
do
{
cv::erode(img, eroded, element);
cv::dilate(eroded, temp, element); // temp = open(img)
cv::subtract(img, temp, temp);
cv::bitwise_or(skel, temp, skel);
eroded.copyTo(img);
done = (cv::countNonZero(img) == 0);
} while (!done);
cv::imshow("Skeleton", skel);
cv::imwrite("18s.png",skel);
cv::waitKey(0);
return 0;
}
code here
And I hve this image :
But I want image like this :
What can I do for this? What is the problem
these steps was done for Image 4 and result is good

Answer 1

Open CV doesn't seem to have a shrink as opposed to an erode operation. At least I can't find it. Try mine
https://github.com/MalcolmMcLean/binaryimagelibrary/blob/master/medialaxistransform.c
Try just using the function "thin" instead of eroding your image.