Process on curve/path skeleton binary image

Question

I am trying to develop a code that can process on path/curve of a skeleton of an image . I want to have a vector of points from the skeleton between two points .

This code ends after adding some points .I didn't find a solution for it .

 #include "opencv2/highgui/highgui.hpp"
 #include "opencv2/imgproc/imgproc.hpp"
 #include "opencv2/imgproc/imgproc_c.h"
 #include <opencv2/legacy/compat.hpp>
 #include <opencv2/core/core.hpp>
 #include <iostream>
 #include <cmath>
 #include <list>
 using namespace cv;
 using namespace std;

  //this method to find the 8-neighbors of a point from image 
 vector<Point> search8Neighbor(cv::Mat mat,Point startPoint)
 {
  vector<Point> segment;
  Point p;
    //uchar p1 = mat.at<uchar>(startPoint.x, startPoint.y);
    uchar p2 = mat.at<uchar>(startPoint.x-1, startPoint.y);
    uchar p3 = mat.at<uchar>(startPoint.x-1, startPoint.y+1);
    uchar p4 = mat.at<uchar>(startPoint.x, startPoint.y+1);
    uchar p5 = mat.at<uchar>(startPoint.x+1, startPoint.y+1);
    uchar p6 = mat.at<uchar>(startPoint.x+1, startPoint.y);
    uchar p7 = mat.at<uchar>(startPoint.x+1, startPoint.y-1);
    uchar p8 = mat.at<uchar>(startPoint.x, startPoint.y-1);
    uchar p9 = mat.at<uchar>(startPoint.x-1, startPoint.y-1);

    //if(p1==255) segment.push_back(startPoint.x, startPoint.y);
    if (p2 == 255) {
            p.x=startPoint.x-1;
            p.y=startPoint.y;
            segment.push_back(p);
    }
    if(p3==255) {
            p.x=startPoint.x-1;
            p.y=startPoint.y+1;
            segment.push_back(p);
    }

    if(p4==255) {
            p.x=startPoint.x;
            p.y=startPoint.y+1;
            segment.push_back(p);
    }

    if(p5==255) {
            p.x=startPoint.x+1;
            p.y=startPoint.y+1;
            segment.push_back(p);
    }

    if(p6==255) {
            p.x=startPoint.x+1;
            p.y=startPoint.y;
            segment.push_back(p);
    }
    if(p7==255) {
            p.x=startPoint.x+1;
            p.y=startPoint.y-1;
            segment.push_back(p);
    }
    if(p8==255){
            p.x=startPoint.x;
            p.y=startPoint.y-1;
            segment.push_back(p);
    }

    if(p9==255) {
            p.x=startPoint.x-1;
            p.y=startPoint.y-1;
            segment.push_back(p);
    }

    return segment ;

 }
  //this method return a vector of points from a skeleton that contains all the points 
 //  between a start point "peak" and an end point 
 //this method use the idea of Breadth first search 
  vector<Point> traceLine(Mat img , Point peak)
{
    vector<Point> vect1;
    vector<Point> vect2;
    img.at<uchar>(peak.x,peak.y)=0;//
    vect1.push_back(peak);//add peak to vect1
    while(vect1.size() != 0)
        {
        Point p=vect1[0];
        vect1.erase(vect1.begin());
        vect2.push_back(p);
        vector<Point> vectN;
        vectN=search8Neighbor(img,p);
        vector<Point>::iterator it;
        it = vect1.begin();
        //cout<<vectN.size()<<endl;
        for(int i=0;i<vectN.size();i++)
            {
                 img.at<uchar>(vectN[i].x,vectN[i].y)=0;
                vect1.insert(it,vectN[i]);

            }
        }
return vect2;
}

int main( int argc, char** argv )
{
cv::Mat im = cv::imread("aa.jpg",0);
if (im.empty())
return -1;
 cv::Mat img;
cv::threshold(im, img, 155, 255, CV_THRESH_BINARY);
vector<Point> vect1;
Point p;
p.x=20;
p.y=30;
if(mat.at<uchar>(p.x-1, p.y)==255)
vect1=traceLine(img,p);
imshow("image",im);
cv::waitKey();
return 0 ;
}

Answer 1

A Mat is indexed in the normal row/col way for matrices, so you want mat.at<uchar>(y, x), not mat.at<uchar>(x, y), or confusion will result.

Where you have:

vector<Point>::iterator it;
it = vect1.begin();
...
for(...)
    vect1.insert(it,vectN[i]);

this will break if the insert causes the buffer to be reallocated, because it then points at some memory that has been deallocated, or perhaps reallocated to something else. Instead use

 vect1.insert(vect1.begin(),vectN[i]);

(which will give slightly different order to what your program seems to do) or use push_back() to actually get breadth first behavior.

The biggest issue is in search8Neighbor(). Where you have uchar p2 = mat.at<uchar>(startPoint.x-1, startPoint.y); what happens if startPoint refers to a pixel at the edge of the image? You will be referring to pixels outside the image and the program will wander off until an exception occurs. Here you need to check where you are in the image and make sure you do not include pixels from outside the image in the neighborhood.

EDIT I didn't believe the code wouldn't work if changed as I said. I have implemented the fixes and it works:

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/imgproc/imgproc_c.h"
#include <opencv2/legacy/compat.hpp>
#include <opencv2/core/core.hpp>
#include <iostream>
#include <cmath>
#include <list>
using namespace cv;
using namespace std;

 // Find the 8-neighbors of a point from image.
 vector<Point> search8Neighbor(cv::Mat mat,Point startPoint)
 {
    vector<Point> neighbors;
    vector<Point> offsets;
    offsets.push_back(Point(-1,  0)); // p2
    offsets.push_back(Point(-1,  1)); // p3
    offsets.push_back(Point( 0,  1)); // p4
    offsets.push_back(Point( 1,  1)); // p5
    offsets.push_back(Point( 1,  0)); // p6
    offsets.push_back(Point( 1, -1)); // p7
    offsets.push_back(Point( 0, -1)); // p8
    offsets.push_back(Point(-1, -1)); // p9

    vector<Point>::iterator it;
    for(it = offsets.begin(); it < offsets.end(); ++it)
    {
        if(!((it->x < 0 && startPoint.x == 0)
            || (it->y < 0 && startPoint.y == 0)
            || (it->x > 0 && startPoint.x == mat.cols - 1)
            || (it->y > 0 && startPoint.y == mat.rows - 1)))
        {
            Point p(startPoint + *it);
            if(mat.at<uchar>(p) == 255)
                neighbors.push_back(p);
        }
    }

    return neighbors;
 }

 //this method return a vector of points from a skeleton that contains all the points 
 //  between a start point "peak" and an end point 
 //this method use the idea of Breadth first search 
  vector<Point> traceLine(Mat img , Point peak)
{
    vector<Point> vect1;
    vector<Point> vect2;
    img.at<uchar>(peak.y, peak.x)=0;//
    vect1.push_back(peak);//add peak to vect1
    while(vect1.size() != 0)
        {
        Point p=vect1[0];
        vect1.erase(vect1.begin());
        vect2.push_back(p);
        vector<Point> vectN;
        vectN = search8Neighbor(img, p);

        //cout<< " vectN.size()=" << vectN.size()<<endl;

        for(int i = 0; i < int(vectN.size()); ++i)
            {
                img.at<uchar>(vectN[i].y, vectN[i].x)=0;
                vect1.push_back(vectN[i]);
            }
        }
    return vect2;
}

int main( int argc, char** argv )
{
    cv::Mat im = cv::imread("aa.jpg",0);
    if (im.empty())
        return -1;
    cv::Mat img;
    cv::threshold(im, img, 155, 255, CV_THRESH_BINARY);
    imshow("thresholded image",im);
    vector<Point> vect1;

    Point p(3, 32);
    uchar u = img.at<uchar>(p.x, p.y);
    if(img.at<uchar>(p) == 255)
        vect1 = traceLine(img, p);

    Mat output(im.rows, im.cols, CV_8UC3, Scalar(0, 0, 0));
    vector<Point>::iterator it;

    for(it = vect1.begin(); it < vect1.end(); ++it)
    {
        Vec3b green(0, 255, 0);
        output.at<Vec3b>(*it) = green;
    }
    imshow("output", output);
    cv::waitKey();
    return 0 ;
}