Detect holes, ends and beginnings of a line using openCV?
Question: I'm trying to create a Python script that detects holes, ends and beginnings
of a line. I thought that openCV would be great to achieve this.

So for example everything starts with this image:

[![enter image description
here](http://i.stack.imgur.com/sojmN.png)](http://i.stack.imgur.com/sojmN.png)

finally what I want to achieve is this:

[![enter image description
here](http://i.stack.imgur.com/abiRm.png)](http://i.stack.imgur.com/abiRm.png)

So I began with importing the image into Python and converting it in
grayscale. Now I came to the idea to track the holes by using the
`goodFeaturesToTrack()` method. It's normally used to find corners in the
image.

However that didn't work so well because after that the script knows the
points, but it doesn't know if a point is from a hole or if it's the beginning
or end of the line. Another problem is that if I use another image this method
detects more points than just the holes, beginnings and ends of the line.

Here is my full code to understand my problem a bit better:

    
    
    import cv2
    import numpy as np
    import matplotlib.pyplot as plt
    
    # lodes in img
    
    img = cv2.imread('png1.png', cv2.IMREAD_COLOR)
    img_gray = cv2.cvtColor(img, cv2.COLOR_RGB2GRAY)
    
    corners = cv2.goodFeaturesToTrack(img_gray, 200, 0.05, 10)
    
    for corner in corners:
        x, y = corner.ravel()
        cv2.circle(img, (x,y), 7, (255,255,0), -1)
    
    cv2.imshow('img',img)
    

I have no idea to get around this problem.


Answer: I added a func getLandmarks() it returns all the wholes. So here I assume that
it will be counted as a hole if there are 2 corners in a radius of 30 pix

    
    
    if abs(x1-x2)<=30 and abs(y1-y2)<=30:
    

This line defines the range.

    
    
    import cv2
    import numpy as np
    
    def getLandmarks(corners):
        holes=[]
        for i in range(0,len(corners)):
            for j in range(i+1,len(corners)):
                x1,y1=corners[i].ravel()
                x2,y2=corners[j].ravel()
                if abs(x1-x2)<=30 and abs(y1-y2)<=30:
                    holes.append((int((x1+x2)/2),int((y1+y2)/2)))
        return holes
    
    # lodes in img
    
    img = cv2.imread('img.png', cv2.IMREAD_COLOR)
    img_gray = cv2.cvtColor(img, cv2.COLOR_RGB2GRAY)
    
    corners = cv2.goodFeaturesToTrack(img_gray, 200, 0.05, 10)
    
    holes=getLandmarks(corners)
    print len(holes)
    for corner in holes:
        cv2.circle(img, (corner), 7, (255,255,0), -1)
    
    cv2.imshow('img',img)
    cv2.waitKey(0)
    

# output

[![enter image description
here](http://i.stack.imgur.com/5MI5Z.png)](http://i.stack.imgur.com/5MI5Z.png)

Now for the Start and end You can easily sort the corners in either X(if the
Path is along left to right) or Y(If the path is along top to down) and the
min and max will be your start and end!

