How to detect horizontal lines in an image and obtain its y-coordinates using python and opencv?

Question

I am using the find contours method and then approximating a line by using fitline function. below is the code:

img = cv2.imread('lines.jpg')
imgray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
ret,dst = cv2.threshold(imgray,127,255,0)
im2,cnts, hierarchy =cv2.findContours(dst,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
rows,cols = img.shape[:2]
[vx,vy,x,y] = cv2.fitLine(cnts[0], cv2.DIST_L2,0,0.01,0.01)
lefty = int((-x*vy/vx) + y)
righty = int(((cols-x)*vy/vx)+y)
cv2.line(img,(cols-1,righty),(0,lefty),(0,255,0),2)
print img.shape[:2]
cv2.imshow('image1',img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Following is the image I am getting as output

I was expecting it to detect each of the black stripes in the image whereas it is detecting only the first line from the bottom

Answer 1

This modified code is what you may have been looking for. I have left most of your lines and it is pretty verbose, but this might help you understand it without further explanation.

I think that you got trapped in two different of problems:

• cnts[0] in your code refers only to first contour. I have introduced a parameter contnumber which you may have to loop over. My current code only executes for contour number 4

• You may not have detected the lines as foreground, but the white areas, this is why I negated the images (thresh = (255-thresh))

  import numpy as np
  import cv2
  
  im = cv2.imread('lines.jpg')
  rows,cols = im.shape[:2]
  imgray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY)
  ret,thresh = cv2.threshold(imgray,125,255,0)
  thresh = (255-thresh)
  thresh2=thresh.copy()
  im2, contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
  
  cv2.imshow('image1',im)
  cv2.imshow('image3',thresh2)
  #cv2.drawContours(im, contours, -1, (0,255,0), 3) #draw all contours
  contnumber=4
  cv2.drawContours(im, contours, contnumber, (0,255,0), 3) #draw only contour contnumber
  cv2.imshow('contours', im)
  
  [vx,vy,x,y] = cv2.fitLine(contours[contnumber], cv2.DIST_L2,0,0.01,0.01)
  lefty = int((-x*vy/vx) + y)
  righty = int(((cols-x)*vy/vx)+y)
  cv2.line(im,(cols-1,righty),(0,lefty),(0,255,255),2)
  
  cv2.imshow('result', im)
  
  cv2.waitKey(0)
  cv2.destroyAllWindows()
  

Hope that helps.