Detect thick black lines in image with OpenCV

I have the following image of a lego board with some bricks on it

Now I am trying to detect the thick black lines (connecting the white squares) with OpenCV. I have already experimented a lot with HoughLinesP, converted the image to gray or b/w before, applied blur, ... Nonthing led to usable results.

# Read image
img = cv2.imread('image.jpg', cv2.IMREAD_GRAYSCALE)

# Resize Image
img =  cv2.resize(img, (0,0), fx=0.25, fy=0.25) 

# Initialize output
out = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)

# Median blurring to get rid of the noise; invert image
img = cv2.medianBlur(img, 5)

# Adaptive Treshold
bw = cv2.adaptiveThreshold(img,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C,\
            cv2.THRESH_BINARY,15,8)

# HoughLinesP
linesP = cv2.HoughLinesP(bw, 500, np.pi / 180, 50, None, 50, 10)

# Draw Lines
if linesP is not None:
    for i in range(0, len(linesP)):
        l = linesP[i][0]
        cv2.line(out, (l[0], l[1]), (l[2], l[3]), (0,0,255), 3, cv2.LINE_AA)

The adaptive treshold lets you see edges quite well, but with HoughLinesP you don't get anything usable out of it

What am I doing wrong?

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Thanks, both @fmw42 and @jeru-luke for your great solutions to this problem! I liked isolating / masking the green board, so I combined both:

import cv2
import numpy as np
 
img = cv2.imread("image.jpg")

scale_percent = 50 # percent of original size
width = int(img.shape[1] * scale_percent / 100)
height = int(img.shape[0] * scale_percent / 100)
dim = (width, height)
  
# resize image
img = cv2.resize(img, dim, interpolation = cv2.INTER_AREA)
lab = cv2.cvtColor(img, cv2.COLOR_BGR2LAB)
a_component = lab[:,:,1]

# binary threshold the a-channel
th = cv2.threshold(a_component,127,255,cv2.THRESH_BINARY_INV+cv2.THRESH_OTSU)[1]

# numpy black
black = np.zeros((img.shape[0],img.shape[1]),np.uint8)

# function to obtain the largest contour in given image after filling it
def get_region(image):
    contours, hierarchy = cv2.findContours(image, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
    c = max(contours, key = cv2.contourArea)
    mask = cv2.drawContours(black,[c],0,255, -1)
    return mask

mask = get_region(th)

# turning the region outside the green block white
green_block = cv2.bitwise_and(img, img, mask = mask)
green_block[black==0]=(255,255,255)

# median blur
median = cv2.medianBlur(green_block, 5)

# threshold on black
lower = (0,0,0)
upper = (15,15,15)
thresh = cv2.inRange(median, lower, upper)

# apply morphology open and close
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (3,3))
morph = cv2.morphologyEx(thresh, cv2.MORPH_OPEN, kernel)
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (29,29))
morph = cv2.morphologyEx(morph, cv2.MORPH_CLOSE, kernel)

# filter contours on area
contours = cv2.findContours(morph, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]
result = green_block.copy()
for c in contours:
    area = cv2.contourArea(c)
    if area > 1000:
            cv2.drawContours(result, [c], -1, (0, 0, 255), 2)
  

# view result
cv2.imshow("result", result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Here I am presenting a repeated segmentation approach using color. This answer is based on the usage of LAB color space

---

1. Isolating the green lego block

img = cv2.imread(image_path)
lab = cv2.cvtColor(img, cv2.COLOR_BGR2LAB)
a_component = lab[:,:,1]

# binary threshold the a-channel
th = cv2.threshold(a_component,127,255,cv2.THRESH_BINARY_INV+cv2.THRESH_OTSU)[1]

th

# function to obtain the largest contour in given image after filling it
def get_region(image):
    contours, hierarchy = cv2.findContours(image, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
    c = max(contours, key = cv2.contourArea)
    black = np.zeros((image.shape[0], image.shape[1]), np.uint8)
    mask = cv2.drawContours(black,[c],0,255, -1)
    return mask

mask = get_region(th)

mask

# turning the region outside the green block white
green_block = cv2.bitwise_and(img, img, mask = mask)
green_block[black==0]=(255,255,255)

green_block

---

2. Segmenting the road

• To get an approximate region of the road, I subtracted the mask and th.

cv2.subtract() performs arithmetic subtraction, where cv2 will take care of negative values.

road = cv2.subtract(mask,th)
# `road` contains some unwanted spots/contours which are removed using the function "get_region"
only_road = get_region(road)

only_road

Masking only the road segment with the original image gives

road_colored = cv2.bitwise_and(img, img, mask = only_road)
road_colored[only_road==0]=(255,255,255)

road_colored

From the above image only the black regions (road) are present, which is easy to segment:

# converting to grayscale and applying threshold
th2 = cv2.threshold(road_colored[:,:,1],127,255,cv2.THRESH_BINARY_INV+cv2.THRESH_OTSU)[1]

# using portion of the code from fmw42's answer, to get contours above certain area
contours = cv2.findContours(th2, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]
result = img.copy()
for c in contours:
    area = cv2.contourArea(c)
    if area > 1000:
        cv2.drawContours(result, [c], -1, (0, 0, 255), 4)

result

Note: To clean up the end result, you can apply morphological operations on th2 before drawing contours.