How to approximate jagged edges as lines using Python OpenCV?


I am trying to find accurate locations for the corners on ink blotches as seen below:

My idea is to fit lines to the edges and then find where they intersect. As of now, I’ve tried using cv2.approxPolyDP() with various values of epsilon to approximate the edges, however this doesn’t look like the way to go. My cv2.approxPolyDP code gives the following result:

Ideally, this is what I want to produce (drawn on paint):

Are there CV functions in place for this sort of problem? I’ve considered using Gaussian blurring before the threshold step although that method does not seem like it would be very accurate for corner finding. Additionally, I would like this to be robust to rotated images, so filtering for vertical and horizontal lines won’t necessarily work without other considerations.


import numpy as np
from PIL import ImageGrab
import cv2

def process_image4(original_image):  # Douglas-peucker approximation
    # Convert to black and white threshold map
    gray = cv2.cvtColor(original_image, cv2.COLOR_BGR2GRAY)
    gray = cv2.GaussianBlur(gray, (5, 5), 0)
    (thresh, bw) = cv2.threshold(gray, 128, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)

    # Convert bw image back to colored so that red, green and blue contour lines are visible, draw contours
    modified_image = cv2.cvtColor(bw, cv2.COLOR_GRAY2BGR)
    contours, hierarchy = cv2.findContours(bw, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
    cv2.drawContours(modified_image, contours, -1, (255, 0, 0), 3)

    # Contour approximation
    try:  # Just to be sure it doesn't crash while testing!
        for cnt in contours:
            epsilon = 0.005 * cv2.arcLength(cnt, True)
            approx = cv2.approxPolyDP(cnt, epsilon, True)
            # cv2.drawContours(modified_image, [approx], -1, (0, 0, 255), 3)
    return modified_image

def screen_record():
        screen = np.array(ImageGrab.grab(bbox=(100, 240, 750, 600)))
        image = process_image4(screen)
        cv2.imshow('window', image)
        if cv2.waitKey(25) & 0xFF == ord('q'):

  • A note about my code: I’m using screen capture so that I can process these images live. I have a digital microscope that can display live feed on a screen, so the constant screen recording will allow me to sample from the video feed and locate the corners live on the other half of my screen.


Here’s a potential solution using thresholding + morphological operations:

  1. Obtain binary image. We load the image, blur with cv2.bilateralFilter(), grayscale, then Otsu’s threshold

  2. Morphological operations. We perform a series of morphological open and close to smooth the image and remove noise

  3. Find distorted approximated mask. We find the bounding rectangle coordinates of the object with cv2.arcLength() and cv2.approxPolyDP() then draw this onto a mask

  4. Find corners. We use the Shi-Tomasi Corner Detector already implemented as cv2.goodFeaturesToTrack() for corner detection. Take a look at this for an explanation of each parameter

Here’s a visualization of each step:

Binary image -> Morphological operations -> Approximated mask -> Detected corners

Here are the corner coordinates:

(103, 550)
(1241, 536)

Here’s the result for the other images

(558, 949)
(558, 347)

Finally for the rotated image

(201, 99)
(619, 168)


import cv2
import numpy as np

# Load image, bilaterial blur, and Otsu's threshold
image = cv2.imread('1.png')
mask = np.zeros(image.shape, dtype=np.uint8)
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blur = cv2.bilateralFilter(gray,9,75,75)
thresh = cv2.threshold(blur, 0, 255, cv2.THRESH_BINARY_INV + cv2.THRESH_OTSU)[1]

# Perform morpholgical operations
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (10,10))
opening = cv2.morphologyEx(thresh, cv2.MORPH_OPEN, kernel, iterations=1)
close = cv2.morphologyEx(opening, cv2.MORPH_CLOSE, kernel, iterations=1)

# Find distorted rectangle contour and draw onto a mask
cnts = cv2.findContours(close, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]
rect = cv2.minAreaRect(cnts[0])
box = cv2.boxPoints(rect)
box = np.int0(box)
cv2.fillPoly(mask, [box], (255,255,255))

# Find corners
mask = cv2.cvtColor(mask, cv2.COLOR_BGR2GRAY)
corners = cv2.goodFeaturesToTrack(mask,4,.8,100)
offset = 25
for corner in corners:
    x,y = corner.ravel(),(x,y),5,(36,255,12),-1)
    x, y = int(x), int(y)
    cv2.rectangle(image, (x - offset, y - offset), (x + offset, y + offset), (36,255,12), 3)
    print("({}, {})".format(x,y))
cv2.imshow('image', image)
cv2.imshow('thresh', thresh)
cv2.imshow('close', close)
cv2.imshow('mask', mask)

Note: The idea for the distorted bounding box came from a previous answer in How to find accurate corner positions of a distorted rectangle from blurry image

Answered By – nathancy

Answer Checked By – Mildred Charles (AngularFixing Admin)

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